[Transcriber's Note: Figures 162-167 have been renumbered. In theoriginal, Figure 162 was labeled as 161; 163 as 162; etc. ]

A Practical Physiology

A Text-Book for Higher Schools

By

Albert F. Blaisdell, M. D. 

Author of "Child's Book of Health, " "How to Keep Well, ""Our Bodies and How We Live, " Etc. , Etc. 

Preface. 

The author has aimed to prepare a text-book on human physiology for use inhigher schools. The design of the book is to furnish a practical manual ofthe more important facts and principles of physiology and hygiene, whichwill be adapted to the needs of students in high schools, normal schools, and academies. 

Teachers know, and students soon learn to recognize the fact, that it isimpossible to obtain a clear understanding of the functions of the variousparts of the body without first mastering a few elementary facts abouttheir structure. The course adopted, therefore, in this book, is to devotea certain amount of space to the anatomy of the several organs beforedescribing their functions. 

A mere knowledge of the facts which can be gained in secondary schools, concerning the anatomy and physiology of the human body, is of little realvalue or interest in itself. Such facts are important and of practicalworth to young students only so far as to enable them to understand therelation of these facts to the great laws of health and to apply them todaily living. Hence, it has been the earnest effort of the author in thisbook, as in his other physiologies for schools, to lay special emphasisupon such points as bear upon personal health. 

Physiology cannot be learned as it should be by mere book study. Theresult will be meagre in comparison with the capabilities of the subject. The study of the text should always be supplemented by a series ofpractical experiments. Actual observations and actual experiments are asnecessary to illuminate the text and to illustrate important principles inphysiology as they are in botany, chemistry, or physics. Hence, assupplementary to the text proper, and throughout the several chapters, aseries of carefully arranged and practical experiments has been added. Forthe most part, they are simple and can be performed with inexpensive andeasily obtained apparatus. They are so arranged that some may be omittedand others added as circumstances may allow. 

If it becomes necessary to shorten the course in physiology, the varioussections printed in smaller type may be omitted or used for home study. 

The laws of most of the states now require in our public schools the studyof the effects of alcoholic drinks, tobacco, and other narcotics upon thebodily life. This book will be found to comply fully with all such laws. 

The author has aimed to embody in simple and concise language the latestand most trustworthy information which can be obtained from the standardauthorities on modern physiology, in regard to the several topics. 

In the preparation of this text-book the author has had the editorial helpof his esteemed friend, Dr. J. E. Sanborn, of Melrose, Mass. , and is alsoindebted to the courtesy of Thomas E. Major, of Boston, for assistance inrevising the proofs. 

Albert F. Blaisdell. 

Boston, August, 1897. 

Contents. 

Chapter I. IntroductionChapter II. The BonesChapter III. The MusclesChapter IV. Physical ExerciseChapter V. Food and DrinkChapter VI. DigestionChapter VII. The Blood and Its CirculationChapter VIII. RespirationChapter IX. The Skin and the KidneysChapter X. The Nervous SystemChapter XI. The Special SenseChapter XII. The Throat and the VoiceChapter XIII. Accidents and EmergenciesChapter XIV. In Sickness and in Health Care of the Sick-Room; Poisons and their Antidotes; Bacteria; Disinfectants; Management of Contagious Diseases. Chapter XV. Experimental Work in Physiology Practical Experiments; Use of the Microscope; Additional Experiments; Surface Anatomy and Landmarks. 

Glossary

Index

Chapter I. 

Introduction. 

1. The Study of Physiology. We are now to take up a new study, and ina field quite different from any we have thus far entered. Of all ourother studies, --mathematics, physics, history, language, --not one comeshome to us with such peculiar interest as does physiology, becausethis is the study of ourselves. 

Every thoughtful young person must have asked himself a hundred questionsabout the problems of human life: how it can be that the few articles ofour daily food--milk, bread, meats, and similar things--build up ourcomplex bodies, and by what strange magic they are transformed into hair, skin, teeth, bones, muscles, and blood. 

How is it that we can lift these curtains of our eyes and behold all thewonders of the world around us, then drop the lids, and though at noonday, are instantly in total darkness? How does the minute structure of the earreport to us with equal accuracy the thunder of the tempest, and the humof the passing bee? Why is breathing so essential to our life, and whycannot we stop breathing when we try? Where within us, and how, burns themysterious fire whose subtle heat warms us from the first breath ofinfancy till the last hour of life?

These and scores of similar questions it is the province of this deeplyinteresting study of physiology to answer. 

2. What Physiology should Teach us. The study of physiology is notonly interesting, but it is also extremely useful. Every reasonable personshould not only wish to acquire the knowledge how best to protect andpreserve his body, but should feel a certain profound respect for anorganism so wonderful and so perfect as his physical frame. For our bodiesare indeed not ourselves, but the frames that contain us, --the ships inwhich we, the real selves, are borne over the sea of life. He must beindeed a poor navigator who is not zealous to adorn and strengthen hisship, that it may escape the rocks of disease and premature decay, andthat the voyage of his life may be long, pleasant, and successful. 

But above these thoughts there rises another, --that in studying physiologywe are tracing the myriad lines of marvelous ingenuity and forethought, asthey appear at every glimpse of the work of the Divine Builder. Howeverclosely we study our bodily structure, we are, at our best, but imperfectobservers of the handiwork of Him who made us as we are. 

3. Distinctive Characters of Living Bodies. Even a very meagreknowledge of the structure and action of our bodies is enough to revealthe following distinctive characters: our bodies are continuallybreathing, that is, they take in oxygen from the surrounding air; theytake in certain substances known as food, similar to those composing thebody, which are capable through a process called oxidation, or throughother chemical changes, of setting free a certain amount of energy. 

Again, our bodies are continually making heat and giving it out tosurrounding objects, the production and the loss of heat being so adjustedthat the whole body is warm, that is, of a temperature higher than that ofsurrounding objects. Our bodies, also, move themselves, either one parton another, or the whole body from place to place. The motive power is notfrom the outside world, but the energy of their movements exists in thebodies themselves, influenced by changes in their surroundings. Finally, our bodies are continually getting rid of so-called waste matters, whichmay be considered products of the oxidation of the material used as food, or of the substances which make up the organism. 

4. The Main Problems of Physiology briefly Stated. We shall learn ina subsequent chapter that the living body is continually losing energy, but by means of food is continually restoring its substance andreplenishing its stock of energy. A great deal of energy thus stored up isutilized as mechanical work, the result of physical movements. We shalllearn later on that much of the energy which at last leaves the body asheat, exists for a time within the organism in other forms than heat, though eventually transformed into heat. Even a slight change in thesurroundings of the living body may rapidly, profoundly, and in specialways affect not only the amount, but the kind of energy set free. Thus themere touch of a hair may lead to such a discharge of energy, that a bodypreviously at rest may be suddenly thrown into violent convulsions. Thisis especially true in the case of tetanus, or lockjaw. 

The main problem we have to solve in the succeeding pages is to ascertainhow it is that our bodies can renew their substance and replenish theenergy which they are continually losing, and can, according to the natureof their surroundings, vary not only the amount, but the kind of energywhich they set free. 

5. Technical Terms Defined. All living organisms are studied usuallyfrom two points of view: first, as to their form and structure; second, asto the processes which go on within them. The science which treats of allliving organisms is called biology. It has naturally twodivisions, --morphology, which treats of the form and structure ofliving beings, and physiology, which investigates their functions, orthe special work done in their vital processes. 

The word anatomy, however, is usually employed instead of morphology. It is derived from two Greek words, and means the science of dissection. Human anatomy then deals with the form and structure of the humanbody, and describes how the different parts and organs are arranged, asrevealed by observation, by dissection, and by the microscope. 

Histology is that part of anatomy which treats of the minutestructure of any part of the body, as shown by the microscope. 

Human physiology describes the various processes that go on in thehuman body in health. It treats of the work done by the various parts ofthe body, and of the results of the harmonious action of the severalorgans. Broadly speaking, physiology is the science which treats offunctions. By the word function is meant the special work which anorgan has to do. An organ is a part of the body which does a specialwork. Thus the eye is the organ of sight, the stomach of digestion, andthe lungs of breathing. 

It is plain that we cannot understand the physiology of our bodies withouta knowledge of their anatomy. An engineer could not understand the workingof his engine unless well acquainted with all its parts, and the manner inwhich they were fitted together. So, if we are to understand theprinciples of elementary physiology, we must master the main anatomicalfacts concerning the organs of the body before considering their specialfunctions. 

As a branch of study in our schools, physiology aims to make clear certainlaws which are necessary to health, so that by a proper knowledge of them, and their practical application, we may hope to spend happier and moreuseful, because healthier, lives. In brief, the study of hygiene, orthe science of health, in the school curriculum, is usually associatedwith that of physiology. [1]

6. Chemical Elements in the Body. All of the various complexsubstances found in nature can be reduced by chemical analysis to about 70elements, which cannot be further divided. By various combinations ofthese 70 elements all the substances known to exist in the world of natureare built up. When the inanimate body, like any other substance, issubmitted to chemical analysis, it is found that the bone, muscle, teeth, blood, etc. , may be reduced to a few chemical elements. 

In fact, the human body is built up with 13 of the 70 elements, namely:oxygen, hydrogen, nitrogen, chlorine, fluorine, carbon, phosphorus, sulphur, calcium, potassium, sodium, magnesium, and iron. Besidesthese, a few of the other elements, as silicon, have been found; but theyexist in extremely minute quantities. 

The following table gives the proportion in which these various elementsare present:

 Oxygen 62. 430 per cent Carbon 21. 150 " " Hydrogen 9. 865 " " Nitrogen 3. 100 " " Calcium 1. 900 " " Phosphorus 0. 946 " " Potassium 0. 230 " " Sulphur 0. 162 " " Chlorine 0. 081 " " Sodium 0. 081 " " Magnesium 0. 027 " " Iron 0. 014 " " Fluorine 0. 014 " " ----- 100. 000

As will be seen from this table, oxygen, hydrogen, and nitrogen, which aregases in their uncombined form, make up 3/4 of the weight of the wholehuman body. Carbon, which exists in an impure state in charcoal, formsmore than 1/5 of the weight of the body. Thus carbon and the three gasesnamed, make up about 96 per cent of the total weight of the body. 

7. Chemical Compounds in the Body. We must keep in mind that, withslight exceptions, none of these 13 elements exist in their elementaryform in the animal economy. They are combined in various proportions, theresults differing widely from the elements of which they consist. Oxygenand hydrogen unite to form water, and water forms more than 2/3 of theweight of the whole body. In all the fluids of the body, water acts as asolvent, and by this means alone the circulation of nutrient material ispossible. All the various processes of secretion and nutrition depend onthe presence of water for their activities. 

8. Inorganic Salts. A large number of the elements of the body uniteone with another by chemical affinity and form inorganic salts. Thussodium and chlorine unite and form chloride of sodium, or common salt. This is found in all the tissues and fluids, and is one of the mostimportant inorganic salts the body contains. It is absolutely necessaryfor continued existence. By a combination of phosphorus with sodium, potassium, calcium, and magnesium, the various phosphates are formed. 

The phosphates of lime and soda are the most abundant of the salts of thebody. They form more than half the material of the bones, are found in theteeth and in other solids and in the fluids of the body. The special placeof iron is in the coloring matter of the blood. Its various salts aretraced in the ash of bones, in muscles, and in many other tissues andfluids. These compounds, forming salts or mineral matters that exist inthe body, are estimated to amount to about 6 per cent of the entireweight. 

9. Organic Compounds. Besides the inorganic materials, there existsin the human body a series of compound substances formed of the union ofthe elements just described, but which require the agency of livingstructures. They are built up from the elements by plants, and are calledorganic. Human beings and the lower animals take the organizedmaterials they require, and build them up in their own bodies into stillmore highly organized forms. 

The organic compounds found in the body are usually divided into threegreat classes:

 1. Proteids, or albuminous substances. 2. Carbohydrates (starches, sugars, and gums). 3. Fats. 

The extent to which these three great classes of organic materials of thebody exist in the animal and vegetable kingdoms, and are utilized for thefood of man, will be discussed in the chapter on food (Chapter V. ). TheProteids, because they contain the element nitrogen and the others donot, are frequently called nitrogenous, and the other two are knownas non-nitrogenous substances. The proteids, the type of which is eggalbumen, or the white of egg, are found in muscle and nerve, in glands, inblood, and in nearly all the fluids of the body. A human body is estimatedto yield on an average about 18 per cent of albuminous substances. In thesucceeding chapters we shall have occasion to refer to various and alliedforms of proteids as they exist in muscle (myosin), coagulated blood(fibrin), and bones (gelatin). 

The Carbohydrates are formed of carbon, hydrogen, and oxygen, thelast two in the proportion to form water. Thus we have animal starch, orglycogen, stored up in the liver. Sugar, as grape sugar, is also found inthe liver. The body of an average man contains about 10 per cent ofFats. These are formed of carbon, hydrogen, and oxygen, in which thelatter two are not in the proportion to form water. The fat of the bodyconsists of a mixture which is liquid at the ordinary temperature. 

Now it must not for one moment be supposed that the various chemicalelements, as the proteids, the salts, the fats, etc. , exist in the body ina condition to be easily separated one from another. Thus a piece ofmuscle contains all the various organic compounds just mentioned, but theyare combined, and in different cases the amount will vary. Again, fat mayexist in the muscles even though it is not visible to the naked eye, and amicroscope is required to show the minute fat cells. 

10. Protoplasm. The ultimate elements of which the body is composedconsist of "masses of living matter, " microscopic in size, of a materialcommonly called protoplasm. [2] In its simplest form protoplasmappears to be a homogeneous, structureless material, somewhat resemblingthe raw white of an egg. It is a mixture of several chemical substancesand differs in appearance and composition in different parts of the body. 

Protoplasm has the power of appropriating nutrient material, of dividingand subdividing, so as to form new masses like itself. When not built intoa tissue, it has the power of changing its shape and of moving from placeto place, by means of the delicate processes which it puts forth. Now, while there are found in the lowest realm of animal life, organisms likethe amoeba of stagnant pools, consisting of nothing more than minutemasses of protoplasm, there are others like them which possess a smallcentral body called a nucleus. This is known as nucleated protoplasm. 

[Illustration: Fig. 1. --Diagram of a Cell. 

 A, nucleus; B, nucleolus; C, protoplasm. (Highly magnified)]

11. Cells. When we carry back the analysis of an organized body asfar as we can, we find every part of it made up of masses of nucleatedprotoplasm of various sizes and shapes. In all essential features thesemasses conform to the type of protoplasmic matter just described. Suchbodies are called cells. In many cells the nucleus is finely granular orreticulated in appearance, and on the threads of the meshwork may be oneor more enlargements, called nucleoli. In some cases the protoplasm at thecircumference is so modified as to give the appearance of a limitingmembrane called the cell wall. In brief, then, a cell is a mass ofnucleated protoplasm; the nucleus may have a nucleolus, and the cellmay be limited by a cell wall. Every tissue of the human body is formedthrough the agency of protoplasmic cells, although in most cases thechanges they undergo are so great that little evidence remains of theirexistence. 

There are some organisms lower down in the scale, whose whole activity isconfined within the narrow limits of a single cell. Thus, the amoebabegins its life as a cell split off from its parent. This divides in itsturn, and each half is a complete amoeba. When we come a little higherthan the amoeba, we find organisms which consist of several cells, and aspecialization of function begins to appear. As we ascend in the animalscale, specialization of structure and of function is found continuallyadvancing, and the various kinds of cells are grouped together intocolonies or organs. 

12. Cells and the Human Organism. If the body be studied in itsdevelopment, it is found to originate from a single mass of nucleatedprotoplasm, a single cell with a nucleus and nucleolus. From thisoriginal cell, by growth and development, the body, with all its varioustissues, is built up. Many fully formed organs, like the liver, consistchiefly of cells. Again, the cells are modified to form fibers, such astendon, muscle, and nerve. Later on, we shall see the white bloodcorpuscles exhibit all the characters of the amoeba (Fig. 2). Even suchdense structures as bone, cartilage, and the teeth are formed from cells. 

[Illustration: Fig. 2. --Amoeboid Movement of a Human White BloodCorpuscle. (Showing various phases of movement. )]

In short, cells may be regarded as the histological units of animalstructures; by the combination, association, and modification of thesethe body is built up. Of the real nature of the changes going on withinthe living protoplasm, the process of building up lifeless material intoliving structures, and the process of breaking down by which waste isproduced, we know absolutely nothing. Could we learn that, perhaps weshould know the secret of life. 

13. Kinds of Cells. Cells vary greatly in size, some of the smallestbeing only 1/3500 an inch or less in diameter. They also vary greatly inform, as may be seen in Figs. 3 and 5. The typical cell is usually_globular_ in form, other shapes being the result of pressure or ofsimilar modifying influences. The globular, as well as the large, flatcells, are well shown in a drop of saliva. Then there are the _columnar_cells, found in various parts of the intestines, in which they are closelyarranged side by side. These cells sometimes have on the free surfacedelicate prolongations called cilia. Under the microscope they resemble awave, as when the wind blows over a field of grain (Fig. 5). There arebesides cells known as _spindle, stellate, squamous_ or pavement, andvarious other names suggested by their shapes. Cells are also described asto their contents. Thus _fat_ and _pigment_ cells are alluded to insucceeding sections. Again, they may be described as to their functions orlocation or the tissue in which they are found, as _epithelial_ cells, _blood_ cells (corpuscles, Figs. 2 and 66), _nerve_ cells (Fig. 4), and_connective-tissue_ cells. 

14. Vital Properties of Cells. Each cell has a life of its own. Itmanifests its vital properties in that it is born, grows, multiplies, decays, and at last dies. [3] During its life it assimilates food, works, rests, and is capable of spontaneous motion and frequently of locomotion. The cell can secrete and excrete substance, and, in brief, presents nearlyall the phenomena of a human being. 

Cells are produced only from cells by a process of self-division, consisting of a cleavage of the whole cell into parts, each of whichbecomes a separate and independent organism. Cells rapidly increase insize up to a certain definite point which they maintain during adult life. A most interesting quality of cell life is motion, a beautiful form ofwhich is found in ciliated epithelium. Cells may move actively andpassively. In the blood the cells are swept along by the current, but thewhite corpuscles, seem able to make their way actively through thetissues, as if guided by some sort of instinct. 

[Illustration: Fig. 3. --Various Forms of Cells. 

 A, columnar cells found lining various parts of the intestines (called _columnar epithelium_); B, cells of a fusiform or spindle shape found in the loose tissue under the skin and in other parts (called _connective-tissue cells_); C, cell having many processes or projections--such are found in connective tissue, D, primitive cells composed of protoplasm with nucleus, and having no cell wall. All are represented about 400 times their real size. ]

Some cells live a brief life of 12 to 24 hours, as is probably the casewith many of the cells lining the alimentary canal; others may live foryears, as do the cells of cartilage and bone. In fact each cell goesthrough the same cycle of changes as the whole organism, though doubtlessin a much shorter time. The work of cells is of the most varied kind, andembraces the formation of every tissue and product, --solid, liquid, orgaseous. Thus we shall learn that the cells of the liver form bile, thoseof the salivary glands and of the glands of the stomach and pancreas formjuices which aid in the digestion of food. 

15. The Process of Life. All living structures are subject toconstant decay. Life is a condition of incessant changes, dependent upontwo opposite processes, repair and decay. Thus our bodies are notcomposed of exactly the same particles from day to day, or even from onemoment to another, although to all appearance we remain the sameindividuals. The change is so gradual, and the renewal of that which islost may be so exact, that no difference can be noticed except at longintervals of time. [4] (See under "Bacteria, " Chapter XIV. )

The entire series of chemical changes that take place in the living body, beginning with assimilation and ending with excretion, is included in oneword, metabolism. The process of building up living material, or thechange by which complex substances (including the living matter itself)are built up from simpler materials, is called anabolism. Thebreaking down of material into simple products, or the changes in whichcomplex materials (including the living substance) are broken down intocomparatively simple products, is known as katabolism. This reductionof complex substances to simple, results in the production of animal forceand energy. Thus a complex substance, like a piece of beef-steak, is builtup of a large number of molecules which required the expenditure of forceor energy to store up. Now when this material is reduced by the process ofdigestion to simpler bodies with fewer molecules, such as carbon dioxid, urea, and water, the force stored up in the meat as potential energybecomes manifest and is used as active life-force known as _kineticenergy_. 

16. Epithelium. Cells are associated and combined in many ways toform a simple tissue. Such a simple tissue is called an epithelium orsurface-limiting tissue, and the cells are known as epithelialcells. These are united by a very small amount of a cement substance whichbelongs to the proteid class of material. The epithelial cells, from theirshape, are known as squamous, columnar, glandular, or ciliated. Again, thecells may be arranged in only a single layer, or they may be severallayers deep. In the former case the epithelium is said to be simple; inthe latter, stratified. No blood-vessels pass into these tissues; thecells derive their nourishment by the imbibition of the plasma of theblood exuded into the subjacent tissue. 

[Illustration: Fig. 4. --Nerve Cells from the Gray Matter of theCerebellum. (Magnified 260 diameters. )]

17. Varieties of Epithelium. The squamous or pavement epitheliumconsists of very thin, flattened scales, usually with a small nucleus inthe center. When the nucleus has disappeared, they become mere hornyplates, easily detached. Such cells will be described as forming the outerlayer of the skin, the lining of the mouth and the lower part of thenostrils. 

The columnar epithelium consists of pear-shaped or elongated cells, frequently as a single layer of cells on the surface of a mucous membrane, as on the lining of the stomach and intestines, and the free surface ofthe windpipe and large air-tubes. 

The glandular or spheroidal epithelium is composed of round cells orsuch as become angular by mutual pressure. This kind forms the lining ofglands such as the liver, pancreas, and the glands of the skin. 

The ciliated epithelium is marked by the presence of very finehair-like processes called cilia, which develop from the free end of thecell and exhibit a rapid whip-like movement as long as the cell is alive. This motion is always in the same direction, and serves to carry awaymucus and even foreign particles in contact with the membrane on whichthe cells are placed. This epithelium is especially common in the airpassages, where it serves to keep a free passage for the entrance and exitof air. In other canals a similar office is filled by this kind ofepithelium. 

18. Functions of Epithelial Tissues. The epithelial structures may bedivided, as to their functions, into two main divisions. One is chieflyprotective in character. Thus the layers of epithelium which form thesuperficial layer of the skin have little beyond such an office todischarge. The same is to a certain extent true of the epithelial cellscovering the mucous membrane of the mouth, and those lining the airpassages and air cells of the lungs. 

[Illustration: Fig. 5. --Various Kinds of Epithelial Cells

 A, columnar cells of intestine; B, polyhedral cells of the conjunctiva; C, ciliated conical cells of the trachea; D, ciliated cell of frog's mouth; E, inverted conical cell of trachea; F, squamous cell of the cavity of mouth, seen from its broad surface; G, squamous cell, seen edgeways. ]

The second great division of the epithelial tissues consists of thosewhose cells are formed of highly active protoplasm, and are busily engagedin some sort of secretion. Such are the cells of glands, --the cells of thesalivary glands, which secrete the saliva, of the gastric glands, whichsecrete the gastric juice, of the intestinal glands, and the cells of theliver and sweat glands. 

19. Connective Tissue. This is the material, made up of fibers andcells, which serves to unite and bind together the different organs andtissues. It forms a sort of flexible framework of the body, and sopervades every portion that if all the other tissues were removed, weshould still have a complete representation of the bodily shape in everypart. In general, the connective tissues proper act as packing, binding, and supporting structures. This name includes certain tissueswhich to all outward appearance vary greatly, but which are properlygrouped together for the following reasons: first, they all act assupporting structures; second, under certain conditions one may besubstituted for another; third, in some places they merge into each other. 

All these tissues consist of a ground-substance, or matrix, cells, andfibers. The ground-substance is in small amount in connective tissuesproper, and is obscured by a mass of fibers. It is best seen in hyalinecartilage, where it has a glossy appearance. In bone it is infiltratedwith salts which give bone its hardness, and make it seem so unlike othertissues. The cells are called connective-tissue corpuscles, cartilagecells, and bone corpuscles, according to the tissues in which they occur. The fibers are the white fibrous and the yellow elastic tissues. 

The following varieties are usually described:

 I. Connective Tissues Proper:

 1. White Fibrous Tissue. 2. Yellow Elastic Tissue. 3. Areolar or Cellular Tissue. 4. Adipose or Fatty Tissue. 5. Adenoid or Retiform Tissue. 

 II. Cartilage (Gristle):

 1. Hyaline. 2. White Fibro-cartilage. 3. Yellow Fibro-cartilage. 

 III. Bone and Dentine of Teeth. 

20. White Fibrous Tissue. This tissue consists of bundles of verydelicate fibrils bound together by a small amount of cement substance. Between the fibrils protoplasmic masses (connective-tissue corpuscles)are found. These fibers may be found so interwoven as to form a sheet, asin the periosteum of the bone, the fasciæ around muscles, and the capsulesof organs; or they may be aggregated into bundles and form rope-likebands, as in the ligaments of joints and the tendons of muscles. Onboiling, this tissue yields gelatine. In general, where white fibroustissue abounds, structures are held together, and there is flexibility, but little or no distensibility. 

[Illustration: Fig. 6. --White Fibrous Tissue. (Highly magnified. )]

21. Yellow Elastic Tissue. The fibers of yellow elastic tissueare much stronger and coarser than those of the white. They are yellowish, tend to curl up at the ends, and are highly elastic. It is these fiberswhich give elasticity to the skin and to the coats of the arteries. Thetypical form of this tissue occurs in the ligaments which bind thevertebræ together (Fig. 26), in the true vocal cords, and in certainligaments of the larynx. In the skin and fasciæ, the yellow elastic isfound mixed with white fibrous and areolar tissues. It does not yieldgelatine on boiling, and the cells are, if any, few. 

[Illustration: Fig. 7. --Yellow Elastic Tissue. (Highly magnified. )]

22. Areolar or Cellular Tissue. This consists of bundles of delicatefibers interlacing and crossing one another, forming irregular spaces ormeshes. These little spaces, in health, are filled with fluid that hasoozed out of the blood-vessels. The areolar tissue forms a protectivecovering for the tissues of delicate and important organs. 

23. Adipose or Fatty Tissue. In almost every part of the body theordinary areolar tissue contains a variable quantity of adipose orfatty tissue. Examined by the microscope, the fat cells consist of anumber of minute sacs of exceedingly delicate, structureless membranefilled with oil. This is liquid in life, but becomes solidified afterdeath. This tissue is plentiful beneath the skin, in the abdominal cavity, on the surface of the heart, around the kidneys, in the marrow of bones, and elsewhere. Fat serves as a soft packing material. Being a poorconductor, it retains the heat, and furnishes a store rich in carbon andhydrogen for use in the body. 

24. Adenoid or Retiform Tissue. This is a variety of connectivetissue found in the tonsils, spleen, lymphatic glands, and alliedstructures. It consists of a very fine network of cells of various sizes. The tissue combining them is known as adenoid or gland-like tissue. 

[Illustration: Fig. 8. --Fibro-Cartilage Fibers. (Showing networksurrounded cartilage cells. )]

25. Cartilage. Cartilage, or gristle, is a tough but highly elasticsubstance. Under the microscope cartilage is seen to consist of amatrix, or base, in which nucleated cells abound, either singly or ingroups. It has sometimes a fine ground-glass appearance, when thecartilage is spoken of as hyaline. In other cases the matrix isalmost replaced by white fibrous tissue. This is called whitefibro-cartilage, and is found where great strength and a certainamount of rigidity are required. 

Again, there is between the cells a meshwork of yellow elastic fibers, andthis is called yellow fibro-cartilage (Fig. 8). The hyaline cartilageforms the early state of most of the bones, and is also a permanentcoating for the articular ends of long bones. The white fibro-cartilage isfound in the disks between the bodies of the vertebræ, in the interior ofthe knee joint, in the wrist and other joints, filling the cavities of thebones, in socket joints, and in the grooves for tendons. The yellowfibro-cartilage forms the expanded part of the ear, the epiglottis, andother parts of the larynx. 

26. General Plan of the Body. To get a clearer idea of the generalplan on which the body is constructed, let us imagine its division intoperfectly equal parts, one the right and the other the left, by a greatknife severing it through the median, or middle line in front, backwardthrough the spinal column, as a butcher divides an ox or a sheep intohalves for the market. In a section of the body thus planned the skull andthe spine together are shown to have formed a tube, containing the brainand spinal cord. The other parts of the body form a second tube (ventral)in front of the spinal or dorsal tube. The upper part of the second tubebegins with the mouth and is formed by the ribs and breastbone. Below thechest in the abdomen, the walls of this tube would be made up of the softparts. 

[Illustration: Fig. 9. --Diagrammatic Longitudinal Section of the Trunk andHead. (Showing the dorsal and the ventral tubes. )

 A, the cranial cavity; B, the cavity of the nose; C, the mouth; D, the alimentary canal represented as a simple straight tube; E, the sympathetic nervous system; F, heart; G, diaphragm; H, stomach; K, end of spinal portion of cerebro-spinal nervous system. ]

We may say, then, that the body consists of two tubes or cavities, separated by a bony wall, the dorsal or nervous tube, so calledbecause it contains the central parts of the nervous system; and thevisceral or ventral tube, as it contains the viscera, or generalorgans of the body, as the alimentary canal, the heart, the lungs, thesympathetic nervous system, and other organs. 

The more detailed study of the body may now be begun by a description ofthe skeleton or framework which supports the soft parts. 

Experiments. 

For general directions and explanations and also detailed suggestions forperforming experiments, see Chapter XV. 

 Experiment 1. _To examine squamous epithelium. _ With an ivory paper-knife scrape the back of the tongue or the inside of the lips or cheek; place the substance thus obtained upon a glass slide; cover it with a thin cover-glass, and if necessary add a drop of water. Examine with the microscope, and the irregularly formed epithelial cells will be seen. 

 Experiment 2. _To examine ciliated epithelium. _ Open a frog's mouth, and with the back of a knife blade gently scrape a little of the membrane from the roof of the mouth. Transfer to a glass slide, add a drop of salt solution, and place over it a cover-glass with a hair underneath to prevent pressure upon the cells. Examine with a microscope under a high power. The cilia move very rapidly when quite fresh, and are therefore not easily seen. 

For additional experiments which pertain to the microscopic examination ofthe elementary tissues and to other points in practical histology, seeChapter XV. 

 [NOTE. Inasmuch as most of the experimental work of this chapter depends upon the use of the microscope and also necessarily assumes a knowledge of facts which are discussed later, it would be well to postpone experiments in histology until they can be more satisfactorily handled in connection with kindred topics as they are met with in the succeeding chapters. ]

Chapter II. 

The Bones. 

27. The Skeleton. Most animals have some kind of framework to supportand protect the soft and fleshy parts of their bodies. This frameworkconsists chiefly of a large number of bones, and is called theskeleton. It is like the keel and ribs of a vessel or the frame of ahouse, the foundation upon which the bodies are securely built. 

There are in the adult human body 200 distinct bones, of many sizes andshapes. This number does not, however, include several small bones foundin the tendons of muscles and in the ear. The teeth are not usuallyreckoned as separate bones, being a part of the structure of the skin. 

The number of distinct bones varies at different periods of life. It isgreater in childhood than in adults, for many bones which are thenseparate, to allow growth, afterwards become gradually united. In earlyadult life, for instance, the skull contains 22 naturally separate bones, but in infancy the number is much greater, and in old age far less. 

The bones of the body thus arranged give firmness, strength, andprotection to the soft tissues and vital organs, and also form levers forthe muscles to act upon. 

28. Chemical Composition of Bone. The bones, thus forming theframework of the body, are hard, tough, and elastic. They are twice asstrong as oak; one cubic inch of compact bone will support a weight of5000 pounds. Bone is composed of earthy or mineral matter(chiefly in the form of lime salts), and of animal matter(principally gelatine), in the proportion of two-thirds of the former toone-third of the latter. 

[Illustration: Fig. 10. --The Skeleton. ]

The proportion of earthy to animal matter varies with age. In infancy thebones are composed almost wholly of animal matter. Hence, an infant'sbones are rarely broken, but its legs may soon become misshapen if walkingis allowed too early. In childhood, the bones still contain a largerpercentage of animal matter than in more advanced life, and are thereforemore liable to bend than to break; while in old age, they contain agreater percentage of mineral matter, and are brittle and easily broken. 

 Experiment 3. _To show the mineral matter in bone_. Weigh a large soup bone; put it on a hot, clear fire until it is at a red heat. At first it becomes black from the carbon of its organic matter, but at last it turns white. Let it cool and weigh again. The animal matter has been burnt out, leaving the mineral or earthy part, a white, brittle substance of exactly the same shape, but weighing only about two-thirds as much as the bone originally weighed. 

 Experiment 4. _To show the animal matter in bone_. Add a teaspoonful of muriatic acid to a pint of water, and place the mixture in a shallow earthen dish. Scrape and clean a chicken's leg bone, part of a sheep's rib, or any other small, thin bone. Soak the bone in the acid mixture for a few days. The earthy or mineral matter is slowly dissolved, and the bone, although retaining its original form, loses its rigidity, and becomes pliable, and so soft as to be readily cut. If the experiment be carefully performed, a long, thin bone may even be tied into a knot. 

 [Illustration: Fig. 11. --The fibula tied into a knot, after the hard mineral matter has been dissolved by acid. ]

29. Physical Properties of Bone. If we take a leg bone of a sheep, ora large end of beef shin bone, and saw it lengthwise in halves, we see twodistinct structures. There is a hard and compact tissue, like ivory, forming the outside shell, and a spongy tissue inside having theappearance of a beautiful lattice work. Hence this is called cancelloustissue, and the gradual transition from one to the other is apparent. 

It will also be seen that the shaft is a hollow cylinder, formed ofcompact tissue, enclosing a cavity called the medullary canal, which isfilled with a pulpy, yellow fat called _marrow_. The marrow is richlysupplied with blood-vessels, which enter the cavity through small openingsin the compact tissue. In fact, all over the surface of bone are minutecanals leading into the substance. One of these, especially constant andlarge in many bones, is called the _nutrient foramen_, and transmits anartery to nourish the bone. 

At the ends of a long bone, where it expands, there is no medullary canal, and the bony tissue is spongy, with only a thin layer of dense bone aroundit. In flat bones we find two layers or plates of compact tissue at thesurface, and a spongy tissue between. Short and irregular bones have nomedullary canal, only a thin shell of dense bone filled with cancelloustissue. 

[Illustration: Fig 12. --The Right femur sawed in two, lengthwise. (Showingarrangement of compact and cancellous tissue. )]

 Experiment 5. Obtain a part of a beef shin bone, or a portion of a sheep's or calf's leg, including if convenient the knee joint. Have the bone sawed in two, lengthwise, keeping the marrow in place. Boil, scrape, and carefully clean one half. Note the compact and spongy parts, shaft, etc. 

 Experiment 6. Trim off the flesh from the second half. Note the pinkish white appearance of the bone, the marrow, and the tiny specks of blood, etc. Knead a small piece of the marrow in the palm; note the oily appearance. Convert some marrow into a liquid by heating. Contrast this fresh bone with an old dry one, as found in the fields. Fresh bones should be kept in a cool place, carefully wrapped in a damp cloth, while waiting for class use. 

A fresh or living bone is covered with a delicate, tough, fibrousmembrane, called the periosteum. It adheres very closely to the bone, and covers every part except at the joints and where it is protected withcartilage. The periosteum is richly supplied with blood-vessels, and playsa chief part in the growth, formation, and repair of bone. If a portion ofthe periosteum be detached by injury or disease, there is risk that alayer of the subjacent bone will lose its vitality and be cast off. [5]

30. Microscopic Structure of Bone. If a very thin slice of bone becut from the compact tissue and examined under a microscope, numerousminute openings are seen. Around these are arranged rings of bone, withlittle black bodies in them, from which radiate fine, dark lines. Theseopenings are sections of canals called _Haversian canals_, after Havers, an English physician, who first discovered them. The black bodies areminute cavities called _lacunæ_, while the fine lines are very minutecanals, _canaliculi_, which connect the lacunæ and the Haversian canals. These Haversian canals are supplied with tiny blood-vessels, while thelacunæ contain bone cells. Very fine branches from these cells pass intothe canaliculi. The Haversian canals run lengthwise of the bone; hence ifthe bone be divided longitudinally these canals will be opened along theirlength (Fig. 13). 

Thus bones are not dry, lifeless substances, but are the very type ofactivity and change. In life they are richly supplied with blood from thenutrient artery and from the periosteum, by an endless network ofnourishing canals throughout their whole structure. Bone has, therefore, like all other living structures, a _self-formative_ power, and draws fromthe blood the materials for its own nutrition. 

[Illustration: Fig. 13. 

 A, longitudinal section of bone, by which the Haversian canals are seen branching and communicating with one another; B, cross section of a very thin slice of bone, magnified about 300 diameters--little openings (Haversian canals) are seen, and around them are ranged rings of bones with little black bodies (lacunæ), from which branch out fine dark lines (canaliculi); C, a bone cell, highly magnified, lying in lacuna. ]

The Bones of the Head. 

31. The Head, or Skull. The bones of the skeleton, the bony frameworkof our bodies, may be divided into those of the head, the trunk, and the limbs. 

The bones of the head are described in two parts, --those of thecranium, or brain-case, and those of the face. Taken together, they form the skull. The head is usually said to contain 22 bones, ofwhich 8 belong to the cranium and 14 to the face. In early childhood, thebones of the head are separate to allow the brain to expand; but as wegrow older they gradually unite, the better to protect the delicate braintissue. 

32. The Cranium. The cranium is a dome-like structure, made upin the adult of 8 distinct bones firmly locked together. These bones are:

 One Frontal, Two Parietal, Two Temporal One Occipital, One Sphenoid, One Ethmoid. 

The frontal bone forms the forehead and front of the head. It isunited with the two parietal bones behind, and extends over the foreheadto make the roofs of the sockets of the eyes. It is this bone which, inmany races of man, gives a dignity of person and a beauty of form seen inno other animal. 

The parietal bones form the sides and roof of the skull. They arebounded anteriorly by the frontal bone, posteriorly by the occipital, andlaterally by the temporal and sphenoid bones. The two bones make abeautiful arch to aid in the protection of the brain. 

The temporal bones, forming the temples on either side, are attachedto the sphenoid bone in front, the parietals above, and the occipitalbehind. In each temporal bone is the cavity containing the organs ofhearing. These bones are so called because the hair usually first turnsgray over them. 

The occipital bone forms the lower part of the base of the skull, aswell as the back of the head. It is a broad, curved bone, and rests on thetopmost vertebra (atlas) of the backbone; its lower part is pierced by alarge oval opening called the _foramen magnum_, through which the spinalcord passes from the brain (Fig. 15). 

The sphenoid bone is in front of the occipital, forming a part of thebase of the skull. It is wedged between the bones of the face and those ofthe cranium, and locks together fourteen different bones. It bears aremarkable resemblance to a bat with extended wings, and forms a series ofgirders to the arches of the cranium. 

The ethmoid bone is situated between the bones of the cranium andthose of the face, just at the root of the nose. It forms a part of thefloor of the cranium. It is a delicate, spongy bone, and is so calledbecause it is perforated with numerous holes like a sieve, through whichthe nerves of smell pass from the brain to the nose. 

[Illustration: Fig. 14. --The Skull]

33. The Face. The bones of the face serve, to a marked extent, ingiving form and expression to the human countenance. Upon these bonesdepend, in a measure, the build of the forehead, the shape of the chin, the size of the eyes, the prominence of the cheeks, the contour of thenose, and other marks which are reflected in the beauty or ugliness of theface. 

The face is made up of fourteen bones which, with the exception ofthe lower jaw, are, like those of the cranium, closely interlocked witheach other. By this union these bones help form a number of cavities whichcontain most important and vital organs. The two deep, cup-like sockets, called the orbits, contain the organs of sight. In the cavities of thenose is located the sense of smell, while the buccal cavity, or mouth, isthe site of the sense of taste, and plays besides an important part in thefirst act of digestion and in the function of speech. 

The bones of the face are:

 Two Superior Maxillary, Two Malar, Two Nasal, Two Lachrymal, Two Palate, Two Turbinated, One Vomer, One Lower Maxillary. 

34. Bones of the Face. The superior maxillary or upper jawbonesform a part of the roof of the mouth and the entire floor of the orbits. In them is fixed the upper set of teeth. 

The malar or cheek bones are joined to the upper jawbones, and helpform the sockets of the eyes. They send an arch backwards to join thetemporal bones. These bones are remarkably thick and strong, and arespecially adapted to resist the injury to which this part of the face isexposed. 

The nasal or nose bones are two very small bones between the eyesockets, which form the bridge of the nose. Very near these bones are thetwo small lachrymal bones. These are placed in the inner angles ofthe orbit, and in them are grooves in which lie the ducts through whichthe tears flow from the eyes to the nose. 

The palate bones are behind those of the upper jaw and with them formthe bony part of the roof of the mouth. The inferior turbinated arespongy, scroll-like bones, which curve about within the nasal cavities soas to increase the surface of the air passages of the nose. 

The vomer serves as a thin and delicate partition between the two cavitiesof the nose. It is so named from its resemblance to a ploughshare. 

[Illustration: Fig. 15. --The Base of the Skull. 

 A, palate process of upper jawbone; B, zygoma, forming zygomatic arch; C, condyle for forming articulation with atlas; D, foramen magnum; E, occipital bone. ]

The longest bone in the face is the inferior maxillary, or lower jaw. It has a horseshoe shape, and supports the lower set of teeth. It is theonly movable bone of the head, having a vertical and lateral motion bymeans of a hinge joint with a part of the temporal bone. 

35. Sutures of the Skull. Before leaving the head we must notice thepeculiar and admirable manner in which the edges of the bones of the outershell of the skull are joined together. These edges of the bones resemblethe teeth of a saw. In adult life these tooth-like edges fit into eachother and grow together, suggesting the dovetailed joints used by thecabinet-maker. When united these serrated edges look almost as if sewedtogether; hence their name, sutures. This manner of union gives unityand strength to the skull. 

In infants, the corners of the parietal bones do not yet meet, and thethrobbing of the brain may be seen and felt under these "soft spots, " or_fontanelles_, as they are called. Hence a slight blow to a babe's headmay cause serious injury to the brain (Fig. 14). 

The Bones of the Trunk. 

36. The Trunk. The trunk is that central part of the body whichsupports the head and the upper pair of limbs. It divides itself into anupper cavity, the thorax, or chest; and a lower cavity, theabdomen. These two cavities are separated by a movable, muscularpartition called the diaphragm, or midriff (Figs. 9 and 49). 

The bones of the trunk are variously related to each other, and some ofthem become united during adult life into bony masses which at earlierperiods are quite distinct. For example, the sacrum is in early life madeup of five distinct bones which later unite into one. 

The upper cavity, or chest, is a bony enclosure formed by thebreastbone, the ribs, and the spine. It contains the heart and the lungs(Fig. 86). 

The lower cavity, or abdomen, holds the stomach, liver, intestines, spleen, kidneys, and some other organs (Fig. 59). 

The bones of the trunk may be subdivided into those of the spine, theribs, and the hips. 

The trunk includes 54 bones usually thus arranged:

 I. Spinal Column, 26 bones: 7 Cervical Vertebræ. 12 Dorsal Vertebræ. 5 Lumbar Vertebræ. 1 Sacrum. 1 Coccyx. 

 II. Ribs, 24 bones: 14 True Ribs. 6 False Ribs. 4 Floating Ribs. 

 III. Sternum. 

 IV. Two Hip Bones. 

 V. Hyoid Bone. 

37. The Spinal Column. The spinal column, or backbone, is amarvelous piece of mechanism, combining offices which nothing short ofperfection in adaptation and arrangement could enable it to perform. It isthe central structure to which all the other parts of the skeleton areadapted. It consists of numerous separate bones, called vertebræ. Theseven upper ones belong to the neck, and are called cervicalvertebræ. The next twelve are the dorsal vertebræ; these belong tothe back and support the ribs. The remaining five belong to the loins, andare called lumbar vertebræ. On looking at the diagram of the backbone(Fig. 9) it will be seen that the vertebræ increase in size and strengthdownward, because of the greater burden they have to bear, thus clearlyindicating that an erect position is the one natural to man. 

[Illustration: Fig. 16. --The Spinal Column. ]

This column supports the head, encloses and protects the spinal cord, andforms the basis for the attachment of many muscles, especially those whichmaintain the body in an erect position. Each vertebra has an openingthrough its center, and the separate bones so rest, one upon another, thatthese openings form a continuous canal from the head to the lower part ofthe spine. The great nerve, known as the spinal cord, extends fromthe cranium through the entire length of this canal. All along the spinalcolumn, and between each two adjoining bones, are openings on each side, through which nerves pass out to be distributed to various parts of thebody. 

Between the vertebræ are pads or cushions of cartilage. These act as"buffers, " and serve to give the spine strength and elasticity and toprevent friction of one bone on another. Each vertebra consists of a body, the solid central portion, and a number of projections called processes. Those which spring from the posterior of each arch are the spinousprocesses. In the dorsal region they are plainly seen and felt in thinpersons. 

The bones of the spinal column are arranged in three slight and gracefulcurves. These curves not only give beauty and strength to the bonyframework of the body, but also assist in the formation of cavities forimportant internal organs. This arrangement of elastic pads between thevertebræ supplies the spine with so many elastic springs, which serve tobreak the effect of shock to the brain and the spinal cord from any suddenjar or injury. 

The spinal column rests on a strong three-sided bone called thesacrum, or sacred-bone, which is wedged in between the hip bones andforms the keystone of the pelvis. Joined to the lower end of the sacrum isthe coccyx, or cuckoo-bone, a tapering series of little bones. 

 Experiment 7. Run the tips of the fingers briskly down the backbone, and the spines of the vertebræ will be tipped with red so that they can be readily counted. Have the model lean forward with the arms folded across the chest; this will make the spines of the vertebræ more prominent. 

 Experiment 8. _To illustrate the movement of torsion in the spine, or its rotation round its own axis_. Sit upright, with the back and shoulders well applied against the back of a chair. Note that the head and neck can be turned as far as 60 degrees or 70 degrees. Now bend forwards, so as to let the dorsal and lumbar vertebræ come into play, and the head can be turned 30 degrees more. 

 Experiment 9. _To show how the spinal vertebræ make a firm but flexible column. _ Take 24 hard rubber overcoat buttons, or the same number of two-cent pieces, and pile them on top of each other. A thin layer of soft putty may be put between the coins to represent the pads of cartilage between the vertebræ. The most striking features of the spinal column may be illustrated by this simple apparatus. 

38. How the Head and Spine are Joined together. The head rests uponthe spinal column in a manner worthy of special notice. This consists inthe peculiar structure of the first two cervical vertebræ, known as theaxis and atlas. The atlas is named after the fabled giant whosupported the earth on his shoulders. This vertebra consists of a ring ofbone, having two cup-like sockets into which fit two bony projectionsarising on either side of the great opening (_foramen magnum_) in theoccipital bone. The hinge joint thus formed allows the head to nodforward, while ligaments prevent it from moving too far. 

On the upper surface of the axis, the second vertebra, is a peg orprocess, called the _odontoid process_ from its resemblance to a tooth. This peg forms a pivot upon which the head with the atlas turns. It isheld in its place against the front inner surface of the atlas by a bandof strong ligaments, which also prevents it from pressing on the delicatespinal cord. Thus, when we turn the head to the right or left, the skulland the atlas move together, both rotating on the odontoid process of theaxis. 

39. The Ribs and Sternum. The barrel-shaped framework of the chest isin part composed of long, slender, curved bones called ribs. Thereare twelve ribs on each side, which enclose and strengthen the chest; theysomewhat resemble the hoops of a barrel. They are connected in pairs withthe dorsal vertebræ behind. 

The first seven pairs, counting from the neck, are called the _true_ ribs, and are joined by their own special cartilages directly to the breastbone. The five lower pairs, called the _false_ ribs, are not directly joined tothe breastbone, but are connected, with the exception of the last two, with each other and with the last true ribs by cartilages. These elasticcartilages enable the chest to bear great blows with impunity. A blow onthe sternum is distributed over fourteen elastic arches. The lowest twopairs of false ribs, are not joined even by cartilages, but are quite freein front, and for this reason are called _floating_ ribs. 

The ribs are not horizontal, but slope downwards from the backbone, sothat when raised or depressed by the strong intercostal muscles, the sizeof the chest is alternately increased or diminished. This movement of theribs is of the utmost importance in breathing (Fig. 91). 

The sternum, or breastbone, is a long, flat, narrow bone forming themiddle front wall of the chest. It is connected with the ribs and with thecollar bones. In shape it somewhat resembles an ancient dagger. 

40. The Hip Bones. Four immovable bones are joined together so as toform at the lower extremity of the trunk a basin-like cavity called thepelvis. These four bones are the sacrum and the coccyx, which have been described, and the two hip bones. 

[Illustration: Fig. 17. --Thorax. (Anterior view. )]

The hip bones are large, irregularly shaped bones, very firm andstrong, and are sometimes called the haunch bones or _ossa innominata_(nameless bones). They are united to the sacrum behind and joined to eachother in front. On the outer side of each hip bone is a deep cup, orsocket, called the _acetabulum_, resembling an ancient vinegar cup, intowhich fits the rounded head of the thigh bone. The bones of the pelvis aresupported like a bridge on the legs as pillars, and they in turn containthe internal organs in the lower part of the trunk. 

41. The Hyoid Bone. Under the lower jaw is a little horseshoe shaped bonecalled the hyoid bone, because it is shaped like the Greek letter upsilon([Greek: u]). The root of the tongue is fastened to its bend, and thelarynx is hung from it as from a hook. When the neck is in its naturalposition this bone can be plainly felt on a level with the lower jaw andabout one inch and a half behind it. It serves to keep open the top of thelarynx and for the attachment of the muscles, which move the tongue. (SeeFig. 46. ) The hyoid bone, like the knee-pan, is not connected with anyother bone. 

The Bones of the Upper Limbs. 

42. The Upper Limbs. Each of the upper limbs consist of the upperarm, the forearm, and the hand. These bones are classifiedas follows:

 Upper Arm: Scapula, or shoulder-blade, Clavicle, or collar bone, Humerus, or arm bone, Forearm: Ulna, Radius, Hand: 8 Carpal or wrist bones, 5 Metacarpal bones, 14 Phalanges, or finger bones, making 32 bones in all. 

43. The Upper Arm. The two bones of the shoulder, the scapulaand the clavicle, serve in man to attach the arm to the trunk. Thescapula, or shoulder-blade, is a flat, triangular bone, placed pointdownwards, and lying on the upper and back part of the chest, over theribs. It consists of a broad, flat portion and a prominent ridge or_spine_. At its outer angle it has a shallow cup known as the _glenoidcavity_. Into this socket fits the rounded head of the humerus. Theshoulder-blade is attached to the trunk chiefly by muscles, and is capableof extensive motion. 

The clavicle, or collar bone, is a slender bone with a double curvelike an italic _f_, and extends from the outer angle of the shoulder-bladeto the top of the breastbone. It thus serves like the keystone of an archto hold the shoulder-blade firmly in its place, but its chief use is tokeep the shoulders wide apart, that the arm may enjoy a freer range ofmotion. This bone is often broken by falls upon the shoulder or arm. 

The humerus is the strongest bone of the upper extremity. As alreadymentioned, its rounded head fits into the socket of the shoulder-blade, forming a ball-and-socket joint, which permits great freedom of motion. The shoulder joint resembles what mechanics call a universal joint, forthere is no part of the body which cannot be touched by the hand. 

[Illustration: Fig. 18. --Left Scapula, or Shoulder-Blade. ]

When the shoulder is dislocated the head of the humerus has been forcedout of its socket. The lower end of the bone is grooved to help form ahinge joint at the elbow with the bones of the forearm (Fig. 27). 

44. The Forearm. The forearm contains two long bones, theulna and the radius. The ulna, so called because it formsthe elbow, is the longer and larger bone of the forearm, and is on thesame side as the little finger. It is connected with the humerus by ahinge joint at the elbow. It is prevented from moving too far back by ahook-like projection called the _olecranon process_, which makes the sharppoint of the elbow. 

The radius is the shorter of the two bones of the forearm, and is onthe same side as the thumb. Its slender, upper end articulates with theulna and humerus; its lower end is enlarged and gives attachment in partto the bones of the wrist. This bone radiates or turns on the ulna, carrying the hand with it. 

 Experiment 10. Rest the forearm on a table, with the palm up (an attitude called supination). The radius is on the outer side and parallel with the ulna If now, without moving the elbow, we turn the hand (pronation), as if to pick up something from the table, the radius may be seen and felt crossing over the ulna, while the latter has not moved. 

[Illustration: Fig. 19. --Left Clavicle, or Collar Bone. (Anteriorsurface. )]

45. The Hand. The hand is the executive or essential part of theupper limb. Without it the arm would be almost useless. It consists of 27separate bones, and is divided into three parts, the wrist, thepalm, and the fingers. 

[Illustration: Fig. 20. --Left Humerus. ]

[Illustration: Fig. 21. --Left Radius and Ulna. ]

The carpus, or wrist, includes 8 short bones, arranged in two rows offour each, so as to form a broad support for the hand. These bones areclosely packed, and tightly bound with ligaments which admit of ampleflexibility. Thus the wrist is much less liable to be broken than if itwere to consist of a single bone, while the elasticity from having theeight bones movable on each other, neutralizes, to a great extent, ashock caused by falling on the hands. Although each of the wrist bones hasa very limited mobility in relation to its neighbors, their combinationgives the hand that freedom of action upon the wrist, which is manifest incountless examples of the most accurate and delicate manipulation. 

The metacarpal bones are the five long bones of the back of the hand. They are attached to the wrist and to the finger bones, and may be easilyfelt by pressing the fingers of one hand over the back of the other. Themetacarpal bones of the fingers have little freedom of movement, while thethumb, unlike the others, is freely movable. We are thus enabled to bringthe thumb in opposition to each of the fingers, a matter of the highestimportance in manipulation. For this reason the loss of the thumb disablesthe hand far more than the loss of either of the fingers. This verysignificant opposition of the thumb to the fingers, furnishing thecomplete grasp by the hand, is characteristic of the human race, and iswanting in the hand of the ape, chimpanzee, and ourang-outang. 

The phalanges, or finger bones, are the fourteen small bones arrangedin three rows to form the fingers. Each finger has three bones; eachthumb, two. 

The large number of bones in the hand not only affords every variety ofmovement, but offers great resistance to blows or shocks. These bones areunited by strong but flexible ligaments. The hand is thus given strengthand flexibility, and enabled to accomplish the countless movements sonecessary to our well-being. 

In brief, the hand is a marvel of precise and adapted mechanism, capablenot only of performing every variety of work and of expressing manyemotions of the mind, but of executing its orders with inconceivablerapidity. 

The Bones of the Lower Limbs. 

46. The Lower Limbs. The general structure and number of the bones ofthe lower limbs bear a striking similarity to those of the upper limbs. Thus the leg, like the arm, is arranged in three parts, the thigh, the lower leg, and the foot. The thigh bone corresponds to thehumerus; the tibia and fibula to the ulna and radius; the ankle to thewrist; and the metatarsus and the phalanges of the foot, to the metacarpusand the phalanges of the hand. 

The bones of the lower limbs may be thus arranged:

 Thigh: Femur, or thigh bone, Lower Leg: Patella, or knee cap, Tibia, or shin bone, Fibula, or splint bone, Foot: 7 Tarsal or ankle bones, 5 Metatarsal or instep bones, 14 Phalanges, or toes bones, making 30 bones in all. 

[Illustration: Fig. 22. --Right Femur, or Thigh Bone. ]

47. The Thigh. The longest and strongest of all the bones is thefemur, or thigh bone. Its upper end has a rounded head which fits into the_acetabulum_, or the deep cup-like cavity of the hip bone, forming aperfect ball-and-socket joint. When covered with cartilage, the ball fitsso accurately into its socket that it may be retained by atmosphericpressure alone (sec. 50). 

The shaft of the femur is strong, and is ridged and roughened in placesfor the attachment of the muscles. Its lower end is broad and irregularlyshaped, having two prominences called _condyles_, separated by a groove, the whole fitted for forming a hinge joint with the bones of the lower legand the knee-cap. 

48. The Lower Leg. The lower leg, like the forearm, consists oftwo bones. The tibia, or shin bone, is the long three-sided boneforming the front of the leg. The sharp edge of the bone is easily feltjust under the skin. It articulates with the lower end of the thigh bone, forming with it a hinge joint. 

The fibula, the companion bone of the tibia, is the long, slenderbone on the outer side of the leg. It is firmly fixed to the tibia at eachend, and is commonly spoken of as the small bone of the leg. Its lower endforms the outer projection of the ankle. In front of the knee joint, embedded in a thick, strong tendon, is an irregularly disk-shaped bone, the patella, or knee-cap. It increases the leverage of importantmuscles, and protects the front of the knee joint, which is, from itsposition, much exposed to injury. 

[Illustration: Fig. 23. --Patella, or Knee-Cap. ]

49. The Foot. The bones of the foot, 26 in number, consist ofthe tarsal bones, the metatarsal, and the phalanges. Thetarsal bones are the seven small, irregular bones which make up theankle. These bones, like those of the wrist, are compactly arranged, andare held firmly in place by ligaments which allow a considerable amount ofmotion. 

One of the ankle bones, the _os calcis_, projects prominently backwards, forming the heel. An extensive surface is thus afforded for the attachmentof the strong tendon of the calf of the leg, called the tendon ofAchilles. The large bone above the heel bone, the _astragalus_, articulates with the tibia, forming a hinge joint, and receives the weightof the body. 

The metatarsal bones, corresponding to the metacarpals of the hand, are five in number, and form the lower instep. 

The phalanges are the fourteen bones of the toes, --three in eachexcept the great toe, which, like the thumb, has two. They resemble innumber and plan the corresponding bones in the hand. The bones of the footform a double arch, --an arch from before backwards, and an arch from sideto side. The former is supported behind by the os calcis, and in front bythe ends of the metatarsal bones. The weight of the body fallsperpendicularly on the astragalus, which is the key-bone or crown of thearch. The bones of the foot are kept in place by powerful ligaments, combining great strength with elasticity. 

[Illustration: Fig. 24. --Right Tibia and Fibula (Anterior surface. )]

[Illustration: Fig. 25. --Bones of Right Foot. (Dorsal surface. )]

The Joints. 

50. Formation of Joints. The various bones of the skeleton areconnected together at different parts of their surfaces by joints, orarticulations. Many different kinds of joints have been described, but thesame general plan obtains for nearly all. They vary according to the kindand the amount of motion. 

The principal structures which unite in the formation of a joint are:bone, cartilage, synovial membrane, and ligaments. Bones makethe chief element of all the joints, and their adjoining surfaces areshaped to meet the special demands of each joint (Fig. 27). The joint-endof bones is coated with a thin layer of tough, elastic cartilage. This isalso used at the edge of joint-cavities, forming a ring to deepen them. The rounded heads of bones which move in them are thus more securely heldin their sockets. 

Besides these structures, the muscles also help to maintain thejoint-surfaces in proper relation. Another essential to the action of thejoints is the pressure of the outside air. This may be sufficient to keepthe articular surfaces in contact even after all the muscles are removed. Thus the hip joint is so completely surrounded by ligaments as to beair-tight; and the union is very strong. But if the ligaments be piercedand air allowed to enter the joint, the union at once becomes much lessclose, and the head of the thigh bone falls away as far as the ligamentswill allow it. 

51. Synovial Membrane. A very delicate connective tissue, called thesynovial membrane, lines the capsules of the joints, and covers theligaments connected with them. It secretes the _synovia_, or joint oil, athick and glairy fluid, like the white of a raw egg, which thoroughlylubricates the inner surfaces of the joints. Thus the friction and heatdeveloped by movement are reduced, and every part of a joint is enabled toact smoothly. 

52. Ligaments. The bones are fastened together, held in place, andtheir movements controlled, to a certain extent, by bands of variousforms, called ligaments. These are composed mainly of bundles ofwhite fibrous tissue placed parallel to, or closely interlaced with, oneanother, and present a shining, silvery aspect. They extend from one ofthe articulating bones to another, strongly supporting the joint, whichthey sometimes completely envelope with a kind of cap (Fig. 28). Thisprevents the bones from being easily dislocated. It is difficult, forinstance, to separate the two bones in a shoulder or leg of mutton, theyare so firmly held together by tough ligaments. 

While ligaments are pliable and flexible, permitting free movement, theyare also wonderfully strong and inextensible. A bone may be broken, or itsend torn off, before its ligaments can be ruptured. The wrist end of theradius, for instance, is often torn off by force exerted on its ligamentswithout their rupture. 

The ligaments are so numerous and various and are in some parts sointerwoven with each other, that space does not allow even mention ofthose that are important. At the knee joint, for instance, there are noless than fifteen distinct ligaments. 

53. Imperfect Joints. It is only perfect joints that are fullyequipped with the structures just mentioned. Some joints lack one or more, and are therefore called imperfect joints. Such joints allow little or nomotion and have no smooth cartilages at their edges. Thus, the bones ofthe skull are dovetailed by joints called sutures, which are immovable. The union between the vertebræ affords a good example of imperfect jointswhich are partially movable. 

[Illustration: Fig. 26. --Elastic Tissue from the Ligaments about Joints. (Highly magnified. )]

54. Perfect Joints. There are various forms of perfect joints, according to the nature and amount of movement permitted. They an dividedinto hinge joints, ball-and-socket joints and pivot joints. 

The hinge joints allow forward and backward movements like a hinge. These joints are the most numerous in the body, as the elbow, the ankle, and the knee joints. 

In the ball-and-socket joints--a beautiful contrivance--the roundedhead of one bone fits into a socket in the other, as the hip joint andshoulder joint. These joints permit free motion in almost every direction. 

In the pivot joint a kind of peg in one bone fits into a notch inanother. The best example of this is the joint between the first andsecond vertebræ (see sec. 38). The radius moves around on the ulna bymeans of a pivot joint. The radius, as well as the bones of the wrist andhand, turns around, thus enabling us to turn the palm of the hand upwardsand downwards. In many joints the extent of motion amounts to only aslight gliding between the ends of the bones. 

55. Uses of the Bones. The bones serve many important and usefulpurposes. The skeleton, a general framework, affords protection, support, and leverage to the bodily tissues. Thus, the bones ofthe skull and of the chest protect the brain, the lungs, and the heart;the bones of the legs support the weight of the body; and the long bonesof the limbs are levers to which muscles are attached. 

Owing to the various duties they have to perform, the bones areconstructed in many different shapes. Some are broad and flat;others, long and cylindrical; and a large number very irregularin form. Each bone is not only different from all the others, but is alsocuriously adapted to its particular place and use. 

[Illustration: Fig. 27. --Showing how the Ends of the Bones are shaped toform the Elbow Joint. (The cut ends of a few ligaments are seen. )]

Nothing could be more admirable than the mechanism by which each one ofthe bones is enabled to fulfill the manifold purposes for which it wasdesigned. We have seen how the bones of the cranium are united by suturesin a manner the better to allow the delicate brain to grow, and to affordit protection from violence. The arched arrangement of the bones of thefoot has several mechanical advantages, the most important being that itgives firmness and elasticity to the foot, which thus serves as a supportfor the weight of the body, and as the chief instrument of locomotion. 

The complicated organ of hearing is protected by a winding series ofminute apartments, in the rock-like portion of the temporal bone. Thesocket for the eye has a jutting ridge of bone all around it, to guard theorgan of vision against injury. Grooves and canals, formed in hard bone, lodge and protect minute nerves and tiny blood-vessels. The surfaces ofbones are often provided with grooves, sharp edges, and rough projections, for the origin and insertion of muscles. 

[Illustration: Fig. 28. --External Ligaments of the Knee. ]

56. The Bones in Infancy and Childhood. The bones of the infant, consisting almost wholly of cartilage, are not stiff and hard as in afterlife, but flexible and elastic. As the child grows, the bones become moresolid and firmer from a gradually increased deposit of lime salts. In timethey become capable of supporting the body and sustaining the action ofthe muscles. The reason is that well-developed bones would be of no use toa child that had not muscular strength to support its body. Again, thenumerous falls and tumbles that the child sustains before it is able towalk, would result in broken bones almost every day of its life. As it is, young children meet with a great variety of falls without serious injury. 

But this condition of things has its dangers. The fact that a child'sbones bend easily, also renders them liable to permanent change of shape. Thus, children often become bow-legged when allowed to walk too early. Moderate exercise, however, even in infancy, promotes the health of thebones as well as of the other tissues. Hence a child may be kept too longin its cradle, or wheeled about too much in a carriage, when the full useof its limbs would furnish proper exercise and enable it to walk earlier. 

57. Positions at School. Great care must be exercised by teachersthat children do not form the habit of taking injurious positions atschool. The desks should not be too low, causing a forward stoop; or toohigh, throwing one shoulder up and giving a twist to the spine. If theseats are too low there will result an undue strain on the shoulder andthe backbone; if too high, the feet have no proper support, the thighs maybe bent by the weight of the feet and legs, and there is a prolongedstrain on the hips and back. Curvature of the spine and round shouldersoften result from long-continued positions at school in seats and at deskswhich are not adapted to the physical build of the occupant. 

[Illustration: Fig. 29. --Section of the Knee Joint. (Showing its internalstructure)

 A, tendon of the semi-membranosus muscle cut across; B, F, tendon of same muscle; C, internal condyle of femur; D, posterior crucial ligament; E, internal interarticular fibro cartilage; G, bursa under knee-cap; H, ligament of knee-cap; K, fatty mass under knee-cap; L, anterior crucial ligament cut across; P, patella, or knee-cap]

A few simple rules should guide teachers and school officials in providingproper furniture for pupils. Seats should be regulated according to thesize and age of the pupils, and frequent changes of seats should be made. At least three sizes of desks should be used in every schoolroom, and morein ungraded schools. The feet of each pupil should rest firmly on thefloor, and the edge of the desk should be about one inch higher than thelevel of the elbows. A line dropped from the edge of the desk shouldstrike the front edge of the seat. Sliding down into the seat, bending toomuch over the desk while writing and studying, sitting on one foot orresting on the small of the back, are all ungraceful and unhealthfulpositions, and are often taken by pupils old enough to know better. Thistopic is well worth the vigilance of every thoughtful teacher, especiallyof one in the lower grades. 

58. The Bones in After Life. Popular impression attributes a lessshare of life, or a lower grade of vitality, to the bones than to anyother part of the body. But really they have their own circulation andnutrition, and even nervous relations. Thus, bones are the seat of activevital processes, not only during childhood, but also in adult life, and in fact throughout life, except perhaps in extreme old age. The finalknitting together of the ends of some of the bones with their shafts doesnot occur until somewhat late in life. For example, the upper end of thetibia and its shaft do not unite until the twenty-first year. The separatebones of the sacrum do not fully knit into one solid bone until thetwenty-fifth year. Hence, the risk of subjecting the bones of youngpersons to undue violence from injudicious physical exercise as in rowing, baseball, football, and bicycle-riding. 

The bones during life are constantly going through the process ofabsorption and reconstruction. They are easily modified in their growth. Thus the continued pressure of some morbid deposit, as a tumor or cancer, or an enlargement of an artery, may cause the absorption or distortion ofbones as readily as of one of the softer tissues. The distortion resultingfrom tight lacing is a familiar illustration of the facility with whichthe bones may be modified by prolonged pressure. 

Some savage races, not content with the natural shape of the head, takespecial methods to mould it by continued artificial pressure, so that itmay conform in its distortion to the fashion of their tribe or race. Thiscustom is one of the most ancient and widespread with which we areacquainted. In some cases the skull is flattened, as seen in certainIndian tribes on our Pacific coast, while with other tribes on the samecoast it is compressed into a sort of conical appearance. In such casesthe brain is compelled, of course, to accommodate itself to the change inthe shape of the head; and this is done, it is said, without any seriousresult. 

59. Sprains and Dislocations. A twist or strain of the ligaments andsoft parts about a joint is known as a sprain, and may result from agreat variety of accidents. When a person falls, the foot is frequentlycaught under him, and the twist comes upon the ligaments and tissues ofthe ankle. The ligaments cannot stretch, and so have to endure the wrenchupon the joint. The result is a sprained ankle. Next to the ankle, asprain of the wrist is most common. A person tries, by throwing out hishand, to save himself from a fall, and the weight of the body brings thestrain upon the firmly fixed wrist. As a result of a sprain, the ligamentsmay be wrenched or torn, and even a piece of an adjacent bone may be tornoff; the soft parts about the injured joint are bruised, and theneighboring muscles put to a severe stretch. A sprain may be a slightaffair, needing only a brief rest, or it may be severe and painful enoughto call for the most skillful treatment by a surgeon. Lack of proper carein severe sprains often results in permanent lameness. 

A fall or a blow may bring such a sudden wrench or twist upon theligaments as to force a bone out of place. This displacement is known as adislocation. A child may trip or fall during play and put his elbowout of joint. A fall from horseback, a carriage, or a bicycle may resultin a dislocation of the shoulder joint. In playing baseball a swift balloften knocks a finger out of joint. A dislocation must be reduced at once. Any delay or carelessness may make a serious and painful affair of it, asthe torn and bruised parts rapidly swell and become extremely sensitive. 

60. Broken Bones. The bones, especially those of the upper limbs, areoften fractured or broken. The _simple_ fracture is the most commonform, the bone being broken in a single place with no opening through theskin. When properly adjusted, the bone heals rapidly. Sometimes bones arecrushed into a number of fragments; this is a _comminuted_ fracture. When, besides the break, there is an opening through the soft parts andsurface of the body, we have a _compound_ fracture. This is a seriousinjury, and calls for the best surgical treatment. 

A bone may be bent, or only partly broken, or split. This is called "agreen-stick fracture, " from its resemblance to a half-broken green stick. This fracture is more common in the bones of children. 

Fractures may be caused by direct violence, as when a bone is broken at acertain point by some powerful force, as a blow from a baseball bat or afall from a horse. Again, a bone may be broken by indirect violence, aswhen a person being about to fall, throws out his hand to save himself. The force of the fall on the hand often breaks the wrist, by which ismeant the fracture of the lower end of the radius, often known as the"silver-fork fracture. " This accident is common in winter from a fall orslip on the ice. 

Sometimes bones are broken at a distance from the point of injury, as in afracture of the ribs by violent compression of the chest; or fracture mayoccur from the vibration of a blow, as when a fall or blow upon the top ofthe head produces fracture of the bones at the base of the brain. [6]

61. Treatment for Broken Bones. When a bone is broken a surgeon isneeded to set it, that is, to bring the broken parts into their naturalposition, and retain them by proper appliances. Nature throws out betweenand around the broken ends of bones a supply of repair material known asplastic lymph, which is changed to fibrous tissue, then to cartilage, andfinally to bone. This material serves as a sort of cement to hold thefractured parts together. The excess of this at the point of union can befelt under the skin for some time after the bone is healed. 

With old people a broken bone is often a serious matter, and may cripplethem for life or prove fatal. A trifling fall, for instance, may cause abroken hip (popularly so called, though really a fracture of the neck ofthe femur), from the shock of which, and the subsequent pain andexhaustion, an aged person may die in a few weeks. In young people, however, the parts of a broken bone will knit together in three or fourweeks after the fracture is reduced; while in adults, six or even more maybe required for firm union. After a broken bone is strong enough to beused, it is fragile for some time; and great care must be taken, especially with children, that the injured parts may not be broken againbefore perfect union takes place. [7]

62. The Effect of Alcohol upon the Bones. While the growth of thebones occurs, of course, mainly during the earlier years of life, yet theydo not attain their full maturity until about the twenty-fifth year; andit is stated that in persons devoted to intellectual pursuits, the skullgrows even after that age. It is plainly necessary that during this periodof bone growth the nutrition of the body should be of the best, that thebones may be built up from pure blood, and supplied with all the materialsfor a large and durable framework. Else the body will be feeble andstunted, and so through life fall short of its purpose. 

If this bony foundation be then laid wrong, the defect can never beremedied. This condition is seen in young persons who have been underfedand overworked. But the use of alcoholic liquors produces a similareffect, hindering bone cell-growth and preventing full development. [8]The appetite is diminished, nutrition perverted and impaired, the staturestunted, and both bodily and mental powers are enfeebled. 

63. Effect of Tobacco upon the Bones. Another narcotic, thedestructive influence of which is wide and serious, is tobacco. Itspernicious influence, like that of alcohol, is peculiarly hurtful to theyoung, as the cell development during the years of growth is easilydisturbed by noxious agents. The bone growth is by cells, and a powerfulnarcotic like tobacco retards cell-growth, and thus hinders the buildingup of the bodily frame. The formation of healthy bone demands good, nutritious blood, but if instead of this, the material furnished for theproduction of blood is poor in quality or loaded with poisonous narcotics, the body thus defrauded of its proper building material becomes undergrownand enfeebled. 

Two unfavorable facts accompany this serious drawback: one is, that owingto the insidious nature of the smoky poison[9] (cigarettes are its worstform) the cause may often be unsuspected, and so go on, unchecked; and theother, that the progress of growth once interrupted, the gap can never befully made up. Nature does her best to repair damages and to restoredefects, but never goes backwards to remedy neglects. 

Additional Experiments. 

 Experiment 11. Take a portion of the decalcified bone obtained from Experiment 4, and wash it thoroughly in water: in this it is insoluble. Place it in a solution of carbonate of soda and wash it again. Boil it in water, and from it gelatine will be obtained. 

 Experiment 12. Dissolve in hydrochloric acid a small piece of the powdered bone-ash obtained from Experiment 3. Bubbles of carbon dioxid are given off, indicating the presence of a carbonate. Dilute the solution; add an excess of ammonia, and we find a white precipitate of the phosphate of lime and of magnesia. 

 Experiment 13. Filter the solution in the preceding experiment, and to the filtrate add oxalate of ammonia. The result is a white precipitate of the oxalate of lime, showing there is lime present, but not as a phosphate. 

 Experiment 14. To the solution of mineral matters obtained from Experiment 3, add acetate of soda until free acetic acid is present, recognized by the smell (like dilute vinegar); then add oxalate of ammonia. The result will be a copious white precipitate of lime salts. 

 Experiment 15. _To show how the cancellous structure of bone is able to support a great deal of weight_. Have the market-man saw out a cubic inch from the cancellous tissue of a fresh beef bone and place it on a table with its principal layers upright. Balance a heavy book upon it, and then gradually place upon it various articles and note how many pounds it will support before giving way. 

 Experiment 16. Repeat the last experiment, using a cube of the decalcified bone obtained from Experiment 4. 

 [NOTE. As the succeeding chapters are studied, additional experiments on bones and their relation to other parts of the body, will readily suggest themselves to the ingenious instructor or the thoughtful student. Such experiments may be utilized for review or other exercises. ]

 Review Analysis: The Skeleton (206 bones). 

 / / 1 Frontal, / / 2 Parietal, / I. Cranium | 2 Temporal, / (8 bones) | 1 Occipital, / \ 1 Sphenoid, | \ 1 Ethmoid. | | / 2 Superior Maxillary, The Head | / 2 Malar, (28 bones). | / 2 Nasal, | II. Face | 2 Lachrymal Bones, | (14 bones) | 2 Palate Bones, | \ 2 Turbinated, | \ 1 Vomer, \ \ 1 Lower Maxillary. \ \ / Hammer, \ III. The Ear | Anvil, \ (6 bones) \ Stirrup. 

 / / 7 Cervical Vertebræ. / / 12 Dorsal Vertebræ, / I. Spinal Column | 5 Lumbar Vertebræ, | (26 bones) \ Sacrum, | \ Coccyx. The Trunk | (54 bones). | / 7 True Ribs, | II. The Ribs | 3 False Ribs, | (24 bones) \ 2 Floating Ribs. | \ III. Sternum. \ IV. Two Hip Bones. \ V. Hyoid Bone. 

 / / Scapula, / I. Upper Arm | Clavicle, | \ Humerus. | The Upper Limbs | II. Forearm / Ulna, (64 bones). | \ Radius. | | / 8 Carpal Bones, \ III. Hand | 5 Metacarpal Bones, \ \ 14 Phalanges. 

 / I. Thigh Femur. / | / Patella, The Lower Limbs | II. Lower Leg | Tibia, (60 bones). | \ Fibula. | | / 7 Tarsal Bones, \ III. Foot | 5 Metatarsal Bones, \ \ 14 Phalanges. 

Chapter III. 

The Muscles. 

64. Motion in Animals. All motion of our bodies is produced by meansof muscles. Not only the limbs are moved by them, but even the movementsof the stomach and of the heart are controlled by muscles. Every part ofthe body which is capable of motion has its own special set of muscles. 

Even when the higher animals are at rest it is possible to observe somekind of motion in them. Trees and stones never move unless acted upon byexternal force, while the infant and the tiniest insect can execute agreat variety of movements. Even in the deepest sleep the beating of theheart and the motion of the chest never cease. In fact, the power toexecute spontaneous movement is the most characteristic property ofliving animals. 

65. Kinds of Muscles. Most of the bodily movements, such as affectthe limbs and the body as a whole, are performed by muscles under ourcontrol. These muscles make up the red flesh or lean parts, which, together with the fat, clothe the bony framework, and give to it generalform and proportion. We call these muscular tissues voluntarymuscles, because they usually act under the control of the will. 

The internal organs, as those of digestion, secretion, circulation, andrespiration, perform their functions by means of muscular activity ofanother kind, that is, by that of muscles not under our control. This workgoes on quite independently of the will, and during sleep. We call theinstruments of this activity involuntary muscles. The voluntarymuscles, from peculiarities revealed by the microscope, are also known asstriped or striated muscles. The involuntary from their smooth, regularappearance under the microscope are called the unstriped or non-striatedmuscles. 

The two kinds of muscles, then, are the red, voluntary, striatedmuscles, and the smooth, involuntary, non-striated muscles. 

66. Structure of Voluntary Muscles. The main substance which clothesthe bony framework of the body, and which forms about two-fifths of itsweight, is the voluntary muscular tissue. These muscles do not cover andsurround the bones in continuous sheets, but consist of separate bundlesof flesh, varying in size and length, many of which are capable ofindependent movement. 

Each muscle has its own set of blood-vessels, lymphatics, and nerves. Itis the blood that gives the red color to the flesh. Blood-vessels andnerves on their way to other parts of the body, do not pass through themuscles, but between them. Each muscle is enveloped in its own sheath ofconnective tissue, known as the fascia. Muscles are not usuallyconnected directly with bones, but by means of white, glistening cordscalled tendons. 

[Illustration: Fig. 30. --Striated (voluntary) Muscular Fibers. 

 A, fiber serparating into disks; B, fibrillæ (highly magnified); C, cross section of a disk]

If a small piece of muscle be examined under a microscope it is found tobe made up of bundles of fibers. Each fiber is enclosed within adelicate, transparent sheath, known as the sarcolemma. If one ofthese fibers be further examined under a microscope, it will be seen toconsist of a great number of still more minute fibers calledfibrillæ. These fibers are also seen marked cross-wise with darkstripes, and can be separated at each stripe into disks. These crossmarkings account for the name _striped_ or _striated_ muscle. 

The fibrillæ, then, are bound together in a bundle to form a fiber, whichis enveloped in its own sheath, the sarcolemma. These fibers, in turn, arefurther bound together to form larger bundles called fasciculi, andthese, too, are enclosed in a sheath of connective tissue. The muscleitself is made up of a number of these fasciculi bound together by adenser layer of connective tissue. 

 Experiment 17. _To show the gross structure of muscle. _ Take a small portion of a large muscle, as a strip of lean corned beef. Have it boiled until its fibers can be easily separated. Pick the bundles and fasciculi apart until the fibers are so fine as to be almost invisible to the naked eye. Continue the experiment with the help of a hand magnifying glass or a microscope. 

67. The Involuntary Muscles. These muscles consist of ribbon-shapedbands which surround hollow fleshy tubes or cavities. We might comparethem to India rubber rings on rolls of paper. As they are never attachedto bony levers, they have no need of tendons. 

[Illustration: Fig. 31. --A, Muscular Fiber, showing Stripes, and Nuclei, band c. (Highly magnified. )]

The microscope shows these muscles to consist not of fibers, but of longspindle-shaped cells, united to form sheets or bands. They have nosarcolemma, stripes, or cross markings like those of the voluntarymuscles. Hence their name of _non-striated_, or _unstriped_, and _smooth_muscles. 

The involuntary muscles respond to irritation much less rapidly than dothe voluntary. The wave of contraction passes over them more slowly andmore irregularly, one part contracting while another is relaxing. This mayreadily be seen in the muscular action of the intestines, calledvermicular motion. It is the irregular and excessive contraction of themuscular walls of the bowels that produces the cramp-like pains of colic. 

The smooth muscles are found in the tissues of the heart, lungs, blood-vessels, stomach, and intestines. In the stomach their contractionproduces the motion by which the food is churned about; in the arteriesand veins they help supply the force by which the blood is driven along, and in the intestines that by which the partly digested food is mainlykept in motion. 

Thus all the great vital functions are carried on, regardless of the willof the individual, or of any outward circumstances. If it required aneffort of the will to control the action of the internal organs we couldnot think of anything else. It would take all our time to attend toliving. Hence the care of such delicate and important machinery has wiselybeen put beyond our control. 

Thus, too, these muscles act instinctively without training; but thevoluntary need long and careful education. A babe can use the muscles ofswallowing on the first day of its life as well as it ever can. But as itgrows up, long and patient education of its voluntary muscles is needed toachieve walking, writing, use of musical instruments, and many other actsof daily life. 

[Illustration: Fig. 32. --A Spindle Cell of Involuntary Muscle. (Highlymagnified. )]

 Experiment 18. _To show the general appearance of the muscles. _ Obtain the lower part of a sheep's or calf's leg, with the most of the lean meat and the hoof left on. One or more of the muscles with their bundles of fibers, fascia, and tendons; are readily made out with a little careful dissection. The dissection should be made a few days before it is wanted and the parts allowed to harden somewhat in dilute alcohol. 

68. Properties of Muscular Tissue. The peculiar property of livingmuscular tissue is irritability, or the capacity of responding to astimulus. When a muscle is irritated it responds by contracting. By thisact the muscle does not diminish its bulk to any extent; it simply changesits form. The ends of the muscle are drawn nearer each other and themiddle is thicker. 

Muscles do not shorten themselves all at once, but the contraction passesquickly over them in the form of a wave. They are usually stimulated bynervous action. The delicate nerve fibrils which end in the fiberscommunicate with the brain, the center of the will power. Hence, when thebrain commands, a nervous impulse, sent along the nerve fibers, becomesthe exciting stimulus which acts upon the muscles and makes them shorter, harder, and more rigid. [10]

Muscles, however, will respond to other than this usual stimulus. Thus anelectrical current may have a similar effect. Heat, also, may producemuscular contraction. Mechanical means, such as a sharp blow or pinching, may irritate a muscle and cause it to contract. 

We must remember that this property of contraction is inherent and belongsto the muscle itself. This power of contraction is often independent ofthe brain. Thus, on pricking the heart of a fish an hour after removalfrom its body, obvious contraction will occur. In this case it is not thenerve force from the brain that supplies the energy for contraction. Thepower of contraction is inherent in the muscle substance, and the stimulusby irritating the nerve ganglia of the heart simply affords theopportunity for its exercise. 

Contraction is not, however, the natural state of a muscle. In time it istired, and begins to relax. Even the heart, the hardest-working muscle, has short periods of rest between its beats. Muscles are highly elastic aswell as contractile. By this property muscle yields to a stretching force, and returns to its original length if the stretching has not beenexcessive. 

[Illustration: Fig. 33. --Principal Muscles of the Body. (Anterior view. )]

69. The Object of Contraction. The object of contraction is obvious. Like rubber bands, if one end of a muscle be fixed and the other attachedto some object which is free to move, the contraction of the muscle willbring the movable body nearer to the fixed point. A weight fastened to thefree end of a muscle may be lifted when the muscle contracts. Thus bytheir contraction muscles are able to do their work. They evencontract more vigorously when resistance is opposed to them than when itis not. With increased weight there is an increased amount of work to bedone. The greater resistance calls forth a greater action of the muscle. This is true up to a certain point, but when the limit has been passed, the muscle quickly fails to respond. 

Again, muscles work best with a certain degree of rapidity provided theirritations do not follow each other too rapidly. If, however, thecontractions are too rapid, the muscles become exhausted and fatigueresults. When the feeling of fatigue passes away with rest, the musclerecovers its power. While we are resting, the blood is pouring in freshsupplies of building material. 

 Experiment 19. _To show how muscles relax and contract_. Lay your left forearm on a table; grasp with the right hand the mass of flesh on the front of the upper arm. Now gradually raise the forearm, keeping the elbow on the table. Note that the muscle thickens as the hand rises. This illustrates the contraction of the biceps, and is popularly called "trying your muscle" Reverse the act. Keep the elbow in position, bring the forearm slowly to the table, and the biceps appears to become softer and smaller, --it relaxes. 

 Experiment 20. Repeat the same experiment with other muscles. With the right hand grasp firmly the extended left forearm. Extend and flex the fingers vigorously. Note the effect on the muscles and tendons of the forearm. Grasp with the right hand the calf of the extended right leg, and vigorously flex the leg, bringing it near to the body. Note the contractions and relaxations of the muscles. 

70. Arrangement of Muscles. Muscles are not connected directly withbones. The mass of flesh tapers off towards the ends, where the fiberspass into white, glistening cords known as tendons. The place atwhich a muscle is attached to a bone, generally by means of a tendon, iscalled its origin; the end connected with the movable bone is itsinsertion. 

There are about 400 muscles in the human body, all necessary for itsvarious movements. They vary greatly in shape and size, according to theirposition and use. Some are from one to two feet long, others only afraction of an inch. Some are long and spindle-shaped, others thin andbroad, while still others form rings. Thus some of the muscles of the armand thigh are long and tapering, while the abdominal muscles are thin andbroad because they help form walls for cavities. Again, the muscularfibers which surround and by their contraction close certain orifices, asthose of the eyelids and lips, often radiate like the spokes of a wheel. 

Muscles are named according to their shape, position, division of originor insertion, and their function. Thus we have the _recti_ (straight), andthe _deltoid_ ([Greek: D], delta), the _brachial_ (arm), _pectoral_(breast), and the _intercostals_ (between the ribs), so named from theirposition. Again, we have the _biceps_ (two-headed), _triceps_(three-headed), and many others with similar names, so called from thepoints of origin and insertion. We find other groups named after theirspecial use. The muscles which bend the limbs are called _flexors_ whilethose which straighten them are known as _extensors_. 

After a bone has been moved by the contraction of a muscle, it is broughtback to its position by the contraction of another muscle on the oppositeside, the former muscle meanwhile being relaxed. Muscles thus acting inopposition to each other are called antagonistic. Thus the biceps servesas one of the antagonists to the triceps, and the various flexors andextensors of the limbs are antagonistic to one another. 

71. The Tendons. The muscles which move the bones by theircontraction taper for the most part, as before mentioned, intotendons. These are commonly very strong cords, like belts or straps, made up of white, fibrous tissue. 

Tendons are most numerous about the larger joints, where they permit freeaction and yet occupy but little space. Large and prominent muscles inthese places would be clumsy and inconvenient. If we bend the arm or legforcibly, and grasp the inside of the elbow or knee joint, we can feel thetendons beneath the skin. The numerous tendons in the palm or on the backof the hand contribute to its marvelous dexterity and flexibility. Thethickest and strongest tendon in the body is the tendon of Achilles, which connects the great muscles in the calf of the leg with the heel bone(sec. 49). 

When muscles contract forcibly, they pull upon the tendons which transmitthe movement to the bones to which they are attached. Tendons may becompared to ropes or cords which, when pulled, are made to act upondistant objects to which one end is fastened. Sometimes the tendon runsdown the middle of a muscle, and the fibers run obliquely into it, thetendon resembling the quill in a feather. Again, tendons are spread out ina flat layer on the surface of muscles, in which case they are calledaponeuroses. Sometimes a tendon is found in the middle of a muscle as wellas at each end of it. 

[Illustration: Fig. 34. --The Biceps Muscle dissected to show its Tendons. ]

72. Synovial Sheaths and Sacs. The rapid movement of the tendonsover bony surfaces and prominences would soon produce an undue amount ofheat and friction unless some means existed to make the motion as easy aspossible. This is supplied by sheaths which form a double lining aroundthe tendons. The opposed surfaces are lined with synovialmembrane, [11] the secretion from which oils the sheaths in which thetendons move. 

Little closed sacs, called synovial sacs or bursæ, similarly linedand containing fluid, are also found in special places between twosurfaces where much motion is required. There are two of these bursæ nearthe patella, one superficial, just under the skin; the other deep beneaththe bone (Fig. 29). Without these, the constant motion of the knee-pan andits tendons in walking would produce undue friction and heat andconsequent inflammation. Similar, though smaller, sacs are found over thepoint of the elbow, over the knuckles, the ankle bones, and various otherprominent points. These sacs answer a very important purpose, and areliable to various forms of inflammation. 

 Experiment 21. Examine carefully the tendons in the parts dissected in Experiment 18. Pull on the muscles and the tendons, and note how they act to move the parts. This may be also admirably shown on the leg of a fowl or turkey from a kitchen or obtained at the market. 

 Obtain the hoof of a calf or sheep with one end of the tendon of Achilles still attached. Dissect it and test its strength. 

73. Mechanism of Movement. The active agents of bodily movements, aswe have seen, are the muscles, which by their contraction cause the bonesto move one on the other. All these movements, both of motion and oflocomotion, occur according to certain fixed laws of mechanics. The bones, to which a great proportion of the muscles in the body are attached, actas distinct levers. The muscles supply the power for moving thebones, and the joints act as fulcrums or points of support. The weight ofthe limb, the weight to be lifted, or the force to overcome, is theresistance. 

74. Levers in the Body. In mechanics three classes of levers aredescribed, according to the relative position of the power, the fulcrum, and the resistance. All the movements of the bones can be referred to oneor another of these three classes. 

Levers of the first class are those in which the fulcrum is betweenthe power and the weight. The crowbar, when used to lift a weight at oneend by the application of power at the other, with a block as a fulcrum, is a familiar example of this class. There are several examples of this inthe human body. The head supported on the atlas is one. The joint betweenthe atlas and the skull is the fulcrum, the weight of the head is theresistance. The power is behind, where the muscles from the neck areattached to the back of the skull. The object of this arrangement is tokeep the head steady and balanced on the spinal column, and to move itbackward and forward. 

[Illustration: Fig. 35. --Showing how the Bones of the Arm serve as Levers. 

 P, power; W, weight; F, fulcrum. ]

Levers of the second class are those in which the weight is betweenthe fulcrum and the power. A familiar example is the crowbar when used forlifting a weight while one end rests on the ground. This class of leversis not common in the body. Standing on tiptoe is, however, an example. Here the toes in contact with the ground are the fulcrum, the power is theaction of the muscles of the calf, and between these is the weight of thebody transmitted down the bones of the leg to the foot. 

Levers of the third class are those in which the power is applied ata point between the fulcrum and weight. A familiar example is where aworkman raises a ladder against a wall. This class of levers is common inthe body. In bending the forearm on the arm, familiarly known as "tryingyour muscle, " the power is supplied by the biceps muscle attached to theradius, the fulcrum is the elbow joint at one end of the lever, and theresistance is the weight of the forearm at the other end. 

 Experiment 22. _To illustrate how the muscles use the bones as levers. _ First, practice with a ruler, blackboard pointer, or any other convenient object, illustrating the different kinds of levers until the principles are familiar. Next, illustrate these principles on the person, by making use of convenient muscles. Thus, lift a book on the toes, by the fingers, on the back of the hand, by the mouth, and in other ways. 

 These experiments, showing how the bones serve as levers, may be multiplied and varied as circumstances may require. 

75. The Erect Position. The erect position is peculiar to man. Noother animal naturally assumes it or is able to keep it long. It is theresult of a somewhat complex arrangement of muscles which balance eachother, some pulling backwards and some forwards. Although the wholeskeleton is formed with reference to the erect position, yet this attitudeis slowly learned in infancy. 

In the erect position the center of gravity lies in the joint between thesacrum and the last lumbar vertebra. A line dropped from this point wouldfall between the feet, just in front of the ankle joints. We rarely standwith the feet close together, because that basis of support is too smallfor a firm position. Hence, in all efforts requiring vigorous muscularmovements the feet are kept more or less apart to enlarge the basis ofsupport. 

Now, on account of the large number and flexibility of the joints, thebody could not be kept in an upright position without the cooperation ofcertain groups of muscles. The muscles of the calf of the leg, acting onthe thigh bone, above the knee, keep the body from falling forward, whileanother set in front of the thigh helps hold the leg straight. These thighmuscles also tend to pull the trunk forward, but in turn are balanced bythe powerful muscles of the lower back, which help keep the body straightand braced. 

The head is kept balanced on the neck partly by the central position ofthe joint between the atlas and axis, and partly by means of strongmuscles. Thus, the combined action of these and other muscles serves tobalance the body and keep it erect. A blow on the head, or a sudden shockto the nervous system, causes the body to fall in a heap, because thebrain has for the time lost its power over the muscles, and they cease tocontract. 

[Illustration: Fig. 36. --Diagram showing the Action of the Chief Muscleswhich keep the Body Erect. (The arrows indicate the direction in whichthese muscles act, the feet serving as a fixed basis. ) [After Huxley. ]

_Muscles which tend to keep the body from falling forward. _

 A, muscles of the calf; B, of the back of the thigh; C, of the spinal column. 

_Muscles which tend to keep the body from falling backward. _

 D, muscles of the front of the leg; E, of the front of the thigh; F, of the front of the abdomen; G, of the front of the neck. ]

76. Important Muscles. There are scores of tiny muscles about thehead, face, and eyes, which, by their alternate contractions andrelaxations, impart to the countenance those expressions which reflect thefeelings and passions of the individual. Two important muscles, thetemporal, near the temples, and the masseter, or chewing muscle, are the chief agents in moving the lower jaw. They are very large in thelion, tiger, and other flesh-eating animals. On the inner side of eachcheek is the buccinator, or trumpeter's muscle, which is largelydeveloped in those who play on wind instruments. Easily seen and feltunder the skin in thin persons, on turning the head to one side, is thesterno-cleido-mastoid muscle, which passes obliquely down on eachside of the neck to the collar bone--prominent in sculpture and painting. 

The chest is supplied with numerous muscles which move the ribs up anddown in the act of breathing. A great, fan-shaped muscle, called thepectoralis major, lies on the chest. It extends from the chest to thearm and helps draw the arm inward and forward. The arm is raised from theside by a large triangular muscle on the shoulder, the deltoid, socalled from its resemblance to the Greek letter delta, [Greek: D]. Thebiceps, or two-headed muscle, forms a large part of the fleshy massin front of the arm. Its use is to bend the forearm on the arm, an actfamiliarly known as "trying your muscle. " Its direct antagonist is thethree-headed muscle called the triceps. It forms the fleshy mass onthe back of the arm, its use being to draw the flexed forearm into a rightline. 

On the back and outside of the forearm are the extensors, whichstraighten the wrist, the hand, and the fingers. On the front and insideof the forearm are the flexors, which bend the hand, the wrist, andthe fingers. If these muscles are worked vigorously, their tendons can bereadily seen and felt under the skin. At the back of the shoulder a large, spread-out muscle passes upward from the back to the humerus. From itswide expanse on the back it is known as the latissimus dorsi(broadest of the back). When in action it draws the arm downward andbackward, or, if one hangs by the hands, it helps to raise the body. It isfamiliarly known as the "climbing muscle. "

[Illustration: Fig. 37. --A Few of the Important Muscles of the Back. ]

Passing to the lower extremity, the thigh muscles are the largest and themost powerful in the body. In front a great, four-headed muscle, quadriceps extensor, unites into a single tendon in which theknee-cap is set, and serves to straighten the knee, or when rising from asitting posture helps elevate the body. On the back of the thigh areseveral large muscles which bend the knee, and whose tendons, known as the"hamstrings, " are readily felt just behind the knee. On the back of theleg the most important muscles, forming what is known as the calf, are thegastrocnemius and the soleus. The first forms the largest partof the calf. The soleus, so named from resembling a sole-fish, is a muscleof broad, flattened shape, lying beneath the gastrocnemius. The tendons ofthese two muscles unite to form the tendon of Achilles, as that herois said to have been invulnerable except at this point. The muscles of thecalf have great power, and are constantly called into use in walking, cycling, dancing, and leaping. 

77. The Effect of Alcoholic Drinks upon the Muscles. It is found thata man can do more work without alcohol than with it. After taking it theremay be a momentary increase of activity, but this lasts only ten orfifteen minutes at the most. It is followed by a rapid reduction of powerthat more than outweighs the momentary gain, while the quality of the workis decidedly impaired from the time the alcohol is taken. 

Even in the case of hard work that must be speedily done, alcohol does nothelp, but hinders its execution. The tired man who does not understand theeffects of alcohol often supposes that it increases his strength, when infact it only deadens his sense of fatigue by paralyzing his nerves. Whenput to the test he is surprised at his self-deception. 

Full intoxication produces, by its peculiar depression of the brain andnervous system, an artificial and temporary paralysis of the muscles, asis obvious in the pitifully helpless condition of a man fully intoxicated. But even partial approach to intoxication involves its proportionateimpairment of nervous integrity, and therefore just so much diminution ofmuscular force. All athletes recognize this fact, as while training for acontest, rigid abstinence is the rule, both from liquors and tobacco. Thismuscular weakness is shown also in the unsteady hand, the trembling limbsof the inebriate, his thick speech, wandering eye, and lolling head. 

78. Destructive Effect of Alcoholic Liquors upon Muscular Tissue. Alcoholic liquors retard the natural chemical changes so essential to goodhealth, by which is meant the oxidation of the nutritious elements offood. Careful demonstration has proved also that the amount of carbondioxide escaping from the lungs of intoxicated persons is from thirty tofifty per cent less than normal. This shut-in carbon stifles the nervousenergy, and cuts off the power that controls muscular force. This lostforce is in close ratio to the retained carbon: so much perverted chemicalchange, so much loss of muscular power. Not only the strength but the finedelicacy of muscular action is lost, the power of nice control of the handand fingers, as in neat penmanship, or the use of musical instruments. 

To this perverted chemical action is also due the fatty degeneration socommon in inebriates, affecting the muscles, the heart, and the liver. These organs are encroached upon by globules of fat (a hydrocarbon), which, while very good in their proper place and quantity, become asource of disorder and even of death when they abnormally invade vitalstructures. Other poisons, as phosphorus, produce this fatty decay morerapidly; but alcohol causes it in a much more general way. 

This is proved by the microscope, which plainly shows the conditionmentioned, and the difference between the healthy tissues and those thusdiseased. 

[Illustration: Fig. 38. --Principal Muscles on the Left Side of Neck. 

 A, buccinator; B, masseter; C, depressor anguli oris; D, anterior portion of the digastric; E, mylo-hyoid; F, tendon of the digastric; G, sterno-hyoid; H, sterno-thyroid; K, omo-hyoid; L, sternal origin of sterno-cleido-mastoid muscle; M, superior fibers of deltoid; N, posterior scalenus; O, clavicular origin of sterno-cleido-mastoid; P, sterno-cleido-mastoid; R, trapezius; S, anterior constrictor; T, splenius capitis; V, stylo-hyoid; W, posterior portion of the digastric; X, fasciculi of ear muscles; Z, occipital. ]

 [NOTE. It was proposed during the Civil War to give each soldier in a certain army one gill of whiskey a day, because of great hardship and exposure. The eminent surgeon, Dr. Frank H. Hamilton of New York, thus expressed his views of the question: "It is earnestly desired that no such experiment will ever be repeated in the armies of the United States. In our own mind, the conviction is established, by the experience and observation of a life, that the regular routine employment of alcoholic stimulants by man in health is never, under any circumstances, useful. We make no exceptions in favor of cold or heat or rain. "

 "It seems to me to follow from these Arctic experiences that the regular use of spirits, even in moderation, under conditions of great physical hardship, continued and exhausting labor, or exposure to severe cold cannot be too strongly deprecated. "

 A. W. Greely, retired Brigadier General, U. S. A. , and formerly leader of the Greely Expedition. ]

79. Effect of Tobacco on the Muscles. That other prominent narcotic, tobacco, impairs the energy of the muscles somewhat as alcohol does, byits paralyzing effect upon the nervous system. As all muscular actiondepends on the integrity of the nervous system, whatever lays itsdeadening hand upon that, saps the vigor and growth of the entire frame, dwarfs the body, and retards mental development. This applies especiallyto the young, in the growing age between twelve or fourteen and twenty, the very time when the healthy body is being well knit and compacted. 

Hence many public schools, as well as our national naval and militaryacademies, rigidly prohibit the use of tobacco by their pupils. So alsoyoung men in athletic training are strictly forbidden to use it. [12] Thisloss of muscular vigor is shown by the unsteady condition of the muscles, the trembling hand, and the inability to do with precision and accuracyany fine work, as in drawing or nice penmanship. 

Additional Experiments. 

 Experiment 23. _ To examine the minute structure of voluntary muscular fiber. _ Tease, with two needles set in small handles, a bit of raw, lean meat, on a slip of glass, in a little water. Continue until the pieces are almost invisible to the naked eye. 

 Experiment 24. Place a clean, dry cover-glass of about the width of the slip, over the water containing the torn fragments. Absorb the excess of moisture at the edge of the cover, by pressing a bit of blotting-paper against it for a moment. Place it on the stage of a microscope and examine with highest obtainable power, by light reflected upward from the mirror beneath the stage. Note the apparent size of the finest fibers; the striation of the fibers, or their markings, consisting of alternate dim and bright cross bands. Note the arrangement of the fibers in bundles, each thread running parallel with its neighbor. 

 Experiment 25. _To examine the minute structure of involuntary muscular fiber, a tendon, or a ligament. _ Obtain a very small portion of the muscular coat of a cow's or a pig's stomach. Put it to soak in a solution of one dram of bichromate of potash in a pint of water. Take out a morsel on the slip of glass, and tease as directed for the voluntary muscle. Examine with a high power of the microscope and note: (1) the isolated cells, long and spindle-shaped, that they are much flattened; (2) the arrangement of the cells, or fibers, in sheets, or layers, from the torn ends of which they project like palisades. 

 Experiment 26. Tease out a small portion of the tendon or ligament in water, and examine with a glass of high power. Note the large fibers in the ligament, which branch and interlace. 

 Experiment 27. With the head slightly bent forwards, grasp between the fingers of the right hand the edge of the left sterno-cleido-mastoid, just above the collar bone. Raise the head and turn it from left to right, and the action of this important muscle is readily seen and felt. In some persons it stands out in bold relief. 

 Experiment 28. The tendons which bound the space (popliteal) behind the knee can be distinctly felt when the muscles which bend the knee are in action. On the outer side note the tendons of the biceps of the leg, running down to the head of the fibula. On the inside we feel three tendons of important muscles on the back of the thigh which flex the leg upon the thigh. 

 Experiment 29. _To show the ligamentous action of the muscles. _ Standing with the back fixed against a wall to steady the pelvis, the knee can be flexed so as to almost touch the abdomen. Take the same position and keep the knee rigid. When the heel has been but slightly raised a sharp pain in the back of the thigh follows any effort to carry it higher. Flexion of the leg to a right angle, increases the distance from the lines of insertion on the pelvic bones to the tuberosities of the tibia by two or three inches--an amount of stretching these muscle cannot undergo. Hence the knee must be flexed in flexion of the hip. 

 Experiment 30. A similar experiment may be tried at the wrist. Flex the wrist with the fingers extended, and again with the fingers in the fist. The first movement can be carried to 90 degrees, the second only to 30 degrees, or in some persons up to 60 degrees. Making a fist had already stretched the extensor muscles of the arm, and they can be stretched but little farther. Hence, needless pain will be avoided by working a stiff wrist with the parts loose, or the fingers extended, and not with a clenched fist. 

 Review Analysis: Important Muscles. 

 Location. Name. Chief Function. 

 Head and Neck. 

 Occipito-frontalis. Moves scalp and raises eye brow. Orbicularis palpebrarum. Shuts the eyes. Levator palpebrarum. Opens the eyes. Temporal. Raise the lower jaw. Masseter. " " " " Sterno-cleido-mastoid. Depresses head upon neck and neck upon chest. Platysma myoides. Depresses lower jaw and lower lip. 

 Trunk. 

 Pectoralis major. Draws arm across front of chest. Pectoralis minor. Depresses point of shoulder, Latissimus dorsi. Draws arm downwards and backwards. Serratus magnus. Assists in raising ribs. Trapezius. Rhomboideus. Backward movements of head and shoulder, Intercostals. Raise and depress the ribs. External oblique. /various forward movements Internal oblique. \ of trunk Rectus abdominis. Compresses abdominal viscera and acts upon pelvis. 

 Upper Limbs. 

 Deltoid. Carries arm outwards and upwards. Biceps. Flexes elbow and raises arm. Triceps. Extends the forearm. Brachialis anticus. Flexor of elbow. Supinator longus. Flexes the forearm. Flexor carpi radialis. Flexors of wrist. Flexor carpi ulnaris. " " "

 Lower Limbs. 

 Gluteus maximus. Adducts the thigh. Adductors of thigh. Draw the leg inwards. Sartorius. Crosses the legs. Rectus femoris. Flexes the thigh. Vastus externus. Extensor of leg. Vastus internus. Extensor of leg upon thigh. Biceps femoris. Flexes leg upon thigh. Gracilis. Flexes the leg and adducts thigh. Tibialis anticus. Draws up inner border of foot. Peroneus longus. Raises outer edge of foot, Gastrocnemius. Keep the body erect, and Soleus. Aid in walking and running. 

Chapter IV. 

Physical Exercise. 

80. Importance of Bodily Exercise. Nothing is so essential to successin life as sound physical health. It enables us to work with energy andcomfort, and better to endure unusual physical and mental strains. Whileothers suffer the penalties of feebleness, a lower standard of functionalactivities, and premature decay, the fortunate possessor of a sound mindin a sound body is better prepared, with proper application, to endure thehardships and win the triumphs of life[13]. 

This element of physical capacity is as necessary to a useful andenergetic life, as are mental endowment and intellectual acquirement. Instinct impels us to seek health and pleasure in muscular exercise. Ahealthy and vigorous child is never still except during sleep. Therestless limbs and muscles of school children pent up for several hours, feel the need of movement, as a hungry man craves food. This naturaldesire for exercise, although too often overlooked, is really one of thenecessities of life. One must be in ill health or of an imperfect nature, when he ceases to feel this impulse. Indeed, motion within proper boundsis essential to the full development and perfect maintenance of the bodilyhealth. Unlike other machines, the human body becomes within reasonablelimits, stronger and more capable the more it is used. 

As our tenure of life at best is short, it is our duty to strive to liveas free as possible from bodily ills. It is, therefore, of paramountimportance to rightly exercise every part of the body, and this withoutundue effort or injurious strain. 

Strictly speaking, physical exercise refers to the functionalactivity of each and every tissue, and properly includes the regulation ofthe functions and movements of the entire body. The word exercise, however, is used usually in a narrower sense as applied to those movementsthat are effected by the contraction of the voluntary muscles. 

Brief reference will be made in this chapter only to such natural andsystematic physical training as should enter into the life of everyhealthy person. 

81. Muscular Activity. The body, as we have learned, is built up ofcertain elementary tissues which are combined to make bones, muscles, nerves, and other structures. The tissues, in turn, are made up ofcountless minute cells, each of which has its birth, lives its briefmoment to do its work in the animal economy, is separated from the tissueof which it was a part, and is in due time eliminated by the organs ofexcretion, --the lungs, the skin, or the kidneys. Thus there is acontinuous process of growth, of decay, and removal, among the individualcells of each tissue. 

 [NOTE. The Incessant Changes in Muscular Tissue. "In every tiny block of muscle there is a part which is really alive, there are parts which are becoming alive, there are parts which have been alive, and are now dying or dead; there is an upward rush from the lifeless to the living, a downward rush from the living to the dead. This is always going on, whether the muscle be quiet and at rest, or whether it be active and moving, --some of the capital of living material is being spent, changed into dead waste; some of the new food is always being raised into living capital. But when the muscle is called upon to do work, when it is put into movement, the expenditure is quickened, there is a run upon the living capital, the greater, the more urgent the call for action. "--Professor Michael Foster. ]

These ceaseless processes are greatly modified by the activity of thebodily functions. Every movement of a muscle, for instance, involveschange in its component cells. And since the loss of every atom of thebody is in direct relation to its activity, a second process is necessaryto repair this constant waste; else the body would rapidly diminish insize and strength, and life itself would soon end. This process of repairis accomplished, as we shall learn in Chapters VI. And VII. , by the organsof nutrition, which convert the food into blood. 

[Illustration: Fig. 39. --Showing how the Muscles of the Back may bedeveloped by a Moderate Amount of Dumb-Bell Exercise at Home. (From aphotograph. )]

82. Effect of Exercise upon the Muscles. Systematic exerciseinfluences the growth and structure of the muscles of the body in a mannersomewhat remarkable. Muscular exercise makes muscular tissue; from thelack of it, muscles become soft and wasted. Muscles properly exercised notonly increase in size, both as a whole and in their individual structure, but are better enabled to get rid of material which tends to hamper theirmovements. Thus muscular exercise helps to remove any needlessaccumulation of fat, as well as useless waste matters, which may exist inthe tissues. As fat forms no permanent structural part of the organism, its removal is, within limits, effected with no inconvenience. 

Muscular strength provides the joints with more powerful ligaments andbetter developed bony parts. After long confinement to the bed fromdisease, the joints have wasted ligaments, thin cartilages, and the bonesare of smaller proportions. Duly exercised muscles influence the size ofthe bones upon which they act. Thus the bones of a well-developed man arestronger, firmer, and larger than those of a feeble person. 

He who has been physically well trained, has both a more complete and amore intelligent use of his muscles. He has acquired the art of causinghis muscles to act in concert. Movements once difficult are now carried onwith ease. The power of coordination is increased, so that a desired endis attained with the least amount of physical force and nervous energy. Inlearning to row, play baseball, ride the bicycle, or in any otherexercises, the beginner makes his movements in a stiff and awkward manner. He will use and waste more muscular force in playing one game of ball, orin riding a mile on his wheel, than an expert would in doing ten times thework. He has not yet learned to balance one set of muscles against theirantagonists. 

[Illustration: Fig. 40. --The Standard Special Chest Weight. 

A convenient machine by means of which all the muscles of the body may beeasily and pleasantly exercised with sufficient variations in themovements to relieve it of monotony. 

A space 6 ft wide, 6 ft deep, and 7 ft high nearly in front of the machineis required for exercise. ]

In time, however, acts which were first done only with effort and by aconscious will, become automatic. The will ceases to concern itself. Bywhat is called reflex action, memory is developed in the spinal cord andthe muscular centers (sec. 273). There is thus a great saving of actualbrain work, and one important cause of fatigue is removed. 

83. Effect of Exercise on Important Organs. The importance ofregular exercise is best understood by noting its effects upon theprincipal organs of the body. As the action of the heart is increased bothin force and frequency during exercise, the flow of blood throughout thebody is augmented. This results from the force of the muscularcontractions which play their part in pressing the blood in the veinsonward towards the heart. Exercise also induces a more vigorousrespiration, and under increased breathing efforts the lung capacity isincreased and the size of the chest is enlarged. The amount of airinspired and expired in a given time is much larger than if the body wereat rest. The blood is thus supplied with a much larger amount of oxygenfrom the air inhaled, and gives off to the air a corresponding excess ofcarbon dioxid and water. 

Again, exercise stimulates and strengthens the organs of digestion. Theappetite is improved, as is especially noted after exercise in the openair. The digestion is more complete, absorption becomes more rapid, theperistaltic movements of the bowels are promoted, and the circulationthrough the liver is more vigorous. More food is taken to supply the forcenecessary for the maintenance of the mechanical movements. Ample exercisealso checks the tendency towards a torpid circulation in the largerdigestive organs, as the stomach and the liver, so common with those whoeat heartily, but lead sedentary lives. In short, exercise may be regardedas a great regulator of nutrition. 

Exercise increases the flow of blood through the small vessels of theskin, and thus increases the radiation of heat from the surface. If theexercise be vigorous and the weather hot, a profuse sweat ensues, therapid evaporation of which cools the body. The skin is thus a mostimportant regulator of the bodily temperature, and prevents any rise abovethe normal which would otherwise result from vigorous exercise. (See secs. 226 and 241). 

84. Effect of Exercise upon the Personal Appearance. Judicious andsystematic exercise, if moderately employed, soon gives a more upright andsymmetrical figure, and an easier and more graceful carriage. Roundedshoulders become square, the awkward gait disappears, and there is seen agraceful poise to the head and a bearing of the body which mark thosewhose muscles have been well trained. A perfectly formed skeleton andwell-developed muscles give the graceful contour and perfect outline tothe human body. The lean, soft limbs of those who have never had anyphysical education, often look as if they belonged to persons recoveringfrom sickness. The effects of sound physical exercise are well exhibitedin the aspect of the neck, shoulders, and chest of one who has been welltrained. This is noticeable in gymnasts and others who practice upon thehorizontal bar, with chest weights, dumb-bells, and other apparatus whichdevelop more especially the muscles of the upper half of the trunk. 

[Illustration: Fig. 41. --Young Woman practicing at Home with the "WhitelyExerciser. " (From a photograph)]

Exercise improves the condition of the tissues generally. They become moreelastic, and in all respects sounder. The skin becomes firm, clear, andwholesome. Hence, every part of the surface of the body rapidly takes on achange in contour, and soon assumes that appearance of vigor and soundnesswhich marks those of firm physical condition. The delicate, ruddy aspectof the complexion, the swing about the body and the bearing of the headand shoulders, of young women whose physical training has been efficient, are in marked contrast with those characteristics in persons whoseeducation in this respect has been neglected. 

85. Effect of Unsuitable or Excessive Exercise. But exercise, likeeverything else which contributes to our welfare, may be carried toexcess. The words excessive and unsuitable, when applied to muscularexertion, are relative terms, and apply to the individual rather than toamount of work done. Thus what may be excessive for one person, might besuitable and beneficial to another. Then the condition of the individual, rather than the character of the muscular work, is always a most importantfactor. 

Breathlessness is, perhaps, the most common effect of undue exertion. Leta middle-aged person, who is out of practice, run a certain distance, andhe is soon troubled with his breathing. The respirations become irregular, and there is a sense of oppression in his chest. He pants, and hisstrength gives out. His chest, and not his legs, has failed him. He issaid to be "out of breath. " He might have practiced dumb-bells or rowedfor some time without inconvenience. 

The heart is often overstrained, and at times has been ruptured duringviolent exertion, as in lifting an immense weight. The various forms ofheart-disease are common with those whose occupations involve severemuscular effort, as professional athletes and oarsmen. Hæmorrhages ofvarious kinds, especially from the lungs, or rupture of blood-vessels inthe brain, are not uncommon results of over-exertion. 

Excessive repetition of muscular movements may lead to permanentcontractions of the parts involved. Thus sailors, mechanics, and othersfrequently develop a rigidity of the tendons of the hand which preventsthe full extension of the fingers. So stenographers, telegraphers andwriters occasionally suffer from permanent contractions of certain musclesof the arm, known as writer's cramp, due to their excessive use. But theaccidents which now and then may result from severe physical exertion, should discourage no one from securing the benefits which accrue frommoderate and reasonable exercise. 

86. Muscular Fatigue. We all know how tiresome it is to hold the armoutstretched horizontally even for a few moments. A single muscle, thedeltoid, in this case does most of the work. Even in a vigorous man, thismuscle can act no longer than four to six minutes before the arm dropshelpless. We may prolong the period by a strong effort of the will, but atime soon comes when by no possible effort are we able to hold out thearm. The muscle is said to be fatigued. It has by no means lost itscontractile power, for if we apply a strong electric stimulus to it, thefatigue seems to disappear. Thus we see the functional power of a musclehas a definite limit, and in fatigue that limit is reached. 

[Illustration: Fig. 42. --A Well-Equipped Gymnasium. (From a photograph. )]

The strength of the muscle, its physical condition, the work it has done, and the mental condition of the individual, all modify the state offatigue. In those difficult acts which involve a special effort of thewill, the matter of nerve exhaustion is largely concerned. Thus, theincessant movements in St. Vitus' dance result in comparatively littlefatigue, because there is no association of the brain with the muscularaction. If a strong man should attempt to perform voluntarily the samemovements, he would soon have to rest. None of the movements which areperformed independently of the will, as the heart-beats and breathingmovements, ever involve the sensation of fatigue. As a result of fatiguethe normal irritability of muscular tissue becomes weakened, and its forceof contraction is lessened. There is, also, often noticed in fatigue apeculiar tremor of the muscles, rendering their movements uncertain. Thestiffness of the muscles which comes on during severe exercise, or the dayafter, are familiar results of fatigue. 

This sense of fatigue should put us on guard against danger. It is a kindof regulator which serves in the ordinary actions of life to warn us notto exceed the limits of useful exercise. Fatigue summons us to rest longbefore all the force of the motor organs has been expended, just as thesensation of hunger warns us that we need food, long before the body hasbecome weak from the lack of nourishment. 

We should never forget that it is highly essential to maintain an unusedreserve of power, just as a cautious merchant always keeps at the bank anunexpended balance of money. If he overspends his money he is bankrupt, and the person who overspends his strength is for the time physicallybankrupt. In each case the process of recovery is slow and painful. 

87. Rest for the Muscles. Rest is necessary for the tissues, thatthey may repair the losses sustained by work; that is, a period of restmust alternate with a period of activity. Even the heart, beatingceaselessly, has its periods of absolute rest to alternate with those ofwork. A steam-engine is always slowly, but surely, losing its fitness forwork. At last it stops from the need of repair. Unlike the engine, thebody is constantly renewing itself and undergoing continual repair. Wereit not for this power to repair and renew its various tissues, the bodywould soon be worn out. 

This repair is really a renovation of the structure. Rest and work arerelative terms, directly opposed to each other. Work quickens the pulseand the respiration, while rest slows both. During sleep the voluntarymuscles are relaxed, and those of organic life work with less energy. Thepulse and the respiration are less frequent, and the temperature lowerthan when awake. Hence sleep, "tired Nature's sweet restorer, " may beregarded as a complete rest. 

The periods of rest should vary with the kind of exercise. Thus exercisewhich produces breathlessness requires frequent but short rests. Thetrained runner, finding his respiration embarrassed, stops a moment toregain his breath. Exercises of endurance cause fatigue less quickly thanthose of speed, but require longer rest. Thus a man not used to longdistances may walk a number of hours without stopping, but while fatigueis slow to result, it is also slow to disappear. Hence a lengthy period ofrest is necessary before he is able to renew his journey. 

88. Amount of Physical Exercise Required. The amount of physicalexercise that can be safely performed by each person, is a most importantand practical question. No rule can be laid down, for what one personbears well, may prove very injurious to another. To a certain extent, eachmust be guided by his own judgment. If, after taking exercise, we feelfatigued and irritable, are subject to headache and sleeplessness, or findit difficult to apply the mind to its work, it is plain that we have beentaxing our strength unduly, and the warnings should be heeded. 

Age is an important factor in the problem, as a young man may do withease and safety, what might be injurious to an older person. In youth, when the body is making its most active development, the judicious use ofgames, sports, and gymnastics is most beneficial. In advanced life, boththe power and the inclination for exercise fail, but even then effortshould be made to take a certain reasonable amount of exercise. 

Abundant evidence shows that physical development is most active fromthirteen to seventeen years of age; this manifests itself clearly byincrease in weight. Hence this period of life is of great consequence. Ifat this age a boy or girl is subjected to undue physical strain, thedevelopment may suffer, the growth be retarded, and the foundation laidfor future ill health. 

[Illustration: Fig. 43. --Student exercising in the School Gymnasium on theRowing Machine. (From a photograph. )]

The proper amount of exercise must vary greatly with circumstances. It maybe laid down as a fairly safe rule, that a person of average height andweight, engaged in study or in any indoor or sedentary occupation, shouldtake an amount of exercise equivalent to walking five or six miles a day. Growing children, as a rule, take more exercise than this, while most menworking indoors take far less, and many women take less exercise than men. Exercise may be varied in many ways, the more the better; but for the mostpart it should always be taken in the open air. 

89. Time for Exercise. It is not prudent to do hard work or takesevere exercise, just before or just after a full meal. The best time isone or two hours after a meal. Vigorous exercise while the stomach isbusily digesting food, may prove injurious, and is apt to result sooner orlater in dyspepsia. On the other hand, severe exercise should not be takenon an empty stomach. Those who do much work or study before breakfast, should first take a light lunch, just enough to prevent any faint feeling. With this precaution, there is no better time for moderate exercise thanthe early morning. 

In the case of children, physical exercises should not be undertaken whenthey are overtired or hungry. Neither is it judicious for adults to takevigorous exercise in the evening, after a long and arduous day's work. 

90. Walking, Running, and Jumping. Walking is generally regarded asthe simplest and most convenient mode of taking exercise. Man isessentially a walking animal. When taken with a special object in view, itis the best and most pleasant of all physical activities. It is suited forindividuals of all ages and occupations, and for residents of everyclimate. The child, the athlete, and the aged are all able to indulge inthis simple and effective means of keeping the body in health. 

In walking, the muscles of the entire body are brought into action, and the movements of breathing and the circulation of the blood areincreased. The body should be erect, the chest thrown out, the head andshoulders held back, and the stride long and elastic. It is an excellentcustom to add to the usefulness of this fine exercise, by deep, voluntaryinhalations of pure air. 

Running is an excellent exercise for children and young people, butshould be sparingly indulged in after the age of thirty-five. If it beaccompanied with a feeling of faintness, breathlessness, and palpitationof the heart, the exercise is too severe, and its continuance may doserious harm. Running as an exercise is beneficial to those who have keptthemselves in practice and in sound condition. It brings into play nearlyevery muscle of the body, and thus serves to develop the power ofendurance, as well as strength and capacity for rapid movement. 

Jumping may well be left to boys and young men under twenty, butskipping with a rope, allied to jumping, is an admirable and beneficialform of exercise. It brings into action many muscles without putting unduestrain upon any particular group. 

91. Skating, Swimming, and Rowing. Skating is a delightful andinvigorating exercise. It calls into play a great variety of muscles, andis admirably adapted for almost all ages. It strengthens the ankles andhelps give an easy and graceful carriage to the body. Skating isespecially valuable, as it can be enjoyed when other out-door exercisesare not convenient. 

Every child above ten years of age should be taught to swim. The art, once mastered, is never forgotten. It calls into use a wide combination ofmuscles. This accomplishment, so easily learned, should be a part of oureducation, as well as baseball or bicycling, as it may chance to any oneto save his own life or that of a companion. 

In many respects rowing is one of the most perfect exercises at ourcommand. It expands the chest, strengthens the body, and gives tone to themuscles of the abdomen. It is very suitable for girls and women, as noother exercise is so well adapted to remedy the muscular defects so markedin their sex. Even elderly persons can row day after day withoutdifficulty. The degree of muscular effort required, can be regulated sothat those with weak hearts and weak lungs can adjust themselves to theexercise. 

92. Bicycling as an Exercise. The bicycle as a means of takingexercise has come into popular use with remarkable rapidity. Sharpcompetition bids fair to make the wheel more popular and less expensivethan ever. Its phenomenal use by persons of all ages and in all stationsof life, is proof of the enthusiasm with which this athletic exercise isemployed by women as well as by men. 

Mechanical skill has removed most of the risks to health and person whichonce existed. A good machine, used by its owner with judgment, is the mostconvenient, the safest, and the least expensive means of traveling forpleasure or exercise. It is doing more than any other form of exercise toimprove the bodily condition of thousands whose occupations confine themall day to sedentary work. Dependent upon no one but himself, the cyclisthas his means of exercise always at hand. No preparation is necessary totake a spin of ten miles or so on the road, during a summer evening orbefore breakfast. 

Bicycling brings into active use the muscles of the legs as well as thoseof the trunk and arms. It seems to benefit those who suffer fromdyspepsia, constipation, and functional disorders of the liver. 

A special caution must be used against overdoing in cycling, for thetemptation by rivalry, making a record, by social competition on the road, is stronger in this form of exercise than in any other, especially foryoung folks. Many cases have occurred of permanent injury, and even lossof life, from collapse simply by excessive exertion and exhaustion. 

93. Outdoor Games and Physical Education. While outdoor gamesare not necessary to maintain health, yet we can scarcely overestimate thepart that the great games of baseball, football, tennis, golf, andcroquet, play in the physical development of young people. When played inmoderation and under suitable conditions, they are most useful andbeneficial exercises. They are played in the open air, and demand a greatvariety of vigorous muscular movement, with a considerable amount of skilland adroitness of action. These games not only involve healthful exercise, but develop all those manly and wholesome qualities so essential tosuccess in life. 

A vigorous body is well-nigh essential to success, but equally importantare readiness of action, sound judgment, good temper, personal courage, asense of fair play, and above all, a spirit of honor. Outdoor games, whenplayed in a reasonable and honorable manner, are most efficient andpractical means to develop these qualities in young people. 

94. The School and Physical Education. The advantages to be derived, during the school period, from the proper care and development of thebody, should be understood and appreciated by school officials, teachers, and parents. The school period is the best time to shape the lives ofpupils, not mentally or morally alone, but physically as well. This is thetime, by the use of a few daily exercises at school, to draw back therounding shoulders, to form the habit of sitting and standing erect, tobuild up strong and comely arms and chests, and otherwise to train pupilsto those methods which will serve to ripen them into vigorous andwell-knit men and women. 

Teachers can by a little effort gain the knowledge requisite properly toinstruct their pupils in a few systematic exercises. Gratifying resultswill follow just as the teacher and pupils evince interest and judgment inthe work. It is found by experience that pupils are not only quick tolearn, but look forward eagerly to the physical exercises as aninteresting change from the routine of school life. 

There should be a stated time for these school exercises, as for any otherduty. There can be practiced in the schoolroom a great variety ofinteresting and useful exercises, which call for little or no expense forapparatus. Such exercises should no more interfere with the children'susual games than any other study does. Under no circumstances should theplay hours be curtailed. 

95. Physical Exercises in School. Physical exercises of some sort, then, should be provided for pupils in our schools, especially in largetowns and cities, where there is little opportunity for outdoor games, andthey should form a part of the regular course of study. The object shouldbe the promotion of sound health rather than the development of muscle, orperforming feats of agility or strength. Exercises with dumb-bells andwands, or even without any apparatus, practiced a few times a day, forfive minutes at a time, do a great deal of good. They relax the tension ofbody and mind, and introduce an element of pleasure into the routine ofschool life. They increase the breathing power and quicken the action ofthe heart. 

[Illustration: Fig. 44. --Physical Exercises as carried on in Schools. (From photographs. )]

 [NOTE. "In early boyhood and youth nothing can replace the active sports so much enjoyed at this period; and while no needless restrictions should be placed upon them, consideration should be paid to the amount, and especially to the character, of the games pursued by delicate youth. For these it would be better to develop the weakened parts by means of systematic physical exercises and by lighter sports. "--Dr. John M. Keating on "Physical Development" in Pepper's _Cyclopædia of the Diseases of Children_. ]

If vigorously and systematically carried out, these exercises invigorateall the tissues and organs of the body, and stimulate them to renewedactivity. They serve to offset the lack of proper ventilation, faultypositions at the desks, and the prolonged inaction of the muscles. Tosecure the greatest benefit from physical training in school, it isimportant that the pupils be interested in these exercises, and considerthem a recreation, and not a task[14]. 

96. Practical Points about Physical Exercise. The main object inundertaking systematic and graduated physical exercises is not to learn todo mere feats of strength and skill, but the better to fit the individualfor the duties and the work of life. Exercises should be considered withreference to their availability from the learner's standpoint. The mostbeneficial exercises ordinarily are the gentle ones, in which no strain isput upon the heart and the respiration. The special aim is to secure theequal use of all the muscles, not the development of a few. Theperformance of feats of strength should never come within the scope of anyeducational scheme. Exercises which call for sustained effort, violentexertion, or sudden strain are best avoided by those who have had nopreparation or training. 

Regular exercise, not sudden and occasional prolonged exertion, isnecessary for health. The man or woman who works in an office or store allthe week, and on Sunday or a holiday indulges in a long spin on thebicycle, often receives more harm than good from the exertion. Exerciseshould be taken, so far as is convenient, in the open air, or in a largeand well-ventilated room. [15]

After the more violent exercises, as baseball, football, a long ride onthe bicycle, or even after a prolonged walk, a warm bath should be takenat the first convenient opportunity. Care should be taken to rub downthoroughly, and to change a part or all of the clothing. Exercise iscomparatively valueless until the idea of taking it for health is quiteforgotten in the interest and pleasure excited by the occasion. Noexercise should be carried to such a degree as to cause fatigue orexhaustion. Keep warmly clad after exercise, avoid chills, and always stopexercising as soon as fatigue is felt. 

Wear clothing which allows free play to all the muscles of the body. Theclothing should be light, loose, and made of wool. Care should be takennot to take cold by standing about in clothes which are damp withperspiration. In brisk walking and climbing hills keep the mouth shut, especially in cold weather, and breathe through the nose, regulating thepace so that it can be done without discomfort. 

97. Effect of Alcoholic Liquors and Tobacco upon Physical Culture. Asa result of the unusual attention given to physical culture in the lastfew years, hundreds of special instructors are now employed in trainingyoung people in the theory and practice of physical exercise. These expertteachers, to do their work with thoroughness and discipline, recognize thenecessity of looking after the daily living of their students. The time ofrising and retiring, the hours of sleep, the dress, the care of the diet, and many other details of personal health become an important part of thetraining. 

Recognizing the fact that alcoholic drink and tobacco are so disastrous toefficiency in any system of physical training, these instructors rigidlyforbid the use of these drugs under all circumstances. While thisprinciple is perhaps more rigorously enforced in training for athleticcontests, it applies equally to those who have in view only themaintenance of health. 

Books on Physical Education. There are many excellent books onphysical education, which are easily obtained for reading or forreference. Among these one of the most useful and suggestive is Blackie'swell-known book, "How to Get Strong and how to Stay so. " This little bookis full of kindly advice and practical suggestions to those who may wishto begin to practice health exercises at home with inexpensive apparatus. For more advanced work, Lagrange's "Physiology of Bodily Exercise" and theIntroduction to Maclaren's "Physical Education" may be consulted. Anotable article on "Physical Training" by Joseph H. Sears, an Ex-Captainof the Harvard Football Team, may be found in Roosevelt's "In Sickness andin Health. "

Price lists and catalogues of all kinds of gymnastic apparatus are easilyobtained on application to firms handling such goods. 

Various Systems of Physical Exercises. The recent revival of popularinterest in physical education has done much to call the attention of thepublic to the usefulness and importance of a more thorough and systematicuse of physical exercises, both at home and in the schools. It is notwithin the scope of this book to describe the various systems of gymnasticand calisthenic exercises now in common use in this country. For the mostpart they have been modified and rearranged from other sources, notablyfrom the two great systems, i. E. , Swedish and German. 

For a most comprehensive work on the Swedish system, the teacher isreferred to the "Swedish System of Educational Gymnastics, " with 264illustrations, by Baron Nils Posse. There is also a small manual forteachers, called "Handbook of School Gymnastics of the Swedish Systems, "by the same author. 

Chapter V. 

Food and Drink. 

98. Why we need Food. The body is often compared to a steam-engine ingood working order. An engine uses up fuel and water to obtain from themthe energy necessary to do its work. So, we consume within our bodiescertain nutritious substances to obtain from them the energy necessary forour activities. Just as the energy for the working of the engine isobtained from steam by the combustion of fuel, so the energy possessed byour bodies results from the combustion or oxidation within us of the foodwe eat. Unless this energy is provided for the body it will have butlittle power of doing work, and like an engine without steam, must soonbecome motionless. 

99. Waste and Repair. A steam-engine from the first stroke of itspiston-rod begins to wear out, and before long needs repair. All workinvolves waste. The engine, unless kept in thorough repair, would soonstop. So with our bodies. In their living cells chemical changes areconstantly going on; energy, on the whole, is running down; complexsubstances are being broken up into simpler combinations. So long as lifelasts, food must be brought to the tissues, and waste products carriedaway from them. It is impossible to move a single muscle, or even to thinkfor one moment, without some minute part of the muscular or brain tissuebecoming of no further use in the body. The transformation of dead matterinto living tissue is the ever-present miracle which life presents even inits lowest forms. 

In childhood the waste is small, and the amount of food taken is morethan sufficient to repair the loss. Some of the extra food is used inbuilding up the body, especially the muscles. As we shall learn in ChapterVIII. , food is also required to maintain the bodily heat. Food, then, is necessary for the production of energy, for the repair of the body, forthe building up of the tissues, and for the maintenance of bodily heat. 

100. Nature of the Waste Material. An ordinarily healthy personpasses daily, on an average, by the kidneys about 50 ounces of wastematerial, of which 96 per cent is water, and from the intestines, on anaverage, 5-1/2 ounces, a large proportion of which is water. By the skin, in the shape of sweat and insensible perspiration, there is cast out about23 ounces, of which 99 per cent is water; and by the lungs about 34ounces, 10 of which are water and the remainder carbon dioxid. 

Now if we omit an estimate of the undigestible remains of the food, wefind that the main bulk of what daily leaves the body consists of water, carbon dioxid, and certain solid matters contained in solution inthe renal secretion and the sweat. The chief of these solid matters isurea, a complex product made up of four elements, --carbon, hydrogen, oxygen, and nitrogen. Water contains only two elements, hydrogen andoxygen; and carbon dioxid also has only two, carbon and oxygen. Hence, what we daily cast out of our bodies consists essentially of these fourelements in the form mainly of water, carbon dioxid, and urea. 

These waste products represent the oxidation that has taken place inthe tissues in producing the energy necessary for the bodily activities, just as the smoke, ashes, clinkers, and steam represent the consumption offuel and water in the engine. Plainly, therefore, if we could restore tothe body a supply of these four elements equivalent to that cast out, wecould make up for the waste. The object of food, then, is to restore tothe body an amount of the four elements equal to that consumed. In otherwords, and briefly: The purpose of food is to supply the waste of thetissues and to maintain the normal composition of the blood. 

101. Classification of Foods. Foods may be conveniently divided intofour great classes, to which the name food-stuffs or alimentaryprinciples has been given. They correspond to the chief "proximateprinciples" of which the body consists. To one or the other of theseclasses all available foods belong[16]. The classification of food-stuffsusually given is as follows:

 I. Proteids, or Nitrogenous Foods. II. Starches and Sugars, or Carbohydrates. III. Fats and Oils. IV. Inorganic or Mineral Foods, --Water, Salt. 

102. Proteids; or Nitrogenous Foods. The proteids, frequentlyspoken of as the nitrogenous foods, are rich in one or more of thefollowing organic substances: albumen, casein, fibrin, gelatine, myosin, gluten, and legumin. 

The type of this class of foods is albumen, well known as the white of anegg. The serum of the blood is very rich in albumen, as is lean meat. Thecurd of milk consists mainly of casein. Fibrin exists largely in blood andflesh foods. Gelatine is obtained from the animal parts of bones andconnective tissue by prolonged boiling. One of the chief constituents ofmuscular fiber is myosin. Gluten exists largely in the cereals wheat, barley, oats, and rye. The proteid principle of peas and beans is legumin, a substance resembling casein. 

As the name implies, the proteids, or nitrogenous foods, contain nitrogen;carbohydrates and fats, on the contrary, do not contain nitrogen. Theprincipal proteid food-stuffs are milk, eggs, flesh foods of all kinds, fish, and the cereals among vegetable foods. Peas and beans are rich inproteids. The essential use of the proteids to the tissues is to supplythe material from which the new proteid tissue is made or the old proteidtissue is repaired. They are also valuable as sources of energy to thebody. Now, as the proteid part of its molecule is the most importantconstituent of living matter, it is evident that proteid food is anabsolute necessity. If our diet contained no proteids, the tissues ofthe body would gradually waste away, and death from starvation wouldresult. All the food-stuffs are necessary in one way or another to thepreservation of perfect health, but proteids, together with a certainproportion of water and inorganic salts, are absolutely necessary for thebare maintenance of animal life--that is, for the formation andpreservation of living protoplasm. 

103. Starches and Sugars. The starches, sugars, and gums, also knownas carbohydrates, enter largely into the composition of foods ofvegetable origin. They contain no nitrogen, but the three elements, carbon, hydrogen, and oxygen, the last two in the same proportion as inwater. The starches are widely distributed throughout the vegetablekingdom. They are abundant in potatoes and the cereals, and in arrowroot, rice, sago, and tapioca. Starch probably stands first in importance amongthe various vegetable foods. 

The sugars are also widely distributed substances, and include thecane, grape, malt, maple, and milk sugars. Here also belong the gums andcellulose found in fruit, cereals, and all vegetables which form thebasis of the plant cells and fibers. Honey, molasses, and manna areincluded in this class. 

The physiological value of the starches and sugars lies in the fact thatthey are oxidized in the body, and a certain amount of energy is therebyliberated. The energy of muscular work and of the heat of the body comeslargely from the oxidation, or destruction, of this class of foods. Now, inasmuch as we are continually giving off energy from the body, chiefly inthe form of muscular work and heat, it is evident that material for theproduction of this energy must be taken in the food. The carbohydratesconstitute the bulk of our ordinary food. 

104. Fats and Oils. These include not only the ordinary fats ofmeat, but many animal and vegetable oils. They are alike inchemical composition, consisting of carbon and hydrogen, with a littleoxygen and no nitrogen. The principal kinds of fat used as food are thefat of meat, butter, suet, and lard; but in many parts of the worldvarious vegetable oils are largely used, as the olive, palm, cotton seed, cocoanut, and almond. 

The use of the fats in the body is essentially the same as that of thestarches and sugars. Weight for weight they are more valuable than thecarbohydrates as sources of energy, but the latter are more easilydigested, and more easily oxidized in the body. An important use of fattyfoods is for the maintenance of the bodily heat. The inhabitants of Arcticregions are thus enabled, by large use of the fat and oil from the animalsthey devour, to endure safely the severe cold. Then there is reason tobelieve that fat helps the digestion of other foods, for it is found thatthe body is better nourished when the fats are used as food. When more fatis consumed than is required to keep up the bodily heat and to yieldworking power, the excess is stored up in various parts of the body, making a sort of reserve fuel, which may be drawn upon at any future time. 

105. Saline or Mineral Foods. All food contains, besides thesubstances having potential energy, as described, certain salinematters. Water and salts are not usually considered foods, but the resultsof scientific research, as well as the experience of life, show that thesesubstances are absolutely necessary to the body. The principal mineralfoods are salt, lime, iron, magnesia, phosphorus, potash, and water. Except common salt and water, these substances are usually taken only incombination with other foods. 

These saline matters are essential to health, and when not present in dueproportion nutrition is disturbed. If a dog be fed on food freed from allsalines, but otherwise containing proper nutrients, he soon suffers fromweakness, after a time amounting to paralysis, and often dies inconvulsions. 

About 200 grains of common salt are required daily by an adult, but alarge proportion of this is in our food. Phosphate of lime is obtainedfrom milk and meats, and carbonate of lime from the hard water we drink. Both are required for the bones and teeth. The salts of potash, whichassist in purifying the blood, are obtained from vegetables and fruits. Aniron salt is found in most foods, and sulphur in the yolk of eggs. 

106. Water. Water is of use chiefly as a solvent, and while notstrictly a food, is necessary to life. It enters into the construction ofevery tissue and is constantly being removed from the body by everychannel of waste[17]. 

As a solvent water aids digestion, and as it forms about 80 per centof the blood, it serves as a carrier of nutrient material to all thetissues of the body. 

Important Articles of Diet. 

107. Milk. The value of milk as a food cannot be overestimated. It affords nourishment in a very simple, convenient, and perfect form. Itis the sole food provided for the young of all animals which nourish theiryoung. It is an ideal food containing, in excellent proportions, all thefour elements necessary for growth and health in earlier youth. 

[Table: Composition of Food Materials. Careful analyses have beenmade of the different articles of food, mostly of the raw, or uncookedfoods. As might be expected, the analyses on record differ more or less inthe percentages assigned to the various constituents, but the followingtable will give a fair idea of the fundamental nutritive value of the morecommon foods:

 In 100 parts Water Proteid Fat Carbohydrate Ash Digestible Cellulose Meat 76. 7 20. 8 1. 5 0. 3 -- 1. 3 Eggs 73. 7 12. 6 12. 1 -- -- 1. 1 Cheese 36-60 25-33 7-30 3-7 -- 3. 4 Cow's Milk 87. 7 3. 4 3. 2 4. 8 -- 0. 7 Wheat Flour 13. 3 10. 2 0. 9 74. 8 0. 3 0. 5 Wheat Bread 35. 6 7. 1 0. 2 55. 5 0. 3 1. 1 Rye Flour 13. 7 11. 5 2. 1 69. 7 1. 6 1. 4 Rye bread 42. 3 6. 1 0. 4 49. 2 0. 5 1. 5 Rice 13. 1 7. 0 0. 9 77. 4 0. 6 1. 0 Corn 13. 1 9. 9 4. 6 68. 4 2. 5 1. 5 Macaroni 10. 1 9. 0 0. 3 79. 0 0. 3 0. 5 Peas and Beans 12-15 23-26 1-1/2-2 49-54 4. 7 2-3 Potatoes 75. 5 2. 0 0. 2 20. 6 0. 7 1. 0 Carrots 87. 1 1. 0 0. 2 9. 3 1. 4 0. 9 Cabbage 90 2. 3 0. 5 4-6 1-2 1. 3 Fruit 84 0. 5 -- 10 4 0. 5]

Cheese is the nitrogenous part of milk, which has been coagulated by theuse of rennet. The curd is then carefully dried, salted, and pressed. Cheese is sometimes difficult of digestion, as on account of its solidform it is not easily acted upon by the digestive fluids. 

108. Meats. The flesh of animals is one of our main sources of food. Containing a large amount of proteid, it is admirably adapted for buildingup and repairing the tissues of the body. The proportion of water is alsohigh, varying from 50 to 75 per cent. The most common meats used inthis country are beef, mutton, veal, pork, poultry, and game. 

Beef contains less fat and is more nutritious than either mutton or pork. Mutton has a fine flavor and is easily digested. Veal and lamb, thoughmore tender, are less easily digested. Pork contains much fat, and itsfiber is hard, so that it is the most difficult to digest of all themeats. Poultry and game have usually a small proportion of fat, but arerich in phosphates and are valued for their flavor. 

109. Eggs. Consisting of about two-thirds water and the rest albumenand fat, eggs are often spoken of as typical natural food. The whiteof an egg is chiefly albumen, with traces of fat and salt; the yolk islargely fat and salts. The yellow color is due partly to sulphur. It isthis which blackens a silver spoon. Eggs furnish a convenient andconcentrated food, and if properly cooked are readily digested. 

110. Fish. Fish forms an important and a most nutritious article ofdiet, as it contains almost as much nourishment as butcher's meat. Thefish-eating races and classes are remarkably strong and healthy. Fishis less stimulating than meat, and is thus valuable as a food for invalidsand dyspeptics. To be at its best, fish should be eaten in its season. Asa rule shell-fish, except oysters, are not very digestible. Some personsare unable to eat certain kinds of fish, especially shell-fish, withouteruptions on the skin and other symptoms of mild poisoning. 

111. Vegetable Foods. This is a large and important group of foods, and embraces a remarkable number of different kinds of diet. Vegetablefoods include the cereals, garden vegetables, the fruits, and other lessimportant articles. These foods supply a certain quantity of albumen andfat, but their chief use is to furnish starches, sugars, acids, and salts. The vegetable foods indirectly supply the body with a large amount ofwater, which they absorb in cooking. 

112. Proteid Vegetable Foods. The most important proteid vegetablefoods are those derived from the grains of cereals and certainleguminous seeds, as peas and beans. The grains when ground make thevarious flours or meals. They contain a large quantity of starch, aproteid substance peculiar to them called gluten, and mineral salts, especially phosphate of lime. Peas and beans contain a smaller proportionof starch, but more proteid matter, called legumin, or vegetable casein. Of the cereal foods, wheat is that most generally useful. Wheat, and cornand oatmeal form most important articles of diet. Wheat flour has starch, sugar, and gluten--nearly everything to support life except fat. 

Oatmeal is rich in proteids. In some countries, as Scotland, it forms animportant article of diet, in the form of porridge or oatmeal cakes. 

Corn meal is not only rich in nitrogen, but the proportion of fat is alsolarge; hence it is a most important and nutritious article of food. Rice, on the other hand, contains less proteids than any other cereal grain, andis the least nutritious. Where used as a staple article of food, as inIndia, it is commonly mixed with milk, cheese, or other nutritioussubstances. Peas and beans, distinguished from all other vegetables bytheir large amount of proteids--excel in this respect even beef, mutton, and fish. They take the place of meats with those who believe in avegetable diet. 

113. Non-proteid Vegetable Foods. The common potato is the best typeof non-proteid vegetable food. When properly cooked it is easilydigested and makes an excellent food. It contains about 75 per cent ofwater, about 20 per cent of carbohydrates, chiefly starch, 2 per cent ofproteids, and a little fat and saline matters. But being deficient inflesh-forming materials, it is unfit for an exclusive food, but is bestused with milk, meat, and other foods richer in proteid substances. Sweetpotatoes, of late years extensively used as food, are rich in starch andsugar. Arrowroot, sago, tapioca, and similar foods are nutritious, andeasily digested, and with milk furnish excellent articles of diet, especially for invalids and children. 

Explanation of the Graphic Chart. The graphic chart, on the nextpage, presents in a succinct and easily understood form the composition offood materials as they are bought in the market, including the edible andnon-edible portions. It has been condensed from Dr. W. O. Atwater'svaluable monograph on "Foods and Diet. " This work is known as the Yearbookof the U. S. Department of Agriculture for 1894. 

KEY: 1, percentage of nutrients; 2, fuel value of 1 pound in calories. Theunit of heat, called a _calorie_, or gramme-degree, is the amount of heatwhich is necessary to raise one gramme (15. 43 grains) of water one degreecentigrade (1. 8 degrees Fahr. ). A, round beef; B, sirloin beef; C, ribbeef; D, leg of mutton; E, spare rib of pork; F, salt pork; G, smoked ham;H, fresh codfish; I, oysters; J, milk; K, butter; L, cheese; M, eggs; N, wheat bread; O, corn meal; P, oatmeal; Q, dried beans; R, rice; S, potatoes; T, sugar. 

This table, among other things, shows that the flesh of fish contains morewater than that of warm-blooded animals. It may also be seen that animalfoods contain the most water; and vegetable foods, except potatoes, themost nutrients. Proteids and fats exist only in small proportions in mostvegetables, except beans and oatmeal. Vegetable foods are rich incarbohydrates while meats contain none. The fatter the meat the less theamount of water. Thus very lean meat may be almost four-fifths water, andfat pork almost one-tenth water. 

[Illustration: Fig. 45. --Graphic Chart of the Composition of FoodMaterials. Composition of Food Materials. Nutritive ingredients, refuse, and fuel value. ]

114. Non-proteid Animal Foods. Butter is one of the most digestibleof animal fats, agreeable and delicate in flavor, and is on this accountmuch used as a wholesome food. Various substitutes have recently come intouse. These are all made from animal fat, chiefly that of beef, and areknown as butterine, oleomargarine, and by other trade names. Thesepreparations, if properly made, are wholesome, and may be usefulsubstitutes for butter, from which they differ but little in composition. 

115. Garden Vegetables. Various green, fresh, and succulentvegetables form an essential part of our diet. They are of importancenot so much on account of their nutritious elements, which are usuallysmall, as for the salts they supply, especially the salts of potash. It isa well-known fact that the continued use of a diet from which freshvegetables are excluded leads to a disease known as scurvy. They are alsoused for the agreeable flavor possessed by many, and the pleasant varietyand relish they give to the food. The undigested residue left by all greenvegetables affords a useful stimulus to intestinal contraction, and tendsto promote the regular action of the bowels. 

116. Fruits. A great variety of fruits, both fresh and dry, isused as food, or as luxuries. They are of little nutritive value, containing, as they do, much water and only a small amount of proteid, butare of use chiefly for the sugar, vegetable acids, and salts they contain. 

In moderate quantity, fruits are a useful addition to our regular diet. They are cooling and refreshing, of agreeable flavor, and tend to preventconstipation. Their flavor and juiciness serve to stimulate a weakappetite and to give variety to an otherwise heavy diet. If eaten inexcess, especially in an unripe or an overripe state, fruits may occasiona disturbance of the stomach and bowels, often of a severe form. 

117. Condiments. The refinements of cookery as well as the cravingof the appetite, demand many articles which cannot be classed strictly asfoods. They are called condiments, and as such may be used inmoderation. They give flavor and relish to food, excite appetite andpromote digestion. Condiments increase the pleasure of eating, and bytheir stimulating properties promote secretions of the digestive fluidsand excite the muscular contractions of the alimentary canal. 

The well-known condiments are salt, vinegar, pepper, ginger, nutmeg, cloves, and various substances containing ethereal oils and aromatics. Their excessive use is calculated to excite irritation and disorder of thedigestive organs. 

118. Salt The most important and extensively used of the condimentsis common salt. It exists in all ordinary articles of diet, but inquantities not sufficient to meet the wants of the bodily tissues. Henceit is added to many articles of food. It improves their flavor, promotescertain digestive secretions, and meets the nutritive demands of the body. The use of salt seems based upon an instinctive demand of the system forsomething necessary for the full performance of its functions. Foodwithout salt, however nutritious in other respects, is taken withreluctance and digested with difficulty. 

Salt has always played an important and picturesque part in the history ofdietetics. Reference to its worth and necessity abounds in sacred andprofane history. In ancient times, salt was the first thing placed on thetable and the last removed. The place at the long table, above or belowthe salt, indicated rank. It was everywhere the emblem of hospitality. Inparts of Africa it is so scarce that it is worth its weight in gold, andis actually used as money. Torture was inflicted upon prisoners of statein olden times by limiting the food to water and bread, without salt. Sointense may this craving for salt become, that men have often risked theirliberty and even their lives to obtain it. 

119. Water. The most important natural beverage is pure water; infact it is the only one required. Man has, however, from the earliesttimes preferred and daily used a variety of artificial drinks, among whichare tea, coffee, and cocoa. 

All beverages except certain strong alcoholic liquors, consist almostentirely of water. It is a large element of solid foods, and ourbodies are made up to a great extent of water. Everything taken into thecirculating fluids of the body, or eliminated from them, is done throughthe agency of water. As a solvent it is indispensable in all theactivities of the body. 

It has been estimated that an average-sized adult loses by means of thelungs, skin, and kidneys about eighty ounces of water every twenty-fourhours. To restore this loss about four pints must be taken daily. Aboutone pint of this is obtained from the food we eat, the remaining threepints being taken as drink. One of the best ways of supplying water to thebody is by drinking it in its pure state, when its solvent properties canbe completely utilized. The amount of water consumed depends largely uponthe amount of work performed by the body, and upon the temperature. 

Being one of the essential elements of the body, it is highly importantthat water should be free from harmful impurities. If it contain the germsof disease, sickness may follow its use. Without doubt the most importantfactor in the spread of disease is, with the exception of impure air, impure water. The chief agent in the spread of typhoid fever isimpure water. So with cholera, the evidence is overwhelming that filthywater is an all-powerful agent in the spread of this terrible disease. 

120. Tea, Coffee, and Cocoa. The active principle of tea is calledtheine; that of coffee, caffeine, and of cocoa, theobromine. They alsocontain an aromatic, volatile oil, to which they owe their distinctiveflavor. Tea and coffee also contain an astringent called tannin, whichgives the peculiar bitter taste to the infusions when steeped too long. Incocoa, the fat known as cocoa butter amounts to fifty per cent. 

121. Tea. It has been estimated that one-half of the human race nowuse tea, either habitually or occasionally. Its use is a prolific sourceof indigestion, palpitation of the heart, persistent wakefulness, and ofother disorders. When used at all it should be only in moderation. Personswho cannot use it without feeling its hurtful effects, should leave italone. It should not be taken on an empty stomach, nor sipped after everymouthful of food. 

122. Coffee. Coffee often disturbs the rhythm of the heart and causespalpitation. Taken at night, coffee often causes wakefulness. This effectis so well known that it is often employed to prevent sleep. Immoderateuse of strong coffee may produce other toxic effects, such as musculartremors, nervous anxiety, sick-headache, palpitation, and variousuncomfortable feelings in the cardiac region. Some persons cannot drinkeven a small amount of tea or coffee without these unpleasant effects. These favorite beverages are unsuitable for young people. 

123. Cocoa. The beverage known as cocoa comes from the seeds of thecocoa-tree, which are roasted like the coffee berries to develop thearoma. Chocolate is manufactured cocoa, --sugar and flavors being added tothe prepared seeds. Chocolate is a convenient and palatable form of highlynutritious food. For those with whom tea and coffee disagree, it may be anagreeable beverage. The large quantity of fat which it contains, however, often causes it to be somewhat indigestible. 

124. Alcoholic Beverages. There is a class of liquids which arecertainly not properly food or drink, but being so commonly used asbeverages, they seem to require special notice in this chapter. In viewof the great variety of alcoholic beverages, the prevalence of theiruse, and the very remarkable deleterious effects they produce upon thebodily organism, they imperatively demand our most careful attention, bothfrom a physiological and an hygienic point of view. 

125. Nature of Alcohol. The ceaseless action of minute forms of plantlife, in bringing about the decomposition of the elaborated products oforganized plant or animal structures, will be described in more detail(secs. 394-398). 

All such work of vegetable organisms, whether going on in the mouldingcheese, in the souring of milk, in putrefying meat, in rotting fruit, orin decomposing fruit juice, is essentially one of fermentation, caused by these minute forms of plant life. There are many kinds offermentation, each with its own special form of minute plant life ormicro-organism. 

In this section we are more especially concerned about that fermentationwhich results from the decomposition of sweet fruit, plant, or othervegetable, juices which are composed largely of water containing sugar andflavoring matters. 

This special form of fermentation is known as alcoholic or vinousfermentation, and the micro-organisms that cause it are familiarly termedalcoholic ferments. The botanist classes them as _Saccharomycetes_, ofwhich there are several varieties. Germs of _Saccharomycetes_ are found onthe surfaces and stems of fruit as it is ripening. While the fruit remainswhole these germs have no power to invade the juice, and even when theskins are broken the conditions are less favorable for their work than forthat of the moulds, [18] which are the cause of the rotting of fruit. 

But when fruit is crushed and its juice pressed out, the_Saccharomycetes_ are carried into it where they cannot get the oxygenthey need from the air. They are then able to obtain oxygen by taking itfrom the sugar of the juice. By so doing they cause a breaking up of thesugar and a rearrangement of its elements. Two new substances are formedin this decomposition of sugar, viz. , carbon dioxid, which arisesfrom the liquid in tiny bubbles, and alcohol, a poison whichremains in the fermenting fluid. 

Now we must remember that fermentation entirely changes the nature of thesubstance fermented. For all forms of decomposition this one law holdsgood. Before alcoholic fermentation, the fruit juice was wholesome andbeneficial; after fermentation, it becomes, by the action of the minutegerms, a poisonous liquid known as alcohol, and which forms an essentialpart of all intoxicating beverages. 

Taking advantage of this great law of fermentation which dominates therealm of nature, man has devised means to manufacture various alcoholicbeverages from a great variety of plant structures, as ripe grapes, pears, apples, and other fruits, cane juices, corn, the malt of barley, rye, wheat, and other cereals. 

The process differs according to the substance used and the manner inwhich it is treated, but the ultimate outcome is always the same, viz. , the manufacture of a beverage containing a greater or lessproportion of alcoholic poison. By the process of _distillation_, new andstronger liquor is made. Beverages thus distilled are known as ardentspirits. Brandy is distilled from wine, rum from fermented molasses, andcommercial alcohol mostly from whiskey. 

The poisonous element in all forms of intoxicating drinks, and the one sofraught with danger to the bodily tissues, is the alcohol theycontain. The proportion of the alcoholic ingredient varies, being about 50per cent in brandy, whiskey, and rum, about 20 to 15 per cent in wines, down to 5 per cent, or less, in the various beers and cider; but whetherthe proportion of alcohol be more or less, the same element of danger isalways present. 

126. Effects of Alcoholic Beverages upon the Human System. One of themost common alcoholic beverages is wine, made from the juice of grapes. Asthe juice flows from the crushed fruit the ferments are washed from theskins and stems into the vat. Here they bud and multiply rapidly, producing alcohol. In a few hours the juice that was sweet and wholesomewhile in the grape is changed to a poisonous liquid, capable of injuringwhoever drinks it. One of the gravest dangers of wine-drinking is thepower which the alcohol in it has to create a thirst which demands morealcohol. The spread of alcoholism in wine-making countries is anillustration of this fact. 

Another alcoholic beverage, common in apple-growing districts, is cider. Until the microscope revealed the ferment germ on the "bloom" of theapple-skin, very little was known of the changes produced in cider duringthe mysterious process of "working. " Now, when we see the bubbles of gasin the glass of cider we know what has produced them, and we know too thata poison which we do not see is there also in corresponding amounts. Wehave learned, too, to trace the wrecked hopes of many a farmer's family tothe alcohol in the cider which he provided so freely, supposing itharmless. 

Beer and other malt liquors are made from grain. By sprouting the grain, which changes its starch to sugar, and then dissolving out the sugar withwater, a sweet liquid is obtained which is fermented with yeast, one kindof alcoholic ferment. Some kinds of beer contain only a small percentageof alcohol, but these are usually drunk in proportionately large amounts. The life insurance company finds the beer drinker a precarious risk; thesurgeon finds him an unpromising subject; the criminal court finds himconspicuous in its proceedings. The united testimony from all thesesources is that beer is demoralizing, mentally, morally, and physically. 

127. Cooking. The process through which nearly all food used bycivilized man has to pass before it is eaten is known as cooking. Very few articles indeed are consumed in their natural state, theexceptions being eggs, milk, oysters, fruit and a few vegetables. Man isthe only animal that cooks his food. Although there are savage races thathave no knowledge of cooking, civilized man invariably cooks most of hisfood. It seems to be true that as nations advance in civilization theymake a proportionate advance in the art of cooking. 

Cooking answers most important purposes in connection with our food, especially from its influence upon health. It enables food to be morereadily chewed, and more easily digested. Thus, a piece of meat when rawis tough and tenacious, but if cooked the fibers lose much of theirtoughness, while the connective tissues are changed into a soft andjelly-like mass. Besides, the meat is much more readily masticated andacted upon by the digestive fluids. So cooking makes vegetables and grainssofter, loosens their structure, and enables the digestive juices readilyto penetrate their substance. 

Cooking also improves or develops flavors in food, especially in animalfoods, and thus makes them attractive and pleasant to the palate. Theappearance of uncooked meat, for example, is repulsive to our taste, butby the process of cooking, agreeable flavors are developed which stimulatethe appetite and the flow of digestive fluids. 

Another important use of cooking is that it kills any minute parasites orgerms in the raw food. The safeguard of cooking thus effectually removessome important causes of disease. The warmth that cooking imparts to foodis a matter of no slight importance; for warm food is more readilydigested, and therefore nourishes the body more quickly. 

The art of cooking plays a very important part in the matter of health, and thus of comfort and happiness. Badly cooked and ill-assorted foods areoften the cause of serious disorders. Mere cooking is not enough, but goodcooking is essential. 

Experiments. 

Experiments with the Proteids. 

Experiment 31. As a type of the group of proteids we take the whiteof egg, egg-white or egg-albumen. Break an egg carefully, so as not to mixthe white with the yolk. Drop about half a teaspoonful of the raw white ofegg into half a pint of distilled water. Beat the mixture vigorously witha glass rod until it froths freely. Filter through several folds of muslinuntil a fairly clear solution is obtained. 

Experiment 32. To a small quantity of this solution in a test tubeadd strong nitric acid, and boil. Note the formation of a whiteprecipitate, which turns yellow. After cooling, add ammonia, and note thatthe precipitate becomes orange. 

Experiment 33. Add to the solution of egg-albumen, excess of strongsolution of caustic soda (or potash), and then a drop or two of verydilute solution (one per cent) of copper sulphate. A violet color isobtained which deepens on boiling. 

Experiment 34. Boil a small portion of the albumen solution in a testtube, adding drop by drop dilute acetic acid (two per cent) until a flakycoagulum of insoluble albumen separates. 

Experiments with Starch. 

Experiment 35. Wash a potato and peel it. Grate it on a nutmeg graterinto a tall cylindrical glass full of water. Allow the suspended particlesto subside, and after a time note the deposit. The lowest layer consistsof a white powder, or starch, and above it lie coarser fragments ofcellulose and other matters. 

Experiment 36. Examine under the microscope a bit of the above whitedeposit. Note that each starch granule shows an eccentric hilum withconcentric markings. Add a few drops of very dilute solution of iodine. Each granule becomes blue, while the markings become more distinct. 

Experiment 37. Examine a few of the many varieties of other kinds ofstarch granules, as in rice, arrowroot, etc. Press some dry starch powderbetween the thumb and forefinger, and note the peculiar crepitation. 

Experiment 38. Rub a few bits of starch in a little cold water. Put alittle of the mixture in a large test tube, and then fill with boilingwater. Boil until an imperfect opalescent solution is obtained. 

Experiment 39. Add powdered dry starch to cold water. It isinsoluble. Filter and test the filtrate with iodine. It gives no bluecolor. 

Experiment 40. Boil a little starch with water; if there is enoughstarch it sets on cooling and a paste results. 

Experiment 41. Moisten some flour with water until it forms a tough, tenacious dough; tie it in a piece of cotton cloth, and knead it in avessel containing water until all the starch is separated. There remainson the cloth a grayish white, sticky, elastic "gluten, " made up ofalbumen, some of the ash, and fats. Draw out some of the gluten intothreads, and observe its tenacious character. 

Experiment 42. Shake up a little flour with ether in a test tube, with a tight-fitting cork. Allow the mixture to stand for an hour, shakingit from time to time. Filter off the ether, and place some of it on aperfectly clean watch glass. Allow the ether to evaporate, when a greasystain will be left, thus showing the presence of fats in the flour. 

Experiment 43. Secure a specimen of the various kinds of flour, andmeal, peas, beans, rice, tapioca, potato, etc. Boil a small quantity ofeach in a test tube for some minutes. Put a bit of each thus cooked on awhite plate, and pour on it two or three drops of the tincture of iodine. Note the various changes of color, --blue, greenish, orange, or yellowish. 

Experiments with Milk. 

Experiment 44. Use fresh cow's milk. Examine the naked-eye characterof the milk. Test its reaction with litmus paper. It is usually neutral orslightly alkaline. 

Experiment 45. Examine with the microscope a drop of milk, notingnumerous small, highly refractive oil globules floating in a fluid. 

Experiment 46. Dilute one ounce of milk with ten times its volume ofwater. Add cautiously dilute acetic acid until there is a copious, granular-looking precipitate of the chief proteid of milk (caseinogen), formerly regarded as a derived albumen. This action is hastened byheating. 

Experiment 47. Saturate milk with Epsom salts, or common salt. Theproteid and fat separate, rise to the surface, and leave a clear fluidbeneath. 

Experiment 48. Place some milk in a basin; heat it to about 100 degreesF. , and add a few drops of acetic acid. The mass curdles and separatesinto a solid curd (proteid and fat) and a clear fluid (the whey), whichcontains the lactose. 

Experiment 49. Take one or two teaspoonfuls of fresh milk in a testtube; heat it, and add a small quantity of extract of rennet. Note thatthe whole mass curdles in a few minutes, so that the tube can be invertedwithout the curd falling out. Soon the curd shrinks, and squeezes out aclear, slightly yellowish fluid, the whey. 

Experiment 50. Boil the milk as before, and allow it to cool; thenadd rennet. No coagulation will probably take place. It is more difficultto coagulate boiled milk with rennet than unboiled milk. 

Experiment 51. Test fresh milk with red litmus paper; it should turnthe paper pale blue, showing that it is slightly alkaline. Place aside fora day or two, and then test with blue litmus paper; it will be found to beacid. This is due to the fact that lactose undergoes the lactic acidfermentation. The lactose is converted into lactic acid by means of aspecial ferment. 

Experiment 52. Evaporate a small quantity of milk to dryness in anopen dish. After the dry residue is obtained, continue to apply heat;observe that it chars and gives off pungent gases. Raise the temperatureuntil it is red hot; allow the dish then to cool; a fine white ash will beleft behind. This represents the _inorganic matter_ of the milk. 

Experiments with the Sugars. 

Experiment 53. Cane sugar is familiar as cooking and table sugar. Thelittle white grains found with raisins are grape sugar, or glucose. Milksugar is readily obtained of the druggist. Prepare a solution of thevarious sugars by dissolving a small quantity of each in water. Heat eachsolution with sulphuric acid, and it is seen to darken or char slowly. 

Experiment 54. Place some Fehling solution (which can be readilyobtained at the drug store as a solution, or tablets may be bought whichanswer the same purpose) in a test tube, and boil. If no yellowdiscoloration takes place, it is in good condition. Add a few drops of thegrape sugar solution and boil, when the mixture suddenly turns to anopaque yellow or red color. 

Experiment 55. Repeat same experiment with milk sugar. 

Chapter VI. 

Digestion. 

128. The Purpose of Digestion. As we have learned, our bodies aresubject to continual waste, due both to the wear and tear of theirsubstance, and to the consumption of material for the production of theirheat and energy. The waste occurs in no one part alone, but in all thetissues. 

Now, the blood comes into direct contact with every one of these tissues. The ultimate cells which form the tissues are constantly being bathed bythe myriads of minute blood-vessels which bring to the cells the rawmaterial needed for their continued renewal. These cells are able toselect from the nutritive fluid whatever they require to repair theirwaste, and to provide for their renewed activity. At the same time, theblood, as it bathes the tissues, sweeps into its current and bears awaythe products of waste. 

Thus the waste occurs in the tissues and the means of repair are obtainedfrom the blood. The blood is thus continually being impoverished by havingits nourishment drained away. How, then, is the efficiency of the bloodmaintained? The answer is that while the ultimate purpose of the food isfor the repair of the waste, its immediate destination is the blood. [19]

129. Absorption of Food by the Blood. How does the food pass from thecavity of the stomach and intestinal canal into the blood-vessels? Thereare no visible openings which permit communication. It is done by what inphysics is known as _endosmotic_ and _exosmotic_ action. That is, wheneverthere are two solutions of different densities, separated only by ananimal membrane, an interchange will take place between them through themembrane. 

To illustrate: in the walls of the stomach and intestines there is anetwork of minute vessels filled with blood, --a liquid containing manysubstances in solution. The stomach and intestinal canal also containliquid food, holding many substances in solution. A membrane, made up ofthe extremely thin walls of the blood-vessels and intestines, separatesthe liquids. An exchange takes place between the blood and the contents ofthe stomach and bowels, by which the dissolved substances of food passthrough the separating membranes into the blood. 

[Illustration: Fig. 46. --Cavities of the Mouth, Pharynx, etc. (Section inthe middle line designed to show the mouth in its relations to the nasalfossæ, the pharynx, and the larynx. )

 A, sphenoidal sinus; B, internal orifice of Eustachian tube; C, velum palati; D, anterior pillar of soft palate; E, posterior pillar of soft palate; F, tonsil; H, lingual portion of the pharynx; K, lower portion of the pharynx; L, larynx; M, section of hyoid bone; N, epiglottis; O, palatine arch]

This change, by which food is made ready to pass into the blood, constitutes food-digestion, and the organs concerned in bringingabout this change in the food are the digestive organs. 

130. The General Plan of Digestion. It is evident that the digestiveorgans will be simple or complex, according to the amount of change whichis necessary to prepare the food to be taken up by the blood. If therequisite change is slight, the digestive organs will be few, and theirstructure simple. But if the food is varied and complex in composition, the digestive apparatus will be complex. This condition applies to thefood and the digestion of man. 

[Illustration: Fig. 47. --Diagram of the Structure of Secreting Glands. 

 A, simple tubular gland; B, gland with mouth shut and sac formed; C, gland with a coiled tube; D, plan of part of a racemose gland]

The digestive apparatus of the human body consists of the alimentary canaland tributary organs which, although outside of this canal, communicatewith it by ducts. The alimentary canal consists of the mouth, the pharynx, the oesophagus, the stomach, and the intestines. Other digestive organswhich are tributary to this canal, and discharge their secretions into it, are the salivary glands, [20] the liver, and the pancreas. 

The digestive process is subdivided into three steps, which take place inthe mouth, in the stomach, and in the intestines. 

131. The Mouth. The mouth is the cavity formed by the lips, thecheeks, the palate, and the tongue. Its bony roof is made up of the upperjawbone on each side, and the palate bones behind. This is the _hardpalate_, and forms only the front portion of the roof. The continuation ofthe roof is called the _soft palate_, and is made up of muscular tissuecovered with mucous membrane. 

The mouth continues behind into the throat, the separation between the twobeing marked by fleshy pillars which arch up from the sides to form thesoft palate. In the middle of this arch there hangs from its free edge alittle lobe called the uvula. On each side where the pillars begin toarch is an almond-shaped body known as the tonsil. When we take cold, one or both of the tonsils may become inflamed, and so swollen as toobstruct the passage into the throat. The mouth is lined with mucousmembrane, which is continuous with that of the throat, oesophagus, stomach, and intestines (Fig. 51). 

132. Mastication, or Chewing. The first step of the process ofdigestion is mastication, the cutting and grinding of the food by theteeth, effected by the vertical and lateral movements of the lower jaw. While the food is thus being crushed, it is moved to and fro by the variedmovements of the tongue, that every part of it may be acted upon by theteeth. The advantage of this is obvious. The more finely the food isdivided, the more easily will the digestive fluids reach every part of it, and the more thoroughly and speedily will digestion ensue. 

The act of chewing is simple and yet important, for if hurriedly orimperfectly done, the food is in a condition to cause disturbance in thedigestive process. Thorough mastication is a necessary introduction to themore complicated changes which occur in the later digestion. 

133. The Teeth. The teeth are attached to the upper and lowermaxillary bones by roots which sink into the sockets of the jaws. Eachtooth consists of a _crown_, the visible part, and one or more fangs, buried in the sockets. There are in adults 32 teeth, 16 in each jaw. 

Teeth differ in name according to their form and the uses to which theyare specially adapted. Thus, at the front of the jaws, the incisors, or cutting teeth, number eight, two on each side. They have a single rootand the crown is beveled behind, presenting a chisel-like edge. Theincisors divide the food, and are well developed in rodents, as squirrels, rats, and beavers. 

Next come the canine teeth, or cuspids, two in each jaw, so calledfrom their resemblance to the teeth of dogs and other flesh-eatinganimals. These teeth have single roots, but their crowns are more pointedthan in the incisors. The upper two are often called eye teeth, and thelower two, stomach teeth. Next behind the canines follow, on each side, two bicuspids. Their crowns are broad, and they have two roots. Thethree hindmost teeth in each jaw are the molars, or grinders. Theseare broad teeth with four or five points on each, and usually each molarhas three roots. 

The last molars are known as the wisdom teeth, as they do not usuallyappear until the person has reached the "years of discretion. " All animalsthat live on grass, hay, corn, and the cereals generally, have largegrinding teeth, as the horse, ox, sheep, and elephant. 

The following table shows the teeth in their order:

 Mo. Bi. Ca. In. In. Ca. Bi. Mo. 

 Upper 3 2 1 2 | 2 1 2 3 = 16 | } = 32 Lower 3 2 1 2 | 2 1 2 3 = 16

The vertical line indicates the middle of the jaw, and shows that on eachside of each jaw there are eight teeth. 

134. Development of the Teeth. The teeth just described are thepermanent set, which succeeds the temporary or milk teeth. The latter are twenty in number, ten in each jaw, of which the four in themiddle are incisors. The tooth beyond on each side is an eye tooth, andthe next two on each side are bicuspids, or premolars. 

The milk teeth appear during the first and second years, and last untilabout the sixth or seventh year, from which time until the twelfth orthirteenth year, they are gradually pushed out, one by one, by thepermanent teeth. The roots of the milk teeth are much smaller than thoseof the second set. 

[Illustration: Fig. 48. --Temporary and Permanent Teeth together. 

_Temporary teeth:_ A, central incisors; B lateral incisors; C, canines; D, anterior molars; E, posterior molars

_Permanent teeth:_ F, central incisors; H, lateral incisors; K, canines; L, first bicuspids; M, second biscuspids; N, first molars]

The plan of a gradual succession of teeth is a beautiful provision ofnature, permitting the jaws to increase in size, and preserving therelative position and regularity of the successive teeth. 

[Illustration: Fig. 49. --Showing the Principal Organs of the Thorax andAbdomen _in situ_. (The principal muscles are seen on the left, andsuperficial veins on the right. )]

135. Structure of the Teeth. If we should saw a tooth down throughits center we would find in the interior a cavity. This is the pulpcavity, which is filled with the dental pulp, a delicate substancerichly supplied with nerves and blood-vessels, which enter the tooth bysmall openings at the point of the root. The teeth are thus nourished likeother parts of the body. The exposure of the delicate pulp to the air, dueto the decay of the dentine, gives rise to the pain of toothache. 

Surrounding the cavity on all sides is the hard substance known as thedentine, or tooth ivory. Outside the dentine of the root is asubstance closely resembling bone, called cement. In fact, it is truebone, but lacks the Haversian canals. The root is held in its socketby a dense fibrous membrane which surrounds the cement as the periosteumdoes bone. 

[Illustration: Fig. 50. --Section of Face. (Showing the parotid andsubmaxillary glands. )]

The crown of the tooth is not covered by cement, but by the hardenamel, which forms a strong protection for the exposed part. Whenthe teeth are first "cut, " the surface of the enamel is coated with adelicate membrane which answers to the Scriptural phrase "the skin of theteeth. " This is worn off in adult life. 

136. Insalivation. The thorough mixture of the saliva with the foodis called insalivation. While the food is being chewed, it ismoistened with a fluid called saliva, which flows into the mouth fromsix little glands. There are on each side of the mouth three salivaryglands, which secrete the saliva from the blood. The parotid issituated on the side of the face in front of the ear. The disease, commonin childhood, during which this gland becomes inflamed and swollen, isknown as the "mumps. " The submaxillary gland is placed below and tothe inner side of the lower jaw, and the sublingual is on the floorof the mouth, between the tongue and the gums. Each gland opens into themouth by a little duct. These glands somewhat resemble a bunch of grapeswith a tube for a stalk. 

The saliva is a colorless liquid without taste or smell. Itsprincipal element, besides water, is a ferment called _ptyalin_, which hasthe remarkable property of being able to change starch into a form ofcane-sugar, known as maltose. 

Thus, while the food is being chewed, another process is going on by whichstarch is changed into sugar. The saliva also moistens the food into amass for swallowing, and aids in speech by keeping the mouth moist. 

The activity of the salivary glands is largely regulated by their abundantsupply of nerves. Thus, the saliva flows into the mouth, even at thesight, smell, or thought of food. This is popularly known as "making themouth water. " The flow of saliva may be checked by nervous influences, assudden terror and undue anxiety. 

 Experiment 56. _To show the action of saliva on starch_. Saliva for experiment may be obtained by chewing a piece of India rubber and collecting the saliva in a test tube. Observe that it is colorless and either transparent or translucent, and when poured from one vessel to another is glairy and more or less adhesive. Its reaction is alkaline to litmus paper. 

 Experiment 57. Make a thin paste from pure starch or arrowroot. Dilute a little of the saliva with five volumes of water, and filter it. This is best done through a filter perforated at its apex by a pin-hole. In this way all air-bubbles are avoided. Label three test tubes _A, B_, and _C_. In _A_, place starch paste; in _B_, saliva; and in _C_ one volume of saliva and three volumes of starch paste. Place them for ten minutes in a water bath at about 104 degrees Fahrenheit. 

 Test portions of all three for a reducing sugar, by means of Fehling's solution or tablets. [21] _A_ and _B_ give no evidence of sugar, while _C_ reduces the Fehling, giving a yellow or red deposit of cuprous oxide. Therefore, starch is converted into a reducing sugar by the saliva. This is done by the ferment ptyalin contained in saliva. 

137. The Pharynx and OEsophagus. The dilated upper part of thealimentary canal is called the pharynx. It forms a blind sac abovethe level of the mouth. The mouth opens directly into the pharynx, andjust above it are two openings leading into the posterior passages of thenose. There are also little openings, one on each side, from which beginthe Eustachian tubes, which lead upward to the ear cavities. 

The windpipe opens downward from the pharynx, but this communication canbe shut off by a little plate or lid of cartilage, the epiglottis. During the act of swallowing, this closes down over the entrance to thewindpipe, like a lid, and prevents the food from passing into theair-passages. This tiny trap-door can be seen, by the aid of a mirror, ifwe open the mouth wide and press down the back of the tongue with thehandle of a spoon (Figs. 46, 84, and 85). 

Thus, there are six openings from the pharynx; the oesophagus beingthe direct continuation from it to the stomach. If we open the mouthbefore a mirror we see through the fauces the rear wall of the pharynx. Inits lining membrane is a large number of glands, the secretion from whichduring a severe cold may be quite troublesome. 

The oesophagus, or gullet, is a tube about nine inches long, reaching from the throat to the stomach. It lies behind the windpipe, pierces the diaphragm between the chest and abdomen, and opens into thestomach. It has in its walls muscular fibers, which, by their worm-likecontractions, grasp the successive masses of food swallowed, and pass themalong downwards into the stomach. 

138. Deglutition, or Swallowing. The food, having been well chewedand mixed with saliva, is now ready to be swallowed as a soft, pasty mass. The tongue gathers it up and forces it backwards between the pillars ofthe fauces into the pharynx. 

If we place the fingers on the "Adam's apple, " and then pretend toswallow something, we can feel the upper part of the windpipe and theclosing of its lid (epiglottis), so as to cover the entrance and preventthe passage of food into the trachea. 

There is only one pathway for the food to travel, and that is down theoesophagus. The slow descent of the food may be seen if a horse ordog be watched while swallowing. Even liquids do not fall or flow down thefood passage. Hence, acrobats can drink while standing on their heads, ora horse with its mouth below the level of the oesophagus. The food isunder the control of the will until it has entered the pharynx; all thelater movements are involuntary. 

[Illustration: Fig. 51. --A View into the Back Part of the Adult Mouth. (The head is represented as having been thrown back, and the tongue drawnforward. )

 A, B, incisors; C, canine; D, E, bicuspids; F, H, K, molars; M, anterior pillar of the fauces; N, tonsil; L, uvula; O, upper part of the pharynx; P, tongue drawn forward; R, linear ridge, or raphé. ]

139. The Stomach. The stomach is the most dilated portion of thealimentary canal and the principal organ of digestion. Its form is noteasily described. It has been compared to a bagpipe, which it resemblessomewhat, when moderately distended. When empty it is flattened, and insome parts its opposite walls are in contact. 

We may describe the stomach as a pear-shaped bag, with the large end tothe left and the small end to the right. It lies chiefly on the left sideof the abdomen, under the diaphragm, and protected by the lower ribs. Thefact that the large end of the stomach lies just beneath the diaphragm andthe heart, and is sometimes greatly distended on account of indigestion orgas, may cause feelings of heaviness in the chest or palpitation of theheart. The stomach is subject to greater variations in size than any otherorgan of the body, depending on its contents. Just after a moderate mealit averages about twelve inches in length and four in diameter, with acapacity of about four pints. 

[Illustration: Fig. 52. --The Stomach. A, cardiac end; B, pyloric end, C, lesser curvature, D, greater curvature]

The orifice by which the food enters is called the cardiac opening, because it is near the heart. The other opening, by which the food leavesthe stomach, and where the small intestine begins, is the pyloricorifice, and is guarded by a kind of valve, known as the pylorus, orgatekeeper. The concave border between the two orifices is called the_small curvature_, and the convex as the _great curvature_, of thestomach. 

140. Coats of Stomach. The walls of the stomach are formed by fourcoats, known successively from without as serous, muscular, sub-mucous, and mucous. The outer coat is the serous membranewhich lines the abdomen, --the peritoneum (note, p. 135). The secondcoat is muscular, having three sets of involuntary muscular fibers. Theouter set runs lengthwise from the cardiac orifice to the pylorus. Themiddle set encircles all parts of the stomach, while the inner setconsists of oblique fibers. The third coat is the sub-mucous, made up ofloose connective tissues, and binds the mucous to the muscular coat. Lastly there is the mucous coat, a moist, pink, inelastic membrane, whichcompletely lines the stomach. When the stomach is not distended, themucous layer is thrown into folds presenting a corrugated appearance. 

[Illustration: Fig. 53. --Pits in the Mucous Membrane of the Stomach, andOpenings of the Gastric Glands. (Magnified 20 diameters. )]

141. The Gastric Glands. If we were to examine with a hand lens theinner surface of the stomach, we would find it covered with little pits, or depressions, at the bottom of which would be seen dark dots. These dotsare the openings of the gastric glands. In the form of fine, wavytubes, the gastric glands are buried in the mucous membrane, their mouthsopening on the surface. When the stomach is empty the mucous membrane ispale, but when food enters, it at once takes on a rosy tint. This is dueto the influx of blood from the large number of very minute blood-vesselswhich are in the tissue between the rows of glands. 

The cells of the gastric glands are thrown into a state of greateractivity by the increased quantity of blood supply. As a result, soonafter food enters the stomach, drops of fluid collect at the mouths of theglands and trickle down its walls to mix with the food. Thus these glandsproduce a large quantity of gastric juice, to aid in the digestion offood. 

142. Digestion in the Stomach. When the food, thoroughly mixed withsaliva, reaches the stomach, the cardiac end of that organ is closed aswell as the pyloric valve, and the muscular walls contract on thecontents. A spiral wave of motion begins, becoming more rapid as digestiongoes on. Every particle of food is thus constantly churned about in thestomach and thoroughly mixed with the gastric juice. The action of thejuice is aided by the heat of the parts, a temperature of about 99 degreesFahrenheit. 

The gastric juice is a thin almost colorless fluid with a sour tasteand odor. The reaction is distinctly acid, normally due to freehydrochloric acid. Its chief constituents are two ferments called pepsinand rennin, free hydrochloric acid, mineral salts, and 95 per cent ofwater. 

[Illustration: Fig. 54. --A highly magnified view of a peptic or gastricgland, which is represented as giving off branches. It shows the columnarepithelium of the surface dipping down into the duct D of the gland, fromwhich two tubes branch off. Each tube is lined with columnar epithelialcells, and there is a minute central passage with the "neck" at N. Hereand there are seen other special cells called parietal cells, P, which aresupposed to produce the acid of the gastric juice. The principal cells arerepresented at C. ]

Pepsin the important constituent of the gastric juice, has thepower, in the presence of an acid, of dissolving the proteid food-stuffs. Some of which is converted into what are called _peptones_, both solubleand capable of filtering through membranes. The gastric juice has noaction on starchy foods, neither does it act on fats, except to dissolvethe albuminous walls of the fat cells. The fat itself is thus set free inthe form of minute globules. The whole contents of the stomach now assumethe appearance and the consistency of a thick soup, usually of a grayishcolor, known as chyme. 

It is well known that "rennet" prepared from the calf's stomach has aremarkable effect in rapidly curdling milk, and this property is utilizedin the manufacture of cheese. Now, a similar ferment is abundant in thegastric juice, and may be called _rennin_. It causes milk to clot, anddoes this by so acting on the casein as to make the milk set into a jelly. Mothers are sometimes frightened when their children, seemingly in perfecthealth, vomit masses of curdled milk. This curdling of the milk is, however, a normal process, and the only noteworthy thing is its rejection, usually due to overfeeding. 

 Experiment 58. _To show that pepsin and acid are necessary for gastric digestion. _ Take three beakers, or large test tubes; label them _A_, _B_, _C_. Put into _A_ water and a few grains of powdered pepsin. Fill _B_ two-thirds full of dilute hydrochloric acid (one teaspoonful to a pint), and fill _C_ two-thirds full of hydrochloric acid and a few grains of pepsin. Put into each a small quantity of well-washed fibrin, and place them all in a water bath at 104 degrees Fahrenheit for half an hour. 

 Examine them. In _A_, the fibrin is unchanged; in _B_, the fibrin is clear and swollen up; in _C_, it has disappeared, having first become swollen and clear, and completely dissolved, being finally converted into peptones. Therefore, both acid and ferment are required for gastric digestion. 

 Experiment 59. Half fill with dilute hydrochloric acid three large test tubes, labelled _A_, _B_, _C_. Add to each a few grains of pepsin. Boil _B_, and make _C_ faintly alkaline with sodic carbonate. The alkalinity may be noted by adding previously some neutral litmus solution. Add to each an equal amount--a few threads--of well-washed fibrin which has been previously steeped for some time in dilute hydrochloric acid, so that it is swollen and transparent. Keep the tubes in a water-bath at about 104 degrees Fahrenheit for an hour and examine them at intervals of twenty minutes. 

 After five to ten minutes the fibrin in _A_ is dissolved and the fluid begins to be turbid. In _B_ and _C_ there is no change. Even after long exposure to 100 degrees Fahrenheit there is no change in _B_ and _C_. 

 After a variable time, from one to four hours, the contents of the stomach, which are now called chyme, begin to move on in successive portions into the next part of the intestinal canal. The ring-like muscles of the pylorus relax at intervals to allow the muscles of the stomach to force the partly digested mass into the small intestines. This action is frequently repeated, until even the indigestible masses which the gastric juice cannot break down are crowded out of the stomach into the intestines. From three to four hours after a meal the stomach is again quite emptied. 

A certain amount of this semi-liquid mass, especially the peptones, withany saccharine fluids, resulting from the partial conversion of starch orotherwise, is at once absorbed, making its way through the delicatevessels of the stomach into the blood current, which is flowing throughthe gastric veins to the portal vein of the liver. 

[Illustration: Fig. 55. --A Small Portion of the Mucous Membrane of theSmall Intestine. (Villi are seen surrounded with the openings of thetubular glands. ) [Magnified 20 diameters. ]]

143. The Small Intestine. At the pyloric end of the stomach thealimentary canal becomes again a slender tube called the smallintestine. This is about twenty feet long and one inch in diameter, and is divided, for the convenience of description, into three parts. 

The first 12 inches is called the duodenum. Into this portion opensthe bile duct from the liver with the duct from the pancreas, these havingbeen first united and then entering the intestine as a common duct. 

The next portion of the intestine is called the jejunum, because itis usually empty after death. 

The remaining portion is named the ileum, because of the many foldsinto which it is thrown. It is the longest part of the small intestine, and terminates in the right iliac region, opening into the largeintestine. This opening is guarded by the folds of the membrane formingthe ileo-cæcal valve, which permits the passage of material from thesmall to the large intestine, but prevents its backward movement. 

144. The Coats of the Small Intestine. Like the stomach, the smallintestine has four coats, the serous, muscular, sub-mucous, and mucous. The serous is the peritoneum. [22] The muscular consistsof an outer layer of longitudinal, and an inner layer of circular fibers, by contraction of which the food is forced along the bowel. The sub-mucouscoat is made up of a loose layer of tissue in which the blood-vessels andnerves are distributed. The inner, or mucous, surface has a fine, velvetyfeeling, due to a countless number of tiny, thread-like projections, called villi. They stand up somewhat like the "pile" of velvet. It isthrough these villi that the digested food passes into the blood. 

[Illustration: Fig. 56. --Sectional View of Intestinal Villi. (Black dotsrepresent the glandular openings. )]

The inner coat of a large part of the small intestine is thrown intonumerous transverse folds called _valvulæ conniventes_. These seem toserve two purposes, to increase the extent of the surface of the bowelsand to delay mechanically the progress of the intestinal contents. Buriedin the mucous layer throughout the length, both of the small and largeintestines, are other glands which secrete intestinal fluids. Thus, in thelower part of the ileum there are numerous glands in oval patches known as_Peyer's patches_. These are very prone to become inflamed and to ulcerateduring the course of typhoid fever. 

145. The Large Intestine. The large intestine begins in theright iliac region and is about five or six feet long. It is much largerthan the small intestine, joining it obliquely at short distance from itsend. A blind pouch, or dilated pocket is thus formed at the place ofjunction, called the cæcum. A valvular arrangement called theileo-cæcal valve, which is provided with a button-hole slit, forms a kindof movable partition between this part of the large intestine and thesmall intestine. 

[Illustration: Fig. 57. --Tubular Glands of the Small Intestines. 

A, B, tubular glands seen in vertical section with their orifices at C, opening upon the membrane between the villi, D, villus (Magnified 40diameters)]

Attached to the cæcum is a worm-shaped tube, about the size of a leadpencil, and from three to four inches long, called the _vermiformappendix_. Its use is unknown. This tube is of great surgical importance, from the fact that it is subject to severe inflammation, often resultingin an internal abscess, which is always dangerous and may prove fatal. Inflammation of the appendix is known as _appendicitis_, --a name quitefamiliar on account of the many surgical operations performed of lateyears for its relief. 

The large intestine passes upwards on the right side as the ascendingcolon, until the under side of the liver is reached, where it passesto the left side, as the transverse colon, below the stomach. Itthere turns downward, as the descending colon, and making an S-shapedcurve, ends in the rectum. Thus the large intestine encircles, in theform of a horseshoe, the convoluted mass of small intestines. 

Like the small intestine, the large has four coats. The mucous coat, however, has no folds, or villi, but numerous closely set glands, likesome of those of the small intestine. The longitudinal muscular fibers ofthe large intestine are arranged in three bands, or bundles, which, beingshorter than the canal itself, produce a series of bulgings or pouches inits walls. This sacculation of the large bowel is supposed to be designedfor delaying the onward flow of its contents, thus allowing more time forthe absorption of the liquid material. The blood-vessels and nerves ofthis part of the digestive canal are very numerous, and are derived fromthe same sources as those of the small intestine. 

146. The Liver. The liver is a part of the digestive apparatus, since it forms the bile, one of the digestive fluids. It is a largereddish-brown organ, situated just below the diaphragm, and on the rightside. The liver is the largest gland in the body, and weighs from 50 to 60ounces. It consists of two lobes, the right and the left, the right beingmuch the larger. The upper, convex surface of the liver is very smooth andeven; but the under surface is irregular, broken by the entrance and exitof the various vessels which belong to the organ. It is held in its placeby five ligaments, four of which are formed by double folds of theperitoneum. 

The thin front edge of the liver reaches just below the bony edge of theribs; but the dome-shaped diaphragm rises slightly in a horizontalposition, and the liver passes up and is almost wholly covered by theribs. In tight lacing, the liver is often forced downward out from thecover of the ribs, and thus becomes permanently displaced. As a result, other organs in the abdomen and pelvis are crowded together, and alsobecome displaced. 

147. Minute Structure of the Liver. When a small piece of the liveris examined under a microscope it is found to be made up of masses ofmany-sided cells, each about 1/1000 of an inch in diameter. Each group ofcells is called a _lobule_. When a single lobule is examined under themicroscope it appears to be of an irregular, circular shape, with itscells arranged in rows, radiating from the center to the circumference. Minute, hair-like channels separate the cells one from another, and unitein one main duct leading from the lobule. It is the lobules which give tothe liver its coarse, granular appearance, when torn across. 

[Illustration: Fig. 58. --Diagrammatic Section of a Villus

 A, layer of columnar epithelium covering the villus; B, central lacteal of villus; C, unstriped muscular fibers; D, goblet cell]

Now there is a large vessel called the portal vein that brings to theliver blood full of nourishing material obtained from the stomach andintestines. On entering the liver this great vein conducts itself as if itwere an artery. It divides and subdivides into smaller and smallerbranches, until, in the form of the tiniest vessels, called capillaries, it passes inward among the cells to the very center of the hepaticlobules. 

148. The Bile. We have in the liver, on a grand scale, exactly thesame conditions as obtain in the smaller and simpler glands. Thethin-walled liver cells take from the blood certain materials which theyelaborate into an important digestive fluid, called the bile. [23]This newly manufactured fluid is carried away in little canals, called_bile ducts_. These minute ducts gradually unite and form at last one mainduct, which carries the bile from the liver. This is known as the hepaticduct. It passes out on the under side of the liver, and as itapproaches the intestine, it meets at an acute angle the cystic duct whichproceeds from the gall bladder and forms with it the common bileduct. The common duct opens obliquely into the horseshoe bend of theduodenum. 

The cystic duct leads back to the under surface of the liver, whereit expands into a sac capable of holding about two ounces of fluid, and isknown as the gall bladder. Thus the bile, prepared in the depths ofthe liver by the liver cells, is carried away by the bile ducts, and maypass directly into the intestines to mix with the food. If, however, digestion is not going on, the mouth of the bile duct is closed, and inthat case the bile is carried by the cystic duct to the gall bladder. Hereit remains until such time as it is needed. 

149. Blood Supply of the Liver. We must not forget that the liveritself, being a large and important organ, requires constant nourishmentfor the work assigned to it. The blood which is brought to it by theportal vein, being venous, is not fit to nourish it. The work is done bythe arterial blood brought to it by a great branch direct from the aorta, known as the hepatic artery, minute branches of which in the form ofcapillaries, spread themselves around the hepatic lobules. 

The blood, having done its work and now laden with impurities, is pickedup by minute veinlets, which unite again and again till they at last formone great trunk called the hepatic vein. This carries the impureblood from the liver, and finally empties it into one of the large veinsof the body. 

After the blood has been robbed of its bile-making materials, it iscollected by the veinlets that surround the lobules, and finds its waywith other venous blood into the hepatic vein. In brief, blood is broughtto the liver and distributed through its substance by two distinctchannels, --the portal vein and the hepatic artery, but it leavesthe liver by one distinct channel, --the hepatic vein. 

[Illustration: Fig. 59--Showing the Relations of the Duodenum and OtherIntestinal Organs. (A portion of the stomach has been cut away. )]

150. Functions of the Liver. We have thus far studied the liver onlyas an organ of secretion, whose work is to elaborate bile for future usein the process of digestion. This is, however, only one of its functions, and perhaps not the most important. In fact, the functions of the liverare not single, but several. The bile is not wholly a digestive fluid, butit contains, also, materials which are separated from the blood to becast out of the body before they work mischief. Thus, the liver ranksabove all others as an organ of excretion, that is, it separatesmaterial of no further use to the body. 

Of the various ingredients of the bile, only the bile salts are of use inthe work of digestion, for they act upon the fats in the alimentary canal, and aid somehow in their emulsion and absorption. They appear to bethemselves split up into other substances, and absorbed with the dissolvedfats into the blood stream again. 

The third function of the liver is very different from those alreadydescribed. It is found that the liver of an animal well and regularly fed, when examined soon after death, contains a quantity of a carbohydratesubstance not unlike starch. This substance, extracted in the form of awhite powder, is really an animal starch. It is called glycogen, orliver sugar, and is easily converted into grape sugar. 

The hepatic cells appear to manufacture this glycogen and to store it upfrom the food brought by the portal blood. It is also thought the glycogenthus deposited and stored up in the liver is little by little changed intosugar. Then, as it is wanted, the liver disposes of this stored-upmaterial, by pouring it, in a state of solution, into the hepatic vein. Itis thus steadily carried to the tissues, as their needs demand, to supplythem with material to be transformed into heat and energy. 

151. The Pancreas. The pancreas, or sweetbread, is much smallerthan the liver. It is a tongue-like mass from six to eight inches long, weighing from three to four ounces, and is often compared in appearance toa dog's tongue. It is somewhat the shape of a hammer with the handlerunning to a point. 

The pancreas lies behind the stomach, across the body, from right to left, with its large head embraced in the horseshoe bend of the duodenum. Itclosely resembles the salivary glands in structure, with its main ductrunning from one end to the other. This duct at last enters the duodenumin company with the common bile duct. 

The pancreatic juice, the most powerful in the body, is clear, somewhat viscid, fluid. It has a decided alkaline reaction and is notunlike saliva in many respects. Combined with the bile, this juice actsupon the large drops of fat which pass from the stomach into the duodenumand emulsifies them. This process consists partly in producing a finesubdivision of the particles of fat, called an emulsion, and partly in achemical decomposition by which a kind of soap is formed. In this way theoils and fats are divided into particles sufficiently minute to permit oftheir being absorbed into the blood. 

Again, this most important digestive fluid produces on starch an actionsimilar to that of saliva, but much more powerful. During its short stayin the mouth, very little starch is changed into sugar, and in thestomach, as we have seen, the action of the saliva is arrested. Now, thepancreatic juice takes up the work in the small intestine and changes thegreater part of the starch into sugar. Nor is this all, for it also actspowerfully upon the proteids not acted upon in the stomach, and changesthem into peptones that do not differ materially from those resulting fromgastric digestion. The remarkable power which the pancreatic juicepossesses of acting on all the food-stuffs appears to be due mainly to thepresence of a specific element or ferment, known as _trypsin_. 

 Experiment 60. _To show the action of pancreatic juice upon oils or fats. _ Put two grains of Fairchild's extract of pancreas into a four-ounce bottle. Add half a teaspoonful of warm water, and shake well for a few minutes; then add a tablespoonful of cod liver oil; shake vigorously. 

 A creamy, opaque mixture of the oil and water, called an emulsion, will result. This will gradually separate upon standing, the pancreatic extract settling in the water at the bottom. When shaken it will again form an emulsion. 

 Experiment 61. _To show the action of pancreatic juice on starch_. Put two tablespoonfuls of _smooth_ starch paste into a goblet, and while still so warm as just to be borne by the mouth, stir into it two grains of the extract of pancreas. The starch paste will rapidly become thinner, and gradually change into soluble starch, in a perfectly fluid solution. Within a few minutes some of the starch is converted through intermediary stages into maltose. Use the Fehling test for sugar. 

152. Digestion in the Small Intestines. After digestion in thestomach has been going on for some time, successive portions of thesemi-digested food begin to pass into the duodenum. The pancreas now takeson new activity, and a copious flow of pancreatic juice is poured alongits duct into the intestines. As the food is pushed along over the commonopening of the bile and pancreatic ducts, a great quantity of bile fromthis reservoir, the gall bladder, is poured into the intestines. These twodigestive fluids are now mixed with the chyme, and act upon it in theremarkable manner just described. 

[Illustration: Fig. 60. --Diagrammatic Scheme of Intestinal Absorption. 

 A, mesentery; B, lacteals and mesentery glands; C, veins of intestines; R. C, receptacle of the chyle (receptaculum chyli); P V, portal vein; H V, hepatic veins; S. V. C, superior vena cava; R. A, right auricle of the heart; I. V. C, inferior vena cava. ]

The inner surface of the small intestine also secretes a liquid calledintestinal juice, the precise functions of which are not known. Thechyme, thus acted upon by the different digestive fluids, resembles athick cream, and is now called chyle. The chyle is propelled alongthe intestine by the worm-like contractions of its muscular walls. Afunction of the bile, not yet mentioned, is to stimulate these movements, and at the same time by its antiseptic properties to prevent putrefactionof the contents of the intestine. 

153. Digestion in the Large Intestines. Digestion does not occur toany great extent in the large intestines. The food enters this portion ofthe digestive canal through the ileo-cæcal valve, and travels through itslowly. Time is thus given for the fluid materials to be taken up by theblood-vessels of the mucous membrane. The remains of the food now becomeless fluid, and consist of undigested matter which has escaped the actionof the several digestive juices, or withstood their influence. Drivenonward by the contractions of the muscular walls, the refuse materials atlast reach the rectum, from which they are voluntarily expelled from thebody. 

Absorption. 

154. Absorption. While food remains within the alimentary canal it is asmuch outside of the body, so far as nutrition is concerned, as if it hadnever been taken inside. To be of any service the food must enter theblood; it must be absorbed. The efficient agents in absorption are theblood-vessels, the lacteals, and the lymphatics. The process through whichthe nutritious material is fitted to enter the blood, is calledabsorption. It is a process not confined, as we shall see, simply to thealimentary canal, but one that is going on in every tissue. 

The vessels by which the process of absorption is carried on are calledabsorbents. The story, briefly told, is this: certain food materialsthat have been prepared to enter the blood, filter through the mucousmembrane of the intestinal canal, and also the thin walls of minuteblood-vessels and lymphatics, and are carried by these to larger vessels, and at last reach the heart, thence to be distributed to the tissues. 

155. Absorption from the Mouth and Stomach. The lining of the mouthand oesophagus is not well adapted for absorption. That this doesoccur is shown by the fact that certain poisonous chemicals, like cyanideof potash, if kept in the mouth for a few moments will cause death. Whilewe are chewing and swallowing our food, no doubt a certain amount of waterand common salt, together with sugar which has been changed from starch bythe action of the saliva, gains entrance to the blood. 

In the stomach, however, absorption takes place with great activity. Thesemi-liquid food is separated from the enormous supply of blood-vessels inthe mucous membrane only by a thin porous partition. There is, therefore, nothing to prevent the exchange taking place between the blood and thefood. Water, along with any substances in the food that have becomedissolved, will pass through the partition and enter the blood-current. Thus it is that a certain amount of starch that has been changed intosugar, of salts in solution, of proteids converted into peptones, is takenup directly by the blood-vessels of the stomach. 

156. Absorption by the Intestines. Absorption by the intestines is amost active and complicated process. The stomach is really an organ morefor the digestion than the absorption of food, while the small intestinesare especially constructed for absorption. In fact, the greatest part ofabsorption is accomplished by the small intestines. They have not only avery large area of absorbing surface, but also structures especiallyadapted to do this work. 

157. The Lacteals. We have learned in Section 144 that the mucouslining of the small intestines is crowded with millions of littleappendages called villi, meaning "tufts of hair. " These are onlyabout 1/30 of an inch long, and a dime will cover more than five hundredof them. Each villus contains a loop of blood-vessels, and another vessel, the lacteal, so called from the Latin word _lac_, milk, because of themilky appearance of the fluid it contains. The villi are adaptedespecially for the absorption of fat. They dip like the tiniest fingersinto the chyle, and the minute particles of fat pass through theircellular covering and gain entrance to the lacteals. The milky materialsucked up by the lacteals is not in a proper condition to be poured atonce into the blood current. It is, as it were, in too crude a state, andneeds some special preparation. 

The intestines are suspended to the posterior wall of the abdomen by adouble fold of peritoneum called the mesentery. In this membrane aresome 150 glands about the size of an almond, called mesentericglands. Now the lacteals join these glands and pour in their fluidcontents to undergo some important changes. It is not unlikely that themesenteric glands may intercept, like a filter, material which, if allowedto enter the blood, would disturb the whole body. Thus, while the glandsmight suffer, the rest of the body might escape. This may account for thefact that these glands and the lymphatics may be easily irritated andinflamed, thus becoming enlarged and sensitive, as often occurs in theaxilla. 

Having been acted upon by the mesenteric glands, and passed through them, the chyle flows onward until it is poured into a dilated reservoir for thechyle, known as the receptaculum chyli. This is a sac-like expansionof the lower end of the thoracic duct. Into this receptacle, situated atthe level of the upper lumbar vertebræ, in front of the spinal column, arepoured, not only the contents of the lacteals, but also of the lymphaticvessels of the lower limbs. 

158. The Thoracic Duct. This duct is a tube from fifteen to eighteeninches long, which passes upwards in front of the spine to reach the baseof the neck, where it opens at the junction of the great veins of the leftside of the head with those of the left arm. Thus the thoracic ductacts as a kind of feeding pipe to carry along the nutritive materialobtained from the food and to pour it into the blood current. It is to beremembered that the lacteals are in reality lymphatics--thelymphatics of the intestines. 

[Illustration: Fig. 61. --Section of a Lymphatic Gland. 

 A, strong fibrous capsule sending partitions into the gland; B, partitions between the follicles or pouches of the _cortical_ or outer portion; C, partitions of the _medullary_ or central portion; D, E, masses of protoplasmic matter in the pouches of the gland; F, lymph-vessels which bring lymph _to_ the gland, passing into its center; G, confluence of those leading to the efferent vessel; H, vessel which carries the lymph away _from_ the gland. ]

159. The Lymphatics. In nearly every tissue and organ of the bodythere is a marvelous network of vessels, precisely like the lacteals, called the lymphatics. These are busily at work taking up and makingover anew waste fluids or surplus materials derived from the blood andtissues generally. It is estimated that the quantity of fluid picked upfrom the tissues by the lymphatics and restored daily to the circulationis equal to the bulk of the blood in the body. The lymphatics seem tostart out from the part in which they are found, like the rootlets of aplant in the soil. They carry a turbid, slightly yellowish fluid, calledlymph, very much like blood without the red corpuscles. 

Now, just as the chyle was not fit to be immediately taken up by theblood, but was passed through the mesenteric glands to be properly workedover, so the lymph is carried to the lymphatic glands, where itundergoes certain changes to fit it for being poured into the blood. Nature, like a careful housekeeper, allows nothing to be wasted that canbe of any further service in the animal economy (Figs. 63 and 64). 

The lymphatics unite to form larger and larger vessels, and at last jointhe thoracic duct, except the lymphatics of the right side of the head andchest and right arm. These open by the right lymphatic duct into thevenous system on the right side of the neck. 

The whole lymphatic system may be regarded as a necessary appendage to thevascular system (Chapter VII. ). It is convenient, however, to treat itunder the general topic of absorption, in order to complete the history offood digestion. 

160. The Spleen and Other Ductless Glands. With the lymphatics may beclassified, for convenience, a number of organs called ductless orblood glands. Although they apparently prepare materials for use inthe body, they have no ducts or canals along which may be carried theresult of their work. Again, they are called blood glands because it issupposed they serve some purpose in preparing material for the blood. 

The spleen is the largest of these glands. It lies beneath thediaphragm, and upon the left side of the stomach. It is of a deep redcolor, full of blood, and is about the size and shape of the palm of thehand. 

The spleen has a fibrous capsule from which partitions pass inwards, dividing it into spaces by a framework of elastic tissue, with plainmuscular fibers. These spaces are filled with what is called the spleenpulp, through which the blood filters from its artery, just as a fluidwould pass through a sponge. The functions of the spleen are not known. Itappears to take some part in the formation of blood corpuscles. In certaindiseases, like malarial fever, it may become remarkably enlarged. It maybe wholly removed from an animal without apparent injury. During digestionit seems to act as a muscular pump, drawing the blood onwards withincreased vigor along its large vein to the liver. 

The thyroid is another ductless gland. It is situated beneath themuscles of the neck on the sides of "Adam's apple" and below it. Itundergoes great enlargement in the disease called goitre. 

The thymus is also a blood gland. It is situated around the windpipe, behind the upper part of the breastbone. Until about the end of the secondyear it increases in size, and then it begins gradually to shrivel away. Like the spleen, the thyroid and thymus glands are supposed to work somechange in the blood, but what is not clearly known. 

The suprarenal capsules are two little bodies, one perched on the topof each kidney, in shape not unlike that of a conical hat. Of theirfunctions nothing definite is known. 

Experiments. 

The action produced by the tendency of fluids to mix, or become equallydiffused in contact with each other, is known as _osmosis_, a form ofmolecular attraction allied to that of adhesion. The various physicalprocesses by which the products of digestion are transferred from thedigestive canal to the blood may be illustrated in a general way by thefollowing simple experiments. 

The student must, however, understand that the necessarily crudeexperiments of the classroom may not conform in certain essentials tothese great processes conducted in the living body, which they areintended to illustrate and explain. 

[Illustration: Fig. 62. ]

 Experiment 62. _Simple Apparatus for Illustrating Endosmotic Action. _ "Remove carefully a circular portion, about an inch in diameter, of the shell from one end of an egg, which may be done without injuring the membranes, by cracking the shell in small pieces, which are picked off with forceps. A small glass tube is then introduced through an opening in the shell and membranes of the other end of the egg, and is secured in a vertical position by wax or plaster of Paris, the tube penetrating the yelk. The egg is then placed in a wine-glass partly filled with water. In the course of a few minutes, the water will have penetrated the exposed membrane, and the yelk will rise in the tube. "--Flint's _Human Physiology_, page 293. 

 Experiment 63. Stretch a piece of moist bladder across a glass tube, --a common lamp-chimney will do. Into this put a strong saline solution. Now suspend the tube in a wide mouthed vessel of water. After a short time it will be found that a part of the salt solution has passed through into the water, while a larger amount of water has passed into the tube and raised the height of the liquid within it. 

161. The Quantity of Food as Affected by Age. The quantity of foodrequired to keep the body in proper condition is modified to a greatextent by circumstances. Age, occupation, place of residence, climate, andseason, as well as individual conditions of health and disease, are alwaysimportant factors in the problem. In youth the body is not only growing, but the tissue changes are active. The restless energy and necessarygrowth at this time of life cannot be maintained without an abundance ofwholesome food. This food supply for young people should be ample enoughto answer the demands of their keen appetite and vigorous digestion. 

In adult life, when the processes of digestion and assimilation areactive, the amount of food may without harm, be in excess of the actualneeds of the body. This is true, however, only so long as active muscularexercise is taken. 

In advanced life the tissue changes are slow, digestion is less active, and the ability to assimilate food is greatly diminished. Growth hasceased, the energy which induced activity is gone, and the proteids are nolonger required to build up worn-out tissues. Hence, as old ageapproaches, the quantity of nitrogenous foods should be steadilydiminished. 

 Experiment 64. Obtain a sheep's bladder and pour into it a heavy solution of sugar or some colored simple elixir, found at any drug store. Tie the bladder carefully and place it in a vessel containing water. After a while it will be found that an interchange has occurred, water having passed into the bladder and the water outside having become sweet. 

 Experiment 65. Make a hole about as big as a five-cent piece in the large end of an egg. That is, break the shell carefully and snip the outer shell membrane, thus opening the space between the outer and inner membranes. Now put the egg into a glass of water, keeping it in an upright position by resting on a napkin-ring. There is only the inner shell membrane between the liquid white of the egg (albumen) and the water. 

 An interchange takes place, and the water passes towards the albumen. As the albumen does not pass out freely towards the water, the membrane becomes distended, like a little bag at the top of the egg. 

162. Ill Effects of a too Generous Diet. A generous diet, even ofthose who take active muscular exercise, should be indulged in only withvigilance and discretion. Frequent sick or nervous headaches, a sense offullness, bilious attacks, and dyspepsia are some of the after-effects ofeating more food than the body actually requires. The excess of food isnot properly acted upon by the digestive juices, and is liable to undergofermentation, and thus to become a source of irritation to the stomach andthe intestines. If too much and too rich food be persistently indulged in, the complexion is apt to become muddy, the skin, especially of the face, pale and sallow, and more or less covered with blotches and pimples; thebreath has an unpleasant odor, and the general appearance of the body isunwholesome. 

An excess of any one of the different classes of foods may lead to seriousresults. Thus a diet habitually too rich in proteids, as with those whoeat meat in excess, often over-taxes the kidneys to get rid of the excessof nitrogenous waste, and the organs of excretion are not able to rid thetissues of waste products which accumulate in the system. From the blood, thus imperfectly purified, may result kidney troubles and various diseasesof the liver and the stomach. 

163. Effect of Occupation. Occupation has an important influence uponthe quantity of food demanded for the bodily support. Those who work longand hard at physical labor, need a generous amount of nutritious food. Aliberal diet of the cereals and lean meat, especially beef, gives thatvigor to the muscles which enables one to undergo laborious and prolongedphysical exertion. On the other hand, those who follow a sedentaryoccupation do not need so large a quantity of food. Brain-workers whowould work well and live long, should not indulge in too generous a diet. The digestion of heavy meals involves a great expenditure of nervousforce. Hence, the forces of the brain-worker, being required for mentalexertion, should not be expended to an unwarranted extent on the task ofdigestion. 

164. Effect of Climate. Climate also has a marked influence on thequantity of food demanded by the system. Much more food of all kinds isconsumed in cold than in warm climates. The accounts by travelers of thequantity of food used by the inhabitants of the frigid zone are almostbeyond belief. A Russian admiral gives an instance of a man who, in hispresence, ate at a single meal 28 pounds of rice and butter. Dr. Hayes, the Arctic traveler, states from personal observation that the dailyration of the Eskimos is 12 to 15 pounds of meat. With the thermometerranging from 60 to 70 degrees F. Below zero, there was a persistentcraving for strong animal diet, especially fatty foods. [24]

[Illustration: Fig. 63. --Lymphatics and Lymphatic Glands of the Axilla. ]

The intense cold makes such a drain upon the heat-producing power of thebody that only food containing the largest proportion of carbon is capableof making up for the loss. In tropical countries, on the other hand, thenatives crave and subsist mainly upon fruits and vegetables. 

165. The Kinds of Food Required. An appetite for plain, well-cookedfood is a safe guide to follow. Every person in good health, taking amoderate amount of daily exercise, should have a keen appetite for threemeals a day and enjoy them. Food should be both nutritious and digestible. It is nutritious in proportion to the amount of material it furnishes forthe nourishment of the tissues. It is digestible in a greater or lessdegree in respect to the readiness with which it yields to the action ofthe digestive fluids, and is prepared to be taken up by the blood. Thisdigestibility depends partly upon the nature of the food in its raw state, partly upon the effect produced upon it by cooking, and to some extentupon its admixture with other foods. Certain foods, as the vegetablealbumens, are both nutritious and digestible. A hard-working man may growstrong and maintain vigorous health on most of them, even if deprived ofanimal food. 

While it is true that the vegetable albumens furnish all that is reallyneeded for the bodily health, animal food of some kind is an economicaland useful addition to the diet. Races of men who endure prolongedphysical exertion have discovered for themselves, without the teaching ofscience, the great value of meat. Hence the common custom of eating meatwith bread and vegetables is a sound one. It is undoubtedly true that thepeople of this country, as a rule, eat meat too often and too much at atime. The judicious admixture of different classes of foods greatly aidstheir digestibility. 

The great abundance and variety of food in this country, permit thisprinciple to be put into practice. A variety of mixed foods, as milk, eggs, bread, and meat, are almost invariably associated to a greater orless extent at every meal. 

Oftentimes where there is of necessity a sameness of diet, there arises acraving for special articles of food. Thus on long voyages, and duringlong campaigns in war, there is an almost universal craving for onions, raw potatoes, and other vegetables. 

166. Hints about Meals. On an average, three meals each day, fromfive to six hours apart, is the proper number for adults. Five hours is byno means too long a time to intervene between consecutive meals, for it isnot desirable to introduce new food into the stomach, until the gastricdigestion of the preceding meal has been completed, and until the stomachhas had time to rest, and is in condition to receive fresh material. Thestomach, like other organs, does its work best at regular periods. [25]

Eating out of mealtimes should be strictly avoided, for it robs thestomach of its needed rest. Food eaten when the body and mind are weariedis not well digested. Rest, even for a few minutes, should be taken beforeeating a full meal. It is well to lie down, or sit quietly and read, fifteen minutes before eating, and directly afterwards, if possible. 

Severe exercise and hard study just after a full meal, are very apt todelay or actually arrest digestion, for after eating heartily, the vitalforces of the body are called upon to help the stomach digest its food. Ifour bodily energies are compelled, in addition to this, to help themuscles or brain, digestion is retarded, and a feeling of dullness andheaviness follows. Fermentative changes, instead of the normal digestivechanges, are apt to take place in the food. 

167. Practical Points about Eating. We should not eat for at leasttwo or three hours before going to bed. When we are asleep, the vitalforces are at a low ebb, the process of digestion is for the time nearlysuspended, and the retention of incompletely digested food in the stomachmay cause bad dreams and troubled sleep. But in many cases ofsleeplessness, a trifle of some simple food, especially if the stomachseems to feel exhausted, often appears to promote sleep and rest. 

 [NOTE. The table on the next page shows the results of many experiments to illustrate the time taken for the gastric digestion of a number of the more common solid foods. There are a good many factors of which the table takes no account, such as the interval since the last meal, state of the appetite, amount of work and exercise, method of cooking, and especially the quantity of food. ]

 Table Showing the Digestibility of the More Common Solid Foods. 

 Food How Time in Cooked Stomach, Hours ------------------------------------------------- Apples, sweet and mellow Raw 1-1/2 Apples, sour and hard " 2-1/2 Apple Dumpling Boiled 3 Bass, striped, fresh Broiled 3 Beans, pod Boiled 2-1/2 Beef, with salt only " 2-3/4 " fresh, lean Raw 3 " " " Fried 4 " " " Roasted 3-1/2 " old, hard, salted Boiled 4-1/4 Beefsteak Broiled 3 Beets Boiled 3-3/4 Bread, corn Baked 3-1/4 " wheat, fresh " 3-1/2 Butter Melted 3-1/2 Cabbage, with vinegar Raw 2 " " " Boiled 4-1/2 " heads Raw 2-1/2 Carrots Boiled 3-1/4 Cheese, old, strong Raw 3-1/2 Chicken, full-grown Fricassee 2-3/4 " soup Boiled 3 Codfish, cured, dried " 2 Corncake Baked 2-3/4 Custard " 2-3/4 Duck, domestic Roasted 4 " wild " 4-1/2 Eggs, fresh, whipped Raw 1-1/2 " " 2 " soft-boiled Boiled 3 " hard-boiled " 3-1/2 " Fried 3-1/2 Fowl, domestic Boiled 4 " " Roasted 4 Gelatin Boiled 2-1/2 Goose Roasted 2-1/2 Green corn and beans Boiled 3-3/4 Hash, meat and vegetables Warmed 2-1/2 Lamb Broiled 2-1/2 Liver " 2 Milk Boiled 2 " Raw 2-1/4 Mutton, fresh Broiled 3 " " Boiled 3 " " Roasted 3-1/4 Oysters, fresh Raw 2-1/2 " " Roasted 3-1/4 " " Stewed 3-1/2 Parsnips Boiled 2-1/2 Pig Roasted 2-1/2 Pig's feet, soused Boiled 1 Pork, recently salted " 4-1/2 " Fried 4-1/4 " Raw 3 " steaks Fried 3-1/4 " Stewed 3 " fat or lean Roasted 5-1/4 Potatoes Baked 2-1/2 " Boiled 3-1/2 " Roasted 2-1/2 Rice Boiled 1 Sago " 1-3/4 Salmon, salted " 4 Soup, barley " 1-1/2 " beans " 3 " beef, vegetables, bread " 4 " marrow bone " 4-1/2 " mutton " 3-1/2 Sponge Cake Baked 2-1/2 Suet, beef, fresh Boiled 5-1/3 " mutton " 4-1/2 Tapioca " 2 Tripe, soused " 1 Trout, salmon, fresh " 1-1/2 " " " Fried 1-1/2 Turkey, wild Roasted 2-1/4 " domestic Boiled 2-1/4 " " Roasted 2-1/2 Turnips Boiled 3-1/2 Veal Roasted 4 " Fried 4-1/2 Venison, steaks Broiled 1-1/2

The state of mind has much to do with digestion. Sudden fear or joy, orunexpected news, may destroy the appetite at once. Let a hungry person beanxiously awaiting a hearty meal, when suddenly a disastrous telegram isbrought him; all appetite instantly disappears, and the tempting food isrefused. Hence we should laugh and talk at our meals, and drive awayanxious thoughts and unpleasant topics of discussion. 

The proper chewing of the food is an important element in digestion. Hence, eat slowly, and do not "bolt" large fragments of food. Ifimperfectly chewed, it is not readily acted upon by the gastric juice, andoften undergoes fermentative changes which result in sour stomach, gastricpain, and other digestive disturbance. 

If we take too much drink with our meals, the flow of the saliva ischecked, and digestion is hindered. It is not desireable to dilute thegastric juice, nor to chill the stomach with large amount of cold liquid. 

Do not take food and drink too hot or too cold. If they are taken toocold, the stomach is chilled, and digestion delayed. If we drink freely ofice-water, it may require half an hour or more for the stomach to regainits natural heat. 

It is a poor plan to stimulate a flagging appetite with highly spiced foodand bitter drinks. An undue amount of pepper, mustard, horseradish, pickles, and highly seasoned meat-sauces may stimulate digestion for thetime, but they soon impair it. 

 [NOTE. The process of gastric digestion was studied many years ago by Dr. Beaumont and others, in the remarkable case of Alexis St. Martin, a French-Canadian, who met with a gun-shot wound which left a permanent opening into his stomach, guarded by a little valve of mucous membrane. Through this opening the lining of the stomach could be seen, the temperature ascertained, and numerous experiments made as to the digestibility of various kinds of food. 

 It was by these careful and convincing experiments that the foundation of our exact knowledge of the composition and action of gastric juice was laid. The modest book in which Dr. Beaumont published his results is still counted among the classics of physiology. The production of artificial fistulæ in animals, a method that has since proved so fruitful, was first suggested by his work. ]

It cannot be too strongly stated that food of a simple character, wellcooked and neatly served, is more productive of healthful living than agreat variety of fancy dishes which unduly stimulate the digestive organs, and create a craving for food in excess of the bodily needs. 

168. The Proper Care of the Teeth. It is our duty not only to takethe very best care of our teeth, but to retain them as long as possible. Teeth, as we well know, are prone to decay. We may inherit poor and softteeth: our mode of living may make bad teeth worse. If an ounce ofprevention is ever worth a pound of cure, it is in keeping the teeth ingood order. Bad teeth and toothless gums mean imperfect chewing of thefood and, hence, impaired digestion. To attain a healthful old age, thepower of vigorous mastication must be preserved. 

One of the most frequent causes of decay of the teeth is the retention offragments of food between and around them. The warmth and moisture of themouth make these matters decompose quickly. The acid thus generatedattacks the enamel of the teeth, causing decay of the dentine. Decayedteeth are often the cause of an offensive breath and a foul stomach. 

[Illustration: Fig. 64. --Lymphatics on the Inside of the Right Hand. ]

To keep the teeth clean and wholesome, they should be thoroughly cleansedat bedtime and in the morning with a soft brush and warm water. Castilesoap, and some prepared tooth-powder without grit, should be used, and thebrush should be applied on both sides of the teeth. 

The enamel, once broken through, is never renewed. The tooth decays, slowly but surely: hence we must guard against certain habits which injurethe enamel, as picking the teeth with pins and needles. We should nevercrack nuts, crush hard candy, or bite off stout thread with the teeth. Stiff tooth-brushes, gritty and cheap tooth-powders, and hot food anddrink, often injure the enamel. 

To remove fragments of food which have lodged between adjacent teeth, aquill or wooden toothpick should be used. Even better than these is theuse of surgeon's floss, or silk, which when drawn between the teeth, effectually dislodges retained particles. If the teeth are not regularlycleansed they become discolored, and a hard coating known as _tartar_accumulates on them and tends to loosen them. It is said that after theage of thirty more teeth are lost from this deposit than from all othercauses combined. In fact decay and tartar are the two great agents thatfurnish work for the dentist. [26]

169. Hints about Saving Teeth. We should exercise the greatest carein saving the teeth. The last resort of all is to lose a tooth byextraction. The skilled dentist will save almost anything in the shape ofa tooth. 

People are often urged and consent to have a number of teeth extractedwhich, with but little trouble and expense, might be kept and do goodservice for years. The object is to replace the teeth with an artificialset. Very few plates, either partial or entire, are worn with realcomfort. They should always be removed before going to sleep, as there isdanger of their being swallowed. 

The great majority of drugs have no injurious effect upon the teeth. Somemedicines, however, must be used with great care. The acids used in thetincture of iron have a great affinity for the lime salts of the teeth. Asthis form of iron is often used, it is not unusual to see teeth very badlystained or decayed from the effects of this drug. The acid used in theliquid preparations of quinine may destroy the teeth in a comparativelyshort time. After taking such medicines the mouth should be thoroughlyrinsed with a weak solution of common soda, and the teeth cleansed. 

170. Alcohol and Digestion. The influence of alcoholic drinks upondigestion is of the utmost importance. Alcohol is not, and cannot beregarded from a physiological point of view as a true food. The receptiongiven to it by the stomach proves this very plainly. It is obviously anunwelcome intruder. It cannot, like proper foods, be transformed into anyelement or component of the human body, but passes on, innutritious andfor the most part unappropriated. Taken even into the mouth, by any personnot hardened to its use, its effect is so pungent and burning as at onceto demand its rejection. But if allowed to pass into the stomach, thatorgan immediately rebels against its intrusion, and not unfrequentlyejects it with indignant emphasis. The burning sensation it producesthere, is only an appeal for water to dilute it. 

The stomach meanwhile, in response to this fiery invitation, secretes fromits myriad pores its juices and watery fluids, to protect itself as muchas possible from the invading liquid. It does not digest alcoholic drinks;we might say it does not attempt to, because they are not materialsuitable for digestion, and also because no organ can perform its normalwork while smarting under an unnatural irritation. 

Even if the stomach does not at once eject the poison, it refuses toadopt it as food, for it does not pass along with the other food material, as chyme, into the intestines, but is seized by the absorbents, borne intothe veins, which convey it to the heart, whence the pulmonary arteryconveys it to the lungs, where its presence is announced in the breath. But wherever alcohol is carried in the tissues, it is always an irritant, every organ in turn endeavoring to rid itself of the noxious material. 

171. Effect of Alcoholic Liquor upon the Stomach. The methods bywhich intoxicating drinks impair and often ruin digestion are various. Weknow that a piece of animal food, as beef, if soaked in alcohol for a fewhours, becomes hard and tough, the fibers having been compacted togetherbecause of the abstraction of their moisture by the alcohol, which has amarvelous affinity for water. In the same way alcohol hardens and toughensanimal food in the stomach, condensing its fibers, and rendering itindigestible, thus preventing the healthful nutrition of the body. So, ifalcohol be added to the clear, liquid white of an egg, it is instantlycoagulated and transformed into hard albumen. As a result of thishardening action, animal food in contact with alcoholic liquids in thestomach remains undigested, and must either be detained there so long asto become a source of gastric disturbance, or else be allowed to passundigested through the pyloric gate, and then may become a cause ofserious intestinal disturbance. [27]

This peculiar property of alcohol, its greedy absorption of water fromobjects in contact with it, acts also by absorbing liquids from thesurface of the stomach itself, thus hardening the delicate glands, impairing their ability to absorb the food-liquids, and so inducinggastric dyspepsia. This local injury inflicted upon the stomach by allforms of intoxicants, is serious and protracted. This organ is, withadmirable wisdom, so constructed as to endure a surprising amount ofabuse, but it was plainly not intended to thrive on alcoholic liquids. Theapplication of fiery drinks to its tender surface produces at first amarked congestion of its blood-vessels, changing the natural pink color, as in the mouth, to a bright or deep red. 

If the irritation be not repeated, the lining membrane soon recovers itsnatural appearance. But if repeated and continued, the congestion becomesmore intense, the red color deeper and darker; the entire surface is thesubject of chronic inflammation, its walls are thickened, and sometimesulcerated. In this deplorable state, the organ is quite unable to performits normal work of digestion. [28]

172. Alcohol and the Gastric Juice. But still another destructiveinfluence upon digestion appears in the singular fact that alcoholdiminishes the power of the gastric juice to do its proper work. Alcoholcoagulates the pepsin, which is the dissolving element in this importantgastric fluid. A very simple experiment will prove this. Obtain a smallquantity of gastric juice from the fresh stomach of a calf or pig, bygently pressing it in a very little water. Pour the milky juice into aclear glass vessel, add a little alcohol, and a white deposit willpresently settle to the bottom. This deposit contains the pepsin of thegastric juice, the potent element by which it does its special work ofdigestion. The ill effect of alcohol upon it is one of the prime factorsin the long series of evil results from the use of intoxicants. 

173. The Final Results upon Digestion. We have thus explained threedifferent methods by which alcoholic drinks exercise a terrible power forharm; they act upon the food so as to render it less digestible; theyinjure the stomach so as seriously to impair its power of digestion; andthey deprive the gastric juice of the one principal ingredient essentialto its usefulness. 

Alcoholic drinks forced upon the stomach are a foreign substance; thestomach treats them as such, and refuses to go on with the process ofdigestion till it first gets rid of the poison. This irritating presenceand delay weaken the stomach, so that when proper food follows, theenfeebled organ is ill prepared for its work. After intoxication, thereoccurs an obvious reaction of the stomach, and digestive organs, againstthe violent and unnatural disturbance. The appetite is extinguished ordepraved, and intense headache racks the frame, the whole system isprostrated, as from a partial paralysis (all these results being the voiceof Nature's sharp warning of this great wrong), and a rest of some daysis needed before the system fully recovers from the injury inflicted. 

It is altogether an error to suppose the use of intoxicants is necessaryor even desirable to promote appetite or digestion. In health, good foodand a stomach undisturbed by artificial interference furnish all theconditions required. More than these is harmful. If it may sometimes seemas if alcoholic drinks arouse the appetite and invigorate digestion, wemust not shut our eyes to the fact that this is only a seeming, and thattheir continued use will inevitably ruin both. In brief, there is no moresure foe to good appetite and normal digestion than the habitual use ofalcoholic liquors. 

174. Effect of Alcoholic Drinks upon the Liver. It is to be notedthat the circulation of the liver is peculiar; that the capillaries of thehepatic artery unite in the lobule with those of the portal vein, and thusthe blood from both sources is combined; and that the portal vein bringsto the liver the blood from the stomach, the intestines, and the spleen. From the fact that alcohol absorbed from the stomach enters the portalvein, and is borne directly to the liver, we would expect to find thisorgan suffering the full effects of its presence. And all the more wouldthis be true, because we have just learned that the liver acts as a sortof filter to strain from the blood its impurities. So the liver isespecially liable to diseases produced by alcoholics. Post mortems ofthose who have died while intoxicated show a larger amount of alcohol inthe liver than in any other organ. Next to the stomach the liver is anearly and late sufferer, and this is especially the case with harddrinkers, and even more moderate drinkers in hot climates. Yellow feveroccurring in inebriates is always fatal. 

The effects produced in the liver are not so much functional as organic;that is, not merely a disturbed mode of action, but a destruction of thefabric of the organ itself. From the use of intoxicants, the liverbecomes at first irritated, then inflamed, and finally seriously diseased. The fine bands, or septa, which serve as partitions between the hepaticlobules, and so maintain the form and consistency of the organ, are thespecial subjects of the inflammation. Though the liver is at firstenlarged, it soon becomes contracted; the secreting cells are compressed, and are quite unable to perform their proper work, which indeed is a veryimportant one in the round of the digestion of food and the purificationof the blood. This contraction of the septa in time gives the whole organan irregularly puckered appearance, called from this fact a hob-nail liveror, popularly, gin liver. The yellowish discoloration, usually fromretained or perverted bile, gives the disease the medical name ofcirrhosis. [29] It is usually accompanied with dropsy in the lowerextremities, caused by obstruction to the return of the circulation fromthe parts below the liver. This disease is always fatal. 

175. Fatty Degeneration Due to Alcohol. Another form of destructivedisease often occurs. There is an increase of fat globules deposited inthe liver, causing notable enlargement and destroying its function. Thisis called fatty degeneration, and is not limited to the liver, but otherorgans are likely to be similarly affected. In truth, this deposition offat is a most significant occurrence, as it means actual destruction ofthe liver tissues, --nothing less than progressive death of the organ. Thiscondition always leads to a fatal issue. Still other forms of alcoholicdisease of the liver are produced, one being the excessive formation ofsugar, constituting what is known as a form of diabetes. 

176. Effect of Tobacco on Digestion. The noxious influence oftobacco upon the process of digestion is nearly parallel to the effects ofalcohol, which it resembles in its irritant and narcotic character. Locally, it stimulates the secretion of saliva to an unnatural extent, andthis excess of secretion diminishes the amount available for normaldigestion. 

Tobacco also poisons the saliva furnished for the digestion of food, andthus at the very outset impairs, in both of these particulars, the generaldigestion, and especially the digestion of the starchy portions of thefood. For this reason the amount of food taken, fails to nourish as itshould, and either more food must be taken, or the body becomes graduallyimpoverished. 

The poisonous _nicotine_, the active element of tobacco, exerts adestructive influence upon the stomach digestion, enfeebling the vigor ofthe muscular walls of that organ. These effects combined producedyspepsia, with its weary train of baneful results. 

The tobacco tongue never presents the natural, clear, pink color, butrather a dirty yellow, and is usually heavily coated, showing a disorderedstomach and impaired digestion. Then, too, there is dryness of the mouth, an unnatural thirst that demands drink. But pure water is stale and flatto such a mouth: something more emphatic is needed. Thus comes theunnatural craving for alcoholic liquors, and thus are taken the firststeps on the downward grade. 

"There is no doubt that tobacco predisposes to neuralgia, vertigo, indigestion, and other affections of the nervous, circulatory anddigestive organs. "--W. H. Hammond, the eminent surgeon of New York cityand formerly Surgeon General, U. S. A. 

Drs. Seaver of Yale University and Hitchcock of Amherst College, instructors of physical education in these two colleges, have clearlydemonstrated by personal examination and recorded statistics that the useof tobacco among college students checks growth in weight, height, chest-girth, and, most of all, in lung capacity. 

Additional Experiments. 

 Experiment 66. Test a portion of _C_ (Experiment 57) with solution of iodine; no blue color is obtained, as all the starch has disappeared, having been converted into a reducing sugar, or maltose. 

 Experiment 67. Make a thick starch paste; place some in test tubes, labeled _A_ and _B_. Keep _A_ for comparison, and to _B_ add saliva, and expose both to about 104 degrees F. _A_ is unaffected, while _B_ soon becomes fluid--within two minutes--and loses its opalescence; this liquefaction is a process quite antecedent to the saccharifying process which follows. 

 Experiment 68. _To show the action of gastric juice on milk_. Mix two teaspoonfuls of fresh milk in a test tube with a few drops of neutral artificial gastric juice;[30] keep at about 100 degrees F. In a short time the milk curdles, so that the tube can be inverted without the curd falling out. By and by _whey_ is squeezed out of the clot. The curdling of milk by the rennet ferment present in the gastric juice, is quite different from that produced by the "souring of milk, " or by the precipitation of caseinogen by acids. Here the casein (carrying with it most of the fats) is precipitated in a neutral fluid. 

 Experiment 69. To the test tube in the preceding experiment, add two teaspoonfuls of dilute hydrochloric acid, and keep at 100 degrees F. For two hours. The pepsin in the presence of the acid digests the casein, gradually dissolving it, forming a straw-colored fluid containing peptones. The peptonized milk has a peculiar odor and bitter taste. 

 Experiment 70. _To show the action of rennet on milk_. Place milk in a test tube, add a drop or two of commercial rennet, and place the tube in a water-bath at about 100 degrees F. The milk becomes solid in a few minutes, forming a _curd_, and by and by the curd of casein contracts, and presses out a fluid, --the _whey_. 

 Experiment 71. Repeat the experiment, but previously boil the rennet. No such result is obtained as in the preceding experiment, because the rennet ferment is destroyed by heat. 

 Experiment 72. _To show the effect of the pancreatic ferment (trypsin) upon albuminous matter_. Half fill three test tubes, _A, B, C_, with one-per-cent solution of sodium carbonate, and add 5 drops of liquor pancreaticus, or a few grains of Fairchild's extract of pancreas, in each. Boil _B_, and make _C_ acid with dilute hydrochloric acid. Place in each tube an equal amount of well-washed fibrin, plug the tubes with absorbent cotton, and place all in a water-bath at about 100 degrees F. 

 Experiment 73. Examine from time to time the three test tubes in the preceding experiment. At the end of one, two, or three hours, there is no change in _B_ and _C_, while in _A_ the fibrin is gradually being eroded, and finally disappears; but it does not swell up, and the solution at the same time becomes slightly turbid. After three hours, still no change is observable in _B_ and _C_. 

 Experiment 74. Filter _A_, and carefully neutralize the filtrate with very dilute hydrochloric or acetic acid, equal to a precipitate of alkali-albumen. Filter off the precipitate, and on testing the filtrate, peptones are found. The intermediate bodies, the albumoses, are not nearly so readily obtained from pancreatic as from gastric digests. 

 Experiment 75. Filter _B_ and _C_, and carefully neutralize the filtrates. They give no precipitate. No peptones are found. 

 Experiment 76. _To show the action of pancreatic juice upon the albuminous ingredients (casein) of milk_. Into a four-ounce bottle put two tablespoonfuls of cold water; add one grain of Fairchild's extract of pancreas, and as much baking soda as can be taken up on the point of a penknife. Shake well, and add four tablespoonfuls of cold, fresh milk. Shake again. 

 Now set the bottle into a basin of hot water (as hot as one can bear the hand in), and let it stand for about forty-five minutes. While the milk is digesting, take a small quantity of milk in a goblet, and stir in ten drops or more of vinegar. A thick curd of casein will be seen. 

 Upon applying the same test to the digested milk, no curd will be made. This is because the pancreatic ferment (trypsin) has digested the casein into "peptone, " which does not curdle. This digested milk is therefore called "peptonized milk. "

 Experiment 77. _To show the action of bile_. Obtain from the butcher some ox bile. Note its bitter taste, peculiar odor, and greenish color. It is alkaline or neutral to litmus paper. Pour it from one vessel to another, and note that strings of mucin (from the lining membrane of the gall bladder) connect one vessel with the other. It is best to precipitate the mucin by acetic acid before making experiments; and to dilute the clear liquid with a little distilled water. 

 Experiment 78. _Test for bile pigments_. Place a few drops of bile on a white porcelain slab. With a glass rod place a drop or two of strong nitric acid containing nitrous acid near the drop of bile; bring the acid and bile into contact. Notice the succession of colors, beginning with green and passing into blue, red, and yellow. 

 Experiment 79. _To show the action of bile on fats_. Mix three teaspoonfuls of bile with one-half a teaspoonful of almond oil, to which some oleic acid is added. Shake well, and keep the tube in a water-bath at about 100 degrees F. A very good emulsion is obtained. 

 Experiment 80. _To show that bile favors filtration and the absorption of fats_. Place two small funnels of exactly the same size in a filter stand, and under each a beaker. Into each funnel put a filter paper; moisten the one with water (_A_) and the other with bile (_B_). Pour into each an equal volume of almond oil; cover with a slip of glass to prevent evaporation. Set aside for twelve hours, and note that the oil passes through _B_, but scarcely any through _A_. The oil filters much more readily through the one moistened with bile, than through the one moistened with water. 

Experiments with the Fats. 

 Experiment 81. Use olive oil or lard. Show by experiment that they are soluble in ether, chloroform and hot water, but insoluble in water alone. 

 Experiment 82. Dissolve a few drops of oil or fat in a teaspoonful of ether. Let a drop of the solution fall on a piece of tissue or rice paper. Note the greasy stain, which does not disappear with the heat. 

 Experiment 83. Pour a little cod-liver oil into a test tube; add a few drops of a dilute solution of sodium carbonate. The whole mass becomes white, making an emulsion. 

 Experiment 84. Shake up olive oil with a solution of albumen in a test tube. Note that an emulsion is formed. 

Chapter VII. 

The Blood and Its Circulation. 

177. The Circulation. All the tissues of the body are traversed byexceedingly minute tubes called capillaries, which receive the blood fromthe arteries, and convey it to the veins. These capillaries form a greatsystem of networks, the meshes of which are filled with the elements ofthe various tissues. That is, the capillaries are closed vessels, and thetissues lie outside of them, as asbestos packing may be used to envelophot-water pipes. The space between the walls of the capillaries and thecells of the tissues is filled with lymph. As the blood flows alongthe capillaries, certain parts of the plasma of the blood filter throughtheir walls into the lymph, and certain parts of the lymph filter throughthe cell walls of the tissues and mingle with the blood current. The lymphthus acts as a medium of exchange, in which a transfer of material takesplace between the blood in the capillaries and the lymph around them. Asimilar exchange of material is constantly going on between the lymph andthe tissues themselves. 

This, then, we must remember, --that in every tissue, so long as the bloodflows, and life lasts, this exchange takes place between the blood withinthe capillaries and the tissues without. 

The stream of blood _to_ the tissues carries to them the material, including the all-important oxygen, with which they build themselves upand do their work. The stream _from_ the tissues carries into the bloodthe products of certain chemical changes which have taken place in thesetissues. These products may represent simple waste matter to be cast outor material which may be of use to some other tissue. 

In brief, the tissues by the help of the lymph live on the blood. Just as our bodies, as a whole, live on the things around us, the food andthe air, so do the bodily tissues live on the blood which bathes them inan unceasing current, and which is their immediate air and food. 

178. Physical Properties of Blood. The blood has been called thelife of the body from the fact that upon it depends our bodily existence. The blood is so essentially the nutrient element that it is calledsometimes very aptly "liquid flesh. " It is a red, warm, heavy, alkalinefluid, slightly salt in taste, and has a somewhat fetid odor. Its colorvaries from bright red in the arteries and when exposed to the air, tovarious tints from dark purple to red in the veins. The color of the bloodis due to the coloring constituent of the red corpuscles, _hæmoglobin_, which is brighter or darker as it contains more or less oxygen. 

[Illustration: Fig. 65. --Blood Corpuscles of Various Animals. (Magnifiedto the same scale. )

 A, from proteus, a kind of newt; B, salamander; C, frog; D, frog after addition of acetic acid, showing the central nucleus; E, bird; F, camel; G, fish; H, crab or other invertebrate animal]

The temperature of the blood varies slightly in different parts of thecirculation. Its average heat near the surface is in health about thesame, _viz_. 98-1/2 degrees F. Blood is alkaline, but outside of the bodyit soon becomes neutral, then acid. The chloride of sodium, or commonsalt, which the blood contains, gives it a salty taste. In a hemorrhagefrom the lungs, the sufferer is quick to notice in the mouth the warm andsaltish taste. The total amount of the blood in the body was formerlygreatly overestimated. It is about 1/13 of the total weight of the body, and in a person weighing 156 pounds would amount to about 12 pounds. 

179. Blood Corpuscles. If we put a drop of blood upon a glass slide, and place upon it a cover of thin glass, we can flatten it out until thecolor almost disappears. If we examine this thin film with a microscope, we see that the blood is not altogether fluid. We find that the liquidpart, or plasma, is of a light straw color, and has floating in it amultitude of very minute bodies, called corpuscles. These are of twokinds, the red and the colorless. The former are much morenumerous, and have been compared somewhat fancifully to countless myriadsof tiny fishes in a swiftly flowing stream. 

180. Red Corpuscles. The red corpuscles are circular disks about1/3200 of an inch in diameter, and double concave in shape. They tend toadhere in long rolls like piles of coins. They are soft, flexible, andelastic, readily squeezing through openings and passages narrower thantheir own diameter, then at once resuming their own shape. 

The red corpuscles are so very small, that rather more than ten millionsof them will lie on a surface one inch square. Their number is so enormousthat, if all the red corpuscles in a healthy person could be arranged in acontinuous line, it is estimated that they would reach four times aroundthe earth! The principal constituent of these corpuscles, next to water, and that which gives them color is _hæmoglobin_, a compound containingiron. As all the tissues are constantly absorbing oxygen, and giving offcarbon dioxid, a very important office of the red corpuscles is to carryoxygen to all parts of the body. 

181. Colorless Corpuscles. The colorless corpuscles are largerthan the red, their average diameter being about 1/2500 of an inch. Whilethe red corpuscles are regular in shape, and float about, and tumblefreely over one another, the colorless are of irregular shape, and stickclose to the glass slide on which they are placed. Again, while the redcorpuscles are changed only by some influence from without, as pressureand the like, the colorless corpuscles spontaneously undergo active andvery curious changes of form, resembling those of the amoeba, a veryminute organism found in stagnant water (Fig. 2). 

The number of both red and colorless corpuscles varies a great deal fromtime to time. For instance, the number of the latter increases aftermeals, and quickly diminishes. There is reason to think both kinds ofcorpuscles are continually being destroyed, their place being supplied bynew ones. While the action of the colorless corpuscles is important to thelymph and the chyle, and in the coagulation of the blood, their realfunction has not been ascertained. 

[Illustration: Fig. 66. --Blood Corpuscles of Man. 

 A, red corpuscles; B, the same seen edgeways; C, the same arranged in rows; D, white corpuscles with nuclei. ]

 Experiment 85. _To show the blood corpuscles_. A moderately powerful microscope is necessary to examine blood corpuscles. Let a small drop of blood (easily obtained by pricking the finger with a needle) be placed upon a clean slip of glass, and covered with thin glass, such as is ordinarily used for microscopic purposes. 

 The blood is thus spread out into a film and may be readily examined. At first the red corpuscles will be seen as pale, disk-like bodies floating in the clear fluid. Soon they will be observed to stick to each other by their flattened faces, so as to form rows. The colorless corpuscles are to be seen among the red ones, but are much less numerous. 

182. The Coagulation of the Blood. Blood when shed from the livingbody is as fluid as water. But it soon becomes viscid, and flows lessreadily from one vessel to another. Soon the whole mass becomes a nearlysolid jelly called a clot. The vessel containing it even can beturned upside down, without a drop of blood being spilled. If carefullyshaken out, the mass will form a complete mould of the vessel. 

At first the clot includes the whole mass of blood, takes the shape ofthe vessel in which it is contained, and is of a uniform color. But in ashort time a pale yellowish fluid begins to ooze out, and to collect onthe surface. The clot gradually shrinks, until at the end of a few hoursit is much firmer, and floats in the yellowish fluid. The white corpusclesbecome entangled in the upper portion of clot, giving it a pale yellowlook on the top, known as the _buffy coat_. As the clot is attached to thesides of the vessel, the shrinkage is more pronounced toward the center, and thus the surface of the clot is hollowed or _cupped_, as it is called. This remarkable process is known as coagulation, or the clotting ofblood; and the liquid which separates from the clot is called serum. The serum is almost entirely free from corpuscles, these being entangledin the fibrin. 

[Illustration: Fig. 67. --Diagram of Clot with Buffy Coat. 

 A, serum; B, cupped upper surface of clot; C, white corpuscles in upper layer of clot; D, lower portion of clot with red corpuscles. ]

This clotting of the blood is due to the formation in the blood, after itis withdrawn from the living body, of a substance called fibrin. [31] It ismade up of a network of fine white threads, running in every directionthrough the plasma, and is a proteid substance. The coagulation of theblood may be retarded, and even prevented, by a temperature below 40degrees F. , or a temperature above 120 degrees F. The addition of commonsalt also prevents coagulation. The clotting of the blood may be hastenedby free access to air, by contact with roughened surfaces, or by keepingit at perfect rest. 

This power of coagulation is of the most vital importance. But for this, a very small cut might cause bleeding sufficient to empty theblood-vessels, and death would speedily follow. In slight cuts, Natureplugs up the wound with clots of blood, and thus prevents excessivebleeding. The unfavorable effects of the want of clotting are illustratedin some persons in whom bleeding from even the slightest wounds continuestill life is in danger. Such persons are called "bleeders, " and surgeonshesitate to perform on them any operation, however trivial, even theextraction of a tooth being often followed by an alarming loss of blood. 

 Experiment 86. A few drops of fresh blood may be easily obtained to illustrate important points in the physiology of blood, by tying a string tight around the finger, and piercing it with a clean needle. The blood runs freely, is red and opaque. Put two or three drops of fresh blood on a sheet of white paper, and observe that it looks yellowish. 

 Experiment 87. Put two or three drops of fresh blood on a white individual butter plate inverted in a saucer of water. Cover it with an inverted goblet. Take off the cover in five minutes, and the drop has set into a jelly-like mass. Take it off in half an hour, and a little clot will be seen in the watery serum. 

 Experiment 88. _To show the blood-clot. _ Carry to the slaughter house a clean, six or eight ounce, wide-mouthed bottle. Fill it with fresh blood. Carry it home with great care, and let it stand over night. The next day the clot will be seen floating in the nearly colorless serum. 

 Experiment 89. Obtain a pint of fresh blood; put it into a bowl, and whip it briskly for five minutes, with a bunch of dry twigs. Fine white threads of fibrin collect on the twigs, the blood remaining fluid. This is "whipped" or defibrinated blood, which has lost the power of coagulating spontaneously. 

183. General Plan of Circulation. All the tissues of the body dependupon the blood for their nourishment. It is evident then that this vitalfluid must be continually renewed, else it would speedily lose all of itslife-giving material. Some provision, then, is necessary not only to havethe blood renewed in quantity and quality, but also to enable it to carryaway impurities. 

So we must have an apparatus of circulation. We need first a centralpump from which branch off large pipes, which divide into smaller andsmaller branches until they reach the remotest tissues. Through thesepipes the blood must be pumped and distributed to the whole body. Then wemust have a set of return pipes by which the blood, after it has carriednourishment to the tissues, and received waste matters from them, shall bebrought back to the central pumping station, to be used again. We musthave also some apparatus to purify the blood from the waste matter it hascollected. 

[Illustration: Fig. 68. --Anterior View of the Heart. 

 A, superior vena cava; B, right auricle; C, right ventricle; D, left ventricle; E, left auricle; F, pulmonary vein; H, pulmonary artery; K, aorta; L, right subclavian artery; M, right common carotid artery; N, left common carotid artery. ]

This central pump is the heart. The pipes leading from it andgradually growing smaller and smaller are the arteries. The veryminute vessels into which they are at last subdivided arecapillaries. The pipes which convey the blood back to the heart arethe veins. Thus, the arteries end in the tissues in fine, hair-likevessels, the capillaries; and the veins begin in the tissues inexceedingly small tubes, --the capillaries. Of course, there can be nobreak in the continuity between the arteries and the vein. The apparatusof circulation is thus formed by the heart, the arteries, thecapillaries, and the veins. 

184. The Heart. The heart is a pear-shaped, muscular organroughly estimated as about the size of the persons closed fist. It lies inthe chest behind the breastbone, and is, lodged between the lobes of thelungs, which partly cover it. In shape the heart resembles a cone, thebase of which is directed upwards, a little backwards, and to the rightside, while the apex is pointed downwards, forwards, and to the left side. During life, the apex of the heart beats against the chest wall inthe space between the fifth and sixth ribs, and about an inch and a halfto the left of the middle line of the body. The beating of the heart canbe readily felt, heard, and often seen moving the chest wall as it strikesagainst it. 

[Illustration: Fig. 69. --Diagram illustrating the Structure of a SerousMembrane. 

 A, the viscus, or organ, enveloped by serous membrane; B, layer of membrane lining cavity; C, membrane reflected to envelop viscus; D, outer layer of viscus, with blood-vessels at E communicating with the general circulation. ]

The heart does not hang free in the chest, but is suspended and kept inposition to some extent by the great vessels connected with it. It isenclosed in a bell-shaped covering called the pericardium. This isreally double, with two layers, one over another. The inner or serouslayer covers the external surface of the heart, and is reflected back uponitself in order to form, like all membranes of this kind, a sac without anopening. [32] The heart is thus covered by the pericardial sac, butis not contained inside its cavity. The space between the two membranes isfilled with serous fluid. This fluid permits the heart and the pericardiumto glide upon one another with the least possible amount of friction. [33]

The heart is a hollow organ, but the cavity is divided into two parts by amuscular partition forming a left and a right side, between which there isno communication. These two cavities are each divided by a horizontalpartition into an upper and a lower chamber. These partitions, however, include a set of valves which open like folding doors between the tworooms. If these doors are closed there are two separate rooms, but if openthere is practically only one room. The heart thus has four chambers, twoon each side. The two upper chambers are called auricles from theirsupposed resemblance to the ear. The two lower chambers are calledventricles, and their walls form the chief portion of the muscularsubstance of the organ. There are, therefore, the right and left auricles, with their thin, soft walls, and the right and left ventricles, with theirthick and strong walls. 

185. The Valves of the Heart. The heart is a valvular pump, whichworks on mechanical principles, the motive power being supplied by thecontraction of its muscular fibers. Regarding the heart as a pump, itsvalves assume great importance. They consist of thin, but strong, triangular folds of tough membrane which hang down from the edges of thepassages into the ventricles. They may be compared to swinging curtainswhich, by opening only one way, allow the blood to flow from the auriclesto the ventricles, but by instantly folding back prevent its return. 

[Illustration: Fig. 70. --Lateral Section of the Right Chest. (Showing therelative position of the heart and its great vessels, the oesophagusand trachea. )

 A, inferior constrictor muscle (aids in conveying food down the oesophagus); B, oesophagus; C, section of the right bronchus; D, two right pulmonary veins; E, great azygos vein crossing oesophagus and right bronchus to empty into the superior vena cava; F, thoracic duct; H, thoracic aorta; K, lower portion of oesophagus passing through the diaphragm; L, diaphragm as it appears in sectional view, enveloping the heart; M, inferior vena cava passing through diaphragm and emptying into auricle; N, right auricle; O, section of right branch of the pulmonary artery; P, aorta; R, superior vena cava; S, trachea. ]

The valve on the right side is called the tricuspid, because itconsists of three little folds which fall over the opening and close it, being kept from falling too far by a number of slender threads calledchordæ tendinæ. The valve on the left side, called the mitral, from its fancied resemblance to a bishop's mitre, consists of two foldswhich close together as do those of the tricuspid valve. 

The slender cords which regulate the valves are only just long enough toallow the folds to close together, and no force of the blood pushingagainst the valves can send them farther back, as the cords will notstretch The harder the blood in the ventricles pushes back against thevalves, the tighter the cords become and the closer the folds are broughttogether, until the way is completely closed. 

From the right ventricle a large vessel called the pulmonary arterypasses to the lungs, and from the left ventricle a large vessel called theaorta arches out to the general circulation of the body. The openingsfrom the ventricles into these vessels are guarded by the semilunarvalves. Each valve has three folds, each half-moon-shaped, hence thename semilunar. These valves, when shut, prevent any backward flow of theblood on the right side between the pulmonary artery and the rightventricle, and on the left side between the aorta and the left ventricle. 

[Illustration: Fig. 71. --Right Cavities of the Heart. 

 A, aorta; B, superior vena cava; C, C, right pulmonary veins; D, inferior vena cava; E, section of coronary vein; F, right ventricular cavity; H, posterior curtain of the tricuspid valve; K, right auricular cavity; M, fossa ovalis, oval depression, partition between the auricles formed after birth. ]

186. General Plan of the Blood-vessels Connected with the Heart. There are numerous blood-vessels connected with the heart, the relativeposition and the use of which must be understood. The two largest veins inthe body, the superior vena cava and the inferior vena cava, open into the right auricle. These two veins bring venous blood from allparts of the body, and pour it into the right auricle, whence it passesinto the right ventricle. 

From the right ventricle arises one large vessel, the pulmonaryartery, which soon divides into two branches of nearly equal size, onefor the right lung, the other for the left. Each branch, having reachedits lung, divides and subdivides again and again, until it ends inhair-like capillaries, which form a very fine network in every part of thelung. Thus the blood is pumped from the right ventricle into the pulmonaryartery and distributed throughout the two lungs (Figs. 86 and 88). 

We will now turn to the left side of the heart, and notice the generalarrangement of its great vessels. Four veins, called the pulmonaryveins, open into the left auricle, two from each lung. These veinsstart from very minute vessels the continuation of the capillaries of thepulmonary artery. They form larger and larger vessels until they becometwo large veins in each lung, and pour their contents into the leftauricle. Thus the pulmonary artery carries venous blood from the rightventricle _to_ the lungs, as the pulmonary veins carry arterial blood_from_ the lungs to the left auricle. 

From the left ventricle springs the largest arterial trunk in the body, over one-half of an inch in diameter, called the aorta. From theaorta other arteries branch off to carry the blood to all parts of thebody, only to be again brought back by the veins to the right side, through the cavities of the ventricles. We shall learn in Chapter VIII. That the main object of pumping the blood into the lungs is to have itpurified from certain waste matters which it has taken up in its coursethrough the body, before it is again sent on its journey from the leftventricle. 

187. The Arteries. The blood-vessels are flexible tubes through whichthe blood is borne through the body. There are three kinds, --thearteries, the veins, and the capillaries, and these differfrom one another in various ways. 

The arteries are the highly elastic and extensible tubes which carrythe pure, fresh blood outwards from the heart to all parts of the body. They may all be regarded as branches of the aorta. After the aorta leavesthe left ventricle it rises towards the neck, but soon turns downwards, making a curve known as the arch of the aorta. 

From the arch are given off the arteries which supply the head and armswith blood. These are the two carotid arteries, which run up on eachside of the neck to the head, and the two subclavian arteries, whichpass beneath the collar bone to the arms. This great arterial trunk nowpasses down in front of the spine to the pelvis, where it divides into twomain branches, which supply the pelvis and the lower limbs. 

The descending aorta, while passing downwards, gives off arteries to thedifferent tissues and organs. Of these branches the chief are thecoeliac artery, which subdivides into three great branches, --oneeach to supply the stomach, the liver, and the spleen; then the renalarteries, one to each kidney; and next two others, the mesentericarteries, to the intestines. The aorta at last divides into two mainbranches, the common iliac arteries, which, by their subdivisions, furnish the arterial vessels for the pelvis and the lower limbs. 

[Illustration: Fig. 72. --Left Cavities of the Heart. 

 A, B, right pulmonary veins; with S, openings of the veins; E, D, C, aortic valves; R, aorta; P, pulmonary artery; O, pulmonic valves; H, mitral valve; K, columnæ carnoeæ; M, right ventricular cavity; N, interventricular septum. ]

The flow of blood in the arteries is caused by the muscular force of theheart, aided by the elastic tissues and muscular fibers of the arterialwalls, and to a certain extent by the muscles themselves. Most of thegreat arterial trunks lie deep in the fleshy parts of the body; but theirbranches are so numerous and become so minute that, with a few exceptions, they penetrate all the tissues of the body, --so much so, that the pointof the finest needle cannot be thrust into the flesh anywhere withoutwounding one or more little arteries and thus drawing blood. 

188. The Veins. The veins are the blood-vessels which carry theimpure blood from the various tissues of the body to the heart. They beginin the minute capillaries at the extremities of the four limbs, andeverywhere throughout the body, and passing onwards toward the heart, receive constantly fresh accessions on the way from myriad other veinsbringing blood from other wayside capillaries, till the central veinsgradually unite into larger and larger vessels until at length they formthe two great vessels which open into the right auricle of the heart. 

These two great venous trunks are the inferior vena cava, bringingthe blood from the trunk and the lower limbs, and the superior venacava, bringing the blood from the head and the upper limbs. These twolarge trunks meet as they enter the right auricle. The four pulmonaryveins, as we have learned, carry the arterial blood from the lungs tothe left auricle. 

[Illustration: Fig. 73. 

 A, part of a vein laid open, with two pairs of valves; B, longitudinal section of a vein, showing the valves closed. ]

A large vein generally accompanies its corresponding artery, but mostveins lie near the surface of the body, just beneath the skin. They may beeasily seen under the skin of the hand and forearm, especially in agedpersons. If the arm of a young person is allowed to hang down a fewmoments, and then tightly bandaged above the elbow to retard the return ofthe blood, the veins become large and prominent. 

The walls of the larger veins, unlike arteries, contain but little ofeither elastic or muscular tissue; hence they are thin, and when emptycollapse. The inner surfaces of many of the veins are supplied withpouch-like folds, or pockets, which act as valves to impede the backwardflow of the blood, while they do not obstruct blood flowing forward towardthe heart. These valves can be shown by letting the forearm hang down, andsliding the finger upwards over the veins (Fig. 73). 

The veins have no force-pump, like the arteries, to propel their contentstowards their destination. The onward flow of the blood in them is due tovarious causes, the chief being the pressure behind of the blood pumpedinto the capillaries. Then as the pocket-like valves prevent the backwardflow of the blood, the pressure of the various muscles of the body urgesalong the blood, and thus promotes the onward flow. 

The forces which drive the blood through the arteries are sufficient tocarry the blood on through the capillaries. It is calculated that theonward flow in the capillaries is about 1/50 to 1/33 of an inch in asecond, while in the arteries the blood current flows about 16 inches in asecond, and in the great veins about 4 inches every second. 

[Illustration: Fig. 74. --The Structure of Capillaries. 

Capillaries of various sizes, showing cells with nuclei]

189. The Capillaries. The capillaries are the minute, hair-liketubes, with very thin walls, which form the connection between the endingof the finest arteries and the beginning of the smallest veins. They aredistributed through every tissue of the body, except the epidermis and itsproducts, the epithelium, the cartilages, and the substance of the teeth. In fact, the capillaries form a network of the tiniest blood-vessels, sominute as to be quite invisible, at least one-fourth smaller than thefinest line visible to the naked eye. 

The capillaries serve as a medium to transmit the blood from the arteriesto the veins; and it is through them that the blood brings nourishment tothe surrounding tissues. In brief, we may regard the whole body asconsisting of countless groups of little islands surrounded byever-flowing streams of blood. The walls of the capillaries are of themost delicate structure, consisting of a single layer of cells looselyconnected. Thus there is allowed the most free interchange between theblood and the tissues, through the medium of the lymph. 

The number of the capillaries is inconceivable. Those in the lungs alone, placed in a continuous line, would reach thousands of miles. The thinwalls of the capillaries are admirably adapted for the importantinterchanges that take place between the blood and the tissues. 

190. The Circulation of the Blood. It is now well to study thecirculation as a whole, tracing the course of the blood from acertain point until it returns to the same point. We may convenientlybegin with the portion of blood contained at any moment in the rightauricle. The superior and inferior venæ cavæ are busily filling theauricle with dark, impure blood. When it is full, it contracts. Thepassage leading to the right ventricle lies open, and through it the bloodpours till the ventricle is full. Instantly this begins, in its turn, tocontract. The tricuspid valve at once closes, and blocks the way backward. The blood is now forced through the open semilunar valves into thepulmonary artery. 

The pulmonary artery, bringing venous blood, by its alternate expansionand recoil, draws the blood along until it reaches the pulmonarycapillaries. These tiny tubes surround the air cells of the lungs, andhere an exchange takes place. The impure, venous blood here gives up its_débris_ in the shape of carbon dioxid and water, and in return takes up alarge amount of oxygen. Thus the blood brought to the lungs by thepulmonary arteries leaves the lungs entirely different in character andappearance. This part of the circulation is often called the lesser orpulmonic circulation. 

The four pulmonary veins bring back bright, scarlet blood, and pour itinto the left auricle of the heart, whence it passes through the mitralvalve into the left ventricle. As soon as the left ventricle is full, itcontracts. The mitral valve instantly closes and blocks the passagebackward into the auricle; the blood, having no other way open, is forcedthrough the semilunar valves into the aorta. Now red in color from itsfresh oxygen, and laden with nutritive materials, it is distributed by thearteries to the various tissues of the body. Here it gives up its oxygen, and certain nutritive materials to build up the tissues, and receivescertain products of waste, and, changed to a purple color, passes from thecapillaries into the veins. 

[Illustration: Fig. 75. --Diagram illustrating the Circulation. 

 1, right auricle; 2, left auricle; 3, right ventricle; 4, left ventricle; 5, vena cava superior; 6, vena cava inferior; 7, pulmonary arteries; 8, lungs; 9, pulmonary veins; 10, aorta; 11, alimentary canal; 12, liver; 13, hepatic artery; 14, portal vein; 15, hepatic vein. ]

All the veins of the body, except those from the lungs and the heartitself, unite into two large veins, as already described, which pour theircontents into the right auricle of the heart, and thus the grand round ofcirculation is continually maintained. This is called the systemiccirculation. The whole circuit of the blood is thus divided into twoportions, very distinct from each other. 

191. The Portal Circulation. A certain part of the systemic orgreater circulation is often called the portal circulation, whichconsists of the flow of the blood from the abdominal viscera through theportal vein and liver to the hepatic vein. The blood brought to thecapillaries of the stomach, intestines, spleen, and pancreas is gatheredinto veins which unite into a single trunk called the portal vein. The blood, thus laden with certain products of digestion, is carried tothe liver by the portal vein, mingling with that supplied to thecapillaries of the same organ by the hepatic artery. From thesecapillaries the blood is carried by small veins which unite into a largetrunk, the hepatic vein, which opens into the inferior vena cava. Theportal circulation is thus not an independent system, but forms a kind ofloop on the systemic circulation. 

The lymph-current is in a sense a slow and stagnant side stream ofthe blood circulation; for substances are constantly passing from theblood-vessels into the lymph spaces, and returning, although after acomparatively long interval, into the blood by the great lymphatic trunks. 

 Experiment 90. _To illustrate the action of the heart, and how it pumps the blood in only one direction_. Take a Davidson or Household rubber syringe. Sink the suction end into water, and press the bulb. As you let the bulb expand, it fills with water; as you press it again, a valve prevents the water from flowing back, and it is driven out in a jet along the other pipe. The suction pipe represents the veins; the bulb, the heart; and the tube end, out of which the water flows, the arteries. 

 [NOTE. The heart is not nourished by the blood which passes through it. The muscular substance of the heart itself is supplied with nourishment by two little arteries called the _coronary arteries_, which start from the aorta just above two of the semilunar valves. The blood is returned to the right auricle (not to either of the venæ cavæ) by the _coronary vein_. ]

The longest route a portion of blood may take from the moment it leavesthe left ventricle to the moment it returns to it, is through the portalcirculation. The shortest possible route is through the substance of theheart itself. The mean time which the blood requires to make a completecircuit is about 23 seconds. 

192. The Rhythmic Action of the Heart. To maintain a steady flow ofblood throughout the body the action of the heart must be regular andmethodical. The heart does not contract as a whole. The two auriclescontract at the same time, and this is followed at once by the contractionof the two ventricles. While the ventricles are contracting, the auriclesbegin to relax, and after the ventricles contract they also relax. Nowcomes a pause, or rest, after which the auricles and ventricles contractagain in the same order as before, and their contractions are followed bythe same pause as before. These contractions and relaxations of thevarious parts of the heart follow one another so regularly that the resultis called the rhythmic action of the heart. 

The average number of beats of the heart, under normal conditions, is from65 to 75 per minute. Now the time occupied from the instant the auriclesbegin to contract until after the contraction of the ventricles and thepause, is less than a second. Of this time one-fifth is occupied by thecontraction of the auricles, two-fifths by the contraction of theventricles, and the time during which the whole heart is at rest istwo-fifths of the period. 

193. Impulse and Sounds of the Heart. The rhythmic action of theheart is attended with various occurrences worthy of note. If the hand belaid flat over the chest wall on the left, between the fifth and sixthribs, the heart will be felt beating. This movement is known as thebeat or impulse of the heart, and can be both seen and felt onthe left side. The heart-beat is unusually strong during active bodilyexertion, and under mental excitement. 

The impulse of the heart is due to the striking of the lower, tense partof the ventricles--the apex of the heart--against the chest wall at themoment of their vigorous contraction. It is important for the physician toknow the exact place where the heart-beat should be felt, for the heartmay be displaced by disease, and its impulse would indicate its newposition. 

Sounds also accompany the heart's action. If the ear be applied over theregion of the heart, two distinct sounds will be heard following oneanother with perfect regularity. Their character may be tolerably imitatedby pronouncing the syllables _lubb_, _dup_. One sound is heard immediatelyafter the other, then there is a pause, then come the two sounds again. The first is a dull, muffled sound, known as the "first sound, " followedat once by a short and sharper sound, known as the "second sound" of theheart. 

The precise cause of the first sound is still doubtful, but it is made atthe moment the ventricles contract. The second sound is, without doubt, caused by the sudden closure of the semilunar valves of the pulmonaryartery and the aorta, at the moment when the contraction of the ventriclesis completed. 

[Illustration: Fig. 76. --Muscular Fibers of the Ventricles. 

 A, superficial fibers common to both ventricles; B, fibers of the left ventricle; C, deep fibers passing upwards toward the base of the heart; D, fibers penetrating the left ventricle]

The sounds of the heart are modified or masked by blowing "murmurs" whenthe cardiac orifices or valves are roughened, dilated, or otherwiseaffected as the result of disease. Hence these new sounds may often affordindications of the greatest importance to physicians in the diagnosis ofheart-disease. 

194. The Nervous Control of the Heart. The regular, rhythmic movementof the heart is maintained by the action of certain nerves. In variousplaces in the substance of the heart are masses of nerve matter, calledganglia. From these ganglia there proceed, at regular intervals, discharges of nerve energy, some of which excite movement, while othersseem to restrain it. The heart would quickly become exhausted if theexciting ganglia had it all their own way, while it would stand still ifthe restraining ganglia had full sway. The influence of one, however, modifies the other, and the result is a moderate and regular activity ofthe heart. 

The heart is also subject to other nerve influences, but from outside ofitself. Two nerves are connected with the heart, the pneumogastricand the sympathetic (secs. 271 and 265). The former appears to beconnected with the restraining ganglia; the latter with the excitingganglia. Thus, if a person were the subject of some emotion which causedfainting, the explanation would be that the impression had been conveyedto the brain, and from the brain to the heart by the pneumogastric nerves. The result would be that the heart for an instant ceases to beat. Deathwould be the result if the nerve influence were so great as to restrainthe movements of the heart for any appreciable time. 

Again, if the person were the subject of some emotion by which the heartwere beating faster than usual, it would mean that there was sent from thebrain to the heart by the sympathetic nerves the impression whichstimulated it to increased activity. 

195. The Nervous Control of the Blood-vessels. The tone and caliberof the blood-vessels are controlled by certain vaso-motor nerves, which are distributed among the muscular fibers of the walls. These nervesare governed from a center in the medulla oblongata, a part of the brain(sec. 270). If the nerves are stimulated more than usual, the muscularwalls contract, and the quantity of the blood flowing through them and thesupply to the part are diminished. Again, if the stimulus is less thanusual, the vessels dilate, and the supply to the part is increased. 

Now the vaso-motor center may be excited to increased activity byinfluences reaching it from various parts of the body, or even from thebrain itself. As a result, the nerves are stimulated, and the vesselscontract. Again, the normal influence of the vaso-motor center may besuspended for a time by what is known as the inhibitory orrestraining effect. The result is that the tone of the blood-vesselsbecomes diminished, and their channels widen. 

The effect of this power of the nervous system is to give it a certaincontrol over the circulation in particular parts. Thus, though the forceof the heart and the general average blood-pressure remain the same, thestate of the circulation may be very different in different parts of thebody. The importance of this local control over the circulation is of theutmost significance. Thus an organ at work needs to be more richlysupplied with blood than when at rest. For example, when the salivaryglands need to secrete saliva, and the stomach to pour out gastric juice, the arteries that supply these organs are dilated, and so the parts areflushed with an extra supply of blood, and thus are aroused to greateractivity. 

Again, the ordinary supply of blood to a part may be lessened, so that theorgan is reduced to a state of inactivity, as occurs in the case of thebrain during sleep. We have in the act of blushing a visible example ofsudden enlargement of the smaller arteries of the face and neck, calledforth by some mental emotion which acts on the vaso-motor center anddiminishes its activity. The reverse condition occurs in the act ofturning pale. Then the result of the mental emotion is to cause thevaso-motor nerves to exercise a more powerful control over thecapillaries, thereby closing them, and thus shutting off the flow ofblood. 

 Experiment 91. Hold up the ear of a white rabbit against the light while the animal is kept quiet and not alarmed. The red central artery can be seen coursing along the translucent organ, giving off branches which by subdivision become too small to be separately visible, and the whole ear has a pink color and is warm from the abundant blood flowing through it. Attentive observation will show also that the caliber of the main artery is not constant; at somewhat irregular periods of a minute or more it dilates and contracts a little. 

[Illustration: Fig. 77. --Some of the Principal Organs of the Chest andAbdomen. (Blood vessels on the left, muscles on the right. )]

In brief, all over the body, the nervous system, by its vaso-motorcenters, is always supervising and regulating the distribution of blood inthe body, sending now more and now less to this or that part. 

[Illustration: Fig. 78. --Capillary Blood-Vessels in the Web of a Frog'sFoot, as seen with the Microscope. ]

196. The Pulse. When the finger is placed on any part of the bodywhere an artery is located near the surface, as, for example, on theradial artery near the wrist, there is felt an intermittent pressure, throbbing with every beat of the heart. This movement, frequently visibleto the eye, is the result of the alternate expansion of the artery by thewave of blood, and the recoil of the arterial walls by their elasticity. In other words, it is the wave produced by throwing a mass of blood intothe arteries already full. The blood-wave strikes upon the elastic wallsof the arteries, causing an increased distention, followed at once bycontraction. This regular dilatation and rigidity of the elastic arteryanswering to the beats of the heart, is known as the pulse. 

The pulse may be easily found at the wrist, the temple, and the inner sideof the ankle. The throb of the two carotid arteries may be plainly felt bypressing the thumb and finger backwards on each side of the larynx. Theprogress of the pulse-wave must not be confused with the actual current ofthe blood itself. For instance, the pulse-wave travels at the rate ofabout 30 feet a second, and takes about 1/10 of a second to reach thewrist, while the blood itself is from 3 to 5 seconds in reaching the sameplace. 

The pulse-wave may be compared to the wave produced by a stiff breeze onthe surface of a slowly moving stream, or the jerking throb sent along arope when shaken. The rate of the pulse is modified by age, fatigue, posture, exercise, stimulants, disease, and many other circumstances. Atbirth the rate is about 140 times a minute, in early infancy, 120 orupwards, in the healthy adult between 65 and 75, the most common numberbeing 72. In the same individual, the pulse is quicker when standing thanwhen lying down, is quickened by excitement, is faster in the morning, andis slowest at midnight. In old age the pulse is faster than in middlelife; in children it is quicker than in adults. 

[Illustration: Fig. 79. --Circulation in the Capillaries, as seen with theMicroscope. ]

As the pulse varies much in its rate and character in disease, it is tothe skilled touch of the physician an invaluable help in the diagnosis ofthe physical condition of his patient. 

 Experiment 92. _To find the pulse_. Grasp the wrist of a friend, pressing with three fingers over the radius. Press three fingers over the radius in your own wrist, to feel the pulse. 

 Count by a watch the rate of your pulse per minute, and do the same with a friend's pulse. Compare its characters with your own pulse. 

 Observe how the character and frequency of the pulse are altered by posture, muscular exercise, a prolonged, sustained, deep inspiration, prolonged expiration, and other conditions. 

197. Effect of Alcoholic Liquors upon the Organs of Circulation. Alcoholic drinks exercise a destructive influence upon the heart, thecirculation, and the blood itself. These vicious liquids can reach theheart only indirectly, either from the stomach by the portal vein to theliver, and thence to the heart, or else by way of the lacteals, and so tothe blood through the thoracic duct. But by either course the route isdirect enough, and speedy enough to accomplish a vast amount of ruinouswork. 

The influence of alcohol upon the heart and circulation is produced mainlythrough the nervous system. The inhibitory nerves, as we have seen, holdthe heart in check, exercise a restraining control over it, very much asthe reins control an active horse. In health this inhibitory influence isprotective and sustaining. But now comes the narcotic invasion ofalcoholic drinks, which paralyze the inhibitory nerves, with the others, and at once the uncontrolled heart, like the unchecked steed, plunges onto violent and often destructive results. 

[Illustration: Fig. 80. --Two Principal Arteries of the Front of the Leg(Anterior Tibial and Dorsalis Pedis). ]

This action, because it is quicker, has been considered also a strongeraction, and the alcohol has therefore been supposed to produce astimulating effect. But later researches lead to the conclusion that theeffect of alcoholic liquors is not properly that of a stimulant, but of anarcotic paralyzant, and that while it indeed quickens, it also reallyweakens the heart's action. This view would seem sustained by the factthat the more the intoxicants are pushed, the deeper are the narcotic andparalyzing effects. After having obstructed the nutritive and reparativefunctions of the vital fluid for many years, their effects at last maybecome fatal. 

This relaxing effect involves not only the heart, but also the capillarysystem, as is shown in the complexion of the face and the color of thehands. In moderate drinkers the face is only flushed, but in drunkards itis purplish. The flush attending the early stages of drinking is, ofcourse, not the flush of health, but an indication of disease. [34]

198. Effect upon the Heart. This forced overworking of the heartwhich drives it at a reckless rate, cuts short its periods of rest andinevitably produces serious heart-exhaustion. If repeated and continued, it involves grave changes of the structure of the heart. The heart muscle, endeavoring to compensate for the over-exertion, may become muchthickened, making the ventricles smaller, and so fail to do its duty inproperly pumping forward the blood which rushes in from the auricle. Orthe heart wall may by exhaustion become thinner, making the ventriclesmuch too large, and unable to send on the current. In still other cases, the heart degenerates with minute particles of fat deposited in itsstructures, and thus loses its power to propel the nutritive fluid. Allthree of these conditions involve organic disease of the valves, and allthree often produce fatal results. 

199. Effect of Alcohol on the Blood-vessels. Alcoholic liquors injurenot only the heart, but often destroy the blood-vessels, chiefly thelarger arteries, as the arch of the aorta or the basilar artery of thebrain. In the walls of these vessels may be gradually deposited a morbidproduct, the result of disordered nutrition, sometimes chalky, sometimesbony, with usually a dangerous dilatation of the tube. 

In other cases the vessels are weakened by an unnatural fatty deposit. Though these disordered conditions differ somewhat, the morbid results inall are the same. The weakened and stiffened arterial walls lose theelastic spring of the pulsing current. The blood fails to sweep on withits accustomed vigor. At last, owing perhaps to the pressure, against theobstruction of a clot of blood, or perhaps to some unusual strain of workor passion, the enfeebled vessel bursts, and death speedily ensues from aform of apoplexy. 

[Illustration: Fig. 81. --Showing the Carotid Artery and Jugular Vein onthe Right Side, with Some of their Main Branches. (Some branches of thecervical plexus, and the hypoglossal nerve are also shown. )]

 [NOTE. "An alcoholic heart loses its contractile and resisting power, both through morbid changes in its nerve ganglia and in its muscle fibers. In typhoid fever, muscle changes are evidently the cause of the heart-enfeeblement; while in diphtheria, disturbances in innervation cause the heart insufficiency. 'If the habitual use of alcohol causes the loss of contractile and resisting power by impairment of both the nerve ganglia and muscle fibers of the heart, how can it act as a heart tonic?'"--Dr. Alfred L. Loomis, Professor of Medicine in the Medical Department of the University of the City of New York. ]

200. Other Results from the Use of Intoxicants. Other disastrousconsequences follow the use of intoxicants, and these upon the blood. Whenany alcohol is present in the circulation, its greed for water induces theabsorption of moisture from the red globules of the blood, theoxygen-carriers. In consequence they contract and harden, thus becomingunable to absorb, as theretofore, the oxygen in the lungs. Then, in turn, the oxidation of the waste matter in the tissues is prevented; thus thecorpuscles cannot convey carbon dioxid from the capillaries, and this factmeans that some portion of refuse material, not being thus changed andeliminated, must remain in the blood, rendering it impure and unfit forits proper use in nutrition. Thus, step by step, the use of alcoholicsimpairs the functions of the blood corpuscles, perverts nutrition, andslowly poisons the blood. 

[Illustration: Fig. 82. --The Right Axillary and Brachial Arteries, withSome of their Main Branches. ]

 [NOTE. "Destroy or paralyze the inhibitory nerve center, and instantly its controlling effect on the heart mechanism is lost, and the accelerating agent, being no longer under its normal restraint, runs riot. The heart's action is increased, the pulse is quickened, an excess of blood is forced into the vessels, and from their becoming engorged and dilated the face gets flushed, all the usual concomitants of a general engorgement of the circulation being the result. "--Dr. George Harley, F. R. S. , an eminent English medical author. 

 "The habitual use of alcohol produces a deleterious influence upon the whole economy. The digestive powers are weakened, the appetite is impaired, and the muscular system is enfeebled. The blood is impoverished, and nutrition is imperfect and disordered, as shown by the flabbiness of the skin and muscles, emaciation, or an abnormal accumulation of fat. "--Dr. Austin Flint, Senior, formerly Professor of the Practice of Medicine in Bellevue Medical College, and author of many standard medical works. 

 "The immoderate use of the strong kind of tobacco, which soldiers affect, is often very injurious to them, especially to very young soldiers. It renders them nervous and shaky, gives rise to palpitation, and is a factor in the production of the irritable or so-called "trotting-heart" and tends to impair the appetite and digestion. "--London _Lancet_. 

 "I never smoke because I have seen the most efficient proofs of the injurious effects of tobacco on the nervous system. "--Dr. Brown-Sequard, the eminent French physiologist. 

 "Tobacco, and especially cigarettes, being a depressant upon the heart, should be positively forbidden. "--Dr. J. M. Keating, on "Physical Development, " in _Cyclopoedia of the Diseases of Children_. ]

201. Effect of Tobacco upon the Heart. While tobacco poisons more orless almost every organ of the body, it is upon the heart that itworks its most serious wrong. Upon this most important organ itsdestructive effect is to depress and paralyze. Especially does this applyto the young, whose bodies are not yet knit into the vigor that can braveinvasion. 

The _nicotine_ of tobacco acts through the nerves that control the heart'saction. Under its baneful influence the motions of the heart areirregular, now feeble and fluttering, now thumping with apparently muchforce: but both these forms of disturbed action indicate an abnormalcondition. Frequently there is severe pain in the heart, often dizzinesswith gasping breath, extreme pallor, and fainting. 

The condition of the pulse is a guide to this state of the heart. In thisthe physician reads plainly the existence of the "tobacco heart, " anaffection as clearly known among medical men as croup or measles. Thereare few conditions more distressing than the constant and impendingsuffering attending a tumultuous and fluttering heart. It is stated thatone in every four of tobacco-users is subject, in some degree, to thisdisturbance. Test examinations of a large number of lads who had usedcigarettes showed that only a very small percentage escaped cardiactrouble. Of older tobacco-users there are very few but have some warningof the hazard they invoke. Generally they suffer more or less from thetobacco heart, and if the nervous system or the heart be naturally feeble, they suffer all the more speedily and intensely. 

Additional Experiments. 

 Experiment 93. Touch a few drops of blood fresh from the finger, with a strip of dry, smooth, neutral litmus paper, highly glazed to prevent the red corpuscles from penetrating into the test paper. Allow the blood to remain a short time; then wash it off with a stream of distilled water, when a blue spot upon a red or violet ground will be seen, indicating its _alkaline_ reaction, due chiefly to the sodium phosphate and sodium carbonate. 

 Experiment 94. Place on a glass slide a thin layer of defibrinated blood; try to read printed matter through it. This cannot be done. 

 Experiment 95. _To make blood transparent or laky_. Place in each of three test tubes two or three teaspoonfuls of defibrinated blood, obtained from Experiment 89, labeled _A, B_, and _C. A_ is for comparison. To _B_ add five volumes of water, and warm slightly, noting the change of color by reflected and transmitted light. By reflected light it is much darker, --it looks almost black; but by transmitted light it is transparent. Test this by looking at printed matter as in Experiment 94. 

 Experiment 96. To fifteen or twenty drops of defibrinated blood in a test tube (labeled _D_) add five volumes of a 10-per-cent solution of common salt. It changes to a very bright, florid, brick-red color. Compare its color with _A, B_, and _C_. It is opaque. 

 Experiment 97. Wash away the coloring matter from the twigs (see Experiment 89) with a stream of water until the fibrin becomes quite white. It is white, fibrous, and elastic. Stretch some of the fibers to show their extensibility; on freeing them, they regain their elasticity. 

 Experiment 98. Take some of the serum saved from Experiment 88 and note that it does not coagulate spontaneously. Boil a little in a test tube over a spirit lamp, and the albumen will coagulate. 

 Experiment 99. _To illustrate in a general way that blood is really a mass of red bodies which give the red color to the fluid in which they float. _ Fill a clean white glass bottle two-thirds full of little red beads, and then fill the bottle full of water. At a short distance the bottle appears to be rilled with a uniformly red liquid. 

 Experiment 100. _To show how blood holds a mineral substance in solution_. Put an egg-shell crushed fine, into a glass of water made acid by a teaspoonful of muriatic acid. After an hour or so the egg-shell will disappear, having been dissolved in the acid water. In like manner the blood holds various minerals in solution. 

 Experiment 101. _To hear the sounds of the heart_. Locate the heart exactly. Note its beat. Borrow a stethoscope from some physician. Listen to the heart-beat of some friend. Note the sounds of your own heart in the same way. 

 Experiment 102. _To show how the pulse may be studied_. "The movements of the artery in the human body as the pulse-wave passes through it may be shown to consist in a sudden dilatation, followed by a slow contraction, interrupted by one or more secondary dilatations. This demonstration may be made by pressing a small piece of looking-glass about one centimeter square (2/3 of an inch) upon the wrist over the radial artery, in such a way that with each pulse beat the mirror may be slightly tilted. If the wrist be now held in such a position that sunlight will fall upon the mirror, a spot of light will be reflected on the opposite side of the room, and its motion upon the wall will show that the expansion of the artery is a sudden movement, while the subsequent contraction is slow and interrupted. "--Bowditch's _Hints for Teachers of Physiology_. 

 [Illustration: Fig. 83. --How the Pulse may be studied by Pressing a Mirror over the Radial Artery. ]

 Experiment 103. _To illustrate the effect of muscular exercise in quickening the pulse_. Run up and down stairs several times. Count the pulse both before and after. Note the effect upon the rate. 

 Experiment 104. _To show the action of the elastic walls of the arteries. _ Take a long glass or metal tube of small caliber. Fasten one end to the faucet of a water-pipe (one in a set bowl preferred) by a very short piece of rubber tube. Turn the water on and off alternately and rapidly, to imitate the intermittent discharge of the ventricles. The water will flow from the other end of the rubber pipe in jets, each jet ceasing the moment the water is shut off. 

 The experiment will be more successful if the rubber bulb attached to an ordinary medicine-dropper be removed, and the tapering glass tube be slipped on to the outer end of the rubber tube attached to the faucet. 

 Experiment 105. Substitute a piece of rubber tube for the glass tube, and repeat the preceding experiment. Now it will be found that a continuous stream flows from the tube. The pressure of water stretches the elastic tube, and when the stream is turned off, the rubber recoils on the water, and the intermittent flow is changed into a continuous stream. 

 Experiment 106. _To illustrate some of the phenomena of circulation. _ Take a common rubber bulb syringe, of the Davidson, Household, or any other standard make. Attach a piece of rubber tube about six or eight feet long to the delivery end of the syringe. 

 To represent the resistance made by the capillaries to the flow of blood, slip the large end of a common glass medicine-dropper into the outer end of the rubber tube. This dropper has one end tapered to a fine point. 

 Place the syringe flat, without kinks or bends, on a desk or table. Press the bulb slowly and regularly. The water is thus pumped into the tube in an intermittent manner, and yet it is forced out of the tapering end of the glass tube in a steady flow. 

 Experiment 107. Take off the tapering glass tube, or, in the place of one long piece of rubber tube, substitute several pieces of glass tubing connected together by short pieces of rubber tubes. The obstacle to the flow has thus been greatly lessened, and the water flows out in intermittent jets to correspond to the compression of the bulb. 

Chapter VIII. 

Respiration. 

202. Nature and Object of Respiration. The blood, as we have learned, not only provides material for the growth and activity of all the tissuesof the body, but also serves as a means of removing from them the productsof their activity. These are waste products, which if allowed to remain, would impair the health of the tissues. Thus the blood becomesimpoverished both by the addition of waste material, and from the loss ofits nutritive matter. 

We have shown, in the preceding chapter, how the blood carries to thetissues the nourishment it has absorbed from the food. We have now toconsider a new source of nourishment to the blood, _viz. _, that which itreceives from the oxygen of the air. We are also to learn one of themethods by which the blood gets rid of poisonous waste matters. In brief, we are to study the set of processes known as respiration, by whichoxygen is supplied to the various tissues, and by which the principalwaste matters, or chief products of oxidation, are removed. 

Now, the tissues are continually feeding on the life-giving oxygen, and atthe same time are continually producing carbon dioxid and other wasteproducts. In fact, the life of the tissues is dependent upon a continualsuccession of oxidations and deoxidations. When the blood leaves thetissues, it is poorer in oxygen, is burdened with carbon dioxid, and hashad its color changed from bright scarlet to purple red. This is thechange from the arterial to venous conditions which has been described inthe preceding chapter. 

Now, as we have seen, the change from venous to arterial blood occurs inthe capillaries of the lungs, the only means of communication between thepulmonary arteries and the pulmonary veins. The blood in the pulmonarycapillaries is separated from the air only by a delicate tissue formed ofits own wall and the pulmonary membrane. Hence a gaseous interchange, the essential step in respiration, very readily takes place between theblood and the air, by which the latter gains moisture and carbon dioxid, and loses its oxygen. These changes in the lungs also restore to the darkblood its rosy tint. 

The only condition absolutely necessary to the purification of the bloodis an organ having a delicate membrane, on one side of which is a thinsheet of blood, while the other side is in such contact with the air thatan interchange of gases can readily take place. The demand for oxygen is, however, so incessant, and the accumulation of carbon dioxid is so rapidin every tissue of the human body, that an All-Wise Creator has provided amost perfect but complicated set of machinery to effect this wonderfulpurification of the blood. 

We are now ready to begin the study of the arrangement and working of therespiratory apparatus. With its consideration, we complete our view of thesources of supply to the blood, and begin our study of its purification. 

[Illustration: Fig. 84. --The Epiglottis. ]

203. The Trachea, or Windpipe. If we look into the mouth of a friend, or into our own with a mirror, we see at the back part an arch which isthe boundary line of the mouth proper. There is just behind this a similarlimit for the back part of the nostrils. The funnel-shaped cavity beyond, into which both the mouth and the posterior nasal passages open, iscalled the pharynx. In its lower part are two openings; thetrachea, or windpipe, in front, and the oesophagus behind. 

The trachea is surmounted by a box-like structure of cartilage, aboutfour and one-half inches long, called the larynx. The upper end ofthe larynx opens into the pharynx or throat, and is provided with a lid, --the epiglottis, --which closes under certain circumstances (secs. 137and 349). The larynx contains the organ of voice, and is more fullydescribed in Chapter XII. 

The continuation of the larynx is the trachea, a tube about three-fourthsof an inch in diameter, and about four inches long. It extends downwardsalong the middle line of the neck, where it may readily be felt in front, below the Adam's apple. 

[Illustration: Fig. 85. --Larynx, Trachea, and the Bronchi. (Front view. )

 A, epiglottis; B, thyroid cartilage; C, cricoid-thyroid membrane, connecting with the cricoid cartilage below, all forming the larynx; D, one of the rings of the trachea. ]

The walls of the windpipe are strengthened by a series of cartilaginousrings, each somewhat the shape of a horseshoe or like the letter C, beingincomplete behind, where they come in contact with the oesophagus. Thus the trachea, while always open for the passage of air, admits of thedistention of the food-passage. 

204. The Bronchial Tubes. The lower end of the windpipe is justbehind the upper part of the sternum, and there it divides into twobranches, called bronchi. Each branch enters the lung of its ownside, and breaks up into a great number of smaller branches, calledbronchial tubes. These divide into smaller tubes, which continuesubdividing till the whole lung is penetrated by the branches, theextremities of which are extremely minute. To all these branches thegeneral name of bronchial tubes is given. The smallest are only aboutone-fiftieth of an inch in diameter. 

[Illustration: Fig. 86. --Relative Position of the Lungs, Heart, and itsGreat Vessels. 

 A, left ventricle; B, right ventricle; C, left auricle; D, right auricle; E, superior vena cava; F, pulmonary artery; G, aorta; H, arch of the aorta; K, innominate artery; L, right common carotid artery; M, right subclavian artery; N, thyroid cartilage forming upper portion of the larynx; O, trachea. ]

Now the walls of the windpipe, and of the larger bronchial tubes wouldreadily collapse, and close the passage for air, but for a wiseprecaution. The horseshoe-shaped rings of cartilage in the trachea and theplates of cartilage in the bronchial tubes keep these passages open. Again, these air passages have elastic fibers running the length of thetubes, which allow them to stretch and bend readily with the movements ofthe neck. 

205. The Cilia of the Air Passages. The inner surfaces of thewindpipe and bronchial tubes are lined with mucous membrane, continuouswith that of the throat, the mouth, and the nostrils, the secretion fromwhich serves to keep the parts moist. 

Delicate, hair-like filaments, not unlike the pile on velvet, calledcilia, spring from the epithelial lining of the air tubes. Theirconstant wavy movement is always upwards and outwards, towards the mouth. Thus any excessive secretion, as of bronchitis or catarrh, is carriedupwards, and finally expelled by coughing. In this way, the lungs are keptquite free from particles of foreign matter derived from the air. Otherwise we should suffer, and often be in danger from the accumulationof mucus and dust in the air passages. Thus these tiny cilia act asdusters which Nature uses to keep the air tubes free and clean (Fig. 5). 

[Illustration: Fig. 87. --Bronchial tube, with its Divisions andSubdivisions. (Showing groups of air cells at the termination of minutebronchial tubes. )]

206. The Lungs. The lungs, the organs of respiration, are twopinkish gray structures of a light, spongy appearance, that fill the chestcavity, except the space taken up by the heart and large vessels. Betweenthe lungs are situated the large bronchi, the oesophagus, the heartin its pericardium, and the great blood-vessels. The base of the lungsrests on the dome-like diaphragm, which separates the chest from theabdomen. This partly muscular and partly tendinous partition is a mostimportant factor in breathing. 

Each lung is covered, except at one point, with an elastic serous membranein a double layer, called the pleura. One layer closely envelops thelung, at the apex of which it is reflected to the wall of the chest cavityof its own side, which it lines. The two layers thus form between them aClosed Sac a serous cavity (see Fig. 69, also note, p. 176). 

[Illustration: Fig. 88. --The Lungs with the Trachea, Bronchi, and LargerBronchial Tubes exposed. (Posterior view. )

 A, division of left bronchus to upper lobe; B, left branch of the Pulmonary artery; C, left bronchus; D, left superior pulmonary vein; E, left inferior pulmonary vein; F, left auricle; K, inferior vena cava; L, division of right bronchus to lower lobe; M, right inferior pulmonary vein; N, right superior pulmonary vein; O, right branch of the pulmonary artery; P, division of right bronchus to upper lobe; R, left ventricle; S, right ventricle. ]

In health the two pleural surfaces of the lungs are always in contact, andthey secrete just enough serous fluid to allow the surfaces to glidesmoothly upon each other. Inflammation of this membrane is called_pleurisy_. In this disease the breathing becomes very painful, as thesecretion of glairy serum is suspended, and the dry and inflamed surfacesrub harshly upon each other. 

The root of the lung, as it is called, is formed by the bronchi, twopulmonary arteries, and two pulmonary veins. The nerves and lymphaticvessels of the lung also enter at the root. If we only remember that allthe bronchial tubes, great and small, are hollow, we may compare the wholesystem to a short bush or tree growing upside down in the chest, of whichthe trachea is the trunk, and the bronchial tubes the branches of varioussizes. 

207. Minute Structure of the Lungs. If one of the smallest bronchialtubes be traced in its tree-like ramifications, it will be found to end inan irregular funnel-shaped passage wider than itself. Around this passageare grouped a number of honeycomb-like sacs, the air cells[35] oralveoli of the lungs. These communicate freely with the passage, andthrough it with the bronchial branches, but have no other openings. Thewhole arrangement of passages and air cells springing from the end of abronchial tube, is called an ultimate lobule. Now each lobule is avery small miniature of a whole lung, for by the grouping together ofthese lobules another set of larger lobules is formed. 

[Illustration: Fig. 89. 

 A, diagrammatic representation of the ending of a bronchial tube in air sacs or alveoli; B, termination of two bronchial tubes in enlargement beset with air sacs (_Huxley_); C, diagrammatic view of an air sac. 

 a lies within sac and points to epithelium lining wall; b, partition between two adjacent sacs, in which run capillaries; c, elastic connective tissue (_Huxley_). ]

In like manner countless numbers of these lobules, bound together byconnective tissue, are grouped after the same fashion to form by theiraggregation the lobes of the lung. The right lung has three suchlobes; and the left, two. Each lobule has a branch of the pulmonary arteryentering it, and a similar rootlet of the pulmonary vein leaving it. Italso receives lymphatic vessels, and minute twigs of the pulmonary plexusof nerves. 

[Illustration: Fig. 90. --Diagram to illustrate the Amounts of Aircontained by the Lungs in Various Phases of Ordinary and of ForcedRespiration. ]

The walls of the air cells are of extreme thinness, consisting of delicateelastic and connective tissue, and lined inside by a single layer of thinepithelial cells. In the connective tissue run capillary vessels belongingto the pulmonary artery and veins. Now these delicate vessels running inthe connective tissue are surrounded on all sides by air cells. It isevident, then, that the blood flowing through these capillaries isseparated from the air within the cells only by the thin walls of thevessels, and the delicate tissues of the air cells. 

This arrangement is perfectly adapted for an interchange between theblood in the capillaries and the air in the air cells. This will be morefully explained in sec. 214. 

208. Capacity of the Lungs. In breathing we alternately take into andexpel from the lungs a certain quantity of air. With each quietinspiration about 30 cubic inches of air enter the lungs, and 30 cubicinches pass out with each expiration. The air thus passing into and out ofthe lungs is called tidal air. After an ordinary inspiration, thelungs contain about 230 cubic inches of air. By taking a deep inspiration, about 100 cubic inches more can be taken in. This extra amount is calledcomplemental air. 

After an ordinary expiration, about 200 cubic inches are left in thelungs, but by forced expiration about one-half of this may be driven out. This is known as supplemental air. The lungs can never be entirelyemptied of air, about 75 to 100 cubic inches always remaining. This isknown as the residual air. 

The air that the lungs of an adult man are capable of containing is thuscomposed:

 Complemental air 100 cubic inches. Tidal " 30 " " Supplemental " 100 " " Residual " 100 " " ---- Total capacity of lungs 330 " "

If, then, a person proceeds, after taking the deepest possible breath, tobreath out as much as he can, he expels:

 Complemental air 100 cubic inches. Tidal " 30 " " Supplemental " 100 " " ---- 230

This total of 230 cubic inches forms what is called the vitalcapacity of the chest (Fig. 90). 

209. The Movements of Breathing. The act of breathing consists of aseries of rhythmical movements, succeeding one another in regular order. In the first movement, inspiration, the chest rises, and there is aninrush of fresh air; this is at once followed by expiration, thefalling of the chest walls, and the output of air. A pause now occurs, andthe same breathing movements are repeated. 

The entrance and the exit of air into the respiratory passages areaccompanied with peculiar sounds which are readily heard on placing theear at the chest wall. These sounds are greatly modified in variouspulmonary diseases, and hence are of great value to the physician inmaking a correct diagnosis. 

In a healthy adult, the number of respirations should be from 16 to 18 perminute, but they vary with age, that of a newly born child being 44 forthe same time. Exercise increases the number, while rest diminishes it. Instanding, the rate is more than when lying at rest. Mental emotion andexcitement quicken the rate. The number is smallest during sleep. Diseasehas a notable effect upon the frequency of respirations. In diseasesinvolving the lungs, bronchial tubes, and the pleura, the rate may bealarmingly increased, and the pulse is quickened in proportion. 

210. The Mechanism of Breathing. The chest is a chamber with bonywalls, the ribs connecting in front with the breastbone, and behind withthe spine. The spaces between the ribs are occupied by the intercostalmuscles, while large muscles clothe the entire chest. The diaphragm servesas a movable floor to the chest, which is an air-tight chamber withmovable walls and floor. In this chamber are suspended the lungs, the aircells of which communicate with the outside through the bronchialpassages, but have no connection with the chest cavity. The thin spacebetween the lungs and the rib walls, called the pleural cavity, is inhealth a vacuum. 

Now, when the diaphragm contracts, it descends and thus increases thedepth of the chest cavity. A quantity of air is now drawn into the lungsand causes them to expand, thus filling up the increased space. As soon asthe diaphragm relaxes, returning to its arched position and reducing thesize of the chest cavity, the air is driven from the lungs, which thendiminish in size. After a short pause, the diaphragm again contracts, andthe same round of operation is constantly repeated. 

The walls of the chest being movable, by the contractions of theintercostals and other muscles, the ribs are raised and the breastbonepushed forward. The chest cavity is thus enlarged from side to side andfrom behind forwards. Thus, by the simultaneous descent of the diaphragmand the elevation of the ribs, the cavity of the chest is increased inthree directions, --downwards, side-ways, and from behind forwards. 

It is thus evident that inspiration is due to a series of muscularcontractions. As soon as the contractions cease, the elastic lungtissue resumes its original position, just as an extended rubber bandrecovers itself. As a result, the original size of the chest cavity isrestored, and the inhaled air is driven from the lungs. Expiration maythen be regarded as the result of an elastic recoil, and not of activemuscular contractions. 

[Illustration: Fig. 91. --Diagrammatic Section of the Trunk. (Showing theexpansion of the chest and the movement of the ribs by action of thelungs. ) [The dotted lines indicate the position during inspiration. ]]

211. Varieties of Breathing. This is the mechanism of quiet, normalrespiration. When the respiration is difficult, additional forces arebrought into play. Thus when the windpipe and bronchial tubes areobstructed, as in croup, asthma, or consumption, many additional musclesare made use of to help the lungs to expand. The position which asthmaticsoften assume, with arms raised to grasp something for support, is from theneed of the sufferer to get a fixed point from which the muscles of thearm and chest may act forcibly in raising the ribs, and thus securing morecomfortable breathing. 

The visible movements of breathing vary according to circumstances. Ininfants the action of the diaphragm is marked, and the movements of theabdomen are especially obvious. This is called abdominal breathing. Inwomen the action of the ribs as they rise and fall, is emphasized morethan in men, and this we call costal breathing. In young persons and inmen, the respiration not usually being impeded by tight clothing, thebreathing is normal, being deep and abdominal. 

Disease has a marked effect upon the mode of breathing. Thus, whenchildren suffer from some serious chest disease, the increased movementsof the abdominal walls seem distressing. So in fracture of the ribs, thesurgeon envelops the overlying part of the chest with long strips of firmadhesive plaster to restrain the motions of chest respiration, that theymay not disturb the jagged ends of the broken bones. Again, in painfuldiseases of the abdomen, the sufferer instinctively suspends the abdominalaction and relies upon the chest breathing. These deviations from thenatural movements of respiration are useful to the physician inascertaining the seat of disease. 

212. The Nervous Control of Respiration. It is a matter of commonexperience that one's breath may be held for a short time, but the need offresh air speedily gets the mastery, and a long, deep breath is drawn. Hence the efforts of criminals to commit suicide by persistent restraintof their breathing, are always a failure. At the very worst, unconsciousness ensues, and then respiration is automatically resumed. Thus a wise Providence defeats the purpose of crime. The movements ofbreathing go on without our attention. In sleep the regularity ofrespiration is even greater than when awake. There is a particular part ofthe nervous system that presides over the breathing function. It issituated in that part of the brain called the medulla oblongata, and isfancifully called the "vital knot" (sec. 270). It is injury to thisrespiratory center which proves fatal in cases of broken neck. 

From this nerve center there is sent out to the nerves that supply thediaphragm and other muscles of breathing, a force which stimulates them toregular contraction. This breathing center is affected by the condition ofthe blood. It is stimulated by an excess of carbon dioxid in the blood, and is quieted by the presence of oxygen. 

 Experiment 108. _To locate the lungs_. Mark out the boundaries of the lungs by "sounding" them; that is, by _percussion_, as it is called. This means to put the forefinger of the left hand across the chest or back, and to give it a quick, sharp rap with two or three fingers. Note where it sounds hollow, resonant. This experiment can be done by the student with only imperfect success, until practice brings some skill. 

 Experiment 109. Borrow a stethoscope, and listen to the respiration over the chest on the right side. This is known as _auscultation_. Note the difference of the sounds in inspiration and in expiration. Do not confuse the heart sounds with those of respiration. The respiratory murmurs may be heard fairly well by applying the ear flat to the chest, with only one garment interposed. 

 Experiment 110. Get a sheep's lungs, with the windpipe attached. Ask for the heart and lungs all in one mass. Take pains to examine the specimen first, and accept only a good one. Parts are apt to be hastily snipped or mangled. Examine the windpipe. Note the horseshoe-shaped rings of cartilage in front, which serve to keep it open. 

 Experiment 111. Examine one bronchus, carefully dissecting away the lung tissue with curved scissors. Follow along until small branches of the bronchial tubes are reached. Take time for the dissection, and save the specimen in dilute alcohol. Put pieces of the lung tissue in a basin of water, and note that they float. 

The labored breathing of suffocation and of lung diseases is due to theexcessive stimulation of this center, caused by the excess of carbondioxid in the blood. Various mental influences from the brain itself, asthe emotions of alarm or joy or distress, modify the action of therespiratory center. 

Again, nerves of sensation on the surface of the body convey influences tothis nerve center and lead to its stimulation, resulting in a vigorousbreathing movement. Thus a dash of cold water on the face or neck of afainting person instantly produces a deep, long-drawn breath. Certaindrugs, as opium, act to reduce the activity of this nerve center. Hence, in opium poisoning, special attention should be paid to keeping up therespiration. The condition of the lungs themselves is made known to thebreathing center, by messages sent along the branches of the greatpneumogastric nerve (page 276), leading from the lungs to the medullaoblongata. 

213. Effects of Respiration upon the Blood. The blood contains threegases, partly dissolved in it and partly in chemical union with certain ofits constituents. These are oxygen, carbon dioxid, and nitrogen. The latter need not be taken into account. The oxygen is thenourishing material which the tissues require to carry on their work. Thecarbon dioxid is a waste substance which the tissues produce by theiractivity, and which the blood carries away from them. 

As before shown, the blood as it flows through the tissues loses most ofits oxygen, and carbon dioxid takes its place. Now if the blood is tomaintain its efficiency in this respect, it must always be receiving newsupplies of oxygen, and also have some mode of throwing off its excess ofcarbon dioxid. This, then, is the double function of the process ofrespiration. Again, the blood sent out from the left side of the heart isof a bright scarlet color. After its work is done, and the blood returnsto the right side of the heart, it is of a dark purple color. This changein color takes place in the capillaries, and is due to the fact that therethe blood gives up most of its oxygen to the tissues and receives fromthem a great deal of carbon dioxid. 

In brief, while passing through the capillaries of the lungs the blood hasbeen changed from the venous to the arterial blood. That is to say, theblood in its progress through the lungs has rid itself of its excess ofcarbon dioxid and obtained a fresh supply of oxygen. [36]

214. Effects of Respiration upon the Air in the Lungs. It is wellknown that if two different liquids be placed in a vessel in contact witheach other and left undisturbed, they do not remain separate, butgradually mix, and in time will be perfectly combined. This is calleddiffusion of liquids. The same thing occurs with gases, though the processis not visible. This is known as the diffusion of gases. It is also truethat two liquids will mingle when separated from each other by a membrane(sec. 129). In a similar manner two gases, especially if of differentdensities, may mingle even when separated from each other by a membrane. 

In a general way this explains the respiratory changes that occur in theblood in the lungs. Blood containing oxygen and carbon dioxid is flowingin countless tiny streams through the walls of the air cells of the lungs. The air cells themselves contain a mixture of the same two gases. A thin, moist membrane, well adapted to allow gaseous diffusion, separates theblood from the air. This membrane is the delicate wall of the capillariesand the epithelium of the air cells. By experiment it has been found thatthe pressure of oxygen in the blood is less than that in the air cells, and that the pressure of carbon dioxid gas in the blood is greater thanthat in the air cells. As a result, a diffusion of gases ensues. Theblood gains oxygen and loses carbon dioxid, while the air cells loseoxygen and gain the latter gas. 

[Illustration: Fig. 92. --Capillary Network of the Air Cells and Origin ofthe Pulmonary Veins. 

 A, small branch of pulmonary artery; B, twigs of the pulmonary artery anastomosing to form peripheral network of the primitive air cells; C, capillary network around the walls of the air sacs; D, branches of network converging for form the veinlets of the pulmonary veins. ]

The blood thus becomes purified and reinvigorated, and at the same time ischanged in color from purple to scarlet, from venous to arterial. It isnow evident that if this interchange is to continue, the air in the cellsmust be constantly renewed, its oxygen restored, and its excess of carbondioxid removed. Otherwise the process just described would be reversed, making the blood still more unfit to nourish the tissues, and morepoisonous to them than before. 

215. Change in the Air in Breathing. The air which we exhale duringrespiration differs in several important particulars from the air weinhale. Both contain chiefly the three gases, though in differentquantities, as the following table shows. 

 Oxygen. Nitrogen. Carbon Dioxid. Inspired air contains 20. 81 79. 15 . 04 Expired air contains 16. 03 79. 58 4. 38

That is, expired air contains about five per cent less oxygen and five percent more carbon dioxid than inspired air. 

The temperature of expired air is variable, but generally is higher thanthat of inspired air, it having been in contact with the warm airpassages. It is also loaded with aqueous vapor, imparted to it likethe heat, not in the depth of the lungs, but in the upper air passages. 

Expired air contains, besides carbon dioxid, various impurities, many ofan unknown nature, and all in small amounts. When the expired air iscondensed in a cold receiver, the aqueous product is found to containorganic matter, which, from the presence of _micro-organisms_, introduced in the inspired air, is apt to putrefy rapidly. Some of theseorganic substances are probably poisonous, either so in themselves, asproduced in some manner in the breathing apparatus, or poisonous as beingthe products of decomposition. For it is known that various animalsubstances give rise, by decomposition, to distinct poisonous productsknown as _ptomaines_. It is possible that some of the constituents of theexpired air are of an allied nature. See under "Bacteria" (Chapter XIV). 

At all events, these substances have an injurious action, for anatmosphere containing simply one per cent of pure carbon dioxid has verylittle hurtful effect on the animal economy, but an atmosphere in whichthe carbon dioxid has been raised one per cent by breathing is highlyinjurious. 

The quantity of oxygen removed from the air by the breathing of an adultperson at rest amounts daily to about 18 cubic feet. About the same amountof carbon dioxid is expelled, and this could be represented by a piece ofpure charcoal weighing 9 ounces. The quantity of carbon dioxid, however, varies with the age, and is increased also by external cold and byexercise, and is affected by the kind of food. The amount of water, exhaled as vapor, varies from 6 to 20 ounces daily. The average dailyquantity is about one-half a pint. 

216. Modified Respiratory Movements. The respiratory column of air isoften used in a mechanical way to expel bodies from the upper airpassages. There are also, in order to secure special ends, a number ofmodified movements not distinctly respiratory. The following peculiarrespiratory acts call for a few words of explanation. 

A sigh is a rapid and generally audible expiration, due to theelastic recoil of the lungs and chest walls. It is often caused bydepressing emotions. Yawning is a deep inspiration with a stretchingof the muscles of the face and mouth, and is usually excited by fatigue ordrowsiness, but often occurs from a sort of contagion. 

Hiccough is a sudden jerking inspiration due to the spasmodiccontraction of the diaphragm and of the glottis, causing the air to rushsuddenly through the larynx, and produce this peculiar sound. Snoringis caused by vibration of the soft palate during sleep, and is habitualwith some, although it occurs with many when the system is unusuallyexhausted and relaxed. 

Laughing consists of a series of short, rapid, spasmodic expirationswhich cause the peculiar sounds, with characteristic movements of thefacial muscles. Crying, caused by emotional states, consists ofsudden jerky expirations with long inspirations, with facial movementsindicative of distress. In sobbing, which often followslong-continued crying, there is a rapid series of convulsive inspirations, with sudden involuntary contractions of the diaphragm. Laughter, andsometimes sobbing, like yawning, may be the result of involuntaryimitation. 

 Experiment 112. _Simple Apparatus to Illustrate the Movements of the Lungs in the Chest_. --T is a bottle from which the bottom has been removed; D, a flexible and elastic membrane tied on the bottle, and capable of being pulled out by the string S, so as to increase the capacity of the bottle. L is a thin elastic bag representing the lungs. It communicates with the external air by a glass tube fitted air-tight through a cork in the neck of the bottle. When D is drawn down, the pressure of the external air causes L to expand. When the string is let go, L contracts again, by virtue of its elasticity. 

 [Illustration: Fig. 93. ]

Coughing is produced by irritation in the upper part of the windpipeand larynx. A deep breath is drawn, the opening of the windpipe is closed, and immediately is burst open with a violent effort which sends a blast ofair through the upper air passages. The object is to dislodge and expelany mucus or foreign matter that is irritating the air passages. 

Sneezing is like coughing; the tongue is raised against the softpalate, so the air is forced through the nasal passages. It is caused byan irritation of the nostrils or eyes. In the beginning of a cold in thehead, for instance, the cold air irritates the inflamed mucous membrane ofthe nose, and causes repeated attacks of sneezing. 

217. How the Atmosphere is Made Impure. The air around us isconstantly being made impure in a great variety of ways. The combustion offuel, the respiration of men and animals, the exhalations from theirbodies, the noxious gases and effluvia of the various industries, togetherwith the changes of fermentation and decomposition to which all organizedmatter is liable, --all tend to pollute the atmosphere. 

The necessity of external ventilation has been foreseen for us. Theforces of nature, --the winds, sunlight, rain, and growing vegetation, --allof great power and universal distribution and application, restore thebalance, and purify the air. As to the principal gases, the air of thecity does not differ materially from that of rural sections. There is, however, a vastly greater quantity of dust and smoke in the air of towns. The breathing of this dust, to a greater or less extent laden withbacteria, fungi, and the germs of disease, is an ever-present and mostpotent menace to public and personal health. It is one of the main causesof the excess of mortality in towns and cities over that of countrydistricts. 

This is best shown in the overcrowded streets and houses of great cities, which are deprived of the purifying influence of sun and air. The fataleffect of living in vitiated air is especially marked in the mortalityamong infants and children living in the squalid and overcrowded sectionsof our great cities. The salutary effect of sunshine is shown by the factthat mortality is usually greater on the shady side of the street. 

218. How the Air is Made Impure by Breathing. It is not the carbondioxid alone that causes injurious results to health, it is moreespecially the organic matter thrown off in the expired air. Thecarbon dioxid which accompanies the organic matter is only the index. Intesting the purity of air it is not difficult to ascertain the amount ofcarbon dioxid present, but it is no easy problem to measure the amount oforganic matter. Hence it is the former that is looked for in factories, churches, schoolrooms, and when it is found to exceed . 07 per cent it isknown that there is a hurtful amount of organic matter present. 

The air as expelled from the lungs contains, not only a certain amount oforganic matter in the form of vapor, but minute solid particles of_débris_ and bacterial micro-organisms (Chap. XIV). The air thusalready vitiated, after it leaves the mouth, putrefies very rapidly. It isat once absorbed by clothing, curtains, carpets, porous walls, and by manyother objects. It is difficult to dislodge these enemies of health even byfree ventilation. The close and disagreeable odor of a filthy orovercrowded room is due to these organic exhalations from the lungs, theskin, and the unclean clothing of the occupants. 

The necessity of having a proper supply of fresh air in enclosedplaces, and the need of removal of impure air are thus evident. If aman were shut up in a tightly sealed room containing 425 cubic feet ofair, he would be found dead or nearly so at the end of twenty-four hours. Long before this time he would have suffered from nausea, headache, dizziness, and other proofs of blood-poisoning. These symptoms are oftenfelt by those who are confined for an hour or more in a room where theatmosphere has been polluted by a crowd of people. The unpleasant effectsrapidly disappear on breathing fresh air. 

219. The Effect on the Health of Breathing Foul Air. People are oftencompelled to remain indoors for many hours, day after day, in shops, factories, or offices, breathing air perhaps only slightly vitiated, butstill recognized as "stuffy. " Such persons often suffer from ill health. The exact form of the disturbance of health depends much upon thehereditary proclivity and physical make-up of the individual. Loss ofappetite, dull headache, fretfulness, persistent weariness, despondency, followed by a general weakness and an impoverished state of blood, oftenresult. 

Persons in this lowered state of health are much more prone to surfer fromcolds, catarrhs, bronchitis, and pneumonia than if they were living in theopen air, or breathing only pure air. Thus, in the Crimean War, thesoldiers who lived in tents in the coldest weather were far more free fromcolds and lung troubles than those who lived in tight and ill-ventilatedhuts. In the early fall when typhoid fever is prevalent, the grounds oflarge hospitals are dotted with canvas tents, in which patients sufferingfrom this fever do much better than in the wards. 

This tendency to inflammatory diseases of the air passages is aggravatedby the overheated and overdried condition of the air in the room occupied. This may result from burning gas, from overheated furnaces and stoves, hot-water pipes, and other causes. Serious lung diseases, such asconsumption, are more common among those who live in damp, overcrowded, orpoorly ventilated homes. 

220. The Danger from Pulmonary Infection. The germ of pulmonaryconsumption, known as the bacillus tuberculosis, is contained in thebreath and the sputa from the lungs of its victims. It is not difficult tounderstand how these bacilli may be conveyed through the air from thelungs of the sick to those of apparently healthy people. Such persons may, however, be predisposed, either constitutionally or by defective hygienicsurroundings, to fall victims to this dreaded disease. Overcrowding, poorventilation, and dampness all tend to increase the risk of pulmonaryinfection. 

It must not be supposed that the tubercle bacillus is necessarilytransmitted directly through the air from the lungs of the sick to beimplanted in the lungs of the healthy. The germs may remain for a time inthe dust turn and _débris_ of damp, filthy, and overcrowded houses. Inthis congenial soil they retain their vitality for a long time, andpossibly may take on more virulent infective properties than theypossessed when expelled from the diseased lungs. [37]

[Illustration: Fig. 94. Example of a Micro-Organism--Bacillus Tuberculosisin Spotum. (Magnified about 500 diameters. )]

221. Ventilation. The question of a practicable and economical systemof ventilation for our homes, schoolrooms, workshops, and publicplaces presents many difficult and perplexing problems. It is perhaps dueto the complex nature of the subject, that ventilation, as an ordinarycondition of daily health, has been so much neglected. The matter ispractically ignored in building ordinary houses. The continuous renewal ofair receives little if any consideration, compared with the provision madeto furnish our homes with heat, light, and water. When the windows areclosed we usually depend for ventilation upon mere chance, --on thechimney, the fireplace, and the crevices of doors and windows. The properventilation of a house and its surroundings should form as prominent aconsideration in the plans of builders and architects as do the grading ofthe land, the size of the rooms, and the cost of heating. 

The object of ventilation is twofold: First, to provide for the removalof the impure air; second, for a supply of pure air. This mustinclude a plan to provide fresh air in such a manner that there shall beno draughts or exposure of the occupants of the rooms to unduetemperature. Hence, what at first might seem an easy thing to do, is, infact, one of the most difficult of sanitary problems. 

222. Conditions of Efficient Ventilation. To secure properventilation certain conditions must be observed. The pure air introducedshould not be far below the temperature of the room, or if so, theentering current should be introduced towards the ceiling, that it may mixwith the warm air. 

Draughts must be avoided. If the circuit from entrance to exit is short, draughts are likely to be produced, and impure air has less chance ofmixing by diffusion with the pure air. The current of air introducedshould be constant, otherwise the balance may occasionally be in favor ofvitiated air. If a mode of ventilation prove successful, it should not beinterfered with by other means of entrance. Thus, an open door may preventthe incoming air from passing through its proper channels. It is desirablethat the inlet be so arranged that it can be diminished in size or closedaltogether. For instance, when the outer air is very cold, or the windblows directly into the inlet, the amount of cold air entering it maylower the temperature of the room to an undesirable degree. 

In brief, it is necessary to have a thorough mixing of pure and impureair, so that the combination at different parts of the room may be fairlyuniform. To secure these results, the inlets and outlets should bearranged upon principles of ventilation generally accepted by authoritieson public health. It seems hardly necessary to say that due attention mustbe paid to the source from which the introduced air is drawn. If it betaken from foul cellars, or from dirty streets, it may be as impure asthat which it is designed to replace. 

Animal Heat. 

223. Animal or Vital Heat. If a thermometer, made for the purpose, beplaced for five minutes in the armpit, or under the tongue, it willindicate a temperature of about 98-1/2 degrees F. , whether the surroundingatmosphere be warm or cold. This is the natural heat of a healthy person, and in health it rarely varies more than a degree or two. But as the bodyis constantly losing heat by radiation and conduction, it is evident thatif the standard temperature be maintained, a certain amount of heat mustbe generated within the body to make up for the loss externally. The heatthus produced is known as animal or vital heat. 

This generation of heat is common to all living organisms. When the massof the body is large, its heat is readily perceptible to the touch and byits effect upon the thermometer. In mammals and birds the heat-productionis more active than in fishes and reptiles, and their temperatures differin degree even in different species of the same class, according to thespecial organization of the animal and the general activity of itsfunctions. The temperature of the frog may be 85 degrees F. In June and 41degrees F. In January. The structure of its tissues is unaltered and theirvitality unimpaired by such violent fluctuations. But in man it isnecessary not only for health, but even for life, that the temperatureshould vary only within narrow limits around the mean of 98-1/2 degrees F. 

We are ignorant of the precise significance of this constancy oftemperature in warm-blooded animals, which is as important and peculiar astheir average height, Man, undoubtedly, must possess a superior delicacyof organization, hardly revealed by structure, which makes it necessarythat he should be shielded from the shocks and jars of varyingtemperature, that less highly endowed organisms endure with impunity. 

224. Sources of Bodily Heat. The heat of the body is generated by thechemical changes, generally spoken of as those of oxidation, which areconstantly going on in the tissues. Indeed, whenever protoplasmicmaterials are being oxidized (the process referred to in sec. 15 askatabolism) heat is being set free. These chemical changes are ofvarious kinds, but the great source of heat is the katabolic process, known as oxidation. 

The vital part of the tissues, built up from the complex classes of food, is oxidized by means of the oxygen carried by the arterial blood, andbroken down into simpler bodies which at last result in urea, carbondioxid, and water. Wherever there is life, this process of oxidation isgoing on, but more energetically in some tissues and organs than inothers. In other words, the minutest tissue in the body is a source ofheat in proportion to the activity of its chemical changes. The moreactive the changes, the greater is the heat produced, and the greater theamount of urea, carbon dioxid, and water eliminated. The waste caused bythis oxidation must be made good by a due supply of food to be built upinto protoplasmic material. For the production of heat, therefore, food isnecessary. But the oxidation process is not as simple and direct as thestatement of it might seem to indicate. Though complicated in its variousstages, the ultimate result is as simple as in ordinary combustion outsideof the body, and the products are the same. 

The continual chemical changes, then, chiefly by oxidation of combustiblematerials in the tissues, produce an amount of heat which is efficient tomaintain the temperature of the living body at about 98-1/2 degrees F. Thisprocess of oxidation provides not only for the heat of the body, but alsofor the energy required to carry on the muscular work of the animalorganism. 

225. Regulation of the Bodily Temperature. While bodily heat is beingcontinually produced, it is also as continually being lost by the lungs, by the skin, and to some extent, by certain excretions. The blood, in itsswiftly flowing current, carries warmth from the tissues where heat isbeing rapidly generated, to the tissues or organs in which it is beinglost by radiation, conduction, or evaporation. Were there no arrangementby which heat could be distributed and regulated, the temperature of thebody would be very unequal in different parts, and would vary at differenttimes. 

The normal temperature is maintained with slight variations throughoutlife. Indeed a change of more than a degree above or below the average, indicates some failure in the organism, or some unusual influence. It isevident, then, that the mechanisms which regulate the temperature of thebody must be exceedingly sensitive. 

The two chief means of regulating the temperature of the body are thelungs and the skin. As a means of lowering the temperature, thelungs and air passages are very inferior to the skin; although, by givingheat to the air we breathe, they stand next to the skin in importance. Asa regulating power they are altogether subordinate to the skin. 

 Experiment 113. _To show the natural temperature of the body_. Borrow a physician's clinical thermometer, and take your own temperature, and that of several friends, by placing the instrument under the tongue, closing the mouth, and holding it there for five minutes. It should be thoroughly cleansed after each use. 

226. The Skin as a Heat-regulator. The great regulator of the bodilytemperature is, undoubtedly, the skin, which performs this functionby means of a self-regulating apparatus with a more or less double action. First, the skin regulates the loss of heat by means of the vaso-motormechanism. The more blood passes through the skin, the greater will bethe loss of heat by conduction, radiation, and evaporation. Hence, anyaction of the vaso-motor mechanism which causes dilatation of thecutaneous capillaries, leads to a larger flow of blood through the skin, and will tend to cool the body. On the other hand, when by the samemechanism the cutaneous vessels are constricted, there will be a smallerflow of blood through the skin, which will serve to check the loss of heatfrom the body (secs. 195 and 270). 

Again, the special nerves of perspiration act directly as regulatorsof temperature. They increase the loss of heat when they promote thesecretion of the skin, and diminish the loss when they cease to promoteit. 

The practical working of this heat-regulating mechanism is well shown byexercise. The bodily temperature rarely rises so much as a degree duringvigorous exercise. The respiration is increased, the cutaneous capillariesbecome dilated from the quickened circulation, and a larger amount ofblood is circulating through the skin. Besides this, the skin perspiresfreely. A large amount of heat is thus lost to the body, sufficient tooffset the addition caused by the muscular contractions. 

It is owing to the wonderful elasticity of the sweat-secreting mechanism, and to the increase in respiratory activity, and the consequent increasein the amount of watery vapor given off by the lungs, that men are able toendure for days an atmosphere warmer than the blood, and even for a shorttime at a temperature above that of boiling water. The temperature of aTurkish bath may be as high as 150 degrees to 175 degrees F. But anatmospheric temperature may be considerably below this, and yet if longcontinued becomes dangerous to life. In August, 1896, for instance, hundreds of persons died in this country, within a few days, from theeffects of the excessive heat. 

A much higher temperature may be borne in dry air than in humid air, orthat which is saturated with watery vapor. Thus, a shade temperature of100 degrees F. In the dry air of a high plain may be quite tolerable, while a temperature of 80 degrees F. In the moisture-laden atmosphere ofless elevated regions, is oppressive. The reason is that in dry air thesweat evaporates freely, and cools the skin. In saturated air at thebodily temperature there is little loss of heat by perspiration, or byevaporation from the bodily surface. 

This topic is again discussed in the description of the skin as aregulator of the bodily temperature (sec. 241). 

227. Voluntary Means of Regulating the Temperature. The voluntaryfactor, as a means of regulating the heat loss in man, is one of greatimportance. Clothing retards the loss of heat by keeping in contact withit a layer of still air, which is an exceedingly bad conductor. When a manfeels too warm and throws off his coat, he removes one of the badlyconducting layers of air, and increases the heat loss by radiation andconduction. The vapor next the skin is thus allowed a freer access to thesurface, and the loss of heat by evaporation of the sweat becomes greater. This voluntary factor by which the equilibrium is maintained must beregarded as of great importance. This power also exists in the loweranimals, but to a much smaller extent. Thus a dog, on a hot day, runs outhis tongue and stretches his limbs so as to increase the surface fromwhich heat is radiated and conducted. 

The production, like the loss, of heat is to a certain extent under thecontrol of the will. Work increases the production of heat, and rest, especially sleep, lessens it. Thus the inhabitants of very hot countriesseek relief during the hottest part of the day by a siesta. The quantityand quality of food also influence the production of heat. A largerquantity of food is taken in winter than in summer. Among the inhabitantsof the northern and Arctic regions, the daily consumption of food is fargreater than in temperate and tropical climates. 

228. Effect of Alcohol upon the Lungs. It is a well recognized factthat alcohol when taken into the stomach is carried from that organ to theliver, where, by the baneful directness of its presence, it produces aspeedy and often disastrous effect. But the trail of its malign power doesnot disappear there. From the liver it passes to the right side of theheart, and thence to the lungs, where its influence is still for harm. 

In the lungs, alcohol tends to check and diminish the breathing capacityof these organs. This effect follows from the partial paralyzing influenceof the stupefying agent upon the sympathetic nervous system, diminishingits sensibility to the impulse of healthful respiration. This diminishedcapacity for respiration is clearly shown by the use of the _spirometer_, a simple instrument which accurately records the cubic measure of thelungs, and proves beyond denial the decrease of the lung space. 

 "Most familiar and most dangerous is the drinking man's inability to resist lung diseases. "--Dr. Adoph Frick, the eminent German physiologist of Zurich. 

 "Alcohol, instead of preventing consumption, as was once believed, reduces the vitality so much as to render the system unusually susceptible to that fatal disease. "--R. S. Tracy, M. D. , Sanitary Inspector of the N. Y. City Health Dept. 

 "In thirty cases in which alcoholic phthisis was present a dense, fibroid, pigmented change was almost invariably present in some portion of the lung far more frequently than in other cases of phthisis. "--_Annual of Medical Sciences_. 

 "There is no form of consumption so fatal as that from alcohol. Medicines affect the disease but little, the most judicious diet fails, and change of air accomplishes but slight real good. .. . In plain terms, there is no remedy whatever for alcoholic phthisis. It may be delayed in its course, but it is never stopped; and not infrequently, instead of being delayed, it runs on to a fatal termination more rapidly than is common in any other type of the disorder. "--Dr. B. W. Richardson in _Diseases of Modern Life_. 

229. Other Results of Intoxicants upon the Lungs. But a more potentinjury to the lungs comes from another cause. The lungs are the arenawhere is carried on the ceaseless interchange of elements that isnecessary to the processes of life. Here the dark venous blood, loadedwith effete material, lays down its carbon burden and, with thebrightening company of oxygen, begins again its circuit. But the enemyintrudes, and the use of alcohol tends to prevent this benign interchange. 

The continued congestion of the lung tissue results in its becomingthickened and hardened, thus obstructing the absorption of oxygen, and theescape of carbon dioxid. Besides this, alcohol destroys the integrity ofthe red globules, causing them to shrink and harden, and impairing theirpower to receive oxygen. Thus the blood that leaves the lungs conveys anexcess of the poisonous carbon dioxid, and a deficiency of the needfuloxygen. This is plainly shown in the purplish countenance of theinebriate, crowded with enlarged veins. This discoloration of the face isin a measure reproduced upon the congested mucous membrane of the lungs. It is also proved beyond question by the decreased amount of carbon dioxidthrown off in the expired breath of any person who has used alcoholics. 

The enfeebled respiration explains (though it is only one of the reasons)why inebriates cannot endure vigorous and prolonged exertion as can ahealthy person. The hurried circulation produced by intoxicants involvesin turn quickened respiration, which means more rapid exhaustion of thelife forces. The use of intoxicants involves a repeated dilatation of thecapillaries, which steadily diminishes their defensive power, renderingthe person more liable to yield to the invasion of pulmonary diseases. [38]

230. Effect of Alcoholics upon Disease. A theory has prevailed, to alimited extent, that the use of intoxicants may act as a preventive ofconsumption. The records of medical science fail to show any proofwhatever to support this impression. No error could be more serious ormore misleading, for the truth is in precisely the opposite direction. Instead of preventing, alcohol tends to develop consumption. Manyphysicians of large experience record the existence of a distinctlyrecognized alcoholic consumption, attacking those constitutions brokendown by dissipation. This form of consumption is steadily progressive, andalways fatal. 

The constitutional debility produced by the habit of using alcoholicbeverages tends to render one a prompt victim to the more severe diseases, as pneumonia, and especially epidemical diseases, which sweep away vastnumbers of victims every year. 

231. Effect of Tobacco upon the Respiratory Passages. The effects oftobacco upon the throat and lungs are frequently very marked andpersistent. The hot smoke must very naturally be an irritant, as the mouthand nostrils were not made as a chimney for heated and narcotic vapors. The smoke is an irritant, both by its temperature and from its destructiveingredients, the carbon soot and the ammonia which it conveys. Itirritates and dries the mucous membrane of the mouth and throat, producingan unnatural thirst which becomes an enticement to the use of intoxicatingliquors. The inflammation of the mouth and throat is apt to extend up theEustachian tube, thus impairing the sense of hearing. 

But even these are not all the bad effects of tobacco. The inhalation ofthe poisonous smoke produces unhealthful effects upon the delicate mucousmembrane of the bronchial tubes and of the lungs. Upon the former theeffect is to produce an irritating cough, with short breath and chronicbronchial catarrh. The pulmonary membrane is congested, taking coldbecomes easy, and recovery from it tedious. Frequently the respiration isseriously disturbed, thus the blood is imperfectly aërated, and so in turnthe nutrition of the entire system is impaired. The cigarette is thedefiling medium through which these direful results frequently invade thesystem, and the easily moulded condition of youth yields readily to thedestructive snare. 

"The first effect of a cigar upon any one demonstrates that tobacco canpoison by its smoke and through the lungs. "--London _Lancet_. 

"The action of the heart and lungs is impaired by the influence of thenarcotic on the nervous system, but a morbid state of the larynx, trachea, and lungs results from the direct action of the smoke. "--Dr. Laycock, Professor of Medicine in the University of Edinburgh. 

Additional Experiments. 

 Experiment 114. _To illustrate the arrangement of the lungs and the two pleuræ. _ Place a large sponge which will represent the lungs in a thin paper bag which just fits it; this will represent the pulmonary layer of the pleura. Place the sponge and paper bag inside a second paper bag, which will represent the parietal layer of the pleura. Join the mouths of the two bags. The two surfaces of the bags which are now in contact will represent the two moistened surfaces of the pleuræ, which rub together in breathing. 

 Experiment 115. _To show how the lungs may be filled with air. _ Take one of the lungs saved from Experiment 110. Tie a glass tube six inches long into the larynx. Attach a piece of rubber to one end of the glass tube. Now inflate the lung several times, and let it collapse. When distended, examine every part of it. 

 Experiment 116. _To take your own bodily temperature or that of a friend. _ If you cannot obtain the use of a physician's clinical thermometer, unfasten one of the little thermometers found on so many calendars and advertising sheets. Hold it for five minutes under the tongue with the lips closed. Read it while in position or the instant it is removed. The natural temperature of the mouth is about 98-1/2 degrees F. 

 Experiment 117. _To show the vocal cords. _ Get a pig's windpipe in perfect order, from the butcher, to show the vocal cords. Once secured, it can be kept for an indefinite time in glycerine and water or dilute alcohol. 

 Experiment 118. _To show that the air we expire is warm. _ Breathe on a thermometer for a few minutes. The mercury will rise rapidly. 

 Experiment 119. _To show that expired air is moist_. Breathe on a mirror, or a knife blade, or any polished metallic surface, and note the deposit of moisture. 

 Experiment 120. _To show that the expired air contains carbon dioxid_. Put a glass tube into a bottle of lime water and breathe through the tube. The A liquid will soon become cloudy, because the carbon dioxid of the expired air throws down the lime held in solution. 

 Experiment 121. "A substitute for a clinical thermometer may be readily contrived by taking an ordinary house thermometer from its tin case, and cutting off the lower part of the scale so that the bulb may project freely. With this instrument the pupils may take their own and each other's temperatures, and it will be found that whatever the season of the year or the temperature of the room, the thermometer in the mouth will record about 99 degrees F. Care must, of course, be taken to keep the thermometer in the mouth till it ceases to rise, and to read while it is still in position. "--Professor H. P. Bowditch. 

 Experiment 122. _To illustrate the manner in which the movements of inspiration cause the air to enter the lungs. _ Fit up an apparatus, as represented in Fig. 95, in which a stout glass tube is provided with a sound cork, B, and also an air-tight piston, D, resembling that of an ordinary syringe. A short tube, A, passing through the cork, has a small India-rubber bag, C, tied to it. Fit the cork in the tube while the piston is near the top. Now, by lowering the piston we increase the capacity of the cavity containing the bag. The pressure outside the bag is thus lowered, and air rushes into it through the tube, A, till a balance is restored. The bag is thus stretched. As soon as we let go the piston, the elasticity of the bag, being free to act, Movements of drives out the air just taken in, and the piston returns to its former place. 

 [Illustration: Fig. 95. Apparatus for Illustrating the Movements of Respiration. ]

 It will be noticed that in this experiment the elastic bag and its tube represent the lungs and trachea; and the glass vessel enclosing it, the thorax. 

For additional experiments on the mechanics of respiration, see ChapterXV. 

Chapter IX. 

The Skin and the Kidneys. 

232. The Elimination of Waste Products. We have traced the food fromthe alimentary canal into the blood. We have learned that various foodmaterials, prepared by the digestive processes, are taken up by thebranches of the portal vein, or by the lymphatics, and carried into theblood current. The nutritive material thus absorbed is conveyed by theblood plasma and the lymph to the various tissues to provide them withnourishment. 

We have learned also that oxygen, taken up in the air cells of the lungs, is being continually carried to the tissues, and that the blood ispurified by being deprived in the lungs of its excess of carbon dioxid. From this tissue activity, which is mainly oxidation, are formed certainwaste products which, as we have seen, are absorbed by the capillaries andlymphatics and carried into the venous circulation. 

In their passage through the blood and tissues, the albumens, sugars, starches, and fats are converted into carbon dioxid, water, and urea, orsome closely allied body. Certain articles of food also contain smallamounts of sulphur and phosphorus, which undergo oxidation into sulphatesand phosphates. We speak, then, of carbon dioxid, salts, and water aswaste products of the animal economy. These leave the body by one ofthe three main channels, --the lungs, the skin, or the kidneys. 

The elimination of these products is brought about by a special apparatuscalled organs of excretion. The worn-out substances themselvesare called excretions, as opposed to secretions, which areelaborated for use in the body. (See note, p. 121. ) As already shown, thelungs are the main channels for the elimination of carbon dioxid, andof a portion of water as vapor. By the skin the body gets rid of asmall portion of salts, a little carbon dioxid, and a largeamount of water in the form of perspiration. From the kidneysare eliminated nearly all the urea and allied bodies, the mainportion of the salts, and a large amount of water. In fact, practically all the nitrogenous waste leaves the body by the kidneys. 

[Illustration: Fig. 96. --Diagrammatic Scheme to illustrate in a veryGeneral Way Absorption and Excretion. 

 A, represents the alimentary canal; L, the pulmonary surface; K, the surface of the renal epithelium; S, the skin; o, oxygen; h, hydrogen, ; n, nitrogen. ]

233. The Skin. The skin is an important and unique organ of thebody. It is a blood-purifying organ as truly as are the lungs and thekidneys, while it also performs other and complex duties. It is not merelya protective covering for the surface of the body. This is indeed the mostapparent, but in some respectes, the lest important, of its functions. This protective duty is necessary and efficient, as is proved by thefamiliar experience of the pain when a portion of the outer skin has beenremoved. 

The skin, being richly supplied with nerves, is an important organ ofsensibility and touch. In some parts it is closely attached tothe structures beneath, while in others it is less firmly adherent andrests upon a variable amount of fatty tissue. It thus assists in relievingthe abrupt projections and depressions of the general surface, and ingiving roundness and symmetry to the entire body. The thickness of theskin varies in different parts of the body. Where exposed to pressure andfriction, as on the soles of the feet and in the palms of the hands, it ismuch thickened. 

The true skin is 1/12 to 1/8 of an inch in thickness, but in certainparts, as in the lips and ear passages, it is often not more than 1/100 ofan inch thick. At the orifices of the body, as at the mouth, ears, andnose, the skin gradually passes into mucous membrane, the structure of thetwo being practically identical. As the skin is an outside covering, so isthe mucous membrane a more delicate inside lining for all cavities intowhich the apertures open, as the alimentary canal and the lungs. 

[Illustration: Fig. 97. --A Layer of the Cuticle from the Palm of the Hand. (Detached by maceration. )]

The skin ranks as an important organ of excretion, its product beingsweat, excreted by the sweat glands. The amount of this excretionevaporated from the general surface is very considerable, and is modifiedas becomes necessary from the varied conditions of the temperature. Theskin also plays an important part in regulating the bodilytemperature(sec. 241). 

234. The Cutis Vera, or True Skin. The skin is remarkably complex inits structure, and is divided into two distinct layers, which may bereadily separated: the deeper layer, --the true skin, dermis, orcorium; and the superficial layer, or outer skin, --the epidermis, cuticle, or scarf skin. 

The true skin consists of elastic and white fibrous tissue, thebundles of which interlace in every direction. Throughout this feltworkstructure which gradually passes into areolar tissue are numerous muscularfibers, as about the hair-follicles and the oil glands. When these tinymuscles contract from cold or by mental emotion, the follicles projectupon the surface, producing what is called "goose flesh. "

The true skin is richly supplied with blood-vessels and nerves, as whencut it bleeds freely, and is very sensitive. The surface of the true skinis thrown into a series of minute elevations called the papillæ, uponwhich the outer skin is moulded. These abound in blood-vessels, lymphatics, and peculiar nerve-endings, which will be described inconnection with the organ of touch (sec. 314). The papillæ are largeand numerous in sensitive places, as the palms of the hands, the soles ofthe feet, and the fingers. They are arranged in parallel curved lines, andform the elevated ridges seen on the surface of the outer skin (Fig. 103). 

235. The Epidermis, or Cuticle. Above the true skin is the epidermis. It is semi-transparent, and under the microscope resembles the scales of afish. It is this layer that is raised by a blister. 

As the epidermis has neither blood-vessels, nerves, nor lymphatics, it may be cut without bleeding or pain. Its outer surface is marked withshallow grooves which correspond to the deep furrows between the papillæof the true skin. The inner surface is applied directly to the papillarylayer of the true skin, and follows closely its inequalities. The outerskin is made up of several layers of cells, which next to the true skinare soft and active, but gradually become harder towards the surface, where they are flattened and scale-like. The upper scales are continuallybeing rubbed off, and are replaced by deeper cells from beneath. There arenew cells continually being produced in the deeper layer, which pushupward the cells already existing, then gradually become dry, and are castoff as fine, white dust. Rubbing with a coarse towel after a hot bathremoves countless numbers of these dead cells of the outer skin. Duringand after an attack of scarlet fever the patient "peels, " that is, shedsan unusual amount of the seal; cells of the cuticle. 

The deeper and more active layer of the epidermis, the _mucosum_, is madeup of cells some of which contain minute granules of pigment, or coloringmatter, that give color to the skin. The differences in the tint, asbrunette, fair, and blond, are due mainly to the amount of coloring matterin these pigment cells. In the European this amount is generally small, while in other peoples the color cells may be brown, yellow, or evenblack. The pinkish tint of healthy skin, and the rosy-red after a bath aredue, not to the pigment cells, but to the pressure of capillaries in thetrue skin, the color of the blood being seen through the semi-transparentouter skin. 

[Illustration: Fig. 98. --Surface of the Palm of the Hand, showing theOpenings of the Sweat Glands and the Grooves between the Papillæ of theSkin. (Magnified 4 diameters. ) [In the smaller figure the same epidermalsurface is shown, as seen with the naked eye. ]]

 Experiment 123. Of course the living skin can be examined only in a general way. Stretch and pull it, and notice that it is elastic. Note any liver spots, white scars, moles, warts, etc. Examine the outer skin carefully with a strong magnifying glass. Study the papillæ on the palms. Scrape off with a sharp knife a few bits of the scarf skin, and examine them with the microscope. 

236. The Hair. Hairs varying in size cover nearly the entire body, except a few portions, as the upper eyelids, the palms of the hands, andthe soles of the feet. 

The length and diameter of the hairs vary in different persons, especiallyin the long, soft hairs of the head and beard. The average number of hairsupon a square inch of the scalp is about 1000, and the number upon theentire head is estimated as about 120, 000. 

Healthy hair is quite elastic, and may be stretched from one-fifth toone-third more than its original length. An ordinary hair from the headwill support a weight of six to seven ounces. The hair may become stronglyelectrified by friction, especially when brushed vigorously in cold, dryweather. Another peculiarity of the hair is that it readily absorbsmoisture. 

237. Structure of the Hair. The hair and the nails are structuresconnected with the skin, being modified forms of the epidermis. A hair isformed by a depression, or furrow, the inner walls of which consist of theinfolded outer skin. This depression takes the form of a sac and is calledthe hair-follicle, in which the roots of the hair are embedded. Atthe bottom of the follicle there is an upward projection of the true skin, a papilla, which contains blood-vessels and nerves. It is coveredwith epidermic cells which multiply rapidly, thus accounting for the rapidgrowth of the hair. Around each papilla is a bulbous expansion, the hairbulb, from which the hair begins to grow. 

[Illustration: Fig. 99. --Epidermis of the Foot. 

It will be noticed that there are only a few orifices of the sweat glandsin this region. (Magnified 8 diameters. )]

The cells on the papillæ are the means by which the hairs grow. As theseare pushed upwards by new ones formed beneath, they are compressed, andthe shape of the follicle determines their cylindrical growth, the shaftof the hair. So closely are these cells welded to form the cylinder, thateven under a microscope the hair presents only a fibrous appearance, except in the center, where the cells are larger, forming themedulla, or pith (Fig. 106). 

The medulla of the hair contains the pigment granules or coloring matter, which may be of any shade between a light yellow and an intense black. Itis this that gives the great variety in color. Generally with old peoplethe pigment is absent, the cells being occupied by air; hence the hairbecomes gray or white. The thin, flat scales on the surface of the hairoverlap like shingles. Connected with the hair-follicles are small bundlesof muscular fibers, which run obliquely in the skin and which, onshortening, may cause the hairs to become more upright, and thus are madeto "stand on end. " The bristling back of an angry cat furnishes a familiarillustration of this muscular action. 

[Illustration: Fig. 100. --Hair and Hair-Follicle. 

 A, root of hair; B, bulb of the hair; C, internal root sheath; D, external root sheath; E, external membrane of follicle; F, muscular fibers attached to the follicle; H, compound sebaceous gland with its duct; K, L, simple sebaceous gland; M, opening of the hair-follicle. ]

Opening into each hair-follicle are usually one or more sebaceous, oroil, glands. These consist of groups of minute pouches lined withcells producing an oily material which serves to oil the hair and keep theskin moist and pliant. 

238. The Nails. The nails are also formed of epidermis cellswhich have undergone compression, much like those forming the shaft of ahair. In other words, a nail is simply a thick layer of horny scales builtfrom the outer part of the scarf skin. The nail lies upon very fine andclosely set papillæ, forming its matrix, or bed. It is covered at itsbase with a fold of the true skin, called its root, from beneathwhich it seems to grow. 

The growth of the nail, like that of the hair and the outer skin, iseffected by the production of new cells at the root and under surface. Thegrowth of each hair is limited; in time it falls out and is replaced by anew one. But the nail is kept of proper size simply by the removal of itsfree edge. 

239. The Sweat Glands. Deep in the substance of the true skin, or inthe fatty tissue beneath it, are the sweat glands. Each glandconsists of a single tube with a blind end, coiled in a sort of ball about1/60 of an inch in diameter. From this coil the tube passes upwardsthrough the dermis in a wavy course until it reaches the cuticle, which itpenetrates with a number of spiral turns, at last opening on the surface. The tubes consist of delicate walls of membrane lined with cells. The coilof the gland is enveloped by minute blood-vessels. The cells of the glandsare separated from the blood only by a fine partition, and draw from itwhatever supplies they need for their special work. 

[Illustration: Fig. 101. --Concave or Adherent Surface of the Nail. 

 A, border of the root; B, whitish portion of semilunar shape (the lunula); C, body of nail. The continuous line around border represents the free edge. ]

[Illustration: Fig. 102. --Nail in Position. 

 A, section of cutaneous fold (B) turned back to show the root of the nail; B, cutaneous fold covering the root of the nail; C, semi lunar whitish portion (lunula); D, free border. ]

With few exceptions every portion of the skin is provided with sweatglands, but they are not equally distributed over the body. They arefewest in the back and neck, where it is estimated they average 400 to thesquare inch. They are thickest in the palms of the hands, where theyamount to nearly 3000 to each square inch. These minute openings occur inthe ridges of the skin, and may be easily seen with a hand lens. Thelength of a tube when straightened is about 1/4 of an inch. The totalnumber in the body is estimated at about 2, 500, 000, thus making the entirelength of the tubes devoted to the secretion of sweat about 10 miles. 

240. Nature and Properties of Sweat. The sweat is a turbid, saltishfluid with a feeble but characteristic odor due to certain volatile fattyacids. Urea is always present in small quantities, and its proportion maybe largely increased when there is deficiency of elimination by thekidneys. Thus it is often observed that the sweat is more abundant whenthe kidneys are inactive, and the reverse is true. This explains theincreased excretion of the kidneys in cold weather. Of the inorganicconstituents of sweat, common salt is the largest and most important. Somecarbon dioxid passes out through the skin, but not more than 1/50 as muchas escapes by the lungs. 

The sweat ordinarily passes off as vapor. If there is no obviousperspiration we must not infer that the skin is inactive, since sweat iscontinually passing from the surface, though often it may not be apparent. On an average from 1-1/2 to 4 pounds of sweat are eliminated daily fromthe skin in the form of vapor. This is double the amount excreted by thelungs, and averages about 1/67 of the weight of the body. 

The visible sweat, or sensible perspiration, becomes abundant duringactive exercise, after copious drinking of cold water, on taking certaindrugs, and when the body is exposed to excessive warmth. Forming morerapidly than it evaporates it collects in drops on the surface. Thedisagreeable sensations produced by humid weather result from the factthat the atmosphere is so loaded with vapor that the moisture of the skinis slowly removed by evaporation. 

 Experiment 124. Study the openings of the sweat glands with the aid of a strong magnifying glass. They are conveniently examined on the palms. 

A man's weight may be considerably reduced within a short time by lossthrough the perspiration alone. This may explain to some extent theweakening effect of profuse perspiration, as from night sweats ofconsumption, convalescence from typhoid fever, or the artificial sweatingfrom taking certain drugs. 

241. The Skin as a Regulator of the Temperature of the Body. We thuslearn that the skin covers and protects the more delicate structuresbeneath it; and that it also serves as an important organ of excretion. Bymeans of the sweat the skin performs a third and a most importantfunction, _viz_. , that of regulating the temperature of the body. 

The blood-vessels of the skin, like those of other parts of the body, areunder the control of the nervous system, which regulates their diameter. If the nervous control be relaxed, the blood-vessels dilate, more bloodflows through them, and more material is brought to the glands of the skinto be acted upon. External warmth relaxes the skin and its blood-vessels. There results an increased flow of blood to the skin, with increasedperspiration. External cold, on the other hand, contracts the skin and itsblood-vessels, producing a diminished supply of blood and a diminishedamount of sweat. 

Now, it is a law of physics that the change from liquid to vapor involvesa loss of heat. A few drops of ether or of any volatile liquid placed onthe skin, produce a marked sense of coldness, because the heat necessaryto change the liquid into vapor has been drawn rapidly from the skin. Thisprinciple holds good for every particle of sweat that reaches the mouth ofa sweat gland. As the sweat evaporates, it absorbs a certain amount ofheat, and cools the body to that extent. 

242. How the Action of the Skin may be Modified. After profusesweating we feel chilly from the evaporation of a large amount ofmoisture, which rapidly cools the surface. When the weather is very warmthe evaporation tends to prevent the bodily temperature from rising. Onthe other hand, if the weather be cold, much less sweat is produced, theloss of heat from the body is greatly lessened, and its temperatureprevented from falling. Thus it is plain why medicine is given and otherefforts are made to sweat the fever patient. The increased activity of theskin helps to reduce the bodily heat. 

The sweat glands are under the control of certain nerve fibers originatingin the spinal cord, and are not necessarily excited to action by anincreased flow of blood through the skin. In other words, the sweat glandsmay be stimulated to increased action both by an increased flow of blood, and also by reflex action upon the vaso-dilator nerves of the parts. Thesetwo agencies, while working in harmony through the vaso-dilators, producephenomena which are essentially independent of each other. Thus a strongemotion, like fear, may cause a profuse sweat to break out, with cold, pallid skin. During a fever the skin may be hot, and its vessels full ofblood, and yet there may be no perspiration. 

[Illustration: Fig. 103. --Papillæ of the Skin of the Palm of the Hand. 

In each papilla are seen vascular loops (dark lines) running up from thevascular network below, the tactile corpuscles with their nerve branches(white lines) which supply the papillæ. ]

The skin may have important uses with which we are not yet acquainted. Death ensues when the heat of the body has been reduced to about 70degrees F. , and suppression of the action of the skin always produces alowering of the temperature. Warm-blooded animals usually die when morethan half of the general surface has been varnished. Superficial burnswhich involve a large part of the surface of the body, generally have afatal result due to shock. 

If the skin be covered with some air-tight substance like a coating ofvarnish, its functions are completely arrested. The bodily heat falls veryrapidly. Symptoms of blood-poisoning arise, and death soon ensues. Thereason is not clearly known, unless it be from the sudden retention ofpoisonous exhalations. 

243. The Skin and the Kidneys. There is a close relationship betweenthe skin and the kidneys, as both excrete organic and saline matter. Inhot weather, or in conditions producing great activity of the skin, theamount of water excreted by the kidneys is diminished. This is shown inthe case of firemen, stokers, bakers, and others who are exposed to greatheat, and drink heavily and sweat profusely, but do not have a relativeincrease in the functions of the kidneys. In cool weather, when the skinis less active, a large amount of water is excreted by the kidneys, as isshown by the experience of those who drive a long distance in severeweather, or who have caught a sudden cold. 

[Illustration: Fig. 104. --Magnified View of a Sweat Gland with its Duct. 

The convoluted gland is seen surrounded with big fat-cells, and may betraced through the dermis to its outlet in the horny layers of theepidermis. ]

244. Absorbent Powers of the Skin. The skin serves to some extent asan organ for absorption. It is capable of absorbing certainsubstances to which it is freely exposed. Ointments rubbed in, areabsorbed by the lymphatics in those parts where the skin is thin, as inthe bend of the elbow or knee, and in the armpits. Physicians usemedicated ointments in this way, when they wish to secure prompt andefficient results. Feeble infants often grow more vigorous by having theirskin rubbed vigorously daily with olive oil. 

A slight amount of water is absorbed in bathing. Sailors deprived offresh water have been able to allay partially their intense thirst bysoaking their clothing in salt water. The extent to which absorptionoccurs through the healthy skin is, however, quite limited. If the outerskin be removed from parts of the body, the exposed surface absorbsrapidly. Various substances may thus be absorbed, and rapidly passed intothe blood. When the physician wishes remedies to act through the skin, hesometimes raises a small blister, and dusts over the surface some drug, afine powder, like morphine. 

The part played by the skin as an organ of touch will be consideredin sections 314 and 315. 

 Experiment 125. _To illustrate the sense of temperature_. Ask the person to close his eyes. Use two test tubes, one filled with cold and the other with hot water, or two spoons, one hot and one cold. Apply each to different parts of the surface, and ask the person whether the touching body is hot or cold. Test roughly the sensibility of different parts of the body with cold and warm metallic-pointed rods. 

 Experiment 126. Touch fur, wood, and metal. The metal feels coldest, although all the objects are at the same temperature. Why?

 Experiment 127. Plunge the hand into water at about 97 degrees F. One experiences a feeling of heat. Then plunge it into water at about 86 degrees F. ; at first it feels cold, because heat is abstracted from the hand. Plunge the other hand direct into water at 86 degrees F. Without previously placing it in water at 97 degrees F. , --it will feel pleasantly warm. 

 Experiment 128. _To illustrate warm and cold spots_. With a blunt metallic point, touch different parts of the skin. Certain points excite the sensation of warmth, others of cold, although the temperatures of the skin and of the instrument remain constant. 

245. Necessity for Personal Cleanliness. It is evident that the skin, with its myriads of blood-vessels, nerves, and sweat and oil glands, is anexceedingly complicated and important structure. The surface iscontinually casting off perspiration, oily material, and dead scales. Byfriction and regular bathing we get rid of these waste materials. If thisbe not thoroughly done, the oily secretion holds the particles of wastesubstances to the surface of the body, while dust and dirt collect, andform a layer upon the skin. When we remember that this dirt consists of agreat variety of dust particles, poisonous matters, and sometimes germs ofdisease, we may well be impressed with the necessity of personalcleanliness. 

This layer of foreign matter on the skin is in several ways injurious tohealth. It clogs the pores and retards perspiration, thus checking theproper action of the skin as one of the chief means of getting rid of thewaste matters of the body. Hence additional work is thrown upon otherorgans, chiefly the lungs and the kidneys, which already have enough todo. This extra work they can do for only a short time. Sooner or laterthey become disordered, and illness follows. Moreover, as this unwholesomelayer is a fertile soil in which bacteria may develop, many skin diseasesmay result from this neglect. It is also highly probable that germs ofdisease thus adherent to the skin may then be absorbed into the system. Parasitic skin diseases are thus greatly favored by the presence of anunclean skin. It is also a fact that uncleanly people are more liable totake cold than those who bathe often. 

The importance of cleanliness would thus seem too apparent to need specialmention, were it not that the habit is so much neglected. The old andexcellent definition that dirt is suitable matter, but in the wrong place, suggests that the place should be changed. This can be done only byregular habits of personal cleanliness, not only of the skin, the hair, the teeth, the nails, and the clothing, but also by the rigid observanceof a proper system in daily living. 

246. Baths and Bathing. In bathing we have two distinct objects inview, --to keep the skin clean and to impart vigor. These are closelyrelated, for to remove from the body worn-out material, which tends toinjure it, is a direct means of giving vigor to all the tissues. Thus acold bath acts upon the nervous system, and calls out, in response to thetemporary abstraction of heat, a freer play of the general vital powers. Bathing is so useful, both locally and constitutionally, that itshould be practiced to such an extent as experience proves to bebeneficial. For the general surface, the use of hot water once a weekfulfills the demands of cleanliness, unless in special occupations. Whether we should bathe in hot or cold water depends upon circumstances. Most persons, especially the young and vigorous, soon become accustomed tocool, and even cold water baths, at all seasons of the year. 

The hot bath should be taken at night before going to bed, as in themorning there is usually more risk of taking cold. The body is readilychilled, if exposed to cold when the blood-vessels of the skin have beenrelaxed by heat. Hot baths, besides their use for the purposes ofcleanliness, have a sedative influence upon the nervous system, tending toallay restlessness and weariness. They are excellent after severe physicalor mental work, and give a feeling of restful comfort like that of sleep. 

[Illustration: Fig. 105. --Epithelial Cells from the Sweat Glands. Thecells are very distinct, with nuclei enclosing pigmentary granulations(Magnified 350 times)]

Cold baths are less cleansing than hot, but serve as an excellenttonic and stimulant to the bodily functions. The best and most convenienttime for a cold bath is in the morning, immediately after rising. To thehealthy and vigorous, it is, if taken at this time, with properprecautions, a most agreeable and healthful luxury. The sensation ofchilliness first felt is caused by the contraction of the skin and itsblood-vessels, so that the blood is forced back, as it were, into thedeeper parts of the body. This stimulates the nervous system, thebreathing becomes quicker and deeper, the heart beats more vigorously, and, as a consequence, the warm blood is sent back to the skin withincreased force. This is known as the stage of reaction, which is bestincreased by friction with a rough towel. This should produce the pleasantfeeling of a warm glow all over the body. 

A cold bath which is not followed by reaction is likely to do more harmthan good. The lack of this reaction may be due to the water being toocold, the bath too prolonged, or to the bather being in a low condition ofhealth. In brief, the ruddy glow which follows a cold bath is the mainsecret of its favorable influence. 

The temperature of the water should be adapted to the age and strength ofthe bather. The young and robust can safely endure cold baths, that wouldbe of no benefit but indeed an injury to those of greater age or of lessvigorous conditions of health. After taking a bath the skin should berapidly and vigorously rubbed dry with a rough towel, and the clothing atonce put on. 

247. Rules and Precautions in Bathing. Bathing in cold water shouldnot be indulged in after severe exercise or great fatigue, whether we areheated or not. Serious results have ensued from cold baths when the bodyis in a state of exhaustion or of profuse perspiration. A daily cold bathwhen the body is comfortably warm, is a safe tonic for almost all personsduring the summer months, and tends especially to restore the appetite. Cold baths, taken regularly, render persons who are susceptible tocolds much less liable to them, and less likely to be disturbed by suddenchanges of temperature. Persons suffering from heart disease or fromchronic disease of an important organ should not indulge in frequent coldbathing except by medical advice. Owing to the relaxing nature of hotbaths, persons with weak hearts or suffering from debility may faint whiletaking them. 

Outdoor bathing should not be taken for at least an hour after afull meal, and except for the robust it is not prudent to bathe with thestomach empty, especially before breakfast. It is a wise rule, in outdooror sea bathing, to come out of the water as soon as the glow of reactionis felt. It is often advisable not to apply cold water very freely to thehead. Tepid or even hot water is preferable, especially by those subjectto severe mental strain. But it is often a source of great relief duringmental strain to bathe the face, neck, and chest freely at bedtime withcold water. It often proves efficient at night in calming thesleeplessness which results from mental labor. 

Hot baths, if taken at bedtime, are often serviceable in preventing athreatened cold or cutting it short, the patient going immediately to bed, with extra clothing and hot drinks. The free perspiration induced helps tobreak up the cold. 

Salt water acts more as a stimulant to the skin than fresh water. Salt-water bathing is refreshing and invigorating for those who arehealthy, but the bather should come out of the water the moment there isthe slightest feeling of chilliness. The practice of bathing in salt watermore than once a day is unhealthful, and even dangerous. Only thestrongest can sustain so severe a tax on their power of endurance. Seabathing is beneficial in many ways for children, as their skin reacts wellafter it. In all cases, brisk rubbing with a rough towel should be hadafterwards. 

[Illustration: Fig. 106. --Magnified Section of the Lower Portion of a Hairand Hair-Follicle. 

 A, membrane of the hair-follicle, cells with nuclei and pigmentary granules; B, external lining of the root sheath; C, internal lining of the root sheath; D, cortical or fibrous portion of the hair shaft; E, medullary portion (pith) of shaft; F, hair-bulb, showing its development from cells from A. ]

The golden rule of all bathing is that it must never be followed by achill. If even a chilliness occur after bathing, it must immediatelybe broken up by some appropriate methods, as lively exercise, briskfriction, hot drinks, and the application of heat. 

Swimming is a most valuable accomplishment, combining bathing andexercise. Bathing of the feet should never be neglected. Cleanliness ofthe hair is also another matter requiring strict attention, especially inchildren. 

248. Care of the Hair and Nails. The hair brush should not be toostiff, as this increases the tendency towards scurfiness of the head. If, however, the hair is brushed too long or too hard, the scalp is greatlystimulated, and an increased production of scurf may result. If the headbe washed too often with soap its natural secretion is checked, and thescalp becomes dry and scaly. The various hair pomades are as a ruleundesirable and unnecessary. 

The nails should be kept in proper condition, else they are not onlyunsightly, but may serve as carriers of germs of disease. The nails areoften injured by too much interference, and should never be trimmed to thequick. The upper surfaces should on no account be scraped. The nail-brushis sufficient to cleanse them without impairing their smooth and polishedsurfaces. 

[Illustration: Fig. 107. --Longitudinal Section of a Finger-Nail. 

 A, last phalanx of the fingers; B, true skin on the dorsal surface of the finger; C, epidermis; D, true skin; E, bed of the nail; F, superficial layer of the nail; H, true skin of the pulp of the finger. ]

249. Use of Clothing. The chief use of clothing, from a hygienicpoint of view, is to assist in keeping the body at a uniform temperature. It also serves for protection against injury, and for personal adornment. The heat of the body, as we have learned, is normally about 98-1/2 degreesF. This varies but slightly in health. A rise of temperature of more thanone degree is a symptom of disturbance. The normal temperature does notvary with the season. In summer it is kept down by the perspiration and itsrapid evaporation. In winter it is maintained by more active oxidation, byextra clothing, and by artificial heat. 

The whole matter of clothing is modified to a great extent by climaticconditions and local environments, --topics which do not comewithin the scope of this book. 

250. Material Used for Clothing. It is evident that if clothing is todo double duty in preventing the loss of heat by radiation, and inprotecting us from the hot rays of the sun, some material must be usedthat will allow the passage of heat in either direction. The idealclothing should be both a bad conductor and a radiator of heat. At thesame time it must not interfere with the free evaporation of theperspiration, otherwise chills may result from the accumulation ofmoisture on the surface of the body. 

Wool is a bad conductor, and should be worn next the skin, both insummer and winter, especially in variable climates. It prevents, betterthan any other material, the loss of heat from the body, and allows freeventilation and evaporation. Its fibers are so lightly woven that theymake innumerable meshes enclosing air, which is one of the best ofnon-conductors. 

Silk ranks next to wool in warmth and porosity. It is much softer andless irritating than flannel or merino, and is very useful for summerwear. The practical objection to its general use is the expense. Furranks with wool as a bad conductor of heat. It does not, however, likewool, allow of free evaporation. Its use in cold countries is universal, but in milder climates it is not much worn. 

Cotton and linen are good conductors of heat, but are notabsorbents of moisture, and should not be worn next the skin. They are, however, very durable and easily cleansed. As an intermediate clothingthey may be worn at all seasons, especially over wool or silk. Waterproofclothing is also useful as a protection, but should not be worn a longertime than necessary, as it shuts in the perspiration, and causes a senseof great heat and discomfort. 

The color of clothing is of some importance, especially if exposeddirectly to the sun's rays. The best reflectors, such as white and lightgray clothing, absorb comparatively little heat and are the coolest, whileblack or dark-colored materials, being poor reflectors and goodabsorbents, become very warm. 

251. Suggestions for the Use of Clothing. Prudence and good senseshould guide us in the spring, in changing winter flannels or clothing forfabrics of lighter weight. With the fickle climate in most sections ofthis country, there are great risks of severe colds, pneumonia, and otherpulmonary diseases from carelessness or neglect in this matter. A changefrom heavy to lighter clothing should be made first in the outer garments, the underclothing being changed very cautiously. 

The two essentials of healthful clothing are cleanliness anddryness. To wear garments that are daily being soiled by perspirationand other cutaneous excretions, is a most uncleanly and unhealthfulpractice. Clothing, especially woolen underclothing, should be frequentlychanged. One of the objections to the use of this clothing is that it doesnot show soiling to the same extent as do cotton and linen. 

Infectious and contagious diseases may be conveyed by the clothing. Hence, special care must be taken that all clothing in contact with sick peopleis burned or properly disinfected. Children especially are susceptible toscarlet fever, diphtheria, and measles, and the greatest care must beexercised to prevent their exposure to infection through the clothing. 

We should never sleep in a damp bed, or between damp sheets. The vitalpowers are enfeebled during sleep, and there is always risk of pneumoniaor rheumatism. The practice of sitting with wet feet and damp clothing ishighly injurious to health. The surface of the body thus chilled may besmall, yet there is a grave risk of serious, if not of fatal, disease. Noharm may be done, even with clothing wet with water or damp withperspiration, so long as exercise is maintained, but the failure orinability to change into dry garments as soon as the body is at rest isfraught with danger. 

Woolen comforters, scarfs, and fur mufflers, so commonly worn around theneck, are more likely to produce throat troubles and local chill than tohave any useful effect. Harm ensues from the fact that the extra coveringinduces local perspiration, which enfeebles the natural defensive power ofthe parts; and when the warmer covering is removed, the perspiring surfaceis readily chilled. Those who never bundle their throats are least liableto suffer from throat ailments. 

252. Ill Effects of Wearing Tightly Fitting Clothing. The injury tohealth caused by tight lacing, when carried to an extreme, is due to thecompression and displacement of various organs by the pressure exerted onthem. Thus the lungs and the heart may be compressed, causing short breathon exertion, palpitation of the heart, and other painful and dangeroussymptoms. The stomach, the liver, and other abdominal organs are oftendisplaced, causing dyspepsia and all its attendant evils. The improper useof corsets, especially by young women, is injurious, as they interferewith the proper development of the chest and abdominal organs. The use oftight elastics below the knee is often injurious. They obstruct the localvenous circulation and are a fruitful source of cold feet and of enlargedor varicose veins. 

Tightly fitting boots and shoes often cause corns, bunions, and ingrowingnails; on the other hand, if too loosely worn, they cause corns fromfriction. Boots too narrow in front crowd the toes together, make themoverlap, and render walking difficult and painful. High-heeled boots throwthe weight of the body forwards, so that the body rests too much on thetoes instead of on the heels, as it should, thus placing an undue strainupon certain groups of muscles of the leg, in order to maintain thebalance, while other groups are not sufficiently exercised. Locomotion isnever easy and graceful, and a firm, even tread cannot be expected. 

The compression of the scalp by a tight-fitting hat interferes with thelocal circulation, and may cause headaches, neuralgia, or baldness, thenutrition of the hair-follicles being diminished by the impairedcirculation. The compression of the chest and abdomen by a tight belt andvarious binders interferes with the action of the diaphragm, --the mostimportant muscle of respiration. 

253. Miscellaneous Hints on the Use of Clothing. Children and oldpeople are less able to resist the extreme changes of temperature than areadults of an average age. Special care should be taken to provide childrenwith woolen underclothing, and to keep them warm and in well-ventilatedrooms. Neither the chest nor limbs of young children should be undulyexposed, as is often done, to the cold blasts of winter or the fickleweather of early spring. Very young children should not be taken out inextremely cold weather, unless quite warmly clad and able to run about. The absurd notion is often entertained that children should be hardened byexposure to the cold. Judicious "hardening" means ample exposure ofwell-fed and well-clothed children. Exposure of children not thus caredfor is simple cruelty. The many sicknesses of children, especiallydiseases of the throat and lungs, may often be traced directly to grosscarelessness, ignorance, or neglect with reference to undue exposure. Thedelicate feet of children should not be injured by wearing ill-fitting orclumsy boots or shoes. Many deformities of the feet, which cause muchvexation and trouble in after years, are acquired in early life. 

No one should sleep in any of the clothes worn during the day, not even inthe same underclothing. All bed clothing should be properly aired, by freeexposure to the light and air every morning. Never wear wet or dampclothing one moment longer than necessary. After it is removed rub thebody thoroughly, put on at once dry, warm clothing, and then exercisevigorously for a few minutes, until a genial glow is felt. Neglect ofthese precautions often results in rheumatism, neuralgia, and diseases ofthe chest, especially among delicate people and young women. 

Pupils should not be allowed to sit in the schoolroom with any outergarments on. A person who has become heated in a warm room should notexpose himself to cold without extra clothing. We must not be in a hurryto put on heavy clothes for winter, but having once worn them, they mustnot be left off until milder weather renders the change safe. The cheaperarticles of clothing are often dyed with lead or arsenic. Hence suchgarments, like stockings and colored underclothing, worn next the skinhave been known to produce severe symptoms of poisoning. As a precaution, all such articles should be carefully washed and thoroughly rinsed beforethey are worn. 

The Kidneys. 

254. The Kidneys. The kidneys are two important organs in theabdomen, one on each side of the spine. They are of a reddish-brown color, and are enveloped by a transparent capsule made up of a fold of theperitoneum. Embedded in fat, the kidneys lie between the upper lumbarvertebræ, and the crest of the hip bone. The liver is above the rightkidney, and the spleen above the left, while both lie close against therear wall of the abdomen, with the intestines in front of them. The humankidneys, though somewhat larger, are exactly of the same shape, color, andgeneral appearance as those of the sheep, so commonly seen in the markets. 

The kidneys are about four inches long, two inches across, one inch thick, and weigh from 41/2 to 51/2 ounces each. The hollow or concave side of thekidneys is turned inwards, and the deep fissure of this side, known as thehilus, widens out to form the pelvis. Through the hilus therenal artery passes into each kidney, and from each hilus passes outwardsthe renal vein, a branch of the inferior vena cava. 

A tube, called the ureter, passes out from the concave border of eachkidney, turns downwards, and enters the bladder in the basin of thepelvis. This tube is from 12 to 14 inches long, about as large as a goosequill, and conveys the secretion of the kidneys to the bladder. 

255. Structure of the Kidneys. The pelvis is surrounded byreddish cones, about twelve in number, projecting into it, called thepyramids of Malpighi. The apices of these cones, known as the_papillæ_, are crowded with minute openings, the mouths of theuriniferous tubules, which form the substance of the kidney. Theselie parallel in the medullary or central structure, but On reaching thecortical or outer layer, they wind about and interlace, ending, at last, in dilated closed sacs called Malpighian capsules. 

[Illustration: Fig. 108. --Vertical Section of the Kidney. 

 A, pyramids of Malpighi; B, apices, or papillæ, of the pyramids, surrounded by subdivisions of the pelvis known as cups or calices; C, pelvis of the kidney; D, upper end of ureter. ]

256. Function of the Kidneys. The Malpighian capsules are really thebeginning of the tubules, for here the work of excretion begins. The thinwall of the capillaries within each capsule separates the blood from thecavity of the tubule. The blood-pressure on the delicate capillary wallscauses the exudation of the watery portions of the blood through the cellwalls into the capsule. The epithelial cell membrane allows the water ofthe blood with certain salts in solution to pass, but rejects the albumen. From the capsules, the excretion passes through the tubules into thepelvis, and on through the ureters to the bladder. But the delicateepithelial walls of the tubules through which it passes permit the inflowof urea and other waste products from the surrounding capillaries. By thistwofold process are separated from the blood the fluid portions of therenal secretion with soluble salts, and the urea with other wastematerial. 

257. How the Action of the Kidneys may be Modified. The action of thekidneys is subject to very marked and sudden modifications, especiallythose operating through the nervous system. Thus whatever raises theblood-pressure in the capillaries of the capsules, will increase thequantity of fluid filtering through them. That is, the watery portion ofthe secretion will be increased without necessarily adding to its solids. So anything which lowers the blood-pressure will diminish the wateryportion of the secretion, that is, the secretion will be scanty, butconcentrated. 

The Renal Secretion. --The function of the kidneys is to secrete afluid commonly known as the urine. The average quantity passed in 24 hoursby an adult varies from 40 to 60 fluid ounces. Normal urine consists ofabout 96 per cent of water and 4 per cent of solids. The latter consistchiefly of certain nitrogenous substances known as urea and uric acid, aconsiderable quantity of mineral salts, and some coloring matter. Urea, the most important and most abundant constituent of urine, contains thefour elements, but nitrogen forms one-half its weight. While, therefore, the lungs expel carbon dioxid chiefly, the kidneys expel nitrogen. Both ofthese substances express the result of oxidations going on in the body. The urea and uric acids represent the final result of the breaking down inthe body of nitrogenous substances, of which albumen is the type. 

Unusual constituents of the urine are _albumen, sugar_, and _bile_. Whenalbumen is present in urine, it often indicates some disease of thekidneys, to which the term _albuminuria_ or Bright's Disease is applied. The presence of grape sugar or glucose indicates the disease known asdiabetes. Bile is another unusual constituent of the urine, appearing in_jaundice_. 

The bladder is situated in the pelvic cavity or in the lowest part ofthe abdomen. When full, the bladder is pear-shaped; when empty, it iscollapsed and lies low in the pelvis. The functions of the bladder are tocollect and retain the urine, which has reached it drop by drop from thekidneys through the ureters, until a certain quantity accumulates, andthen to expel it from the body. 

[Illustration: Fig. 109. --Vertical Section of the Back. (Showing kidneys_in situ_ and the relative position of adjacent organs and vessels. )[Posterior view. ]

 A, 12th dorsal vertebra; B, diaphragm; C, receptaculum chyli; D, small intestines]

In the kidneys, as elsewhere, the vaso-motor nerves are distributedto the walls of the blood-vessels, and modify the quantity and thepressure of blood in these organs. Thus, some strong emotion, like fear orundue anxiety, increases the blood-pressure, drives more blood to thekidneys, and causes a larger flow of watery secretion. When the atmosphereis hot, there is a relaxation of the vessels of the skin, with a morethan ordinary flow of blood, which is thus withdrawn from the deeperorgans. The blood-pressure in the kidneys is not only diminished, but thetotal quantity passing through them in a given time is much lessened. As aresult, the secretion of the kidneys is scanty, but it contains an unusualpercentage of solids. 

When the atmosphere is cold, the reverse is true. The cutaneous vesselscontract, the blood is driven to the deeper organs with increasedpressure, and there is a less amount of sweat, but an increased renalsecretion, containing a smaller proportion of solids. Certain drugs havethe power of increasing or diminishing the renal secretion. As the wastematters eliminated by the kidneys are being constantly produced in thetissues, the action of the renal organs is continuous, in marked contrastwith the intermittent flow of most of the secretions proper, asdistinguished from the excretions. 

258. Effects of Alcoholic Drinks upon the Kidneys. The kidneys differfrom some of the other organs in this: those can rest a while without anyharm to themselves, or to the body. We can keep the eyes closed for a fewdays, if necessary, without injury, and in fact often with benefit; or, wecan abstain from food for some days, if need be, and let the stomach rest. But the kidneys cannot, with safety, cease their work. Their duty inridding the blood of waste products, and of any foreign or poisonousmaterial introduced, must be done not only faithfully, but continually, orthe whole body at once suffers from the evil effects of the retained wastematters. 

This vital fact is the key to the injurious results developed in thekidneys by the use of alcoholic drinks. These two organs have largeblood-vessels conveying full amounts of blood to and from theirstructures, and they feel very quickly the presence of alcohol. Alcoholicliquors excite and irritate the delicate renal membranes, and speedilydisturb and eventually destroy their capacity to excrete the propermaterials from the blood. 

The continued congestion of the minute structure of the kidney cuts offthe needed nutrition of the organ, and forms the primary step in theseries of disasters. Sometimes from this continued irritation, with theresulting inflammation, and sometimes from change of structure of thekidney by fatty degeneration, comes the failure to perform its properfunction. Then, with this two-edged sword of disaster, the urea, whichbecomes a poisonous element, and should be removed, is retained in thesystem, while the albumen, which is essential to healthy blood, isfiltered away through the diseased kidney. 

259. Alcoholic Liquors as a Cause of Bright's Disease. Theunfortunate presence of albumen in the urine is often a symptom of thatinsidious and fatal malady known as _albuminuria_ or Bright's disease, often accompanied with dropsy and convulsions. One of the most constantcauses of this disease is the use of intoxicants. It is not at allnecessary to this fatal result that a person be a heavy drinker. Steady, moderate drinking will often accomplish the work. Kidney diseases producedby alcoholic drinks, are less responsive to medical treatment and morefatal than those arising from any other known cause. [39]

 Experiment 129. Obtain a sheep's kidney in good order. Observe that its shape is something like that of a bean, and note that the concave part (hilus), when in its normal position, is turned towards the backbone. Notice that all the vessels leave and enter the kidney at the hilus. Observe a small thick-walled vessel with open mouth from which may be pressed a few drops of blood. This is the renal artery. Pass a bristle down it. With the forceps, or even with a penknife, lift from the kidney the fine membrane enclosing it. This is the kidney capsule. 

 Divide the kidney in halves by a section from its outer to near its inner border. Do not cut directly through the hilus. Note on the cut surfaces, on the outer side, the darker cortical portion, and on the inner side, the smooth, pale, medullary portion. Note also the pyramids of Malpighi. 

Chapter X. 

The Nervous System. 

260. General View of the Nervous System. Thus far we have learnedsomething of the various organs and the manner in which they do theirwork. Regarding our bodily structure as a kind of living machine, we havestudied its various parts, and found that each is designed to perform somespecial work essential to the well-being of the whole. As yet we havelearned of no means by which these organs are enabled to adjust theiractivities to the needs of other tissues and other organs. We are nowprepared to study a higher, a more wonderful and complex agency, --thenervous system, the master tissue, which controls, regulates, anddirects every other tissue of the human body. 

The nervous system, in its properties and mode of action, is distinct fromall the other systems and organs, and it shares with no other organ ortissue the power to do its special work. It is the medium through whichall impressions are received. It connects all the parts of the body intoan organism in which each acts in harmony with every other part for thegood of the whole. It animates and governs all movements, voluntary orinvoluntary, --secretion, excretion, nutrition; in fact all the processesof organic life are subject to its regulating power. The different organsof the body are united by a common sympathy which regulates their action:this harmonious result is secured by means of the nervous system. 

This system, in certain of its parts, receives impressions, and generatesa force peculiar to itself. We shall learn that there can be no physicalcommunication between or coördination of the various parts of organs, orharmonious acts for a desire result, without the nerves. Generalimpressions, as in ordinary sensation, or special impressions, as insight, smell, taste, or hearing, --every instinct, every act of the will, and every thought are possible only through the action of the nervecenters. 

261. Nerve Cells. However complicated the structure of nerve tissuein man seems to be, it is found to consist of only two different elements, nerve cells and nerve fibers. These are associated and combinedin many ways. They are arranged in distinct masses called nervecenters, or in the form of cords known as nerves. The former aremade up of nerve fibers; the latter of both cells and fibers. 

[Illustration: Fig. 110. Nerve Cells from the Spinal Cord. ]

Nerve cells, which may be regarded as the central organs of the nervefibers, consist of masses of cell protoplasm, with a large _nucleus_ and_nucleolus_. They bear a general resemblance to other cells, but vary muchin size and shape. Nerve cells grow, become active, and die, as do othercells. A number of processes branch off from them, some cells giving oneor two, others many. The various kinds of nerve cells differ much in theshape and number of processes. One of the processes is a strand whichbecomes continuous with the axis cylinder of the nerve fibers; that is, the axis cylinders of all nerve fibers are joined in one place or anotherwith at least one cell. 

Each part of this system has its own characteristic cell. Thus we have inthe spinal cord the large, irregular cells with many processes, and in thebrain proper the three-sided cells with a process jutting out from eachcorner. So characteristic are these forms of cells, that any particularpart of nerve structure may be identified by the kind of cells seen underthe microscope. Nerve cells and nerve fibers are often arranged ingroups, the various cells of the groups communicating with one another. This clustered arrangement is called a nerve center. 

262. Nerve Fibers. The nerve fibers, the essential elements ofthe nerves, somewhat resemble tubes filled with a clear, jelly-likesubstance. They consist of a rod, or central core, continuous throughoutthe whole length of the nerve, called the axis cylinder. This core issurrounded by the white substance of Schwann, or medullary sheath, whichgives the nerve its characteristic ivory-white appearance. The whole isenclosed in a thin, delicate sheath, known as neurilemma. 

[Illustration: Fig. 111. --Nerve Cells from the Gray Matter of the Brain. ]

The axis cylinder generally passes without any break from the nervecenters to the end of the fibers. [40] The outer sheath (neurilemma) isalso continuous throughout the length of the fibers. The medullary sheath, on the other hand, is broken at intervals of about 1/25 of an inch, and atthe same intervals nuclei are found along the fiber, around each ofwhich is a minute protoplasmic mass. Between each pair of nuclei thesheath is interrupted. This point is known as the _node of Ranvier_. 

Some nerve fibers have no inner sheath (medullary), the outer aloneprotecting the axis cylinder. These are known as the non-medullary fibers. They are gray, while the ordinary medullary fibers are white inappearance. The white nerve fibers form the white part of the brainand of the spinal cord, and the greater part of the cerebro-spinal nerves. The gray fibers occur chiefly in branches from the sympatheticganglia, though found to some extent in the nerves of the cerebro-spinalsystem. 

In a general way, the nerve fibers resemble an electric cable wire withits central rod of copper, and its outer non-conducting layer of silk orgutta percha. Like the copper rod, the axis cylinder along which the nerveimpulse travels is the essential part of a nerve fiber. In a cut nervethis cylinder projects like the wick of a candle. It is really thecontinuation of a process of a nerve cell. Thus the nerve cells and nervefibers are related, in that the process of one is the axis cylinder andessential part of the other. 

The separate microscopic threads or fibers, bound together in cords ofvariable size, form the nerves. Each strand or cord is surrounded andprotected by its own sheath of connective tissue, made up of nerves. According to its size a nerve may have one or many of these strands. Thewhole nerve, not unlike a minute tendon in appearance, is covered by adense sheath of fibrous tissue, in which the blood-vessels and lymphaticsare distributed to the nerve fibers. 

[Illustration: Fig. 112. --Medullated Nerve Fibers. 

 A, a medullated nerve fiber, showing the subdivision of the medullary sheath into cylindrical sections imbricated with their ends, a nerve corpuscle with an oval nucleus is seen between the neurilemma and the medullary sheath; B, a medullated nerve fiber at a node or constriction of Ranvier, the axis cylinder passes uninterruptedly from one segment into the other, but the medullary sheath is interrupted. ]

263. The Functions of the Nerve Cells and Nerve Fibers. The nervecells are a highly active mass of living material. They find theirnourishment in the blood, which is supplied to them in abundance. Theblood not only serves as nourishment, but also supplies new material, asit were, for the cells to work over for their own force or energy. Thuswe may think of the nerve cells as a sort of a miniature manufactory, deriving their material from the blood, and developing from it nervousenergy. 

The nerve fibers, on the other hand, are conductors of nervous energy. They furnish a pathway along which the nerve energy generated by the cellsmay travel. Made up as they are of living nerve substance, the fibers canalso generate energy, yet it is their special function to conductinfluences to and from the cells. 

[Illustration: Fig. 113. --Non-Medullated Fibers. 

Two nerve fibers, showing the nodes or constrictions of Ranvier and theaxis cylinder. The medullary sheath has been dissolved away. The deeplystained oblong nuclei indicate the nerve corpuscles within theneurilemma. ]

264. The Nervous System Compared to a Telegraphic System. In men andother highly organized animals, nerves are found in nearly every tissueand organ of the body. They penetrate the most minute muscular fibers;they are closely connected with the cells of the glands, and are found inthe coats of even the smallest blood-vessels. They are among the chieffactors of the structure of the sense organs, and ramify through the skin. Thus the nervous system is the system of organs through the functions ofwhich we are brought into relation with the world around us. When we hear, our ears are bringing us into relation with the outer world. So sightopens up to us another gateway of knowledge. 

It will help us the better to understand the complicated functions of thenervous system, if we compare it to a telegraph line. The brain is themain office, and the multitudes of nerve fibers branching off to all partsof the body are the wires. By means of these, nerve messages areconstantly being sent to the brain to inform it of what is going on invarious parts of the body, and asking what is to be done in each case. Thebrain, on receiving the intelligence, at once sends back the requiredinstructions. Countless messages are sent to and fro with unerringaccuracy and marvelous rapidity. 

Thus, when we accidentally pick up something hot, it is instantlydropped. A nerve impulse passes from the nerves of touch in the fingers tothe brain, which at once hurries off its order along another set of nervesfor the hand to drop the burning object. These examples, so common indaily life, may be multiplied to any extent. Almost every voluntary act weperform is executed under the direction of the nervous system, althoughthe time occupied is so small that it is beyond our power to estimate it. The very frequency with which the nerves act tends to make us forget theirbeneficent work. 

265. Divisions of the Nervous System. This system in man consists oftwo great divisions. The first is the great nerve center of the body, thecerebro-spinal system, which rules the organs of animal life. Thisincludes the brain, the spinal cord, and the cerebro-spinalnerves. Nerves are given off from the brain and the cord, and form themediums of communication between the external parts of the body, themuscles or the sense organs, and the brain. 

The second part is the sympathetic system, which regulates theorganic life. This consists of numerous small nerve centers arranged inoval masses varying greatly in size, called ganglia or knots. Theseare either scattered irregularly through the body, or arranged in a doublechain of knots lying on the front of the spine, within the chest andabdomen. From this chain large numbers of nerves are given off, which endchiefly in the organs of digestion, circulation, and respiration. Thesympathetic system serves to bring all portions of the animal economy intodirect sympathy with one another. 

266. The Brain as a Whole. The brain is the seat of theintellect, the will, the affections, the emotions, the memory, andsensation. It has also many other and complex functions. In it areestablished many reflex, automatic, and coordinating centers, which are asindependent of consciousness as are those of the spinal cord. 

The brain is the largest and most complex mass of nerve tissue in thebody, made up of an enormous collection of gray cells and nerve fibers. This organ consists of a vast number of distinct ganglia, or separatemasses of nerve matter, each capable of performing separate functions, butunited through the cerebral action into a harmonious whole. 

[Illustration: Fig. 114. --The Upper Surface of the Cerebrum. (Showing itsdivision into two hemispheres, and also the convolutions)]

The average weight of the adult human brain is about 50 ounces for men and45 ounces for women. Other things being equal, the size and weight of thebrain bear a general relation to the mental power of the individual. As arule, a large, healthy brain stands for a vigorous and superior intellect. The brains of many eminent men have been found to be 8 to 12 ounces abovethe average weight, but there are notable exceptions. The brains ofidiots are small; indeed, any weight under a certain size, about 30ounces, seems to be invariably associated with an imbecile mind. 

The human brain is absolutely heavier than that of any other animal, except the whale and elephant. Comparing the size of these animals withthat of man, it is instructive to notice how much larger in proportion tothe body is man's brain. The average proportion of the weight of the brainto the weight of the body is greater in man than in most animals, beingabout 1 to 36. In some small birds, in the smaller monkeys, and in somerodents, the proportional weight of the brain to that of the body is evengreater than in man. 

267. The Cerebrum. The three principal masses which make up the brainwhen viewed as a whole are:

 1. The cerebrum, or brain proper. 2. The cerebellum, or lesser brain. 3. The medulla oblongata. 

The cerebrum comprises nearly seven-eighths of the entire mass, andfills the upper part of the skull. It consists of two halves, the rightand left cerebral hemispheres. These are almost separated from eachother by a deep median fissure. The hemispheres are united at the bottomof the fissure by a mass of white fibers passing from side to side. Eachof these hemispheres is subdivided into three lobes, so that the entirecerebrum is made up of six distinct lobes. 

The cerebrum has a peculiar convoluted appearance, its deep folds beingseparated by fissures, some of them nearly an inch in depth. 

It is composed of both white and gray matter. The former comprises thegreater part of the mass, while the latter is spread over the surface in alayer of about 1/8 of an inch thick. The gray matter is the portion havingthe highest functions, and its apparent quantity is largely increased bybeing formed in convolutions. 

The convolutions of the cerebrum are without doubt associated with allthose higher actions which distinguish man's life; but all theconvolutions are not of equal importance. Thus it is probable that onlythe frontal part of the brain is the intellectual region, while certainconvolutions are devoted to the service of the senses. 

The cerebrum is the chief seat of the sensations, the intellect, the will, and the emotions. A study of cerebral injuries and diseases, andexperiments upon the lower animals, prove that the hemispheres, and moreespecially the gray matter, are connected with mental states. Theconvolutions in the human brain are more prominent than in that of thehigher animals, most nearly allied to man, although some species ofanimals, not especially intelligent, have marked cerebral convolutions. The higher races of men have more marked convolutions than those lesscivilized. 

A view of the under surface of the brain, which rests on the floor of theskull, shows the origin of important nerves, called the cranialnerves, the cerebellum, the structure connecting the opticnerves (optic commissure), the bridge of nervous matter (ponsVarolii) connecting the two hemispheres of the cerebellum, and lastlynumerous and well-marked convolutions. 

268. The Cerebellum. The cerebellum, or lesser brain, lies inthe back of the cranium, and is covered over in man by the posterior lobeof the cerebrum. It is, at it were, astride of the back of thecerebro-spinal axis, and consists of two hemispheres joined by a centralmass. On its under surface is a depression which receives the medullaoblongata. The cerebellum is separated from the cerebrum by ahorizontal partition of membrane, a portion of the dura mater. In someanimals, as in the cat, this partition is partly bone. 

The cerebellum is connected with other parts of the nervous system bystrands of white matter on each side, radiating from the center anddivided into numerous branches. Around these branches the gray matter isarranged in a beautiful manner, suggesting the leaves of a tree: hence itsname, arbor vitæ, or the tree of life. 

The functions of the cerebellum are not certainly known. It appears toinfluence the muscles of the body so as to regulate their movements; thatis, it serves to bring the various muscular movements into harmoniousaction. The mechanism by which it does this has not yet been clearlyexplained. In an animal from which the cerebellum has been removed, thefunctions of life do not appear to be destroyed, but all power of eitherwalking or flying straight is lost. 

[Illustration: Fig. 115. --A Vertical Section of the Brain. 

 A, frontal lobe of the cerebrum; B, parietal lobe; C, parieto occipital lobe with fissure between this lobe and D, the occipital lobe; E, cerebellum; F, arbor vitæ; H, pons Varolu; K, medulla oblongata; L, portion of lobe on the opposite side of brain. 

The white curved band above H represents the corpus callosum. ]

Disease or injury of the cerebellum usually produces blindness, giddiness, a tendency to move backwards, a staggering, irregular gait, anda feeling of insecurity in maintaining various positions. There is no lossof consciousness, or other disturbance of the mental functions. 

269. The Membranes of the Brain. The brain and spinal cord areprotected by three important membranes, known as the meninges, --thedura mater, the arachnoid, and the pia mater. 

The outer membrane, the dura mater, is much thicker and stronger thanthe others, and is composed of white fibrous and elastic connectivetissue. It closely lines the inner surface of the skull, and forms aprotective covering for the brain. Folds of it pass between the severaldivisions of the brain and serve to protect them. 

The arachnoid is a thin membrane which lies beneath the dura mater. It secretes a serous fluid which keeps the inner surfaces moist. 

The pia mater is a very delicate, vascular membrane which covers theconvolutions, dips into all the fissures, and even penetrates into theinterior of the brain. It is crowded with blood-vessels, which divide andsubdivide very minutely before they penetrate the brain. The membranes ofthe brain are sometimes the seat of inflammation, a serious and painfuldisease, commonly known as brain fever. 

270. The Medulla Oblongata. This is the thick upper part of thespinal cord, lying within the cavity of the skull. It is immediately underthe cerebellum, and forms the connecting link between the brain and thespinal cord. It is about an inch and a quarter long, and from one-half tothree-fourths of an inch wide at its upper part. The medullaoblongata consists, like the spinal cord, of columns of white fibersand masses of gray matter, but differently arranged. The gray matter isbroken up into masses which serve as centers of origin for various nerves. The functions of the medulla oblongata are closely connected with thevital processes. It is a great nerve tract for transmitting sensory andmotor impressions, and also the seat of a number of centers for reflexactions of the highest importance to life. Through the posterior part ofthe medulla the sensory impressions pass, that is, impressions from belowupwards to the brain resulting in sensation or feeling. In the anteriorpart of the medulla, pass the nerves for motor transmission, that is, nerve influences from above downwards that shall result in muscularcontractions in some part of the body. 

The medulla is also the seat of a number of reflex centers connected withthe influence of the nervous system on the blood-vessels, the movements ofthe heart, of respiration, and of swallowing, and on the secretion ofsaliva. This spot has been called the "vital knot. " In the medulla alsoare centers for coughing, vomiting, swallowing, and the dilatation of thepupil of the eye. It is also in part the deep origin of many of the'important cranial nerves. 

[Illustration: Fig. 116. --Illustrating the General Arrangement of theNervous System. (Posterior view. )]

271. The Cranial Nerves. The cranial or cerebral nervesconsist of twelve pairs of nerves which pass from the brain throughdifferent openings in the base of the skull, and are distributed over thehead and face, also to some parts of the trunk and certain internalorgans. These nerves proceed in pairs from the corresponding parts of eachside of the brain, chiefly to the organs of smell, taste, hearing, andsight. 

The cranial nerves are of three kinds: sensory, motor, and bothcombined, _viz_. , mixed. 

Distribution and Functions of the Cranial Nerves. The cranial nervesare thus arranged in pairs:

The first pair are the olfactory nerves, which pass down throughthe ethmoid bone into the nasal cavities, and are spread over the innersurface of the nose. They are sensory, and are the special nerves ofsmell. 

The second pair are the optic nerves, which, under the name ofthe _optic tracts_, run down to the base of the brain, from which an opticnerve passes to each eyeball. These are sensory nerves, and are devoted tosight. 

The third, fourth, and sixth pairs proceed to the muscles of theeyes and control their movements. These are motor nerves, the movers ofthe eye. 

Each of the fifth pair of nerves is in three branches, and proceedsmainly to the face. They are called tri-facial, and are mixed nerves, partly sensory and partly motor. The first branch is purely sensory, andgives sensibility to the eyeball. The second gives sensibility to thenose, gums, and cheeks. The third (mixed) gives the special sensation oftaste on the front part of the tongue, and ordinary sensation on the innerside of the cheek, on the teeth, and also on the scalp in front of theear. The motor branches supply the chewing muscles. 

The seventh pair, the facial, proceed to the face, where theyspread over the facial muscles and control their movements. Theeighth pair are the auditory, or nerves of hearing, and aredistributed to the special organs of hearing. 

The next three pairs of nerves all arise from the medulla, and escapefrom the cavity of the skull through the same foramen. They are sometimesdescribed as one pair, namely, the eighth, but it is more convenient toconsider them separately. 

The ninth pair, the glosso-pharyngeal, are partly sensory andpartly motor. Each nerve contains two roots: one a nerve of taste, whichspreads over the back part of the tongue; the other a motor nerve, whichcontrols the muscles engaged in swallowing. 

The tenth pair, the pneumogastric, also known as the vagusor wandering nerves, are the longest and most complex of all the cranialnerves. They are both motor and sensory, and are some of the mostimportant nerves in the body. Passing from the medulla they descend nearthe oesophagus to the stomach, sending off, on their way, branches tothe throat, the larynx, the lungs, and the heart. Some of their branchesrestrain the movements of the heart, others convey impressions to thebrain, which result in quickening or slowing the movements of breathing. Other branches pass to the stomach, and convey to the brain impressionswhich inform us of the condition of that organ. These are the nerves bywhich we experience the feelings of pain in the stomach, hunger, nausea, and many other vague impressions which we often associate with that organ. 

[Illustration: Fig. 117. --Anterior View of the Medulla Oblongata. 

 A, chiasm of the optic nerves; B, optic tracts; C, motor oculi communis; D, fifth nerve; E, motor oculi externus; F, facial nerve; H, auditory nerve; I, glosso-pharyngeal nerve; K, pneumogastric; L, spinal accessory; M, cervical nerves; N, upper extremity of spinal cord; O, decussation of the anterior pyramids; R, anterior pyramids of the medulla oblongata; S, pons Varolii. ]

The eleventh pair, the spinal accessory, are strictly motor, andsupply the muscles of the neck and the back. 

The twelfth pair, the hypoglossal, are also motor, pass to themuscles of the tongue, and help control the delicate movements in the actof speech. 

272. The Spinal Cord. This is a long, rod-like mass of white nervefibers, surrounding a central mass of gray matter. It is a continuation ofthe medulla oblongata, and is lodged in the canal of the spinal column. Itextends from the base of the skull to the lower border of the first lumbarvertebra, where it narrows off to a slender filament of gray substance. 

The spinal cord is from 16 to 18 inches long, and has about thethickness of one's little finger, weighing about 1-1/2 ounces. Like thebrain, it is enclosed in three membranes, which in fact are thecontinuation of those within the skull. They protect the delicate cord, and convey vessels for its nourishment. The space between the two innermembranes contains a small quantity of fluid, supporting the cord, as itwere in a water-bath. It is thus guarded against shocks. 

The cord is suspended and kept in position in the canal by delicateligaments at regular intervals between the inner and outer membranes. Finally, between the canal, enclosed by its three membranes, and the bonywalls of the spinal canal, there is considerable fatty tissue, a sort ofpacking material, imbedded in which are some large blood-vessels. 

273. Structure of the Spinal Cord. The arrangement of the parts ofthe spinal cord is best understood by a transverse section. Two fissures, one behind, the other in front, penetrate deeply into the cord, verynearly dividing it into lateral halves. In the middle of the isthmus whichjoins the two halves, is a very minute opening, the _central canal_ of thecord. This tiny channel, just visible to the naked eye, is connected withone of the openings of the medulla oblongata, and extends, as do theanterior and posterior fissures, the entire length of the cord. 

The spinal cord, like the brain, consists of gray and white matter, butthe arrangement differs. In the brain the white matter is within, and thegray matter is on the surface. In the cord the gray matter is arranged intwo half-moon-shaped masses, the backs of which are connected at thecentral part. The white matter, consisting mainly of fibers, running forthe most part in the direction of the length of the cord, is outside ofand surrounds the gray crescents. Thus each half or side of the cord hasits own gray crescent, the horns of which point one forwards and the otherbackwards, called respectively the anterior and posterior cornua or horns. 

It will also be seen that the white substance itself, in each half of thecord, is divided by the horns of the gray matter and by fibers passingfrom them into three parts, which are known as the anterior, posterior, and lateral columns. 

 Experiment 130. Procure at the market an uninjured piece of the spinal cord from the loin of mutton or the sirloin or the rib of beef. After noting its general character while fresh, put it to soak in dilute alcohol, until it is sufficiently hard to be cut in sections. 

274. The Spinal Nerves. From the gray matter on each side of thespinal cord 31 spinal nerves are given off and distributed chiefly tothe muscles and the skin. They pass out at regular intervals on each sideof the canal, by small openings between the vertebræ. Having escaped fromthe spine, they pass backwards and forwards, ramifying in the soft partsof the body. The first pair pass out between the skull and the atlas, thenext between the atlas and the axis, and so on down the canal. The eighthpair, called _cervical_, pass out in the region of the neck; twelve, called _dorsal_, in the region of the ribs; five are _lumbar_, and five_sacral_, while the last pair leave the cord near the coccyx. 

Each spinal nerve has two roots, one from the anterior, the otherfrom the posterior portion of the cord. These unite and run side byside, forming as they pass between the vertebræ one silvery thread, ornerve trunk. Although bound up in one bundle, the nerve fibers of the tworoots remain quite distinct, and perform two entirely different functions. 

After leaving the spinal cord, each nerve divides again and again intofiner and finer threads. These minute branches are distributed through themuscles, and terminate on the surface of the body. The anterior rootsbecome motor nerves, their branches being distributed to certainmuscles of the body, to control their movements. The posterior rootsdevelop into sensory nerves, their branches being distributed throughthe skin and over the surface of the body to become nerves of touch. Inbrief, the spinal nerves divide and subdivide, to reach with their twigsall parts of the body, and provide every tissue with a nerve center, astation from which messages may be sent to the brain. 

[Illustration: Fig. 118. --Side View of the Spinal Cord. (Showing thefissures and columns. )

 A, anterior median fissure; B, posterior median fissure; C, anterior lateral fissure; D, posterior lateral fissure; E, lateral column; F, anterior column; G, posterior column; H, posterior median column; K, anterior root; L, posterior root; M, ganglion of N, a spinal nerve. ]

275. The Functions of the Spinal Nerves. The messages which passalong the spinal nerves to and from the brain are transmitted mostlythrough the gray matter of the cord, but some pass along the white matteron the outer part. As in the brain, however, all the active powers of thecord are confined to the gray matter. The spinal nerves themselves havenothing to do with sensation or will. They are merely conductors to carrymessages to and fro. They neither issue commands nor feel a sensation. Hence, they consist entirely of white matter. 

276. Functions of the Spinal Cord. The spinal cord is the principalchannel through which all impulses from the trunk and extremities pass tothe brain, and all impulses to the trunk and extremities pass from thebrain. That is, the spinal cord receives from various parts of the bodyby means of its sensory nerves certain impressions, and conveys them tothe brain, where they are interpreted. 

The cord also transmits by means of its motor nerves the commands of thebrain to the voluntary muscles, and so causes movement. Thus, when thecord is divided at any point, compressed, as by a tumor or broken bone, ordisorganized by disease, the result is a complete loss of sensation andvoluntary movement below the point of injury. If by accident a man has hisspinal cord injured at some point, he finds he has lost all sensation andpower of motion below that spot. The impulse to movement started in hisbrain by the will does not reach the muscles he wishes to move, becausetraveling _down_ the spinal cord, it cannot pass the seat of injury. 

So the impression produced by pricking the leg with a pin, which, beforepain can be felt, must travel up the spinal cord to the brain, cannotreach the brain because the injury obstructs the path. The telegraph wirehas been cut, and the current can no longer pass. 

277. The Spinal Cord as a Conductor of Impulses. The identity instructure of the spinal nerves, whether motor or sensory, and the vastnumber of nerves in the cord make it impossible to trace for any distancewith the eye, even aided by the microscope and the most skillfuldissection, the course of nerve fibers. The paths by which the motorimpulses travel down the cord are fairly well known. These impulsesoriginate in the brain, and passing down keep to the same side of thecord, and go out by nerves to the same side of the body. 

The sensory impulses, however, soon after they enter the cord by thenerve of one side, cross in the cord to the opposite side, up which theytravel to the brain. Thus the destruction of one lateral half of the cordcauses paralysis of motion on the _same side_ as the injury, but loss ofsensation on the _opposite side_, because the posterior portion destroyedconsists of fibers which have crossed from the opposite side. 

Experiment proves that if both roots of a spinal nerve be cut, all thoseparts of the body to which they send branches become paralyzed, and haveneither sense of pain nor power of voluntary movement. The parts mighteven be cut or burned without pain. It is precisely like cutting atelegraph wire and stopping the current. 

[Illustration: Fig. 119. --The Base of the Brain. 

 A, anterior lobe of the cerebrum; B, olfactory nerve; C, sphenoid portion of the posterior lobe; D, optic chiasm; E, optic tract; F, abducens; H, M, hemispheres of the cerebellum; K, occipital portion of the occipital lobe; L, fissure separating the hemispheres; N, medulla oblongata; O, olivary body; P, antenor pyramids; R, pons Valoru; S, section of olfactory nerve, with the trunk removed to show sulcus in which it is lodged; T, anterior extremity of median fissure]

Experiment also proves that if only the posterior root of a spinal nervebe cut, all sensation is lost in the parts to which the nerve passes, butthe power of moving these parts is retained. But if the anterior rootalone be divided, all power of motion in the parts supplied by that nerveis lost, but sensation remains. From these and many other experiments, itis evident that those fibers of a nerve which are derived from theanterior root are motor, and those from the posterior root sensory, fibers. Impulses sent _from_ the brain and spinal cord to muscles will, therefore, pass along the anterior roots through those fibers of thenerves which are derived from these (motor) roots. On the other hand, impressions or sensations passing _to_ the brain will enter the spinalcord and reach the brain through the posterior or sensory roots. 

278. The Spinal Cord as a Reflex Center. Besides this function of thespinal cord as a great nerve conductor to carry sensations to the brain, and bring back its orders, it is also an independent center for whatis called reflex action. By means of its sensory nerves it receivesimpressions from certain parts of the body, and on its own authority sendsback instructions to the muscles by its motor nerves, without consultingthe brain. This constitutes reflex action, so called because theimpulse sent to the spinal cord by certain sensory nerves is at oncereflected or sent back as a motor impulse to the muscles. 

This reflex action is a most important function of the spinal cord. Thispower is possessed only by the gray matter of the cord, the whitesubstance being simply a conductor. 

The cells of gray matter are found all along the cord, but are groupedtogether in certain parts, notably in the cervical and lumbar regions. Thecells of the anterior horns are in relation with the muscles by means ofnerve fibers, and are also brought into connection with the skin and othersensory surfaces, by means of nerve fibers running in the posterior partof the cord. Thus there is established in the spinal cord, withoutreference to the brain at all, a reflex mechanism. 

279. Reflex Centers. For the purpose of illustration, we mightconsider the body as made up of so many segments piled one on another, each segment presided over by a similar segment of spinal cord. Eachbodily segment would have sensory and motor nerves corresponding to itsconnection with the spinal cord. The group of cells in each spinal segmentis intimately connected with the cells of the segments above and below. Thus an impression reaching the cells of one spinal segment might be sostrong as to overflow into the cells of other segments, and thus causeother parts of the body to be affected. 

Take as an example the case of a child who has eaten improper food, whichirritates its bowels. Sensory nerves of the bowels are disturbed, andpowerful impressions are carried up to a center in the spinal cord. Theseimpressions may now overflow into other centers, from which spasmodicdischarges of nerve energy may be liberated, which passing to the muscles, throw them into violent and spasmodic contraction. In other words, thechild has a fit, or convulsion. All this disturbance being the result ofreflex action (the spasmodic motions being quite involuntary, as the braintakes no part in them), the child meanwhile is, of course, entirelyunconscious and, however it may seem to be distressed, really suffers nopain. 

Scattered along the entire length of the spinal cord, especially in theupper part, are groups of nerve cells which preside over certain specificfunctions of animal life; that is, definite collections of cells whichcontrol definite functions. Thus there are certain centers for maintainingthe action of the heart, and the movements of breathing; and low down inthe cord, in the lumbar regions, are centers for the control of thevarious abdominal organs. 

Numerous other reflex centers are described by physiologists, but enoughhas been said to emphasize the great importance of the spinal cord as anindependent nerve center, besides its function as a conductor ofnervous impulses to and from the brain. 

280. The Brain as a Reflex Center. The brain, as we have juststated, is the seat of consciousness and intelligence. It is also the seatof many reflex, automatic, and coordinating centers. These give rise tocertain reflex actions which are as entirely independent of consciousnessas are those of the spinal cord. These acts take place independently ofthe will, and often without the consciousness of the individual. Thus, asudden flash of light causes the eyes to blink, as the result of reflexaction. The optic nerves serve as the sensory, and the facial nerves asthe motor, conductors. The sudden start of the whole body at some loudnoise, the instinctive dodging a threatened blow, and the springing backfrom sudden danger, are the results of reflex action. The result ensues inthese and in many other instances, without the consciousness of theindividual, and indeed beyond his power of control. 

281. The Importance of Reflex Action. Reflex action is thus amarvelous provision of nature for our comfort, health, and safety. Itsvast influence is not realized, as its numberless acts are so continuallygoing on without our knowledge. In fact, the greater part of nerve poweris expended to produce reflex action. The brain is thus relieved of a vastamount of work. It would be impossible for the brain to serve as a"thinking center" to control every act of our daily life. If we had toplan and to will every heart-beat or every respiration, the struggle forlife would soon be given up. 

The fact that the gray cells of the spinal cord can originate a countlessnumber of reflex and automatic activities is not only of great importancein protecting the body from injury, but increases vastly the range of theactivities of our daily life. 

Even walking, riding the bicycle, playing on a piano, and numberless othersuch acts may be reflex movements. To learn how, requires, of course, theaction of the brain, but with frequent repetition the muscles become soaccustomed to certain successive movements, that they are continued bythe cord without the control of the brain. Thus we may acquire a sort ofartificial reflex action, which in time becomes in a way a part of ourorganization, and is carried on without will power or even consciousness. 

So, while the hands are busily doing one thing, the brain can be intentlythinking of another. In fact, any attempt to control reflex action is moreapt to hinder than to help. In coming rapidly down stairs, the descentwill be made with ease and safety if the spinal cord is allowed entirecharge of the act, but the chances of stumbling or of tripping are verymuch increased if each step be taken as the result of the will power. Thereflex action of the cord may be diminished, or inhibited as it is called, but this power is limited. Thus, we can by an effort of the will stopbreathing for a certain time, but beyond that the reflex mechanismovercomes our will and we could not, if we would, commit suicide byholding our breath. When we are asleep, if the palm of the hand betickled, it closes; when we are awake we can prevent it. 

[Illustration: Fig. 120. --Dr. Waller's Diagrammatic Illustration of theReflex Process. 

From the sentient surface (1) an afferent impulse passes along (2) to theposterior root of the spinal cord, the nerve fibers of the posterior rootending in minute filaments among the small cells of this part of the cord(3). In some unknown way this impulse passes across the gray part of thecord to the large cells of the anterior root (5), the cells of this partbeing connected by their axis-cylinder with the efferent fibers (6). Theseconvey the stimulus to the fibers of the muscle (7), which accordinglycontract. Where the brain is concerned in the action the circuit is longerthrough S and M. ]

 Experiment 131. _To illustrate reflex action by what is called knee-jerk. _ Sit on a chair, and cross the right leg over the left one. With the tips of the fingers or the back of a book, strike the right ligamentum patellæ. The right leg will be raised and thrown forward with a jerk, owing to the contraction of the quadriceps muscles. An appreciable time elapses between the striking of the tendon and the jerk. The presence or absence of the knee-jerk may be a most significant symptom to the physician. 

282. The Sympathetic System. Running along each side of the spine, from the base of the skull to the coccyx, is a chain of nerve knots, organglia. These ganglia, twenty-four on each side, and their branchesform the sympathetic system, as distinguished from the cerebro-spinalsystem consisting of the brain and spinal cord and the nerves springingfrom them. The ganglia of the sympathetic system are connected with eachother and with the sensory roots of the spinal nerves by a network of graynerve fibers. 

At the upper end the chain of each side passes up into the cranium and isclosely connected with the cranial nerves. In the neck, branches pass tothe lungs and the heart. From the ganglia in the chest three nerves form acomplicated network of fibers, from which branches pass to the stomach, the liver, the intestines, the kidneys, and other abdominal organs. Asimilar network of fibers is situated lower down in the pelvis, from whichbranches are distributed to the pelvic organs. At the coccyx the twochains unite into a single ganglion. 

Thus, in general, the sympathetic system, while intimately connected withthe cerebro-spinal, forms a close network of nerves which speciallyaccompany the minute blood-vessels, and are distributed to the muscles ofthe heart, the lungs, the stomach, the liver, the intestines, and thekidneys--that is, the hollow organs of the body. 

283. The Functions of the Sympathetic System. This system exercises asuperintending influence over the greater part of the internal organs ofthe body, controlling to a certain extent the functions of digestion, nutrition, circulation, and respiration. The influence thusespecially connected with the processes of organic life is generallydifferent from, or even opposed to, that conveyed to the same organs byfibers running in the spinal or cranial nerves. These impulses are beyondthe control of the will. 

[Illustration: Fig. 121. --The Cervical and Thoracic Portion of theSympathetic Nerve and its Main Branches. 

 A, right pneumogastric; B, spinal accessory; C, glosso-pharyngeal; D, right bronchus; E, right branch of pulmonary artery; F, one of the intercostal nerves; H, great splanchnic nerve; K, solar plexus; L, left pneumogastric; M, stomach branches of right pneumogastric; N, right ventricle; O, right auricle; P, trunk of pulmonary artery; R, aorta; S, cardiac nerves; T, recurrent laryngeal nerve; U, superior laryngeal nerve; V, submaxillary ganglion; W, lingual branch of the 5th nerve; X, ophthalmic ganglion; Y, motor oculi externus. ]

Hence, all these actions of the internal organs just mentioned that arenecessary to the maintenance of the animal life, and of the harmony whichmust exist between them, are controlled by the sympathetic system. But forthis control, the heart would stop beating during sleep, digestion wouldcease, and breathing would be suspended. Gentle irritation of thesenerves, induced by contact of food in the stomach, causes that organ tobegin the churning motion needed for digestion. Various mental emotionsalso have a reflex action upon the sympathetic system. Thus, terrordilates the pupils, fear acts upon the nerves of the small blood-vesselsof the face to produce pallor, and the sight of an accident, or even theemotions produced by hearing of one, may excite nausea and vomiting. 

The control of the blood-vessels, as has been stated (sec. 195), isone of the special functions of the sympathetic system. Through the nervesdistributed to the muscular coats of the arteries, the caliber of thesevessels can be varied, so that at one moment they permit a large quantityof blood to pass, and at another will contract so as to diminish thesupply. This, too, is beyond the control of the will, and is brought aboutby the vaso-motor nerves of the sympathetic system through a reflexarrangement, the center for which is the medulla oblongata. 

284. Need of Rest. The life of the body, as has been emphasized inthe preceding chapters, is subject to constant waste going on everymoment, from the first breath of infancy to the last hour of old age. Weshould speedily exhaust our life from this continual loss, but for itsconstant renewal with fresh material. This exhaustion of life is increasedby exertion, and the process of repair is vastly promoted by rest. Thus, while exercise is a duty, rest is equally imperative. 

The eye, when exactingly used in fine work, should have frequent intervalsof rest in a few moments of darkness by closing the lids. The brain, whenurged by strenuous study, should have occasional seasons of rest by adash of cold water upon the forehead, and a brief walk with slow and deepinspirations of fresh air. The muscles, long cramped in a painfulattitude, should be rested as often as may be, by change of posture or bya few steps around the room. 

It is not entirely the amount of work done, but the continuity ofstrain that wears upon the body. Even a brief rest interrupts thisstrain; it unclogs the wheels of action. Our bodies are not designed forcontinuous toil. An alternation of labor and rest diminishes the waste oflife. The benign process of repair cannot go on, to any extent, duringstrenuous labor, but by interposing frequent though brief periods of rest, we lessen the amount of exhaustion, refresh the jaded nerves, and theremaining labor is more easily endured. 

285. Benefits of Rest. There is too little repose in our Americannature and in our modes of life. A sense of fatigue is the mute appeal ofthe body for a brief respite from labor, and the appeal should, ifpossible, be heeded. If this appeal be not met, the future exertionexhausts far more than if the body had been even slightly refreshed. Ifthe appeal be met, the brief mid-labor rest eases the friction of toil, and the remaining labor is more easily borne. The feeling that afive-minute rest is so much time lost is quite an error. It is a gain ofphysical strength, of mental vigor, and of the total amount of work done. 

The merchant burdened with the cares of business life, the soldier on thelong march, the ambitious student over-anxious to win success in hisstudies, the housewife wearied with her many hours of exacting toil, eachwould make the task lighter, and would get through it with less loss ofvital force, by occasionally devoting a few minutes to absolute rest inentire relaxation of the strained muscles and overtaxed nerves. 

286. The Sabbath as a Day of Physiological Rest. The divineinstitution of a Sabbath of rest, one day in seven, is based upon thehighest needs of our nature. Rest, to be most effective, should alternatein brief periods with labor. 

It is sound physiology, as well as good morals and manners, to ceasefrom the usual routine of six days of mental or physical work, and restboth the mind and the body on the seventh. Those who have succeeded bestin what they have undertaken, and who have enjoyed sound health during along and useful life, have studiously lived up to the mandates of thisgreat physiological law. It is by no means certain that the tendencynowadays to devote the Sabbath to long trips on the bicycle, tiresomeexcursions by land and sea, and sight-seeing generally, affords that realrest from a physiological point of view which nature demands after sixdays of well-directed manual or mental labor. 

287. The Significance of Sleep as a Periodical Rest. Of the chiefcharacteristics of all living beings none is so significant as theirperiodicity. Plants as well as animals exhibit this periodiccharacter. Thus plants have their annual as well as daily periods ofactivity and inactivity. Hibernating animals pass the winter in acondition of unconsciousness only to have their functions of activityrestored in early spring. Human beings also present many instances of aperiodic character, many of which have been mentioned in the precedingpages. Thus we have learned that the heart has its regular alternatingperiods of work and rest. After every expiration from the lungs there is apause before the next inspiration begins. 

Now sleep is just another manifestation of this periodic andphysiological rest by which Nature refreshes us. It is during the periodsof sleep that the energy expended in the activities of the waking hours ismainly renewed. In our waking moments the mind is kept incessantly activeby the demands made on it through the senses. There is a never-ceasingexpenditure of energy and a consequent waste which must be repaired. Atime soon comes when the brain cells fail to respond to the demand, andsleep must supervene. However resolutely we may resist this demand, Nature, in her relentless way, puts us to sleep, no matter what objectsare brought before the mind with a view to retain its attention. [41]

288. Effect of Sleep upon the Bodily Functions. In all the higheranimals, the central nervous system enters once at least in thetwenty-four hours into the condition of rest which we call sleep. Inasmuch as the most important modifications of this function are observedin connection with the cerebro-spinal system, a brief consideration of thesubject is properly studied in this chapter. In Chapter IV. We learnedthat repose was as necessary as exercise to maintain muscular vigor. Soafter prolonged mental exertion, or in fact any effort which involves anexpenditure of what is often called nerve-force, sleep becomes anecessity. The need of such a rest is self-evident, and the loss of it isa common cause of the impairment of health. While we are awake and active, the waste of the body exceeds the repair; but when asleep, the waste isdiminished, and the cells are more actively rebuilding the structure forto-morrow's labor. The organic functions, such as are under the directcontrol of the sympathetic nervous system, --circulation, respiration, anddigestion, --are diminished in activity during sleep. The pulsations of theheart and the respiratory movements are less frequent, and thecirculation is slower. The bodily temperature is reduced, and the cerebralcirculation is diminished. The eyes are turned upward and inward, and thepupils are contracted. 

The senses do not all fall to sleep at once, but drop off successively:first the sight, then the smell, the taste, the hearing and lastly thetouch. The sleep ended, they awake in an inverse order, touch, hearing, taste, smell, and sight. 

289. The Amount of Sleep Required. No precise rule can be laid downconcerning the amount of sleep required. It varies with age, occupation, temperament, and climate to a certain extent. An infant whose mainbusiness it is to grow spends the greater part of its time in sound sleep. Adults of average age who work hard with their hands or brain, underperfectly normal physiological conditions, usually require at least eighthours of sleep. Some need less, but few require more. Personalpeculiarities, and perhaps habit to a great extent, exert a markedinfluence. Some of the greatest men, as Napoleon I. , have been verysparing sleepers. Throughout his long and active life, Frederick the Greatnever slept more than five or six hours in the twenty-four. On the otherhand, some of the busiest brain-workers who lived to old age, as WilliamCullen Bryant and Henry Ward Beecher, required and took care to secure atleast eight or nine hours of sound sleep every night. 

In old age, less sleep is usually required than in adult life, while theaged may pass much of their time in sleep. In fact, each person learns byexperience how much sleep is necessary. There is no one thing which moreunfits one for prolonged mental or physical effort than the loss ofnatural rest. 

290. Practical Rules about Sleep. Children should not be played withboisterously just before the bedtime hour, nor their minds excited withweird goblin stories, or a long time may pass before the wide-open eyesand agitated nerves become composed to slumber. Disturbed or insufficientsleep is a potent factor towards producing a fretful, irritable child. 

At all ages the last hour before sleep should, if possible, be spentquietly, to smooth the way towards sound and refreshing rest. The sleepinduced by medicine is very often troubled and unsatisfactory. Medicinesof this sort should not be taken except on the advice of a physician. 

While a hearty meal should not usually be taken just before bedtime, it isnot well to go to bed with a sense of positive faintness and hunger. Rather, one should take a very light lunch of quite simple food as asupport for the next eight hours. 

[Illustration: Fig. 122. --Trunk of the Left Pneumogastric. 

(Showing its distribution by its branches and ganglia to the larynx, pharynx, heart, lungs, and other parts. )]

It is better, as a rule, not to engage in severe study during the hoursjust before bedtime. Neither body nor mind being at its best after thefatigues of the day, study at that time wears upon the system more, andthe progress is less than at earlier hours. One hour of morning or daystudy is worth a much longer time late at night. It is, therefore, aneconomy both of time and of nerve force to use the day hours and the earlyevening for study. 

The so-called "cat naps" should never be made to serve as a substitute fora full night's sleep. They are largely a matter of habit, and aredetrimental to some as well as beneficial to others. Late hours areusually associated with exposure, excitement, and various other drainsupon the nerve force, and hence are injurious. 

It is better to sleep on one or other side than on the back. The headshould be somewhat raised, and a mattress is better than a feather bed. The bedclothes should be sufficient, but not too heavy. Light tends toprevent sleep, as do loud or abrupt sounds, but monotonous sounds aid it. 

291. Alcohol and the Brain. The unfortunate effects which alcoholicdrinks produce upon the brain and nervous system differ from thedestructive results upon other parts of the body in this respect, thatelsewhere the consequences are usually both less speedy and less obvious. The stomach, the liver, and even the heart may endure for a while thetrespass of the narcotic poison, and not betray the invasion. But thenervous system cannot, like them, suffer in silence. 

In the other parts of the body the victim may (to a certain extent)conceal from others the suffering of which he himself is painfullyconscious. But the tortured brain instantly reveals the calamity and theshame, while the only one who may not fully realize it is the victimhimself. Besides this, the injuries inflicted upon other organs affectonly the body, but here they drag down the mind, ruin the morals, anddestroy the character. 

The brain is indeed the most important organ of the body, as it presidesover all the others. It is the lofty seat of power and authority. Here theking is on his throne. But if, by this malignant adversary, the kinghimself be dethroned, his whole empire falls to ruins. 

292. How Alcohol Injures the Brain. The brain, the nerve centers, and the nerves are all made up of nerve pulp, the softest and mostdelicate tissue in the whole bodily structure. Wherever this fragilematerial occurs in our bodies, --in the skull, the spine, the trunk, or thelimbs, --the all-wise Architect has carefully protected it from violence, for a rough touch would injure it, or even tender pressure would disturbits function. 

It is a further indication of the supreme importance of the brain, thatabout one-fifth of the entire blood of the body is furnished to it. Manifestly, then, this vital organ must be tenderly cared for. It mustindeed be well nourished, and therefore the blood sent to it must behighly nutrient, capable of supplying oxygen freely. This condition isessential to successful brain action. But intoxicants bring to it bloodsurcharged with a poisonous liquid, and bearing only a limited supply ofoxygen. 

Another condition of a healthy brain is that the supply of blood to itshall be equable and uniform. But under the influence of strong drink, theblood pours into the paralyzed arteries a surging tide that floods thehead, and hinders and may destroy the use of the brain and the senses. Still another requirement is that whatever is introduced into the cerebraltissues, having first passed through the stomach walls and thence into theblood, shall be bland, not irritating. But in the brain of the inebriateare found not only the distinct odor but the actual presence of alcohol. Thus we plainly see how all these three vital conditions of a healthybrain are grossly violated by the use of intoxicants. 

 "I think there is a great deal of injury being done by the use of alcohol in what is supposed by the consumer to be a most moderate quantity, to persons who are not in the least intemperate, and to people supposed to be fairly well. It leads to degeneration of the tissues; it damages the health; it injures the intellect. Short of drunkenness, that is, in those effects of it which stop short of drunkenness, I should say from my experience that alcohol is the most destructive agent we are aware of in this country. "--Sir William Gull, the most eminent English physician of our time. 

293. Why the Brain Suffers from the Alcoholic Habit. We do not findthat the alcoholic habit has produced in the brain the same coarseinjuries that we see in other organs, as in the stomach, the liver, or theheart. Nor should we expect to find them; for so delicate and so sensitiveis the structure of this organ, that a very slight injury here goes agreat way, --a disturbance may be overwhelming to the brain that would beonly a trifle to some of the less delicate organs. 

Alcohol has different degrees of affinity for different organs of thebody, but much the strongest for the cerebral tissues. Therefore the brainfeels more keenly the presence of alcohol than does any other organ. Almost the moment that the poison is brought into the stomach, the nervessend up the alarm that an invading foe has come. At once there follows ashock to the brain, and very soon its paralyzed blood-vessels aredistended with the rush of blood. This first effect is, in a certainsense, exhilarating, and from this arousing influence alcohol has beenerroneously considered a stimulant; but the falsity of this view ispointed out elsewhere in this book. 

294. Alcohol, the Enemy of Brain Work. The healthy brain contains alarger proportion of water than does any other organ. Now alcohol, withits intense affinity for water, absorbs it from the brain, and thuscondenses and hardens its structure. One of the important elements of thebrain is its albumen; this also is contracted by alcohol. The nerve cellsand fibers gradually become shriveled and their activity is lowered, theelasticity of the arteries is diminished, the membranes enveloping thebrain are thickened, and thus all proper brain nutrition is impaired. Theentire organ is slowly hardened, and becomes unfitted for the properperformance of its delicate duties. In brief, alcohol in any and everyform is the enemy of successful and long-continued brain work. 

[Illustration: Fig. 123. --Nerve Trunks of the Right Arm. ]

295. Other Physical Results of Intoxicants. What are some of thephysical results observed? First, we note the failure of the vaso-motornerves to maintain the proper tone of the blood-vessels, as in the turgidface and the congested cornea of the eye. Again, we observe the loss ofmuscular control, as is shown by the drop of the lower lip, the thickenedspeech, and the wandering eye. The spinal cord, too, is often affected andbecomes unable to respond to the demand for reflex action, as appears fromthe trembling hands, the staggering legs, the swaying body, and thegeneral muscular uncertainty. All these are varied results of thetemporary paralysis of the great nerve centers. 

Besides, the sensibility of the nerves is deadened. The inebriate mayseize a hot iron and hardly know it, or wound his hand painfully and neverfeel the injury. The numbness is not of the skin, but of the brain, forthe drunken man may be frozen or burned to death without pain. The senses, too, are invaded and dulled. Double vision is produced, the eyes not beingso controlled as to bring the image upon corresponding points of theretina. 

296. Diseases Produced by Alcohol. The diseases that follow in thetrain of the alcoholic habit are numerous and fatal. It lays itsparalyzing hand upon the brain itself, and soon permanently destroys theintegrity of its functions. In some the paralysis is local only, perhapsin one of the limbs, or on one side of the body; in others there is ageneral muscular failure. The vitality of the nerve centers is sothoroughly impaired that general paralysis often ensues. A condition ofinsomnia, or sleeplessness, often follows, or when sleep does come, it isin fragments, and is far from refreshing to the jaded body. 

In time follows another and a terrible disease known as _deliriumtremens_; and this may occur in those who claim to be only moderatedrinkers, rarely if ever intoxicated. It accompanies an utter breakdown ofthe nervous system. Here reason is for the time dethroned, while at sometimes wild and frantic, or at others a low, mumbling delirium occurs, witha marked trembling from terror and exhaustion. 

There is still another depth of ruin in this downward course, and that is_insanity_. In fact, every instance of complete intoxication is a case oftemporary insanity, that is, of mental unsoundness with loss ofself-control. Permanent insanity may be one of the last results ofintemperance. Alcoholism sends to our insane asylums a large proportion oftheir inmates, as ample records testify. 

297. Mental and Moral Ruin Caused by Alcoholism. Alcoholism, the evilprince of destroyers, also hastens to lay waste man's mental and moralnature. Just as the inebriate's senses, sight, hearing, and touch, fail toreport correctly of the outer world, so the mind fails to preside properlyover the inner realm. Mental perceptions are dulled. The stupefiedfaculties can hardly be aroused by any appeal. Memory fails. Thus the manis disqualified for any responsible labor. No railroad company, nomercantile house, will employ any one addicted to drinking. The mind ofthe drunkard is unable to retain a single chain of thought, but gropesabout with idle questionings. The intellect is debased. Judgment isimpossible, for the unstable mind cannot think, compare, or decide. 

The once active power of the will is prostrate, and the victim can nolonger resist the feeblest impulse of temptation. The grand faculty ofself-control is lost; and as a result, the baser instincts of our lowernature are now uppermost; greed and appetite rule unrestrained. 

But the moral power is also dragged down to the lowest depths. All thefiner sensibilities of character are deadened; all pride of personalappearance, all nice self-respect and proper regard for the good opinionof others, every sense of decorum, and at last every pretence of decency. Dignity of behavior yields to clownish silliness, and the person latelyrespected is now an object of pity and loathing. The great centralconvictions of right and wrong now find no place in his nature; conscienceis quenched, dishonesty prevails. This is true both as to the solemnpromises, which prove mere idle tales, and also as to property, for heresorts to any form of fraud or theft to feed the consuming craving formore drink. 

298. Evil Results of Alcoholism Inherited. But the calamity does notend with the offender. It may follow down the family line, and fastenitself upon the unoffending children. These often inherit the craving fordrink, with the enfeebled nature that cannot resist the craving, and soare almost inevitably doomed to follow the appalling career of theirparents before them. 

Nor does this cruel taint stop with the children. Even their descendantsare often prone to become perverse. As one example, careful statistics ofa large number of families, more than two hundred descended fromdrunkards, show that a very large portion of them gave undoubted proof ofwell-marked degeneration. This was plain in the unusual prevalence ofinfant mortality, convulsions, epilepsy, hysteria, fatal brain diseases, and actual imbecility. [42]

It is found that the long-continued habitual user of alcoholic drinks, theman who is never intoxicated, but who will tell you that he has drunkwhiskey all his life without being harmed by it, is more likely totransmit the evil effects to his children than the man who has occasionaldrunken outbreaks with intervals of perfect sobriety between. By hisfrequently repeated small drams he keeps his tissues constantly"alcoholized" to such an extent that they are seldom free from some of themore or less serious consequences. His children are born with organismswhich have received a certain bias from which they cannot escape; they arefreighted with some heredity, or predisposition to particular forms ofdegeneration, to some morbid tendency, to an enfeebled constitution, tovarious defective conditions of mind and body. Let the children of such aman attempt to imitate the drinking habits of the father and they quicklyshow the effects. Moderate drinking brings them down. 

Among other consequences of an alcoholic inheritance which have beentraced by careful observers are: Morbid changes in the nerve centers, consisting of inflammatory lesions, which vary according to the age inwhich they occur; alcoholic insanity; congenital malformations; and a muchhigher infant death rate, owing to lack of vitality, than among thechildren of normal parents. 

Where the alcoholic inheritance does not manifest itself in some definitedisease or disorder, it can still be traced in the limitations to be foundin the drinking man's descendants. They seem to reach a level from whichthey cannot ascend, and where from slight causes they deteriorate. Theparents, by alcoholic poisoning, have lowered the race stock of vitalitybeyond the power of ascent or possibility to rise above or overcome thedownward tendency. 

Of course these effects of alcoholics differ widely according to thedegree of intoxication. Yet, we must not forget that the real nature ofinebriety is always the same. The end differs from the beginning only indegree. He who would avoid a life of sorrow, disgrace, and shame mustcarefully shun the very first glass of intoxicants. 

299. Opium. Opium is a gum-like substance, the dried juice of theunripe capsule of the poppy. The head of the plant is slit with fineincisions, and the exuding white juice is collected. When it thickens andis moulded in mass, it becomes dark with exposure. _Morphine_, a whitepowder, is a very condensed form of opiate; _laudanum_, an alcoholicsolution of marked strength; and _paregoric_, a diluted and flavored formof alcoholic tincture. 

300. Poisonous Effects of Opium. Some persons are drawn into the useof opium, solely for its narcotic and intoxicating influence. Every early consent to its use involves a lurking pledge to repeat thepoison, till soon strong cords of the intoxicant appetite bind the nowyielding victim. 

Opium thus used lays its benumbing hand upon the brain, the mind isbefogged, thought and reasoning are impossible. The secretions of thestomach are suspended, digestion is notably impaired, and the gastricnerves are so deadened that the body is rendered unconscious of its needs. 

The moral sense is extinguished, persons once honest resort to fraud andtheft, if need be, to obtain the drug, till at last health, character, andlife itself all become a pitiful wreck. 

301. The Use of Opium in Patent Medicines. Some forms of this drugare found in nearly all the various patent medicines so freely sold as acure-all for every mortal disease. Opiates are an ingredient in differentforms and proportions in almost all the soothing-syrups, cough medicines, cholera mixtures, pain cures, and consumption remedies, so widely andunwisely used. Many deaths occur from the use of these opiates, which atfirst seem indeed to bring relief, but really only smother the prominentsymptoms, while the disease goes on unchecked, and at last proves fatal. 

These patent medicines may appear to help one person and be fraught withdanger to the next, so widely different are the effects of opiates upondifferent ages and temperaments. But it is upon children that these fatalresults oftenest fall. Beyond doubt, thousands of children have beensoothed and soothed out of existence. [43]

302. The Victim of the Opium Habit. Occasionally persons convalescingfrom serious sickness where anodynes were taken, unwisely cling to themlong after recovery. Other persons, jaded with business or with worry, andunable to sleep, unwisely resort to some narcotic mixture to procure rest. In these and other similar cases, the use of opiates is always mostpernicious. The amount must be steadily increased to obtain the elusiverepose, and at best the phantom too often escapes. 

Even if the desired sleep is procured, it is hardly the coveted rest, buta troubled and dreamy slumber, leaving in the morning the body quiteunrefreshed, the head aching, the mouth dry, and the stomach utterlydevoid of appetite. But far worse than even this condition is the slavishyielding to the habit, which soon becomes a bondage in which life is shornof its wholesome pleasures, and existence becomes a burden. 

303. Chloral. There are other preparations which have becomeinstruments of direful and often fatal injury. Chloral is a powerfuldrug that has been much resorted to by unthinking persons to producesleep. Others, yielding to a morbid reluctance to face the problems oflife, have timidly sought shelter in artificial forgetfulness. To all suchit is a false friend. Its promises are treason. It degrades the mind, tramples upon the morals, overpowers the will, and destroys life itself. 

304. Cocaine, Ether, Chloroform, and Other Powerful Drugs. Anotherdangerous drug is Cocaine. Ether and chloroform, those pricelessblessings to the human race if properly controlled, become instruments ofdeath when carelessly trifled with. Persons who have been accustomed toinhale the vapor in slight whiffs for neuralgia or similar troubles do soat imminent hazard, especially if lying down. They are liable to becomeslowly unconscious, and so to continue the inhalation till life is ended. 

There is still another class of drugs often carelessly used, whose effect, while less directly serious than those mentioned, is yet far fromharmless. These drugs, which have sprung into popular use since thedisease _la grippe_ began its dreaded career, include _phenacetine_, _antipyrine_, _antifebrine_, and other similar preparations. These drugshave been seized by the public and taken freely and carelessly for allsorts and conditions of trouble. The random arrow may yet do serious harm. These drugs, products of coal-oil distillation, are powerful depressants. They lower the action of the heart and the tone of the nervous centers. Thus the effect of their continued use is to so diminish the vigor of thesystem as to aggravate the very disorder they are taken to relieve. 

305. Effect of Tobacco on the Nervous System. That the use of tobaccoproduces a pernicious effect upon the nervous system is obvious from theindignant protest of the entire body against it when it is first used. Itspoisonous character is amply shown by the distressing prostration andpallor, the dizziness and faintness, with extreme nausea and vomiting, which follow its employment by a novice. 

The morbid effects of tobacco upon the nervous system of those whohabitually use it are shown in the irregular and enfeebled action of theheart, with dizziness and muscular tremor. The character of the pulseshows plainly the unsteady heart action, caused by partial paralysis ofthe nerves controlling this organ. Old, habitual smokers often show anirritable and nervous condition, with sleeplessness, due doubtless to lackof proper brain nutrition. 

All these results tend to prove that tobacco is really a nerve poison, andthere is reason to suspect that the nervous breakdown of many men inmature life is often due to the continued use of this depressing agent. This is shown more especially in men of sedentary life and habits, as menof active habits and out-door life, experience less of the ill effects oftobacco. 

Few, if any, habitual users of tobacco ever themselves approve of it. Theyall regret the habit, and many lament they are so enslaved to it that theycannot throw it off. They very rarely advise any one to follow theirexample. 

306. Effects of Tobacco on the Mind. With this continuouslydepressing effect of tobacco upon the brain, it is little wonder that themind may become enfeebled and lose its capacity for study or successfuleffort. This is especially true of the young. The growth and developmentof the brain having been once retarded, the youthful user of tobacco(especially the foolish cigarette-smoker) has established a permanentdrawback which may hamper him all his life. 

The young man addicted to the use of tobacco is often through its useretarded in his career by mental languor or weakening will power, and bymental incapacity. The keenness of mental perception is dulled, and theability to seize and hold an abstract thought is impaired. True, theseeffects are not sharply obvious, as it would be impossible to contrast thepresent condition of any one person with what it might have been. But thecomparison of large numbers conveys an instructive lesson. Scholars whostart well and give promise of a good future fail by the way. The honorsof the great schools, academies, and colleges are very largely taken bythe tobacco abstainers. This is proved by the result of repeated andextensive comparisons of the advanced classes in a great number ofinstitutions in this country and in Europe. So true is this that any youngman who aspires to a noble career should bid farewell either to hishonorable ambition or to his tobacco, for the two very rarely traveltogether. Consequently our military and naval academies and very manyseminaries and colleges prohibit the use of tobacco by their students. Forthe same reasons the laws of many states very properly forbid the sale toboys of tobacco, and especially of cigarettes. 

307. Effect of Tobacco upon Character. Nor does tobacco spare themorals. The tobacco-user is apt to manifest a selfish disregard of thecourtesies due to others. He brings to the presence of others a repulsivebreath, and clothing tainted with offensive odors. He poisons theatmosphere that others must inhale, and disputes their rights to breathe apure, untainted air. The free use of tobacco by young people dulls theacuteness of the moral senses, often leads to prevarication and deceit inthe indulgence, and is apt to draw one downward to bad associates. It isnot the speed but the direction that tells on the future character anddestiny of young men. 

Additional Experiments. 

 Experiment 132. _To illustrate the cooperation of certain parts of the body. _ Tickle the inside of the nose with a feather. This does not interfere with the muscles of breathing, but they come to the help of the irritated part, and provoke sneezing to clear and protect the nose. 

 Experiment 133. Pretend to aim a blow at a person's eye. Even if he is warned beforehand, the lids will close in spite of his effort to prevent them. 

 Experiment 134. _To illustrate how sensations are referred to the ends of the nerves_. Strike the elbow end of the ulna against anything hard (commonly called "hitting the crazy bone") where the ulna nerve is exposed, and the little finger and the ring finger will tingle and become numb. 

 Experiment 135. _To show that every nerve is independent of any other. _ Press two fingers closely together. Let the point of the finest needle be carried ever so lightly across from one finger to another, and we can easily tell just when the needle leaves one finger and touches the other. 

 Experiment 136. _To paralyze a nerve temporarily_. Throw one arm over the sharp edge of a chair-back, bringing the inner edge of the biceps directly over the edge of the chair. Press deep and hard for a few minutes. The deep pressure on the nerve of the arm will put the arm "asleep, " causing numbness and tingling. The leg and foot often "get asleep" by deep pressure on the nerves of the thigh. 

 Experiment 137. Press the ulnar nerve at the elbow, the prickling sensation is referred to the skin on the ulnar side of the hand. 

 Experiment 138. Dip the elbow in ice-cold water; at first one feels the sensation of cold, owing to the effect on the cutaneous nerve-endings. Afterwards, when the trunk of the ulnar nerve is affected, pain is felt in the skin of the ulnar side of the hand, where the nerve terminates. 

Chapter XI. 

The Special Senses. 

308. The Special Senses. In man certain special organs are set apartthe particular duty of which is to give information of the nature of therelations which he sustains to the great world of things, and of which heis but a mere speck. The special senses are the avenues by which we obtainthis information as to our bodily condition, the world around us, and themanner in which it affects us. 

Animals high in the scale are affected in so many different ways, and byso many agencies, that a subdivision of labor becomes necessary that thesense avenues may be rigidly guarded. One person alone may be a sufficientwatch on the deck of a sloop, but an ocean steamer needs a score or moreon guard, each with his special duty and at his own post. Or the sensesare like a series of disciplined picket-guards, along the outposts of themind, to take note of events, and to report to headquarters anyinformation which may be within the range of their duty. 

Thus it is that we are provided with a number of special senses, bymeans of which information is supplied regarding outward forces andobjects. These are touch, taste, smell, seeing, and hearing, towhich may be added the muscular sense and a sense of temperature. 

309. General Sensations. The body, as we have learned, is made up ofa great number of complicated organs, each doing its own part of thegeneral work required for the life and vigor of the human organism. Theseorgans should all work in harmony for the good of the whole. We must havesome means of knowing whether this harmony is maintained, and of receivingtimely warning if any organ fails to do its particular duty. 

Such information is supplied by the common or general sensations. Thus we have a feeling of hunger or thirst indicating the need of food, and a feeling of discomfort when imperfectly clad, informing us of theneed of more clothing. 

To these may be added the sensation of pain, tickling, itching, and so on, the needs of which arise from the complicated structure of the human body. The great majority of sensations result from some stimulus oroutward agency; and yet some sensations, such as those of faintness, restlessness, and fatigue seem to spring up within us in some mysteriousway, without any obvious cause. 

310. Essentials of a Sense Organ. Certain essentials are necessaryfor a sensation. First, there is a special structure adapted to aparticular kind of influence. Thus the ear is formed specially for beingstimulated by the waves of sound, while the eye is not influenced bysound, but responds to the action of light. These special structures arecalled terminal organs. 

Again, a nerve proceeds from the special structure, which is in directcommunication with nerve cells in the brain at the region ofconsciousness. This last point is important to remember, for if onsome account the impression is arrested in the connecting nerve, nosensation will result. Thus a man whose spine has been injured may notfeel a severe pinch on either leg. The impression may be quite sufficientto stimulate a nerve center in a healthy cord, so as to produce a markedreflex act, but he has no sensation, because the injury has prevented theimpression from being carried up the cord to the higher centers in thebrain. 

311. The Condition of Sensation. It is thus evident that while animpression may be made upon a terminal organ, it cannot strictly be calleda sensation until the person becomes conscious of it. The consciousnessof an impression is, therefore, the essential element of a sensation. 

It follows that sensation may be prevented in various ways. In the senseof sight, for example, one person may be blind because the terminal organ, or eye, is defective or diseased. Another may have perfect eyes and yethave no sight, because a tumor presses on the nerve between the eye andthe brain. In this case, the impression fails because of the break in thecommunication. Once more, the eye may be perfect and the nerve connectionunbroken, and yet the person cannot see, because the center in the brainitself is injured from disease or accident, and cannot receive theimpression. 

312. The Functions of the Brain Center in the Perception of anImpression. Sensation is really the result of a change which occurs ina nerve center in the brain, and yet we refer impressions to the variousterminal organs. Thus, when the skin is pinched, the sensation is referredto the skin, although the perception is in the brain. We may think it isthe eyes that see objects; in reality, it is only the brain that takesnote of them. 

This is largely the result of education and habit. From a blowon the head one sees flashes of light as vividly as if torches actuallydance before the eyes. Impressions have reached the seeing-center in thebrain from irritation of the optic nerve, producing the same effect asreal lights would cause. In this case, however, knowing the cause of thecolors, the person is able to correct the erroneous conclusion. 

As a result of a depraved condition of blood, the seeing-center itself maybe unduly stimulated, and a person may see objects which appear real. Thusin an attack of delirium tremens, the victim of alcoholic poisoning seeshorrible and fantastic creatures. The diseased brain refers them as usualto the external world; hence they appear real. As the sufferer's judgmentis warped by the alcoholic liquor, he cannot correct the impressions, andis therefore deceived by them. 

313. Organs of Special Sense. The organs of special sense, the meansby which we are brought into relation with surrounding objects, areusually classed as five in number. They are sometimes fancifully called"the five gateways of knowledge"--the skin, the organ of touch; thetongue, of taste; the nose, of smell; the eye, of sight;and the ear, of hearing. 

[Illustration: Fig. 124. --Magnified View of a Papilla of the Skin, with aTouch Corpuscle. ]

314. The Organ of Touch. The organ of touch, or tactile sensibility, is the most widely extended of all the special senses, and perhaps thesimplest. It is certainly the most precise and certain in its results. Itis this sense to which we instinctively appeal to escape from theillusions into which the other senses may mislead us. It has its seat inthe skin all over the body, and in the mucous membrane of the nostrils. All parts of the body, however, do not have this sense in an equal degree. 

In Chapter IX. We learned that the superficial layers of the skin coversand dips in between the papillæ. We also learned that these papillæ arerichly provided with blood-vessels and sensory nerve fibers (sec. 234). Now these nerve fibers terminate in a peculiar way in those parts of thebody which are endowed with a very delicate sense of touch. In everypapilla are oval-shaped bodies about 1/300 of an inch long, around whichthe nerve fibers wind, and which they finally enter. These are calledtouch-bodies, or tactile corpuscles, and are found in greatnumbers on the feet and toes, and more scantily in other places, as on theedges of the eyelids. 

Again, many of the nerve fibers terminate in corpuscles, the largest about1/20 of an inch long, called Pacinian corpuscles. These are mostnumerous in the palm of the hand and the sole of the foot. In the papillæof the red border of the lips the nerves end in capsules which enclose oneor more fibers, and are called end-bulbs. 

The great majority of the nerve fibers which supply the skin do not end insuch well-defined organs. They oftener divide into exceedingly delicatefilaments, the terminations of which are traced with the greatestdifficulty. 

315. The Sense of Touch. Touch is a sensation of contact referred tothe surface of the body. It includes three things, --the sense ofcontact, the sense of pressure, and the sense of heat andcold. 

The sense of contact is the most important element in touch. By it welearn of the form, size, and other properties of objects, as theirsmoothness and hardness. As we all know, the sense of touch varies indifferent parts of the skin. It is most acute where the outer skin isthinnest. The tips of the fingers, the edges of the lips, and the tip ofthe tongue are the most sensitive parts. 

Even the nails, the teeth, and the hair have the sense of touch in aslight degree. When the scarf skin is removed, the part is not moresensitive to sense of contact. In fact, direct contact with theunprotected true skin occasions pain, which effectually masks the feelingof touch. The sense of touch is capable of education, and is generallydeveloped to an extraordinary degree in persons who are deprived of someother special sense, as sight or hearing. We read of the famous blindsculptor who was said to model excellent likenesses, guided entirely bythe sense of touch. An eminent authority on botany was a blind man, ableto distinguish rare plants by the fingers, and by the tip of the tongue. The blind learn to read with facility by passing their fingers over raisedletters of a coarse type. It is impossible to contemplate, even for amoment, the prominence assigned to the sense of touch in the physicalorganism, without being impressed with the manifestations of design--thework of an all-wise Creator. 

316. Muscular Sense; Sense of Temperature; Pain. When a heavy objectis laid upon certain parts of the body, it produces a sensation ofpressure. By it we are enabled to estimate differences of weight. Ifan attempt be made to raise this object, it offers resistance which themuscles must overcome. This is known as the muscular sense. Itdepends on sensory nerves originating in the muscles and carryingimpressions from them to the nerve centers. 

The skin also judges, to a certain extent, of heat and cold. These sensations can be felt only by the skin. Direct irritation of anerve does not give rise to them. Thus, the exposed pulp of a diseasedtooth, when irritated by cold fluids, gives rise to pain, and not to asensation of temperature. Various portions of the body have differentdegrees of sensibility in this respect. The hand will bear a degree ofheat which would cause pain to some other parts of the body. Then, again, the sensibility of the outer skin seems to affect the sensibility to heat, for parts with a thin skin can bear less heat than portions with a thickcuticle. 

 Experiment 139. _To illustrate how the sense of touch is a matter of habit or education_. Shut both eyes, and let a friend run the tips of your fingers first lightly over a hard plane surface; then press hard, then lightly again, and the surface will seem to be concave. 

 Experiment 140. Cross the middle over the index finger, roll a small marble between the fingers; one has a distinct impression of two marbles. Cross the fingers in the same way, and rub them against the point of the nose. A similar illusion is experienced. 

 Experiment 141. _To test the sense of locality_. Ask a person to shut his eyes, touch some part of his body lightly with the point of a pin, and ask him to indicate the part touched. 

As to the general temperature, this sense is relative and is muchmodified by habit, for what is cold to an inhabitant of the torrid zonewould be warm to one accustomed to a very cold climate. 

Pain is an excessive stimulation of the sensory nerves, and in it allfiner sensations are lost. Thus, when a piece of hot iron burns the hand, the sensation is the same as when the iron is very cold, and extreme coldfeels like intense heat. 

317. The Organ of Taste. The sense of taste is located chieflyin the tongue, but may also be referred even to the regions of the fauces. Taste, like touch, consists in a particular mode of nerve termination. 

The tongue is a muscular organ covered with mucous membrane, and isrichly supplied with blood-vessels and nerves. By its complicatedmovements it is an important factor in chewing, in swallowing, and inarticulate speech. The surface of the tongue is covered with irregularprojections, called papillæ, --fine hair-like processes, about 1/12 ofan inch high. Interspersed with these are the fungiform papillæ. These are shaped something like a mushroom, and may often be detected bytheir bright red points when the rest of the tongue is coated. 

Towards the root of the tongue is another kind of papillæ, thecircumvallate, eight to fifteen in number, arranged in the form ofthe letter V, with the apex directed backwards. These are so calledbecause they consist of a fungiform papilla surrounded by a fold of mucousmembrane, presenting the appearance of being walled around. 

In many of the fungiform and most of the circumvallate papillæ arepeculiar structures called taste buds or taste goblets. Theseexist in great numbers, and are believed to be connected with nervefibers. These taste buds are readily excited by savory substances, andtransmit the impression along the connected nerve. 

The tongue is supplied with sensory fibers by branches from the fifth andeighth pairs of cranial nerves. The former confers taste on the front partof the tongue, and the latter on the back part. Branches of the latteralso pass to the soft palate and neighboring parts and confer taste onthem. The motor nerve of the tongue is the ninth pair, the hypoglossal. 

[Illustration: Fig. 125. --The Tongue. 

 A, epiglottis; B, glands at the base of tongue; C, tonsil; D, median circumvallate papilla, E, circumvallate papillæ; F, filiform papillæ; H, furrows on border of the tongue; K, fungiform papillæ. ]

318. The Sense of Taste. The sense of taste is excited by stimulationof the mucous membrane of the tongue and of the palate, affecting the endsof the nerve fibers. Taste is most acute in or near the circumvallatepapillæ. The middle of the tongue is scarcely sensitive to taste, whilethe edges and the tip are, as a rule, highly sensitive. 

Certain conditions are necessary that the sense of taste may beexercised. First, the substance to be tasted must be in _solution_, or besoluble in the fluids of the mouth. Insoluble substances are tasteless. Ifwe touch our tongue to a piece of rock crystal, there is a sensation ofcontact or cold, but no sense of taste. On the other hand, when we bringthe tongue in contact with a piece of rock salt, we experience thesensations of contact, coolness, and saline taste. 

Again, the mucous membrane of the mouth must be _moist_. When the mouth isdry, and receives substances not already in solution, there is no salivaready to dissolve them; hence, they are tasteless. This absence of tasteis common with the parched mouth during a fever. 

The tongue assists in bringing the food in contact with the nerves, bypressing it against the roof of the mouth and the soft palate, and thus isproduced the fullest sense of taste. 

319. Physiological Conditions of Taste. The tongue is the seat ofsensations which are quite unlike each other. Thus, besides the sense oftaste, there is the sensation of touch, pressure, heat and cold, burningor acrid feelings, and those produced by the application of the tongue toan interrupted electric current. These are distinct sensations, due tosome chemical action excited probably in the touch cells, although thetrue tastes may be excited by causes not strictly chemical. Thus a smarttap on the tongue may excite the sensation of taste. 

In the majority of persons the back of the tongue is most sensitive tobitters, and the tip to sweets. Saline matters are perceived mostdistinctly at the tip, and acid substances at the sides. The nerves oftaste are sensitive in an extraordinary degree to some articles of foodand certain drugs. For example, the taste of the various preparations ofquinine, peppermint, and wild cherry is got rid of with difficulty. 

Like the other special senses, that of taste may become fatigued. Therepeated tasting of one substance rapidly deadens the sensibility, probably by over-stimulation. Some savors so impress the nerves of tastethat others fail to make any impression. This principle is used to makedisagreeable medicine somewhat tasteless. Thus a few cloves, or grains ofcoffee, or a bit of pepper, eaten before a dose of castor oil, renders itless nauseous. 

Flavor is something more than taste. It is in reality a mixedsensation, in which smell and taste are both concerned, as is shown by thecommon observation that one suffering from a cold in the head, whichblunts his sense of smell, loses the proper flavor of his food. So if aperson be blindfolded, and the nose pinched, he will be unable todistinguish between an apple and an onion, if one be rubbed on the tongueafter the other. As soon as the nostrils are opened the difference is atonce perceived. 

 Experiment 142. Put a drop of vinegar on a friend's tongue, or on your own. Notice how the papillæ of the tongue start up. 

 Experiment 143. Rub different parts of the tongue with the pointed end of a piece of salt or gum-aloes, to show that the _back_ of the tongue is most sensitive to salt and bitter substances. 

 Experiment 144. Repeat the same with some sweet or sour substances, to show that the _edges_ of the tongue are the most sensitive to these substances. 

 Experiment 145. We often fail to distinguish between the sense of taste and that of smell. Chew some pure, roasted coffee, and it seems to have a distinct taste. Pinch the nose hard, and there is little taste. Coffee has a powerful odor, but only a feeble taste. The same is true of garlic, onions, and various spices. 

 Experiment 146. Light helps the sense of taste. Shut the eyes, and palatable foods taste insipid. Pinch the nose, close the eyes, and see how palatable one half of a teaspoonful of cod-liver oil becomes. 

 Experiment 147. Close the nostrils, shut the eyes, and attempt to distinguish by taste alone between a slice of an apple and one of a potato. 

320. Modifications of the Sense of Taste. Taste is modified to agreat extent by habit, education, and other circumstances. Articles offood that are unpleasant in early life often become agreeable in lateryears. There is occasionally a craving, especially with people of apeculiar nervous organization, for certain unnatural articles (as chalkand laundry starch) which are eaten without the least repugnance. Again, the most savory dishes may excite disgust, while the simplest articles mayhave a delicious flavor to one long deprived of them. The taste forcertain articles is certainly acquired. This is often true of rawtomatoes, olives, and especially of tobacco. 

The organs of taste and smell may be regarded as necessary accessories ofthe general apparatus of nutrition, and are, therefore, more or lessessential to the maintenance of animal life. While taste and smell aregenerally maintained until the close of life, sight and hearing are oftenimpaired by time, and may be altogether destroyed, the other vitalfunctions remaining unimpaired. 

321. Effect of Tobacco and Alcohol upon Taste. It would be remarkableif tobacco should fail to injure the sense of taste. The effect producedupon the tender papillæ of the tongue by the nicotine-loaded juices andthe acrid smoke tends to impair the delicate sensibility of the entiresurface. The keen appreciation of fine flavors is destroyed. The onceclear and enjoyable tastes of simple objects become dull and vapid; thushighly spiced and seasoned articles of food are in demand, and thenfollows continued indigestion, with all its suffering. 

Again, the burning, almost caustic effect of the stronger alcoholicdrinks, and the acrid pungency of tobacco smoke, are disastrous to thefiner perceptions of both taste and odors. 

322. Smell. The sense of smell is lodged in the delicatemembrane which lines the nasal cavities. The floor, sides, and roof ofthese cavities are formed by certain bones of the cranium and the face. Man, in common with all air-breathing animals, has two nasal cavities. They communicate with the outer air by two nostrils opening in front, while two other passages open into the pharynx behind. 

To increase the area of the air passages, the two light, spongy turbinatedbones, one on each side, form narrow, winding channels. The mucousmembrane, with the branches of the olfactory nerve, lines the dividingwall and the inner surfaces of these winding passages. Below all thesebones the lower turbinated bones may be said to divide the olfactorychamber above from the ordinary air passages. 

[Illustration: Fig. 126. --Distribution of Nerves over the Interior of theNostrils. (Outer wall. )

 A, branches of the nerves of smell--olfactory nerve, or ganglion; B, nerves of common sensation to the nostril; E, F, G, nerves to the, palate springing from a ganglion at C; H, vidian nerve, from which branches D, I, and J spring to be distributed to the nostrils. ]

The nerves which supply the nasal mucous membrane are derived from thebranches of the fifth and the first pair of cranial nerves, --theolfactory. The latter, however, are the nerves of smell proper, and arespread out in a kind of thick brush of minute nerve filaments. It is inthe mucous membrane of the uppermost part of the cavity of the nostrilthat the nerve endings of smell proper reside. The other nerves whichsupply the nostrils are those of common sensation (sec. 271). 

323. The Sense of Smell. The sense of smell is excited by the contactof odorous particles contained in the air, with the fibers of theolfactory nerves, which are distributed over the delicate surface ofthe upper parts of the nasal cavities. In the lower parts are the endingsof nerves of ordinary sensation. These latter nerves may be irritated bysome substance like ammonia, resulting in a powerfully pungent sensation. This is not a true sensation of smell, but merely an irritation of a nerveof general sensation. 

In ordinary quiet breathing, the air simply flows along the lower nasalpassages into the pharynx, scarcely entering the olfactory chamber at all. This is the reason why, when we wish to perceive a faint odor, we sniff upthe air sharply. By so doing, the air which is forcibly drawn into thenostrils passes up even into the higher olfactory chamber, where some ofthe floating particles of the odorous material come into contact with thenerves of smell. 

One of the most essential conditions of the sense of smell is that thenasal passages be kept well bathed in the fluid secreted by the liningmembrane. At the beginning of a cold in the head, this membrane becomesdry and swollen, thus preventing the entrance of air into the upperchamber, deadening the sensibility of the nerves, and thus the sense ofsmell is greatly diminished. 

The delicacy of the sense of smell varies greatly in different individualsand in different animals. It is generally more acute in savage races. Itis highly developed in both the carnivora and the herbivora. Many animalsare more highly endowed with this sense than is man. The dog, for example, appears to depend on the sense of smell almost as much as on sight. It iswell known, also, that fishes have a sense of smell. Fragments of baitthrown into the water soon attract them to a fishing ground, and at depthswhich little or no light can penetrate. Deer, wild horses, and antelopesprobably surpass all other animals in having a vivid sense of smell. 

Smell has been defined as "taste at a distance, " and it is obvious thatthese two senses not only form a natural group, but are clearlyassociated in their physical action, especially in connection with theperception of the flavor of food. The sense of odor gives us informationas to the quality of food and drink, and more especially as to the qualityof the air we breathe. Taste is at the gateway of the alimentary canal, while smell acts as the sentinel of the respiratory tract. Just as tasteand flavor influence nutrition by affecting the digestive process, so theagreeable odors about us, even those of the perfumes, play an importantpart in the economy of life. 

324. The Sense of Sight. The sight is well regarded as thehighest and the most perfect of all our senses. It plays so common and sobeneficent a part in the animal economy that we scarcely appreciate thismarvelous gift. Sight is essential not only to the simplest matters ofdaily comfort and necessity, but is also of prime importance in theculture of the mind and in the higher forms of pleasure. It opens to usthe widest and the most varied range of observation and enjoyment. Thepleasures and advantages it affords, directly and indirectly, have neithercessation nor bounds. 

Apart from its uses, the eye itself is an interesting and instructiveobject of study. It presents beyond comparison the most beautiful exampleof design and artistic workmanship to be found in the bodily structure. Itis the watchful sentinel and investigator of the external world. Unlikethe senses of taste and smell we seem, by the sense of vision, to becomeaware of the existence of objects which are entirely apart from us, andwhich have no direct or material link connecting them with our bodies. Andyet we are told that in vision the eye is affected by something which isas material as any substance we taste or smell. 

 [NOTE. "The higher intelligence of man is intimately associated with the perfection of the eye. Crystalline in its transparency, sensitive in receptivity, delicate in its adjustments, quick in its motions, the eye is a fitting servant for the eager soul, and, at times, the truest interpreter between man and man of the spirit's inmost workings. The rainbow's vivid hues and the pallor of the lily, the fair creations of art and the glance of mutual affection, all are pictured in its translucent depths, and transformed and glorified by the mind within. Banish vision, and the material universe shrinks for us to that which we may touch; sight alone sets us free to pierce the limitless abyss of space. "--M'Kendrick and Snodgrass's _Physiology of the Senses_. ]

Physicists tell us that this material, known as the _luminiferous ether_, permeates the universe, and by its vibrations transmits movements whichaffect the eye, giving rise to the sensation of light, and the perceptionof even the most distant objects. Our eyes are so constructed as torespond to the vibrations of this medium for the transmission of light. 

325. The Eye. The eye, the outer instrument of vision, is a mostbeautiful and ingenious machine. All its parts are arranged with such adelicate adjustment to one another, and such an exquisite adaptation ofevery part to the great object of the whole, that the eye is properlyregarded as one of the wonders of nature. 

The eyeball is nearly spherical in shape, but is slightly elongatedfrom before backwards. The front part is clear and transparent, and bulgessomewhat prominently to allow the entrance of the rays of light. The eyerests in a bowl-shaped socket, called the orbit, formed by parts ofvarious bones of the head and face. The margins of this cavity are formedof strong bone which can withstand heavy blows. The socket is padded withloose, fatty tissue, and certain membranes, which serve as a soft andyielding bed in which the eyeball can rest and move without injury. In asevere sickness this fatty tissue is absorbed, and this fact explains thesunken appearance of the eyes. 

The orbit is pierced through its posterior surface by an opening throughwhich the nerve of sight, the optic, passes to the eyeball. We may thinkof the optic nerve holding the eyeball much as the stem holds theapple. It is the function of this most important nerve to transmitretinal impressions to the seat of consciousness in the brain, where theyare interpreted. 

The eye is bathed with a watery fluid, and protected by the eyelids andthe eyebrows; it is moved in various directions, by muscles, all of whichwill soon be described. 

[Illustration: Fig. 127. --Section of the Human Eye. ]

326. The Coats of the Eyeball. The eyeball proper is elastic butfirm, and is composed of three coats, or layers, each of whichperforms important functions. These coats are the sclerotic, thechoroid, and the retina. 

The sclerotic coat is the outside layer and enclosing membrane of theeyeball. It is a tough, fibrous coat for the protection and maintenance ofthe shape of the eye. It is white and glistening in appearance, and is inpart visible, to which the phrase, "the white of the eye, " is applied. Tothis coat, which serves as a kind of framework for the eye, are attachedthe muscles which move the eyeball. In front of the globe, the scleroticpasses into a transparent circular portion forming a window through whichone can see into the interior. This is the cornea. 

The cornea, a clear, transparent, circular disk, fits into thesclerotic, somewhat as the crystal fits into the metallic case of a watch, forming a covering for its dial. It projects from the general contour ofthe eyeball, not unlike a rounded bay-window, and is often spoken of asthe "window of the eye. "

Lining the inner surface of the sclerotic is the second coat, thechoroid. It is dark in color and fragile in structure, and is made upalmost entirely of blood-vessels and nerves. As the choroid approaches thefront part of the eyeball, its parts become folded upon themselves into aseries of ridges, called ciliary processes. These folds graduallybecome larger, and at last merge into the ciliary or accommodationmuscle of the eye. The circular space thus left in front by thetermination of the choroid is occupied by the iris, a thin, circularcurtain, suspended in the aqueous humor behind the cornea and in front ofthe crystalline lens. In its center is a round opening for the admissionof light. 

This is the pupil, which appears as if it were a black spot. The backof the iris is lined with dark pigment, and as the coloring matter is moreor less abundant, we may have a variety of colors. This pigment layer andthat of the choroid and retina absorb the light entering the eye, so thatlittle is reflected. 

The pupil appears black, just as the open doorway to a dark closet seemsblack. The margin of the iris is firmly connected with the eyeball allround, at the junction of the sclerotic and the cornea. 

327. The Retina. The third and innermost coat of the eyeball is theretina. This is the perceptive coat, without which it would be impossibleto see, and upon which the images of external objects are received. Itlines nearly the whole of the inner surface of the posterior chamber, resting on the inner surface of the choroid. It is with the retina, therefore, that the vitreous humor is in contact. 

The retina is a very thin, delicate membrane. Although very thin, itis made up of ten distinct layers, and is so complicated in structure thatnot even a general description will be attempted in this book. It does notextend quite to the front limits of the posterior chamber, but stops shortin a scalloped border, a little behind the ciliary processes. This is thenerve coat of the eye, and forms the terminal organ of vision. It isreally an expansion of the ultimate fibers of the optic nerve, by means ofwhich impressions are sent to the brain. 

The retina contains curious structures which can be seen only with the aidof the microscope. For instance, a layer near the choroid is made up ofnerve cells arranged in innumerable cylinders called "rods and cones, " andpacked together not unlike the seeds of a sunflower. These rods and conesare to be regarded as the peculiar modes of termination of the nervefilaments of the eye, just as the taste buds are the modes of terminationof the nerve of taste in the tongue, and just as the touch corpuscles arethe terminations of the nerves in the skin. 

 Experiment 148. Close one eye and look steadily at the small a in the figure below. The other letters will also be visible at the same time. If now the page be brought slowly nearer to the eye while the eye is kept steadily looking at the small a, the large A will disappear at a certain point, reappearing when the book is brought still nearer. 

 [Illustration: a oAx]

 On the reappearance of the A it will be noted that it comes into view from the inner side, the x being seen before it. If now we move the book towards its original place, the A will again disappear, coming again into view from the outer side when the o is seen before it. 

328. Inner Structure of the Eye. Let us imagine an eyeball dividedthrough the middle from above downwards. Let us now start in front andobserve its parts (Fig. 127). We come first to the cornea, which hasjust been described. The iris forms a sort of vertical partition, dividing the cavity of the eyeball into two chambers. 

[Illustration: Fig. 128. --Diagram illustrating the Manner in which theImage of an Object is brought to a Focus on the Retina. ]

The anterior chamber occupies the space between the cornea and theiris, and is filled with a thin, watery fluid called the aqueoushumor. 

The portion behind the iris forms the posterior chamber, and containsthe crystalline lens and a transparent, jelly-like fluid, thevitreous humor. This fluid is never renewed, and its loss ispopularly described by the phrase, "when the eye runs out. "

 Experiment 149. The retina is not sensitive where the optic nerve enters the eyeball. This is called the "blind spot. " Put two ink-bottles about two feet apart, on a table covered with white paper. Close the left eye, and fix the right steadily on the left-hand inkstand, gradually varying the distance from the eye to the ink-bottle. At a certain distance the right-hand bottle will disappear; but nearer or farther than that, it will be plainly seen. 

The vitreous humor fills about four-fifths of the eyeball and prevents itfrom falling into a shapeless mass. It also serves to hold the choroid andthe retina in position, and to maintain the proper relations of the innerstructures of the eye. 

The iris consists of a framework of connective tissue, the surface ofwhich is lined by cells containing pigment, which gives color to the eye. 

Bundles of involuntary muscular fibers are found in the substance of theiris. Some are arranged in a ring round the margin of the pupil; othersradiate from it like the spokes of a wheel. When the circular fiberscontract, the pupil is made smaller, but if these fibers relax, theradiating fibers cause the pupil to dilate more or less widely. 

329. The Crystalline Lens. Just behind the pupil and close to the iris isa semi-solid, double-convex body, called the crystalline lens. It isshaped like a magnifying glass, convex on each side, but with theposterior surface more convex than the anterior. In health it is perfectlyclear and transparent, and highly elastic. When the lens becomes opaque, from change in old age, or from ulcers or wounds, we have the diseaseknown as _cataract_. 

[Illustration: Fig. 129. --Diagram showing the Change in the Lens duringAccommodation. 

On the right the lens is arranged for distant vision, the ciliary muscleis relaxed and the ligament D is tense, so flattening by its compressionthe front of the lens C; on the left the muscle A is acting, and thisrelaxes the ligament and allows the lens B to become more convex, and sofitted for the vision of near objects. ]

The lens is not placed loosely in the eyeball, but is enclosed in atransparent and elastic capsule suspended throughout its circumference bya ligament called the suspensory ligament. This ligament not onlyretains the lens in place, but is capable of altering its shape. Inordinary conditions of the eye, this ligament is kept tense so that thefront part of the lens is flattened somewhat by the pressure on it. 

All around the edge, where the cornea, sclerotic, and choroid meet, is aring of involuntary muscular fibers, forming the ciliary muscle. Whenthese fibers contract, they draw forwards the attachment of the suspensoryligament of the lens, the pressure of which on the lens is consequentlydiminished. The elasticity of the lens causes it at once to bulgeforwards, and it becomes more convex. 

The ciliary muscle is thus known as the muscle of accommodation, because it has the power to accommodate the eye to near and distantobjects. In this respect it corresponds in its use to the adjusting screwin the opera-glass and the microscope. 

330. The Eye Compared to the Photographic Camera. As an opticalinstrument, the eye may be aptly compared, in many particulars, to thephotographic camera. The latter, of course, is much simpler instructure. The eyelid forms the cap, which being removed, the light fromthe object streams through the eye and passes across the dark chamber tothe retina behind, which corresponds to the sensitive plate of the camera. The transparent structures through which the rays of light pass representthe lenses. To prevent any reflected light from striking the plate andinterfering with the sharpness of the picture, the interior of thephotographic camera box is darkened. The pigmented layer of the choroidcoat represents this blackened lining. 

In the camera, the artist uses a thumb-screw to bring to a focus on thesensitive plate the rays of light coming from objects at differentdistances. Thus the lens of the camera may be moved nearer to or fartherfrom the object. In order to obtain clear images, the same result must beaccomplished by the eye. When the eye is focused for near objects, thoseat a distance are blurred, and when focused for distant objects, thosenear at hand are indistinct. Now, in the eye there is no arrangement toalter the position of the lenses, as in the camera, but the same result isobtained by what is called "accommodation. "

Again, every camera has an arrangement of diaphragms regulating the amountof light. This is a rude contrivance compared with the iris, which bymeans of its muscular fibers can in a moment alter the size of the pupil, thus serving a similar purpose. 

[Illustration: Fig. 130. --Illustrating the manner in which the Image of anObject is brought to a Focus in a Photographer's Camera. ]

331. The Refractive Media of the Eye. The eye is a closed chamberinto which no light can pass but through the cornea. All the rays thatenter the eye must also pass through the crystalline lens, which bringsthem to a focus, as any ordinary lens would do. 

Now, if the media through which the light from an object passes to reachthe retina were all of the same density as the air, and were also planesurfaces, an impression would be produced, but the image would not bedistinct. The action of the lens is aided by several refractive mediain the eye. These media are the cornea, the aqueous humor, and thevitreous humor. By reason of their shape and density these media refractthe rays of light, and bring them to a focus upon the retina, thus aidingin producing a sharp and distinct image of the object. Each point of theimage being the focus or meeting-place of a vast number of rays comingfrom the corresponding point of the object is sufficiently bright tostimulate the retina to action. [44]

Thus, the moment rays of light enter the eye they are bent out of theircourse. By the action of the crystalline lens, aided by the refractivemedia, the rays of light that are parallel when they fall upon the normaleye are brought to a focus on the retina. 

If the entire optical apparatus of the eye were rigid and immovable, oneof three things would be necessary, in order to obtain a clear image of anobject; for only parallel rays (that is, rays coming from objects distantabout thirty feet or more), are brought to a focus in the average normaleye, unless some change is brought about in the refractive media. First, the posterior wall of the eye must be moved further back, or the lenswould have to be capable of movement, or there must be some way ofincreasing the focusing power of the lens. In the eye it is the convexityof the lens that is altered so that the eye is capable of adjusting itselfto different distances. [45]

[Illustration: Fig. 131. --The Actual Size of the Test-Type, which shouldbe seen by the Normal Eye at a Distance of Twenty Feet. ]

332. The More Common Defects of Vision. The eye may be free fromdisease and perfectly sound, and yet vision be indistinct, because therays of light are not accurately brought to a focus on the retina. "Oldsight, " known as presbyopia, is a common defect of vision inadvancing years. This is a partial loss of the power to accommodate theeye to different distances. This defect is caused by an increase in thedensity of the crystalline lens, and an accompanying diminution in theability to change its form. The far point of vision is not changed, butthe near point is removed so far from the eye, that small objects are nolonger visible. 

[Illustration: Fig. 132. --Diagram illustrating the Hypermetropic(far-sighted) Eye. 

The image P' of a point P falls behind the retina in the unaccommodatedeye. By means of a convex lens it may be focused on the retina withoutaccommodation (dotted lines). (To save space P is placed much too near theeye. )]

Hence, when a person about forty-five years of age complains of dim light, poor print, and tired eyes, the time has come to seek the advice of anoptician. A convex lens may be needed to aid the failing power to increasethe convexity of the lens, and to assist it in bringing the divergent raysof light to a focus. 

In "long sight, " or hypermetropia both the near and far point ofvision are concerned, and there is no distinct vision at any distancewithout a strain. It is a defect in the focus, dependent upon the form ofthe eyes, and exists in childhood. The axis of the eyeball is too short, and the focus falls beyond the retina, which is too near the cornea. Inchildhood this strain may pass unnoticed, but, sooner or later itmanifests itself by a sense of fatigue, dizziness, and a blurred andindistinct vision. The remedy is in the use of convex glasses to convergeparallel rays of light before they enter the eye. The muscles ofaccommodation are thus relieved of their extra work. 

"Short sight, " known as myopia, is one of the commonest defects ofvision. In this defect the axis of the eye, or the distance between thecornea and the retina, is too long and the rays of light are brought to afocus in front of the retina. The tendency to short-sightedness exists inmany cases at birth, and is largely hereditary. It is alarmingly commonwith those who make a severe demand upon the eyes. During childhood thereis a marked increase of near-sightedness. The results of imprudence andabuse, in matters of eyesight, are so disastrous, especially during schoollife, that the question of short sight becomes one of paramountimportance. 

 Experiment 150. With a hand-mirror reflect the sunlight on a white wall. Look steadily at the spot for a full minute, and then let the mirror suddenly be removed. The "complementary" color--a dark spot--will appear. 

 Experiment 151. _To show that impressions made upon the retina do not disappear at once_. Look steadily at a bright light for a moment or two, and then turn away suddenly, or shut the eyes. A gleam of light will be seen for a second or two. 

 Look steadily at a well-lighted window for a few seconds, and then turn the eyes suddenly to a darkened wall. The window frame may be plainly seen for a moment. 

 Glance at the sun for a moment, close the eyes and the image of the sun may be seen for a few seconds. 

 Experiment 152. Take a round piece of white cardboard the size of a saucer, and paint it in alternate rings of red and yellow, --two primary colors. Thrust a pin through the center and rotate it rapidly. The eye perceives neither color, but orange, --the secondary color. 

 Experiment 153. To note the shadows cast upon the retina by opaque matters in the vitreous humor (popularly known as floating specks, or gossamer threads), look through a small pin-hole in a card at a bright light covered by a ground-glass shade. 

 Experiment 154. _To illustrate accommodation_. Standing near a source of light, close one eye, hold up both forefingers not quite in a line, keeping one finger about six or seven inches from the other eye, and the other forefinger about sixteen to eighteen inches from the eye. Look at the _near_ finger; a distinct image is obtained of it, while the far one is blurred or indistinct. Look at the far image; it becomes distinct, while the near one becomes blurred. Observe that in accommodating for the near object, one is conscious of a distinct effort. 

In many cases near-sightedness becomes a serious matter and demandsskillful advice and careful treatment. To remedy this defect, somethingmust be done to throw farther back the rays proceeding from an object sothat they will come to a focus exactly on the retina. This is done bymeans of concave glasses, properly adjusted to meet the conditions of theeyes. The selection of suitable glasses calls for great care, as much harmmay be done by using glasses not properly fitted to the eye. 

[Illustration: Fig. 133. --Diagram illustrating the Myopic (near-sighted)Eye. 

The image P' of a distant object P falls in front of the retina evenwithout accommodation. By means of a concave lens (L) the image may bemade to fall on the retina (dotted lines). (To save space P is placed muchtoo near the eye). ]

There is an optical condition of the eye known as astigmatism, inwhich the cornea is usually at fault. In this defect of vision thecurvature of the cornea is greater in one meridian than in another. As aresult the rays from an object are not all brought to the same focus. Objects appear distorted or are seen with unequal clearness. Glasses of apeculiar shape are required to counteract this defect. 

333. The Movements of the Eyes. In order that our eyes may beefficient instruments of vision, it is necessary that they have the powerof moving independently of the head. The mechanical arrangement by whichthe eyeballs are moved in different directions is quite simple. It is doneby six little muscles, arranged in three pairs, which, with one exception, originate in the back of the cavity in which the eye rests. Four of thesemuscles run a straight course and are called the _recti_. The remainingtwo muscles bend in their course and are called _oblique_. Thecoördination of these tiny muscles is marvellous in its delicacy, accuracy, and rapidity of action. 

When, for any cause, the coördination is faulty, "cross eye, " technicallycalled strabismus, is produced. Thus, if the internal rectus isshortened, the eye turns in; if the external rectus, the eye turns out, producing what is known as "wall eye. " It is thus evident that the beautyof the internal mechanism of the eye has its fitting complement in theprecision, delicacy, and range of movement conferred upon it by itsmuscles. 

334. The Eyelids and Eyebrows. The eye is adorned and protected bythe eyelids, eyelashes, and eyebrows. 

[Illustration: Fig. 134. --Muscles of the Eyeball. 

 A, attachment of tendon connected with the three recti muscles; B, external rectus, divided and turned downward, to expose the internus rectus; C, inferior rectus; D, internal rectus; E, superior rectus; F, superior oblique; H, pulley and reflected portion of the superior oblique; K, inferior oblique; L, levator palpebri superioris; M, middle portion of the same muscle (L); N, optic nerve. ]

The eyelids, two in number, move over the front of the eyeball andprotect it from injury. They consist of folds of skin lined with mucousmembrane, kept in shape by a layer of fibrous material. Near the innersurface of the lids is a row of twenty or thirty glands, known as the_Meibomian glands_, which open on the free edges of each lid. When one ofthese glands is blocked by its own secretion, the inflammation whichresults is called a "sty. "

The inner lining membrane of the eyelids is known as the conjunctiva;it is richly supplied with blood-vessels and nerves. After lining the lidsit is reflected on to the eyeballs. It is this membrane which isoccasionally inflamed from taking cold. 

The free edges of the lids are bordered with two or more rows of hairscalled the eyelashes, which serve both for ornament and for use. Theyhelp to protect the eyes from dust, and to a certain extent to shade them. Their loss gives a peculiar, unsightly look to the face. 

The upper border of the orbit is provided with a fringe of short, stiffhairs, the eyebrows. They help to shade the eyes from excessivelight, and to protect the eyelids from perspiration, which would otherwisecause serious discomfort. 

335. The Lacrymal Apparatus. Nature provides a special secretion, thetears, to moisten and protect the eye. The apparatus producing thissecretion consists of the lacrymal or tear gland and lacrymalcanals or tear passages (Fig. 136). 

Outside of the eyeball, in the loose, fatty tissue of the orbit, in theupper and outer corner is the lacrymal or tear gland. It isabout the size of a small almond and from it lead several little canalswhich open on the inner surface of the upper lid. The fluid from the glandflows out by these openings over the eyeball, and is collected at theinner or nasal corner. Here in each lid is a little reddish elevation, or_lacrymal caruncle_, in which is an opening, communicating with a smallcanal in the lid which joins the lacrymal sac, lodged between theorbit and the bridge of the nose (Fig. 137). 

From this sac there passes a channel, the nasal duct, about one-halfof an inch long, leading into the lower portion of the nostril. The fluidwhich has flowed over the eye is drained off by these canals into thenose. During sleep this secretion is much diminished. When the eyes areopen the quantity is sufficient to moisten the eyeball, the excess beingcarried into the nose so gradually that the attention is not attracted toit. 

The lacrymal canals are at times blocked by inflammation of the nasalduct, and the fluid collects in the corners of the eyelids and overflowsdown the cheeks, producing much inconvenience. The lining membrane of theeyelids through these canals is continuous with that of the nostrils. Hence, when the lining membrane of the eye is red and swollen, as during acold, the nasal passages are also irritated, and when the nasal membraneis inflamed, the irritation is apt to pass upwards and affect the eyelids. 

336. The Tears. The lacrymal or tear gland is under the control of thenervous system. Thus, if anything irritates the eyelids, the sensorynerves are stimulated and the impression is carried to the brain. Thencethe nerve impulses travel to the lacrymal glands, leading to an increasedflow of their secretion. The irritation of the sensory nerves in the nasalpassages by smelling such substances as onions, or pungent salts, oftencauses a copious flow of tears. 

[Illustration: Fig. 135. --Lacrymal Gland and Ducts. 

 A, lachrymal gland, the size of a small almond lodged in a shallow depression in the bones of the orbit; B, lachrymal ducts (usually seven), which form a row of openings into the conjunctival fold. ]

Various mental emotions, as joy and grief, may produce similar results. Inthese cases the glands secrete the fluid in such quantities that it cannotescape by the lacrymal canals, and the excess rolls over the cheeks astears. Excessive grief sometimes acts on the nerve centers in exactly theopposite manner, so that the activity of the glands is arrested and lessfluid is secreted. This explains why some people do not shed tears intimes of deep grief. 

 Experiment 155. Gently turn the inner part of your lower eyelid down. Look in a mirror, and the small lacrymal point, or opening into the nasal duct, may be observed. 

337. Color-blindness. There is an abnormal condition of vision calledcolor-blindness, in which the power of discrimination between differentcolors is impaired. Experiment shows that ninety-six out of every onehundred men agree as to the identity or the difference of color, while theremaining four show a defective perception of color. 

The first may be said to have _normal vision_; the second are called_color-blind_. It is a curious fact that ten times more men than women arecolor-blind. 

In its true sense, color-blindness is always congenital, oftenhereditary. This condition of abnormal vision is totally incurable. Aperson may be color-blind and not know it until the defect is accidentallyrevealed. The common form of defective color-vision is the inability todistinguish between _red_ and _green_. As green lights mean safety, andred lights danger, on railroads, on shipboard, and elsewhere, it becomesof paramount importance that no one who is color-blind should be employedin such service. Various tests are now required by statute law in manystates to be used for the detection of such defects of vision amongemployees in certain occupations. 

338. School Life and the Eyesight. The eyes of children need morecare than those of adults, because their eyes are still in the course ofdevelopment. The eyes, like any other organ which is yet to attain itsfull growth, require more care in their use than one which has alreadyreached its full size. They are peculiarly liable to be affected byimproper or defective light. Hence the care of the eyes during school lifeis a matter of the most practical importance. 

In no matter of health can the teacher do a more distinct service than inlooking after the eyesight of the pupils. Children suffering fromdefective vision are sometimes punished by teachers for supposedstupidity. Such pupils, as well as the deaf, are peculiarly sensitive totheir defects. Every schoolroom should have plenty of light; it shouldcome from either side or the rear, and should be regulated with suitableshades and curtains. 

Pupils should not be allowed to form the bad habit of reading with thebook held close to the eyes. The long search on maps for obscure namesprinted in letters of bad and trying type should be discouraged. Strainingthe eyes in trying to read from slates and blackboards, in the last hourof the afternoon session, or in cloudy weather, may do a lifelong injuryto the eyesight. Avoid the use, so far as possible, especially in adefective light, of text-books which are printed on battered type and wornplates. 

The seat and desk of each scholar should be carefully arranged to suit theeyesight, as well as the bones and muscles. Special pains should be takenwith the near-sighted pupils, and those who return to school after anattack of scarlet fever, measles, or diphtheria. 

 Experiment 156. _To test color-blindness. _ On no account is the person being tested to be asked to name a color. In a large class of students one is pretty sure to find some who are more or less color-blind. The common defects are for red and green. 

Place worsteds on a white background in a good light. Select, as a testcolor, a skein of light green color, such as would be obtained by mixing apure green with white. Ask the examinee to select and pick out from theheap all those skeins which appear to him to be of the same color, whetherof lighter or darker shades. A color-blind person will select amongstothers some of the confusion-colors, _e. G. _, pink, yellow. A colored plateshowing these should be hung up in the room. Any one who selects all thegreens and no confusion-colors has normal color vision. If, however, oneor more confusion-colors be selected, proceed as follows: select as a testcolor a skein of pale rose. If the person be red-blind, he will chooseblue and violet; if green-blind, gray and green. 

Select a bright red skein. The red-blind will select green and brown; thegreen-blind picks out reds or lighter brown. 

339. Practical Hints on the Care of the Eyes. The eye is anexceedingly delicate and sensitive organ. While it is long-suffering, itsendurance has a limit. Like all the other organs of the body, the eyes arebetter for moderate and rational use. More than any other organ theyrequire attention to the general health, as the condition of the skin, exercise in the open air, good food, and proper habits of daily living. 

The tissues of the eyes are peculiarly sensitive to any general influence. Certain constitutional diseases, like rheumatism, lead-poisoning, diphtheria, and measles often affect the eyes. Special care should betaken with children's eyes during and after an attack of measles andscarlet fever. The eyes of young infants should not be exposed to glaringlights or to the direct rays of the sun, as when taken out in babycarriages. 

[Illustration: Fig. 136. --Showing the Relative Position of the LacrymalApparatus, the Eyeball, and the Eyelids. 

 A, lacrymal canals, with the minute orifices represented as two black dots (puncta lacrymalia) to the right; B, tendon of the orbicularis palpebrarum muscle; apparently under B is seen the lacrymal sac. The minute openings of the Meibomian glands are seen on the free margins of the eyelids. 

Below A is seen a small conical elevation, with black dots (the lacrymalpapilla or caruncle). ]

Glasses should be worn when they are needed. A failure to do this ususallycauses much unnecessary suffering. It is far from wise to postpone as longas possible the first use of glasses. The selection and proper fitting ofglasses call for the combined skill of both the physician and theoptician. Obstinate headaches are often caused by defective vision, andmay disappear after discontinuing improper glasses. 

The habit of reading, in the cars or elsewhere, the daily paper andpoorly printed books, with their blurred and indistinct type, is a severestrain on the accommodation apparatus of the eyes. It is a dangerouspractice to read in bed at night, or while lying down in a darkened orshaded room. This is especially true during recovery from illness. Themuscles of the eyes undergo excessive strain in accommodating themselvesto the unnatural position. The battered type, wood-pulp paper, and poorpresswork, now so commonly used in the cheap editions of books andperiodicals, are often injurious to the eyesight. 

Reading-matter should not be held nearer to the eyes than is necessary tomake the print appear perfectly sharp and distinct. No print should beread continuously that cannot be seen clearly at about eighteen inches. Those who read music are especially liable to strain the eyes, becauseexact vision is required to follow the notes. Persons who wear glasses forreading should be careful to use them while reading music, and good lightis necessary to avoid any undue strain. 

After reading steadily for some time, the eyes should be rested by closingthem a short period or by looking at some distant object, even if only fora few moments. The book, the sewing, and work generally, should be held asfar from the eyes as is compatible with good vision. The natural tendencyis to reverse this rule. We should never read, write, sew, stitch, orotherwise use the eyes when they smart or tingle, or when the sight is dimor blurred. The eyes are then tired and need a rest. Much injury may bedone by reading in twilight, or by artificial light in the early morning, and by reading and working in badly lighted and ill-ventilated rooms. 

Good artificial light is much to be preferred to insufficient sunlight. The artificial light should be sufficiently bright and steady; a fickeringlight is always bad. Riding against a strong wind, especially on abicycle, may prove hurtful, at least for eyes that are inclined to anykind of inflammation. The light reflected from snow is a common source ofinjury to the eyes. It is a wise caution in passing from a dark room toavoid looking immediately at the sun, an incandescent light, theglistening snow, or other bright objects. 

The eyes should never be rubbed, or the fingers thrust into them, [46] andmuch less when they are irritated by any foreign substance. The sooner theoffending substance is removed the better. 

[Illustration: Fig. 137. --Lacrymal Canals, Lacrymal Sac, and Nasal ducts, opened by their Anterior Portion. ]

340. Effect of Alcohol upon the Eye. The earlier and slighter formsof injury done to the eye by the use of intoxicants are quite familiar:the watery condition of the eye and of the lids, and the red and blearedaspect of the organ. Both are the result of chronic inflammation, whichcrowds the blood into the vessels of the cornea, making them bloodshot andvisible. The nerves controlling the circulation of the eye are partiallyparalyzed, and thus the relaxed vessels become distended. 

But more serious results ensue. Long use of intoxicants produces diseasesof the retina, involving in many cases marked diminution of acuteness aswell as quickness of vision, and at times distorted images upon thesurface of the retina. In other instances, the congestion of the opticnerve is so serious as to involve a progressive wasting of that organ, producing at first a hazy dimness of vision which gradually becomes worseand worse, till total blindness may ensue. 

It is beyond question that a wide comparison of cases by carefulobservers proves that a large fraction of those who indulge in strongdrink suffer from some form of disease of the eye. 

341. Effect of Tobacco upon Vision. Tobacco, in its distribution ofevil effects, does not neglect the senses and especially the eye. Avariety of vicious results is produced. The pungent smoke inflames thelids. The narcotic dilates the pupil, causing dimness and confusion ofvision. A diseased condition occurs with severe pain in the eye followedby impaired vision. 

Oculists speak impressively of the ill effects of tobacco, and especiallyof cigarettes, upon the eyes of the young. They mention a well-knowndisease, tobacco blindness, usually beginning with color-blindness, andprogressing occasionally with increasing dimness of vision to entire lossof sight. [47]

342. The Sense of Hearing. The structure of the human ear is muchmore complicated than is generally supposed. It is an apparatusconstructed to respond to the waves of sound. As a whole, it may beconsidered a peculiar form of nerve-ending. 

The external ear forms only a part of a most elaborate apparatus wherebysound waves may be transmitted inwards to the real organ of hearing. Thereally sensitive part of the ear, in which the auditory nerve ends, isburied for protection deep out of sight in the bones of the head; so deepthat sounds cannot directly affect it. Some arrangement, therefore, isrequired for conducting the sounds inwards to this true organ. 

[Illustration: Fig. 138. --The Pinna, or Auricle. ]

In studying the structure of the ear, and how it is fitted to respond tosonorous vibrations, we may divide it into three parts: thesound-conducting part, known as the external ear, the middleear, and the deeply placed nerve portion, the inner ear. 

343. The External Ear. The external ear consists of an expandedportion known as the pinna or _auricle_, and of a passage, theauditory canal or _meatus_, leading inwards from it. The surface ofthe auricle is convoluted to collect and transmit the vibrations of air bywhich sound is produced the auditory canal conducts these vibrations tothe tympanic membrane. Many animals move the auricle in the direction ofthe sound. Thus the horse pricks up its ears when it hears a noise, thebetter to judge of the direction of sounds. [48]

The external auditory meatus, the passage to the middle ear, is curvedand is about an inch and a quarter long. Near its outer portion are anumber of fine hairs slanting outwards to prevent the entrance of insects. Embedded in the deeper parts of the canal are glands which secrete the_cerumen_, or ear-wax, which keeps the canal moist, and helps to protectit against foreign bodies and insects. As the result of a cold, this waxmay collect in sufficient quantities to block the passage, and to diminishto a considerable extent the power of hearing. 

344. The Middle Ear. At the inner end of the outer ear passage is thetympanum, known as "the drum of the ear. " It is a thin, oval membrane, stretched at an angle across the deep end of the passage, which itcompletely closes. The tympanum is thus a partition between thepassage of the outer ear and the cavity of the middle ear. On its innerside is a small air chamber in the petrous portion of the temporal bone, called the cavity of the tympanum. Its bony walls are lined withmucous membrane similar to that lining the nose, mouth, and throat. On theinner wall of the tympanum are two openings, the round window, or _foramenrotundum_, and the oval window, or _foramen ovale_. 

The tympanic cavity communicates with the back part of the throat, by theEustachian tube. This tube is about one and a half inches long andlined with mucous membrane similar to that of the tympanic chamber and thethroat. This passage is usually closed, but is opened in the act ofswallowing. In health there is no communication between the chamber of themiddle ear and the outside, except by the Eustachian tube. Thus a throatcold, with redness and swelling of the mucous membrane, is usuallyaccompanied with some degree of deafness, because the swelling may blockthe lumen of the tube, and thus prevent the free passage of air to andfro. 

[Illustration: Fig. 139. --General View of the Organ of Hearing. 

 A, pinna; B, cavity of the concha, showing the orifices of a great number of sebaceous glands; C, external auditory meatus; D, membrana tympani; F, incus; H, malleus; K, handle of malleus applied to the internal surface of the membrana tympani; L, tensor tympani muscle; between M and K is the tympanic cavity; N, Eustachian tube; O, P, semicircular canals; R, internal auditory canal; S, large nerve given off from the facial ganglion; T, facial and auditory nerves. ]

A most curious feature of the ear is the chain of tiny movable bones whichstretch across the cavity of the middle ear. They connect the tympanicmembrane with the labyrinth, and serve to convey the vibrationscommunicated to the membrane across the cavity of the tympanum to theinternal ear. These bones are three in number, and from their shape arecalled the malleus, or _hammer_, incus, or _anvil_; andstapes, or _stirrup_. 

The hammer is attached by its long handle to the inner surface of the drumof the ear. The round head is connected with the anvil by a movable joint, while the long projection of the anvil is similarly connected with thestirrup bone. The plate of the stirrup is fixed by a membrane into theoval window of the inner wall of the tympanic chamber. 

These little bones are connected with each other and the tympanum byligaments and moved by three tiny muscles. Two are attached to the hammer, and tighten and relax the drum; the other is attached to the stirrup, andprevents it from being pushed too deeply into the oval window. 

[Illustration: Fig. 140. --Ear-Bones. (Anterior View. )

 1, malleus, or hammer; 2, incus, or anvil; 3, stapes, or stirrup. ]

345. The Internal Ear. This forms one of the most delicate andcomplex pieces of mechanism in the whole body. It is that portion of theorgan which receives the impression of sound, and carries it directly tothe seat of consciousness in the brain. We are then able to say that wehear. 

The internal ear, or bony labyrinth, consists of three distinct parts, orvariously shaped chambers, hollowed out in the temporal bone, --thevestibule, the semicircular canals, and the cochlea, or snail's shell. 

[Illustration: Fig. 141. --A Cast of the External Auditory Canal. (Posterior view)]

The vestibule is the common cavity with which all the other portionsof the labyrinth connect. It is an oval-shaped chamber, about 1/3 of aninch in diameter, occupying the middle part of the internal ear. It is onthe inner side of the oval window, which was closed, as we have seen, bythe stirrup bone. From one side of this vestibule, or central hall, thethree semicircular canals pass off, and from the other side, the cochlea. 

The three semicircular canals, so called from their shape, aresimply bony tubes about 1/20 of an inch in width, making a curve of about1/4 of an inch in diameter. They pass out from the vestibule, and afterbending around somewhat like a hoop, they return again to the vestibule. Each bony canal contains within it a membranous canal, at the end of whichit is dilated to form an _ampulla_. 

 Experiment 157. _To vibrate the tympanic membrane and the little ear-bones. _ Shut the mouth, and pinch the nose tightly. Try to force air through the nose. The air dilates the Eustachian tube, and is forced into the ear-drum. The distinct crackle, or clicking sound, is due to the movement of the ear-bones and the tympanic membrane. 

The cochlea, or snail's shell, is another chamber hollowed out in thesolid bone. It is coiled on itself somewhat like a snail's shell. There isa central pillar, around which winds a long spiral canal. One passage fromthe cochlea opens directly into the vestibule; the other leads to thechamber of the middle ear, and is separated from it by the little roundwindow already described. 

The cochlea contains thousands of the most minute cords, known as thefibers or _organ of Corti_. [49] Under the microscope they present theappearance of the keyboard of a piano. These fibers appear to vibrate insympathy with the countless shades of sounds which daily penetrate theear. From the hair-like processes on these tightly stretched fibers, auditory impulses appear to be transmitted to the brain. 

The tubes and chambers of the inner ear enclose and protect a delicatemembranous sac of exactly the same shape as themselves. Between the bonywalls of the passages and the membranous bag inside is a thin, clearfluid, the _perilymph_. The membranous bag itself contains a similarfluid, the _endolymph_. In this fluid are found some minute crystals oflime like tiny particles of sand, called _otoliths_, or ear-stones. Everymovement of the fluid itself throws these grains from side to side. 

[Illustration: Fig. 142. --Bony internal Ear of Right Side. (Magnified; theupper figure of the natural size. )

 A, oval window (foramen ovale); B, C, D, semicircular canals; * represents the bulging part (ampulla) of each canal; E, F, G cochlea, H, round window (foramen rotundum). ]

The auditory nerve, or nerve of hearing, passes to the inner ear, through a passage in the solid bone of the skull. Its minute filamentsspread at last over the inner walls of the membranous labyrinth in twobranches, --one going to the vestibule and the ampullæ at the ends of thesemicircular canals, the other leading to the cochlea. 

346. Mechanism of Hearing. Waves of sound reach the ear, and aredirected by the concha to the external passage, at the end of which theyreach the tympanic membrane. When the sound-waves beat upon this thinmembrane, it is thrown into vibration, reproducing in its movements thecharacter of the air-vibrations that have fallen upon it. 

Now the vibrations of the tympanic membrane are passed along the chain ofbones attached to its inner surface and reach the stirrup bone. Thestirrup now performs a to-and-fro movement at the oval window, passing theauditory impulse inwards to the internal ear. 

Every time the stirrup bone is pushed in and drawn out of the ovalwindow, the watery fluid (the perilymph) in the vestibule and inner ear isset in motion more or less violently, according to the intensity of thesound. The membranous labyrinth occupies the central portion of thevestibule and the passages leading from it. When, therefore, the perilymphis shaken it communicates the impulse to the fluid (endolymph) containedin the inner membranous bag. The endolymph and the tiny grains of ear-sandnow perform their part in this marvelous and complex mechanism. They aredriven against the sides of the membranous bag, and so strike the ends ofthe nerves of hearing, which transmit the auditory impulses to the seat ofsensation in the brain. 

It is in the seat of sensation in the brain called the _sensorium_ thatthe various auditory impulses received from different parts of the innerear are fused into one, and interpreted as sounds. It is the extent of thevibrations that determines the loudness of the sound; the number of themthat determines the pitch. 

 Experiment 158. Hold a ticking watch between the teeth, or touch the upper incisors with a vibrating tuning-fork; close both ears, and observe that the ticking or vibration is heard louder. Unstop one ear, and observe that the ticking or vibration is heard loudest in the stopped ear. 

 Experiment 159. Hold a vibrating tuning-fork on the incisor teeth until you cannot hear it sounding. Close one or both ears, and you will hear it. 

 Experiment 160. Listen to a ticking watch or a tuning-fork kept vibrating electrically. Close the mouth and nostrils, and take either a deep inspiration or deep expiration, so as to alter the tension of the air in the tympanum; in both cases the sound is diminished. 

 Experiment 161. With a blindfolded person test his sense of the direction of sound, _e. G. _, by clicking two coins together. It is very imperfect. Let a person press both auricles against the side of the head, and hold both hands vertically in front of each meatus. On a person making a sound in front, the observed person will refer it to a position behind him. 

347. Practical Hints on the Care of the Ear. This very delicate andcomplicated organ is often neglected when skilled treatment is urgentlyneeded, and it is often ignorantly and carelessly tampered with when itshould be let alone. 

Never insert into the ear canal the corners of towels, ear spoons, theends of toothpicks, hairpins, or any other pointed instruments. It is aneedless and dangerous practice, usually causing, in time, some form ofinflammation. The abrasion of the skin in the canal thus produced affordsa favorable soil for the growth of vegetable parasites. 

[Illustration: Fig. 143. --Diagram of the Middle and Internal Ear. ]

This, in turn, may lead to a chronic inflammation of the canal and of thetympanic membrane. Again, there is always risk that the elbow may bejogged and the instrument pushed through the drum-head. There is, ofcourse, a natural impulse to relieve the itching of the ear. This shouldbe done with the tips of the fingers or not at all. 

The popular notion that something should be put into the ear to curetoothache is erroneous. This treatment does not cure a toothache, and maylead to an injury to the delicate parts of the ear. A piece of absorbentcotton, carefully inserted into the ear, may be worn out of doors, whenthe cold air causes pain, but should be removed on coming into the house. 

Frequent bathing in the cold water of ponds and rivers is liable toinjure both the ears and the general health. In salt-water bathing, theforce of the waves striking against the ears often leads to earache, long-continued inflammation, or defective hearing; to diminish this risk, insert into the ears a small plug of absorbent cotton. 

The ears are often carelessly exposed to cold water and inclement weather. Very cold water should never be used to bathe the ears and nostrils. Bathemoderately and gently in lukewarm water, using a wash-rag in preference toa sponge; dry gently and thoroughly. Children's ears are often rudelywashed, especially in the auditory canal. This is not at all necessary tocleanliness, and may result in a local inflammation. 

Never shout suddenly in a person's ear. The ear is not prepared for theshock, and deafness has occasionally resulted. A sudden explosion, thenoise of a cannon, may burst the drum-head, especially if the Eustachiantube be closed at the time. During heavy cannonading, soldiers are taughtto keep the mouth open to allow an equal tension of air. 

[Illustration: Fig. 144. --Section of Cochlea. 

From A straight downwards is the direction of the central column, to whichE points. B points to the projecting ridge, almost dividing the canal ofthe tube into an upper compartment (D), and a lower (C). ]

Insects may gain entrance to the ears and occasion annoyance, pain, andfright, perhaps leading to vomiting, even to convulsions, with nervouschildren. A lighted lamp held at the entrance of the ear will often inducethe offending insect to crawl out towards the light. A few drops of warmwater, sweet oil, or molasses, dropped into the ear, will help remove theintruder. 

When a discharge occurs from the ears, it is not best to plug them withcotton wads. It only keeps in what should be got rid of. Do not go tosleep with the head on a window sill or in any position, with the earsexposed to draughts of cold or damp air. 

No effort should be made to remove the ear wax unless it accumulatesunduly. The skin of the canal grows outward, and the extra wax and dustwill be naturally carried out, if let alone. Never employ any of the manyarticles or "drops, " advertised to cure deafness. Neuralgic pain in thecanal, usually classed as earache, may be due to decayed or improperlyfilled teeth. 

Quinine, so generally used in its many preparations for malaria, causes apeculiar ringing or buzzing in the ears. This is a warning that it shouldbe taken in smaller doses, or perhaps stopped for a time. In some casesquinine may produce temporary deafness. 

The practice of snuffing up cold water into the nostrils is occasionallyfollowed by an acute inflammation of the middle ear, some of the waterfinding its way through the Eustachian tube into this part of the organ ofhearing. The nasal douche, so often advised as a home remedy for nasalcatarrh, should be used only with great caution, and always in accordancewith detailed directions from a physician. 

348. Effect of Tobacco upon the Hearing. The sense of hearing isoften injured by the use of tobacco. The irritating smoke filling all theinner cavity of the mouth and throat, readily finds its way up theEustachian tube, dries the membrane, and irritates or inflames thedelicate mechanism of the inner ear. Thus may be produced a variety ofserious aural disturbances, such as unnatural noises, whistling, androaring, followed oftentimes by a partial loss of hearing. 

Hearing may be impaired by the use of alcoholic beverages. Alcoholinflames the mucous membrane of the throat, then by its nearness thelining of the Eustachian tube, and finally may injure the delicateapparatus of the internal ear. 

Additional Experiments. 

 Experiment 162. Use a small pair of wooden compasses, or an ordinary pair of dividers with their points guarded by a small piece of cork. Apply the points of the compasses lightly and simultaneously to different parts of the body, and ascertain at what distance apart the points are felt as two. The following is the order of sensibility: tip of tongue, tip of the middle finger, palm, forehead, and back of hand. 

 Experiment 163. Test as in preceding experiment the skin of the arm, beginning at the shoulder and passing downwards. Observe that the sensibility is greater as one tests towards the fingers, and also in the transverse than in the long axis of the limb. In all cases compare the results obtained on both sides of the body. 

 Experiment 164. By means of a spray-producer, spray the back of the hand with ether, and observe how the sensibility is abolished. 

 Experiment 165. Touch your forehead with your forefinger; the finger appears to feel the contact, but on rubbing the forefinger rapidly over the forehead, it is the latter which is interpreted as "feeling" the finger. 

 Experiment 166. Generally speaking, the sensation of touch is referred to the cutaneous surfaces. In certain cases, however, it is referred even beyond this. Holding firmly in one hand a cane or a pencil, touch an object therewith; the sensation is referred to the extremity of the cane or pencil. 

 If, however, the cane or pencil be held loosely in one's hand, one experiences two sensations: one corresponding to the object touched, and the other due to the contact of the rod with the skin. The process of mastication affords a good example of the reference of sensations to and beyond the periphery of the body. 

 Experiment 167. Prepare a strong solution of sulphate of quinine with the aid of a little sulphuric acid to dissolve it (_bitter_), a five-per-cent solution of sugar (_sweet_), a ten-per-cent solution of common salt (_saline_), and a one-per-cent solution of acetic acid (_acid_). Wipe the tongue dry, and lay on its tip a crystal of sugar. It is not tasted until it is dissolved. 

 Experiment 168. Apply a crystal of sugar to the tip, and another to the back of the tongue. The sweet taste is more pronounced at the tip. 

 Experiment 169. Repeat the process with sulphate of quinine in solution. It is scarcely tasted on the tip, but is tasted immediately on the back part of the tongue. Test where salines and acids are tasted most acutely. 

 Experiment 170. _To illustrate the muscular sense_. Take two equal iron or lead weights; heat one and leave the other cold. The cold weight will feel the heavier. 

 Experiment 171. Place a thin disk of _cold_ lead, the size of a silver dollar, on the forehead of a person whose eyes are closed; remove the disk, and on the same spot place two warm disks of equal size. The person will judge the latter to be about the same weight, or lighter, than the single cold disk. 

 Experiment 172. Compare two similar wooden disks, and let the diameter of one be slightly greater than that of the other. Heat the smaller one to over 120 degrees F. , and it will be judged heavier than the larger cold one. 

 Experiment 173. _To illustrate the influence of excitation of one sense organ on the other sense organs_. Small colored patches the shape and color of which are not distinctly visible may become so when a tuning-fork is kept vibrating near the ears. In other individuals the visual impressions are diminished by the same process. 

 On listening to the ticking of a watch, the ticking sounds feebler or louder on looking at a source of light through glasses of different colors. 

 If the finger be placed in cold or warm water the temperature appears to rise when a red glass is held in front of the eyes. 

 Experiment 174. _Formation of an inverted image on the retina_. Take a freshly removed ox-eye; dissect the sclerotic from that part of its posterior segment near the optic nerve. Roll up a piece of blackened paper in the form of a tube, black surface innermost, and place the eye in it with the cornea directed forward. Look at an object--_e. G. _, a candle-flame--and observe the inverted image of the flame shining through the retina and choroid, and notice how the image moves when the candle is moved. 

 Experiment 175. Focus a candle-flame or other object on the ground-glass plate of an ordinary photographic camera, and observe the small inverted image. 

 Experiment 176. _To illustrate spherical aberration_. Make a pin-hole in a blackened piece of cardboard; look at a light placed at a greater distance than the normal distance of accommodation. One will see a radiate figure with four to eight radii. The figures obtained from opposite eyes will probably differ in shape. 

 Experiment 177. Hold a thin wooden rod or pencil about a foot from the eyes and look at a distant object. Note that the object appears double. Close the right eye; the left image disappears, and _vice versa_. 

 Experiment 178. _To show the movements of the iris_. It is an extremely beautiful experiment, and one that can easily be made. Look through a pin-hole in a card at a uniform white surface as the white shade of an ordinary reading-lamp. With the right eye look through the pin-hole, the left eye being closed. Note the size of the (slightly dull) circular visual field. Open the left eye, the field becomes brighter and smaller (contraction of pupil); close the left eye, after an appreciable time, the field (now slightly dull) is seen gradually to expand. One can thus see and observe the rate of movements of his own iris. 

 [Illustration: Fig. 145. ]

 Experiment 179. _To show the blind spot_. The left eye being shut, let the right eye be fixed upon the cross as in Fig. 145. When the book is held at arm's length, both cross and round spot will be visible; but if the book be brought to about 8 inches from the eye, the gaze being kept steadily upon the cross, the round spot will at first disappear, but as the book, is brought still nearer both cross and round spot will again be seen. 

 Experiment 180. _To illustrate the duration of retinal impressions_. On a circular white disk, about halfway between the center and circumference, fix a small, black, oblong disk, and rapidly rotate it by means of a rotating wheel. There appears a ring of gray on the black, showing that the impression on the retina lasts a certain time. 

 [Illustration: Fig. 146. --Optic Disks. 

 The disk A, having black and white sectors, when rotated rapidly gives an even gray tint as in B. ]

 Experiment 181. Mark off a round piece of cardboard into black and white sectors as in A (Fig. 146). Attach it so as to rotate it rapidly, as on a sewing machine. An even gray tint will be produced as in B. 

 Experiment 182. _To illustrate imperfect visual judgments_. Make three round black dots, A, B, C, of the same size, in the same line, and let A and C be equidistant from B. Between A and B make several more dots of the same size. A and B will then appear to be farther apart than B and C. 

 [Illustration: * * * * * * * A B C ]

 For the same reason, of two squares absolutely identical in size, one marked with alternately clear and dark cross-bands, and the other with alternately clear and dark upright markings, the former will appear broader and the latter higher than the other. 

 Experiment 183. Make on a white card two squares of equal size. Across the one draw _horizontal_ lines at equal distances, and in the other make similar _vertical_ lines. Hold them at some distance. The one with horizontal lines appears higher than it really is, while the one with vertical lines appears broader, i. E. , both appear oblong. 

 Experiment 184. Look at the row of letters (S) and figures (8). To

 [Illustration: S S S S S S S S 8 8 8 8 8 8 8 8 ]

 some the upper halves of the letters and figures may appear to be of the same size as the lower halves, to others the lower halves may appear larger. Hold the figure upside down, and observe that there is a considerable difference between the two, the lower halves being considerably larger. 

 Experiment 185. _To illustrate imperfect visual judgment_. The length of a line appears to vary according to the angle and direction of certain other lines in relation to it (Fig. 147). The length of the two vertical lines is the same, yet B appears much longer than A. 

 [Illustration: Fig. 147. --To show False Estimate of Size. 

 \ / \ / /|\ | / | \ | | | A | B | | | \ | / | \|/ | / \ / \ ]

 Experiment 186. In indirect vision the appreciation of direction is still more imperfect. While leaning on a large table, fix a point on the table, and then try to arrange three small pieces of colored paper in a straight line. Invariably, the papers, being at a distance from the fixation-point, and being seen by indirect vision, are arranged, not in a straight line, but in the arc of a circle with a long radius. 

Chapter XII. 

The Throat and the Voice. 

349. The Throat. The throat is a double highway, as it were, through which the air we breathe traverses the larynx on its way to thelungs, and through which the food we swallow reaches the oesophaguson its passage to the stomach. It is, therefore, a very important regionof the body, being concerned in the great acts of respiration anddigestion. 

The throat is enclosed and protected by various muscles and bonystructures, along which run the great blood-vessels that supply the head, and the great nerve trunks that pass from the brain to the parts below. 

We have already described the food passages (Chapter VI. ) and theair passages (Chapter VIII. ). 

To get a correct idea of the throat we should look into the wide-openmouth of some friend. Depressing the tongue we can readily see the backwall of the pharynx, which is common to the two main avenues leadingto the lungs and the stomach. Above, we notice the air passages, whichlead to the posterior cavities of the nose. We have already described thehard palate, the soft palate, the uvula, and the tonsils (Fig. 46). 

On looking directly beyond these organs, we see the beginning of thedownward passage, --the pharynx. If now the tongue be forcibly drawnforward, a curved ridge may be seen behind it. This is theepiglottis, which, as we have already learned shuts down, like thelid of a box, over the top of the larynx (secs. 137 and 203). 

The throat is lined with mucous membrane covered with ciliated epithelium, which secretes a lubricating fluid which keeps the parts moist andpliable. An excess of this secretion forms a thick, tenacious mass ofmucus, which irritates the passages and gives rise to efforts of hawkingand coughing to get rid of it. 

350. The Larynx. The larynx, the essential organ of voice, formsthe box-like top of the windpipe. It is built of variously shapedcartilages, connected by ligaments. It is clothed on the outside withmuscles; on the inside it is lined with mucous membrane, continuous withthat of the other air passages. 

[Illustration: Fig. 148. --View of the Cartilages in front project and formthe lages and Ligaments of the "Adam's apple, " plainly seen and Larynx. (Anterior view. )

 A, hyoid bone; B, thyro-hyoid membrane; C, thyroid cartilage; D, erico-thyroid membrane; E, cricoid cartilage, lateral ligaments seen on each side; F, upper ring of the trachea. ("Adam's apple" is in the V-shaped groove on a line with B and C. )]

The larynx has for a framework two cartilages, the thyroid and thecricoid, one above the other. The larger of these, called thethyroid, from a supposed resemblance to a shield, consists of twoextended wings which join in front, but are separated by a wide intervalbehind. The united edges in front project and form the "Adam's apple"plainly seen and easily felt on most people, especially on very lean men. 

Above and from the sides rise two horns connected by bands to the hyoidbone from which the larynx is suspended. This bone is attached bymuscles and ligaments to the skull. It lies at the base of the tongue, andcan be readily felt by the finger behind the chin at the angle of the jawand the neck (sec. 41 and Fig. 46). From the under side of the thyroid twohorns project downwards to become jointed to the cricoid. The thyroid thusrests upon, and is movable on, the cricoid cartilage. 

The cricoid cartilage, so called from its fancied resemblance to asignet-ring, is smaller but thicker and stronger than the thyroid, andforms the lower and back part of the cavity of the larynx. This cartilageis quite sensitive to pressure from the fingers, and is the cause of thesharp pain felt when we try to swallow a large and hard piece of food notproperly chewed. 

[Illustration: Fig. 149. --Diagram of a Sectional of Nasal and ThroatPassages. 

 C, nasal cavities; T, tongue; L, lower jaw; M, mouth; U, uvula; E, epiglottis; G, larynx; O, oesophagus. ]

On the upper edge of the cricoid cartilage are perched a pair of verysingular cartilages, pyramidal in shape, called the arytenoid, whichare of great importance in the production of the voice. These cartilagesare capped with little horn-like projections, and give attachment at theiranterior angles to the true vocal cords, and at their posteriorangles to the muscles which open and close the glottis, or upperopening of the windpipe. When in their natural position the arytenoidcartilages resemble somewhat the mouth of a pitcher, hence their name. 

351. The Vocal Cords. The mucous membrane which lines the variouscartilages of the larynx is thrown into several folds. Thus, one fold, thefree edge of which is formed of a band of elastic fibers, passeshorizontally outwards from each side towards the middle line, at the levelof the base of the arytenoid cartilages. These folds are called the truevocal cords, by the movements of which the voice is produced. 

Above them are other folds of mucous membrane called the false vocalcords, which take no part in the production of the voice. Thearrangement of the true vocal cords, projecting as they do towards themiddle line, reduces to a mere chink the space between the part of thelarynx above them and the part below them. This constriction of the larynxis called the glottis. 

[Illustration: Fig. 150. --View of the Cartilages and Ligaments of Larynx. (Posterior view. )

 A, epiglottis; B, thyroid cartilage; C, arytenoid cartilage; D, ligament connecting lower cornu of the thyroid with the back of the cricoid cartilage; E, cricoid cartilage; F, upper ring of the trachea. ]

352. The Mechanism of the Voice. The mechanism of the voice may bemore easily understood by a study of Fig. 150. We have here the larynx, viewed from behind, with all the soft parts in connection with it. Onlooking down, the folds forming the true vocal cords are seen enclosing aV-shaped aperture (the glottis), the narrow part being in front. 

The form of this aperture may be changed by the delicately coordinateactivities of the muscles of the larynx. For instance, the vocal cords maybe brought so closely together that the space becomes a mere slit. Airforced through the slit will throw the edges of the folds into vibrationand a sound will be produced. 

The Variations in the form of the opening will determine the variations inthe sound. Now, if the various muscles of the larynx be relaxed, theopening of the glottis is wider. Thus the air enters and leaves the larynxduring breathing, without throwing the cords into vibration enough toproduce any sound. 

We may say that the production of the voice is effected by an arrangementlike that of some musical instruments, the sounds produced by thevibrations of the vocal cords being modified by the tubes above and below. All musical sounds are due to movements or vibrations occurring with acertain regularity, and they differ in loudness, pitch, and quality. Loudness of the sound depends upon the extent of the vibrations, pitch onthe rapidity of the vibrations, and quality on the admixture of tonesproduced by vibrations of varying rates of rapidity, related to oneanother. 

[Illustration: Fig. 151. --Longitudinal Section of the Larynx. (Showing thevocal cords. )

 A, epiglottis; B, section of hyoid bone; C, superior vocal cord; D, ventricle of the larynx; E, inferior vocal cord; F, section of the thyroid cartilage; H, section of anterior portion of the cricoid cartilage; K, trachea; L, section of the posterior portion of the cricoid cartilage; M, arytenoid cartilage; N, section of the arytenoid muscle. ]

353. Factors in the Production of the Voice. Muscles which pass fromthe cricoid cartilage to the outer angle of the arytenoids act to bringthe vocal cords close together, and parallel to one another, so that thespace between them is narrowed to a slit. A strong expiration now drivesthe air from the lungs through the slit, between the cords, and throwsthem into vibration. The vibration is small in amount, but very rapid. Other muscles are connected with the arytenoid cartilages which serve toseperate the vocal cords and to widely open the glottis. The force of theoutgoing current of air determines the extent of the movement of thecords, and thus the loudness of the sound will increase with greater forceof expiration. 

We have just learned that the pitch of sound depends on the rapidity ofthe vibrations. This depends on the length of cords and their tightnessfor the shorter and tighter a string is, the higher is the note which itsvibration produces. The vocal cords of women are about one-third shorterthan those of men, hence the higher pitch of the notes they produce. Inchildren the vocal cords are shorter than in adults. [50] The cords oftenor singers are also shorter than those of basses and baritones. Themuscles within the larynx, of course, play a very important part inaltering the tension of the vocal cords. Those qualities of the voicewhich we speak of as sweet, harsh, and sympathetic depend to a greatextent upon the peculiar structure of the vocal cords of the individual. 

Besides the physical condition of the vocal cords, as their degree ofsmoothness, elasticity, thickness, and so on, other factors determine thequality of an individual's voice. Thus, the general shape and structure ofthe trachea, the larynx, the throat, and mouth all influence the qualityof voice. In fact, the air passages, both below and above the vibratingcords, act as resonators, or resounding chambers, and intensify and modifythe sounds produced by the cords. It is this fact that prompts skillfulteachers of music and elocution to urge upon their pupils the necessity ofthe mouth being properly opened during speech, and especially duringsinging. 

 Experiment 187. _To show the anatomy of the throat_. Study the general construction of the throat by the help of a hand mirror. Repeat the same on the throat of some friend. 

 Experiment 188. _To show the construction of the vocal organs_. Get a butcher to furnish two windpipes from a sheep or a calf. They differ somewhat from the vocal organs of the human body, but will enable us to recognize the different parts which have been described, and thus to get a good idea of the gross anatomy. 

 One specimen should be cut open lengthwise in the middle line in front, and the other cut in the same way from behind. 

354. Speech. Speech is to be distinguished from voice. It may existwithout voice, as in a whisper. Speech consists of articulatedsounds, produced by the action of various parts of the mouth, throat, andnose. Voice is common to most animals, but speech is the peculiarprivilege of man. 

[Illustration: Fig. 152. --Diagramatic Horizontal Section of Larynx to showthe Direction of Pull of the Posterior Crico-Arytenoid Muscles, whichabduct the Vocal Cords. (Dotted lines show position in abduction. )]

The organ of speech is perhaps the most delicate and perfect _motor_apparatus in the whole body. It has been calculated that upwards of 900movements per minute can be made by the movable organs of speech duringreading, speaking, and singing. It is said that no less than a hundreddifferent muscles are called into action in talking. Each part of thisdelicate apparatus is so admirably adjusted to every other that all partsof this most complex machinery act in perfect harmony. 

There are certain articulate sounds called vowel or vocal, from the factthat they are produced by the vocal cords, and are but slightly modifiedas they pass out of the mouth. The true vowels, _a, e, i, o, u_, canall be sounded alone, and may be prolonged in expiration. These are thesounds chiefly used in singing. The differences in their characters areproduced by changes in the position of the tongue, mouth, and lips. 

Consonants are sounds produced by interruptions of the outgoingcurrent of air, but in some cases have no sound in themselves, and servemerely to modify vowel sounds. Thus, when the interruption to the outgoingcurrent takes place by movements of the lips, we have the _labial_consonants, _p_, _b_, _f_, and _v_. When the tongue, in relation with theteeth or hard palate, obstructs the air, the _dental_ consonants, _d_, _t_, _l_, and _s_ are produced. _Gutturals_, such as _k_, _g_, _ch_, _gh_, and _r_, are due to the movements of the root of the tongue in connectionwith the soft palate or pharynx. 

To secure an easy and proper production of articulate sounds, the mouth, teeth, lips, tongue, and palate should be in perfect order. Themodifications in articulation occasioned by a defect in the palate, or inthe uvula, by the loss of teeth, from disease, and from congenitaldefects, are sufficiently familiar. We have seen that speech consistsessentially in a modification of the vocal sounds by the accessory organs, or by parts above the larynx, the latter being the essential vocalinstrument. 

Many animals have the power of making articulated sounds; a few haverisen, like man, to the dignity of sentences, but these are only byimitation of the human voice. Both vowels and consonants can bedistinguished in the notes of birds, the vocal powers of which aregenerally higher than those of mammals. The latter, as a rule, produceonly vowels, though some are also able to form consonants. 

Persons idiotic from birth are incapable of producing any other vocalsounds than inarticulate cries, although supplied with all the internalmeans of articulation. Persons deaf and dumb are in the same situation, though from a different cause; the one being incapable of imitating, andthe other being deprived of hearing the sounds to be imitated. 

[Illustration: Fig. 153. --Direction of Pull of the LateralCrico-Arytenoids, which adduct the Vocal Cords. (Dotted lines showposition in adduction. )]

In _whispering_, the larynx takes scarcely any part in the production ofthe sounds; the vocal cords remain apart and comparatively slack, and theexpiratory blast rushes through without setting them in vibration. 

In _stammering_, spasmodic contraction of the diaphragm interrupts theeffort of expiration. The stammerer has full control of the mechanism ofarticulation, but not of the expiratory blast. His larynx and his lips areat his command, but not his diaphragm. To conquer this defect he musttrain his muscles of respiration to calm and steady action during speech. The _stutterer_, on the other hand, has full control of the muscles ofexpiration. His diaphragm is well drilled, but his lips and tongue areinsubordinate. 

355. The Care of the Throat and Voice. The throat, exposed as it isto unwholesome and overheated air, irritating dust of the street, factories, and workshops, is often inflamed, resulting in that commonailment, _sore throat_. The parts are red, swollen, and quite painful onswallowing. Speech is often indistinct, but there is no hoarseness orcough unless the uvula is lengthened and tickles the back part of thetongue. Slight sore throat rarely requires any special treatment, asidefrom simple nursing. 

The most frequent cause of throat trouble is the action of cold upon theheated body, especially during active perspiration. For this reason a coldbath should not be taken while a person is perspiring freely. The musclesof the throat are frequently overstrained by loud talking, screaming, shouting, or by reading aloud too much. People who strain or misuse thevoice often suffer from what is called "clergyman's sore throat. " Attacksof sore throat due to improper methods of breathing and of using the voiceshould be treated by judicious elocutionary exercises and a system ofvocal gymnastics, under the direction of proper teachers. 

Persons subject to throat disease should take special care to wearsuitable underclothing, adapted to the changes of the seasons. Frequentbaths are excellent tonics to the skin, and serve indirectly to protectone liable to throat ailments from changes in the weather. It is notprudent to muffle the neck in scarfs, furs, and wraps, unless perhapsduring an unusual exposure to cold. Such a dress for the neck only makesthe parts tender, and increases the liability to a sore throat. 

Every teacher of elocution or of vocal music, entrusted with the trainingof a voice of some value to its possessor, should have a good, practicalknowledge of the mechanism of the voice. Good voices are often injured byinjudicious management on the part of some incompetent instructor. It isalways prudent to cease speaking or singing in public the moment there isany hoarseness or sore throat. 

The voice should not be exercised just after a full meal, for a fullstomach interferes with the free play of the diaphragm. A sip of watertaken at convenient intervals, and held in the mouth for a moment or two, will relieve the dryness of the throat during the use of the voice. 

356. Effect of Alcohol upon the Throat and Voice. Alcoholic beveragesseriously injure the throat, and consequently the voice, by causing achronic inflammation of the membrane lining the larynx and the vocalcords. The color is changed from the healthful pink to red, and thenatural smooth surface becomes roughened and swollen, and secretes a toughphlegm. 

The vocal cords usually suffer from this condition. They are thickened, roughened, and enfeebled, the delicate vibration of the cords is impaired, the clearness and purity of the vocal tones are gone, and instead thevoice has become rough and husky. So well known is this result thatvocalists, whose fortune is the purity and compass of their tones, arescrupulously careful not to impair these fine qualities by convivialindulgences. 

357. Effect of Tobacco upon the Throat and Voice. The effect oftobacco is often specially serious upon the throat, producing a diseasewell known to physicians as "the smoker's sore throat. " Still further, itproduces inflammation of the larynx, and thus entails disorders of thevocal cords, involving rough voice and harsh tones. For this reasonvocalists rarely allow themselves to come under the narcotic influence oftobacco smoke. It is stated that habitual smokers rarely have a normalcondition of the throat. 

Additional Experiments. 

 Experiment 189. _To illustrate the importance of the resonating cavity of the nose in articulation_. Pinch the nostrils, and try to pronounce slowly the words "Lincoln, " "something, " or any other words which require the sound of _m_, _ln_, or _ng_. 

 [Illustration: Fig. 154. ]

 Experiment 190. _To illustrate the passage of air through the glottis. _ Take two strips of India rubber, and stretch them over the open end of a boy's "bean-blower, " or any kind of a tube. Tie them tightly with thread, so that a chink will be left between them, as shown in Fig. 154. Force the air through such a tube by blowing hard, and if the strips are not too far apart a sound will be produced. The sound will vary in character, just as the bands are made tight or loose. 

 Experiment 191. "A very good illustration of the action of the vocal bands in the production of the voice may be given by means of a piece of bamboo or any hollow wooden tube, and a strip of rubber, about an inch or an inch and a half wide, cut from the pure sheet rubber used by dentists. 

 "One end of the tube is to be cut sloping in two directions, and the strip of sheet rubber is then to be wrapped round the tube, so as to leave a narrow slit terminating at the upper corners of the tube. 

 "By blowing into the other end of the tube the edges of the rubber bands will be set in vibration, and by touching the vibrating membrane at different points so as to check its movements it may be shown that the pitch of the note emitted depends upon the length and breadth of the vibrating portion of the vocal bands. "[51]--Dr. H. P. Bowditch. 

 [NOTE. The limitations of a text-book on physiology for schools do not permit so full a description of the voice as the subject deserves. For additional details, the student is referred to Cohen's _The Throat and the Voice_, a volume in the "American Health Primer Series. " Price 40 cents. ]

Chapter XIII. 

Accidents and Emergencies. 

358. Prompt Aid to the Injured. A large proportion of the accidents, emergencies, and sudden sicknesses that happen do not call for medical orsurgical attention. For those that do require the services of a physicianor surgeon, much can be often done before the arrival of professionalhelp. Many a life has been saved and much suffering and anxiety preventedby the prompt and efficient help of some person with a cool head, a steadyhand, and a practical knowledge of what to do first. Many of us can recallwith mingled admiration and gratitude the prompt services rendered ourfamilies by some neighbor or friend in the presence of an emergency orsudden illness. 

In fact, what we have studied in the preceding chapters becomes tenfoldmore interesting, instructive, and of value to us, if we are able tosupplement such study with its practical application to the treatment ofthe more common and less serious accidents and emergencies. 

While no book can teach one to have presence of mind, a cool head, or torestrain a more or less excitable temperament in the midst of suddendanger, yet assuredly with proper knowledge for a foundation, a certainself-confidence may be acquired which will do much to prevent hastyaction, and to maintain a useful amount of self-control. 

Space allows us to describe briefly in this chapter only a few of thesimplest helps in the more common accidents and emergencieswhich are met with in everyday life. [52]

 359. Hints as to what to Do First. Retain so far as possible yourpresence of mind, or, in other words, keep cool. This is an all-importantdirection. Act promptly and quietly, but not with haste. Whatever you do, do in earnest; and never act in a half-hearted manner in the presence ofdanger. Of course, a knowledge of what to-do and how to do it willcontribute much towards that self-control and confidence that commandsuccess. Be sure and send for a doctor at once if the emergency calls forskilled service. All that is expected of you under such circumstancesis to tide over matters until the doctor comes. 

[Illustration: Fig. 155. --Showing how Digital Compression should beapplied to the Brachial Artery. ]

Do not presume upon any smattering of knowledge you have, to assume anyrisk that might lead to serious results. Make the sufferer comfortable bygiving him an abundance of fresh air and placing him in a restfulposition. Do all that is possible to keep back the crowd of curiouslookers-on, whom a morbid curiosity has gathered about the injured person. Loosen all tight articles of clothing, as belts, collars, corsets, andelastics. Avoid the use of alcoholic liquors. They are rarely of any realservice, and in many instances, as in bleeding, may do much harm. 

360. Incised and Lacerated Wounds. An incised or cut woundis one made by a sharp instrument, as when the finger is cut with aknife. Such a wound bleeds freely because the clean-cut edges do not favorthe clotting of blood. In slight cuts the bleeding readily ceases, and thewound heals by primary union, or by "first intention, " as surgeons callit. 

Lacerated and contused wounds are made by a tearing or bruisinginstrument, for example, catching the finger on a nail. Such wounds bleedbut little, and the edges and surfaces are rough and ragged. 

If the incised wound is deep or extensive, a physician is necessary tobring the cut edges together by stitches in order to get primary union. Oftentimes, in severe cuts, and generally in lacerations, there is a lossof tissue, so that the wound heals by "second intention"; that is, thewound heals from the bottom by a deposit of new cells called_granulations_, which gradually fill it up. The skin begins to grow fromthe edges to the center, covering the new tissue and leaving a cicatrix orscar with which every one is familiar. 

361. Contusion and Bruises. An injury to the soft tissues, caused bya blow from some blunt instrument, or a fall, is a contusion, orbruise. It is more or less painful, followed by discoloration due tothe escape of blood under the skin, which often may not be torn through. Ablack eye, a knee injured by a fall from a bicycle, and a finger hurt by abaseball, are familiar examples of this sort of injury. Such injuriesordinarily require very simple treatment. 

The blood which has escaped from the capillaries is slowly absorbed, changing color in the process, from blue black to green, and fading into alight yellow. Wring out old towels or pieces of flannel in hot water, andapply to the parts, changing as they become cool. For cold applications, cloths wet with equal parts of water and alcohol, vinegar, and witch-hazelmay be used. Even if the injury is apparently slight it is always safe torest the parts for a few days. 

When wounds are made with ragged edges, such as those made by brokenglass and splinters, more skill is called for. Remove every bit offoreign substance. Wash the parts clean with one of the manyantiseptic solutions, bring the torn edges together, and hold them inplace with strips of plaster. Do not cover such an injury all over withplaster, but leave room for the escape of the wound discharges. For anoutside dressing, use compresses made of clean cheese-cloth or strips ofany clean linen cloth. The antiseptic _corrosive-sublimate gauze_ on saleat any drug store should be used if it can be had. 

Wounds made by toy pistols, percussion-caps, and rusty nails and tools, ifneglected, often lead to serious results from blood-poisoning. A hotflaxseed poultice may be needed for several days. Keep such wounds cleanby washing or syringing them twice a day with hot _antiseptics_, which arepoisons to _bacteria_ and kill them or prevent their growth. Bacteria arewidely distributed, and hence the utmost care should be taken to haveeverything which is to come in contact with a wounded surface free fromthe germs of inflammation. In brief, such injuries must be kept_scrupulously neat_ and _surgically clean_. 

[Illustration: Fig. 156. --Dotted Line showing the Course of the BrachialArtery. ]

The injured parts should be kept at rest. Movement and disturbance hinderthe healing process. 

362. Bites of Mad Dogs. Remove the clothing at once, if only from thebitten part, and apply a temporary ligature _above_ the wound. Thisinterrupts the activity of the circulation of the part, and to thatextent delays the absorption of the poisonous saliva by the blood-vesselsof the wound. A dog bite is really a lacerated and contused wound, andlying in the little roughnesses, and between the shreds, is the poisonoussaliva. If by any means these projections and depressions affording thelodgment can be removed, the poison cannot do much harm. If done with aknife, the wound would be converted, practically, into an incised wound, and would require treatment for such. 

If a surgeon is at hand he would probably cut out the injured portion, orcauterize it thoroughly. Professional aid is not always at our command, and in such a case it would be well to take a poker, or other suitablepiece of iron, heat it _red_ hot in the fire, wipe off and destroy theentire surface of the wound. As fast as destroyed, the tissue becomeswhite. An iron, even at a white heat, gives less pain and at once destroysthe vitality of the part with which it comes in contact. 

If the wound is at once well wiped out, and a stick of solid nitrate ofsilver (lunar caustic) rapidly applied to the entire surface of the wound, little danger is to be apprehended. Poultices and warm fomentations shouldbe applied to the injury to hasten the sloughing away of the part whosevitality has been intentionally destroyed. 

Any dog, after having bitten a person, is apt, under a mistaken belief, tobe at once killed. This should not be done. There is no more danger from adog-bite, unless the dog is suffering from the disease called _rabies_ oris "mad, " than from any other lacerated wound. The suspected animal shouldbe at once placed in confinement and watched, under proper safeguards, forthe appearance of any symptoms that indicate rabies. 

Should no pronounced symptoms indicate this disease in the dog, a greatdeal of unnecessary mental distress and worry can be saved both on thepart of the person bitten and his friends. 

363. Injuries to the Blood-vessels. It is very important to know thedifference between the bleeding from an artery and that from a vein. 

If an artery bleeds, the blood leaps in spurts, and is of abright scarlet color. 

If a vein bleeds, the blood flows in a steady stream, and is of adark purple color. 

If the capillaries are injured the blood merely oozes. 

Bleeding from an artery is a dangerous matter in proportion to the size ofthe vessel, and life itself may be speedily lost. Hemorrhage from a veinor from the capillaries is rarely troublesome, and is ordinarily easilychecked, aided, if need be, by hot water, deep pressure, the applicationof some form of iron styptic, or even powdered alum. When an artery isbleeding, always remember to make deep pressure between the wound and theheart. In all such cases send at once for the doctor. 

[Illustration: Fig. 157. --Showing how Digital Compression should beapplied to the Femoral Artery. ]

Do not be afraid to act at once. A resolute grip in the right place withfirm fingers will do well enough, until a twisted handkerchief, stoutcord, shoestring, suspender, or an improvised tourniquet[53] is ready totake its place. If the flow of blood does not stop, change the pressureuntil the right spot is found. 

Sometimes it will do to seize a handful of dry earth and crowd it downinto the bleeding wound, with a firm pressure. Strips of an oldhandkerchief, underclothing, or cotton wadding may also be used as acompress, provided pressure is not neglected. 

In the after-treatment it is of great importance that the wound and thedressing should be kept free from bacteria by keeping everythingsurgically clean. 

364. Where and how to Apply Pressure. The principal places in whichto apply pressure when arteries are injured and bleeding should always bekept in mind. 

 Experiment 192. _How to tie a square knot_. If the student would render efficient help in accidents and emergencies, to say nothing of service on scores of other occasions, he must learn how to tie a square or "reef" knot. This knot is secure and does not slip as does the "granny" knot. The square knot is the one used by surgeons in ligating vessels and securing bandages. Unless one knew the difference, the insecure "granny" knot might be substituted. 

 [Illustration: Fig. 158. --Showing how a Square Knot may be tied with a Cord and a Handkerchief. ]

 A square knot is tied by holding an end of a bandage or cord in each hand, and then passing the end in the _right_ hand over the one in the left and tying; the end now in the _left_ hand is passed over the one in the right and again tied. 

If in the finger, grasp it with the thumb and forefinger, and pinchit firmly on each side; if in the hand, press on the bleeding spot, or press with the thumb just above and in front of the wrist. 

For injuries below the elbow, grasp the upper part of the arm withthe hands, and squeeze hard. The main artery runs in the middle line ofthe bend of the elbow. Tie the knotted cord here, and bend the forearm soas to press hard against the knot. 

For the upper arm, press with the fingers against the bone on theinner side, and just on the edge of the swell of the biceps muscle. Now weare ready for the knotted cord. Take a stout stick of wood, about a footlong, and twist the cord hard with it, bringing the knot firmly over theartery. 

For the foot or leg, pressure as before, in the hollow behindthe knee, just above the calf of the leg. Bend the thigh towards theabdomen and bring the leg up against the thigh, with the knot in the bendof the knee. 

365. Bleeding from the Stomach and Lungs. Blood that comes from thelungs is bright red, frothy, or "soapy. " There is rarely much; it usuallyfollows coughing, feels warm, and has a salty taste. This is a gravesymptom. Perfect rest on the back in bed and quiet must be insisted upon. Bits of ice should be eaten freely. Loosen the clothing, keep theshoulders well raised, and the body in a reclining position and absolutelyat rest. Do not give alcoholic drinks. 

Blood from the stomach is not frothy, has a sour taste, and isusually dark colored, looking somewhat like coffee grounds. It is more inquantity than from the lungs, and is apt to be mixed with food. Employ thesame treatment, except that the person should be kept flat on the back. 

366. Bleeding from the Nose. This is the most frequent and the leastdangerous of the various forms of bleeding. Let the patient sit upright;leaning forward with the head low only increases the hemorrhage. Raise thearm on the bleeding side; do not blow the nose. Wring two towels out ofcold water; wrap one around the neck and the other properly folded overthe forehead and upper part of the nose. 

Add a teaspoonful of powdered _alum_ to a cup of water, and snuff it upfrom the hand. If necessary, soak in alum water a piece of absorbentcotton, which has been wound around the pointed end of a pencil orpenholder; plug the nostril by pushing it up with a twisting motion untilfirmly lodged. 

367. Burns or Scalds. Burns or scalds are dangerous in proportion totheir extent and depth. A child may have one of his fingers burned offwith less danger to life than an extensive scald of his back and legs. Adeep or extensive burn or scald should always have prompt medicalattendance. 

In burns by acids, bathe the parts with an alkaline fluid, as dilutedammonia, or strong soda in solution, and afterwards dress the burn. 

In burns caused by lime, caustic potash, and other alkalies, soak theparts with vinegar diluted with water; lemon juice, or any other dilutedacid. 

Remove the clothing with the greatest care. Do not pull, but carefully cutand coax the clothes away from the burned places. Save the skin unbrokenif possible, taking care not to break the blisters. The secret oftreatment is to prevent friction, and to keep out the air. Ifthe burn is slight, put on strips of soft linen soaked in a strongsolution of baking-soda and water, one heaping table spoonful to a cupfulof water. This is especially good for scalds. 

[Illustration: Fig. 159. --Dotted Line showing the Course of the FemoralArtery. ]

_Carron oil_ is one of the best applications. It is simply halflinseed-oil and half lime-water shaken together. A few tablespoonfuls ofcarbolic acid solution to one pint may be added to this mixture to helpdeaden the pain. Soak strips of old linen or absorbent cotton in thistime-honored remedy, and gently apply. 

If carbolized or even plain _vaseline_ is at hand, spread it freely onstrips of old linen, and cover well the burnt parts, keeping out the airwith other strips carefully laid on. Simple cold water is better thanflour, starch, toilet powder, cotton batting, and other things which areapt to stick, and make an after-examination very painful. 

[Illustration: Fig. 160. --Showing how Hemorrhage from the Femoral Arterymay be arrested by the Use of an Improvised Apparatus (technically calleda _Tourniquet_). ]

368. Frost Bites. The ears, toes, nose, and fingers are occasionallyfrozen, or frost-bitten. No warm air, warm water, or fire should beallowed near the frozen parts until the natural temperature is nearlyrestored. Rub the frozen part vigorously with snow or snow-water in a coldroom. Continue this until a burning, tingling pain is felt, when allactive treatment should cease. 

Pain shows that warmth and circulation are beginning to return. The aftereffects of a frost bite are precisely like those of a burn, and requiresimilar treatment. Poultices made from scraped raw potatoes afford muchcomfort for an after treatment. 

369. Catching the Clothing on Fire. When the clothing catches fire, throw the person down on the ground or floor, as the flames will tendless to rise toward the mouth and nostrils. Then without a moment's delay, roll the person in a carpet or hearth-rug, so as to stifle the flames, leaving only the head out for breathing. 

If no carpet or rug can be had, then take off your coat, shawl, or cloakand use it instead. Keep the flame as much as possible from the face, soas to prevent the entrance of the hot air into the lungs. This can be doneby beginning at the neck and shoulders with the wrapping. 

370. Foreign Bodies in the Throat. Bits of food or other smallobjects sometimes get lodged in the throat, and are easily extracted bythe forefinger, by sharp slaps on the back, or expelled by vomiting. If itis a sliver from a toothpick, match, or fishbone, it is no easy matter toremove it; for it generally sticks into the lining of the passage. If theobject has actually passed into the windpipe, and is followed by suddenfits of spasmodic coughing, with a dusky hue to the face and fingers, surgical help must be called without delay. 

If a foreign body, like coins, pencils, keys, fruit-stones, etc. , isswallowed, it is not wise to give a physic. Give plenty of hard-boiledeggs, cheese, and crackers, so that the intruding substance maybe enfoldedin a mass of solid food and allowed to pass off in the natural way. 

371. Foreign Bodies in the Nose. Children are apt to push beans, peas, fruit-stones, buttons, and other small objects, into the nose. Sometimes we can get the child to help by blowing the nose hard. At othertimes, a sharp blow between the shoulders will cause the substance to fallout. If it is a pea or bean, which is apt to swell with the warmth andmoisture, call in medical help at once. 

372. Foreign Bodies in the Ear. It is a much more difficult matter toget foreign bodies out of the ear than from the nose. Syringe in a littlewarm water, which will often wash out the substance. If live insects getinto the ear, drop in a little sweet oil, melted vaseline, salt and water, or even warm molasses. 

If the tip of the ear is pulled up gently, the liquid will flow in morereadily. If a light is held close to the outside ear, the insect may becoaxed to crawl out towards the outer opening of the ear, being attractedby the bright flame. 

373. Foreign Bodies in the Eye. Cinders, particles of dust, and othersmall substances, often get into the eye, and cause much pain. It willonly make bad matters worse to rub the eye. Often the copious flow oftears will wash the substance away. It is sometimes seen, and removedsimply by the twisted corner of a handkerchief carefully used. If it isnot removed, or even found, in this way, the upper lid must be turnedback. 

[Illustration: Fig. 161. --Showing how the Upper Eyelid may be everted witha Pencil or Penholder. ]

This is done usually as follows: Seize the lashes between the thumb andforefinger, and draw the edge of the lid away from the eyeball. Now, telling the patient to look down, press a slender lead-pencil or penholderagainst the lid, parallel to and above the edge, and then pull the edgeup, and turn it over the pencil by means of the lashes. 

The eye is now readily examined, and usually the foreign body is easilyseen and removed. Do not increase the trouble by rubbing the eye after youfail, but get at once skilled help. After the substance has been removed, bathe the eye for a time with hot water. 

If lime gets into the eye, it may do a great amount of mischief, andgenerally requires medical advice, or permanent injury will result. Untilsuch advice can be had, bathe the injured parts freely with a weaksolution of vinegar and hot water. 

374. Broken Bones. Loss of power, pain, and swelling are symptoms ofa broken bone that may be easily recognized. Broken limbs should always behandled with great care and tenderness. If the accident happens in thewoods, the limb should be bound with handkerchiefs, suspenders, or stripsof clothing, to a piece of board, pasteboard, or bark, padded with moss orgrass, which will do well enough for a temporary splint. Always put abroken arm into a sling after the splints are on. 

[Illustration: Fig. 162. --Showing how an Umbrella may be used as aTemporary Splint in Fracture of the Leg. ]

Never move the injured person until the limb is made safe from furtherinjuries by putting on temporary splints. If you do not need to move theperson, keep the limb in a natural, easy position, until the doctor comes. 

Remember that this treatment for broken bones is only to enable thepatient to be moved without further injury. A surgeon is needed at once toset the broken bone. 

[Illustration: Fig. 163. --Showing how a Pillow may be used as a TemporarySplint in Fracture of the Leg. ]

375. Fainting. A fainting person should be laid flat at once. Giveplenty of fresh air, and dash cold water, if necessary, on the head andneck. Loosen all tight clothing. Smelling-salts may be held to the nose, to excite the nerves of sensation. 

376. Epileptic and Hysterical Fits, Convulsions of Children. Sufferers from "fits" are more or less common. In _epilepsy_, the suffererfalls with a peculiar cry; a loss of consciousness, a moment of rigidity, and violent convulsions follow. There is foaming at the mouth, the eyesare rolled up, and the tongue or lips are often bitten. When the fit isover the patient remains in a dazed, stupid state for some time. It is amistake to struggle with such patients, or to hold them down and keep themquiet. It does more harm than good. 

See that the person does not injure himself; crowd a pad made from afolded handkerchief or towel between the teeth, to prevent biting of thelips or tongue. Do not try to make the sufferer swallow any drink. Unfasten the clothes, especially about the neck and chest. Persons who aresubject to such fits should rarely go out alone, and never into crowded orexcited gatherings of any kind. 

_Hysterical fits_ almost always occur in young women. Such patients neverbite their tongue nor hurt themselves. Placing a towel wrung out in coldwater across the face, or dashing a little cold water on the face orneck, will usually cut short the fit, speaking firmly to the patient atthe same time. Never sympathize too much with such patients; it will onlymake them a great deal worse. 

377. Asphyxia. Asphyxia is from the Greek, and means an "absence ofpulse. " This states a fact, but not the cause. The word is now commonlyused to mean _suspended animation_. When for any reason the proper supplyof oxygen is cut off, the tissues rapidly load up with carbon dioxid. Theblood turns dark, and does not circulate. The healthy red or pink look ofthe lips and finger-nails becomes a dusky purple. The person is sufferingfrom a lack of oxygen; that is, from asphyxia, or suffocation. It is evident there can be several varieties of asphyxia, as in apparentdrowning, strangulation and hanging, inhalation of gases, etc. 

The first and essential thing to do is to give fresh air. Remove theperson to the open air and place him on his back. Remove tight clothingabout the throat and waist, dash on cold water, give a few drops ofammonia in hot water or hot ginger tea. Friction applied to the limbsshould be kept up. If necessary, use artificial respiration by theSylvester method (sec. 380). 

The chief dangers from poisoning by noxious gases come from the fumes ofburning coal in the furnace, stove, or range; from "blowing out" gas, turning it down, and having it blown out by a draught; from the foul airoften found in old wells; from the fumes of charcoal and the foul air ofmines. 

378. Apparent Drowning. Remove all tight clothing from the neck, chest, and waist. Sweep the forefinger, covered with a handkerchief ortowel, round the mouth, to free it from froth and mucus. Turn the body onthe face, raising it a little, with the hands under the hips, to allow anywater to run out from the air passages. Take only a moment for this. 

Lay the person flat upon the back, with a folded coat, or pad of anykind, to keep the shoulders raised a little. Remove all the wet, clingingclothing that is convenient. If in a room or sheltered place, strip thebody, and wrap it in blankets, overcoats, etc. If at hand, use bottles ofhot water, hot flats, or bags of hot sand round the limbs and feet. Watchthe tongue: it generally tends to slip back, and to shut off the air fromthe glottis. Wrap a coarse towel round the tip of the tongue, and keep itwell pulled forward. 

The main thing to do is to keep up artificial respiration until thenatural breathing comes, or all hope is lost. This is the simplest way todo it: The person lies on the back; let some one kneel behind the head. Grasp both arms near the elbows, and sweep them upward above the headuntil they nearly touch. Make a firm pull for a moment. This tends to fillthe lungs with air by drawing the ribs up, and making the chest cavitylarger. Now return the arms to the sides of the body until they press hardagainst the ribs. This tends to force out the air. This makes artificiallya complete act of respiration. Repeat this act about fifteen times everyminute. 

[Illustration: Fig. 164. --The Sylvester Method. (Firstmovement--inspiration. )]

All this may be kept up for several hours. The first sign of recovery isoften seen in the slight pinkish tinge of the lips or finger-nails. Thatthe pulse cannot be felt at the wrist is of little value in itself as asign of death. Life may be present when only the most experienced ear candetect the faintest heart-beat. 

When a person can breathe, even a little, he can swallow. Holdsmelling-salts or hartshorn to the nose. Put one teaspoonful of thearomatic spirits of ammonia, or even of ammonia water, into a half-glassof hot water, and give a few teaspoonfuls of this mixture every fewminutes. Meanwhile do not fail to keep up artificial warmth in the mostvigorous manner. 

379. Methods of Artificial Respiration. There are severalwell-established methods of artificial respiration. The two known as theSylvester and the Marshall Hall methods are generally acceptedas efficient and practical. 

[Illustration: Fig. 165. --The Sylvester Method. (Secondmovement--expiration. )]

380. The Sylvester Method. The water and mucus are supposed to havebeen removed from the interior of the body by the means above described(sec. 378). 

The patient is to be placed on his back, with a roll made of a coat or ashawl under the shoulders; the tongue should then be drawn forward andretained by a handkerchief which is placed across the extended organ andcarried under the chin, then crossed and tied at the back of the neck. Anelastic band or small rubber tube or a suspender may be used for the samepurpose. 

The attendant should kneel at the head and grasp the elbows of thepatient and draw them upward until the hands are carried above the headand kept in this position until one, two, three, can be slowly counted. This movement elevates the ribs, expands the chest, and creates a vacuumin the lungs into which the air rushes, or in other words, the movementproduces _inspiration_. The elbows are then slowly carried downward, placed by the side, and pressed inward against the chest, therebydiminishing the size of the latter and producing _expiration_. 

These movements should be repeated about fifteen times each minute for atleast two hours, provided no signs of animation show themselves. 

381. The Marshall Hall Method. The patient should be placed facedownwards, the head resting on the forearm with a roll or pillow placedunder the chest; he should then be turned on his side, an assistantsupporting the head and keeping the mouth open; after an interval of twoor three seconds, the patient should again be placed face downward andallowed to remain in this position the same length of time. This operationshould be repeated fifteen or sixteen times each minute, and continued(unless the patient recovers) for at least two hours. 

[Illustration: Fig. 166. --The Marshall Hall Method. (First position. )]

If, after using one of the above methods, evidence of recovery appears, such as an occasional gasp or muscular movement, the efforts to produceartificial respiration must not be discontinued, but kept up untilrespiration is fully established. All wet clothing should then be removed, the patient rubbed dry, and if possible placed in bed, where warmth andwarm drinks can be properly administered. A small amount of nourishment, in the form of hot milk or beef tea, should be given, and the patient keptquiet for two or three days. 

[Illustration: Fig. 167. --The Marshall Hall Method. (Second position. )]

382. Sunstroke or Heatstroke. This serious accident, so far-reachingoftentimes in its result, is due to an unnatural elevation of the bodilytemperature by exposure to the direct rays of the sun, or from the extremeheat of close and confined rooms, as in the cook-rooms and laundries ofhotel basements, from overheated workshops, etc. 

There is sudden loss of consciousness, with deep, labored breathing, anintense burning heat of the skin, and a marked absence of sweat. The mainthing is to lower the temperature. Strip off the clothing; apply choppedice, wrapped in flannel to the head. Rub ice over the chest, and placepieces under the armpits and at the sides. If there is no ice, use sheetsor cloths wet with cold water. The body may be stripped, and sprinkledwith ice-water from a common watering-pot. 

If the skin is cold, moist, or clammy, the trouble is due to heatexhaustion. Give plenty of fresh air, but apply no cold to the body. Applyheat, and give hot drinks, like hot ginger tea. Sunstroke or heatstroke isa dangerous affliction. It is often followed by serious and permanentresults. Persons who have once suffered in this way should carefully avoidany risk in the future. 

Chapter XIV. 

In Sickness and in Health. 

383. Arrangement of the Sick-room. This room, if possible, should beon the quiet and sunny side of the house. Pure, fresh air, sunshine, andfreedom from noise and odor are almost indispensable. A fireplace as ameans of ventilation is invaluable. The bed should be so placed that theair may get to it on all sides and the nurse move easily around it. Screens should be placed, if necessary, so as to exclude superfluous lightand draughts. 

The sick-room should be kept free from all odors which affect the sickunpleasantly, as perfumery, highly scented soaps, and certain flowers. Remove all useless ornaments and articles likely to collect dust, asunnecessary pieces of furniture and heavy draperies. A clean floor, with afew rugs to deaden the footsteps, is much better than a woolen carpet. Rocking-chairs should be banished from the sick-room, as they are almostsure to disturb the sick. 

A daily supply of fresh flowers tends to brighten the room. Keep themedicines close at hand, but all poisonous drugs should be kept carefullyby themselves and ordinarily under lock and key. A small table should beplaced at the bedside, and on it the bell, food tray, flowers and othersmall things which promote the comfort of the patient. 

The nurse should not sleep with the patient. Sofas and couches are notcommonly comfortable enough to secure needed rest. A cot bed is at onceconvenient and inexpensive, and can be readily folded and put out of sightin the daytime. It can also be used by the patient occasionally, especially during convalescence. 

384. Ventilation of the Sick-room. Proper ventilation is mostessential to the sick-room, but little provision is ordinarily made for soimportant a matter. It is seldom that one of the windows cannot be letdown an inch or more at the top, a screen being arranged to avoid anydraught on the patient. Remove all odors by ventilation and not byspraying perfumery, or burning pastilles, which merely conceal offensiveodors without purifying the air. During cold weather and in certaindiseases, the patient may be covered entirely with blankets and thewindows opened wide for a few minutes. 

Avoid ventilation by means of doors, for the stale air of the house, kitchen smells, and noises made by the occupants of the house, are apt toreach the sick-room. The entire air of the room should be changed at leasttwo or three times a day, in addition to the introduction of a constantsupply of fresh air in small quantities. 

385. Hints for the Sick-room. Always strive to look cheerful andpleasant before the patient. Whatever may happen, do not appear to beannoyed, discouraged, or despondent. Do your best to keep up the courageof sick persons under all circumstances. In all things keep in constantmind the comfort and ease of the patient. 

Do not worry the sick with unnecessary questions, idle talk, or sillygossip. It is cruel to whisper in the sick-room, for patients are alwaysannoyed by it. They are usually suspicious that something is wrong andgenerally imagine that their condition has changed for the worse. 

Symptoms of the disease should never be discussed before the patient, especially if he is thought to be asleep. He may be only dozing, and anysuch talk would then be gross cruelty. Loud talking must, of course, beavoided. The directions of the physician must be rigidly carried out inregard to visitors in the sick-room. This is always a matter of foremostimportance, for an hour or even a night of needed sleep and rest may belost from the untimely call of some thoughtless visitor. A competentnurse, who has good sense and tact, should be able to relieve the familyof any embarrassment under such circumstances. 

Do not ever allow a kerosene light with the flame turned down to remain inthe sick-room. Use the lamp with the flame carefully shaded, or in anadjoining room, or better still, use a sperm candle for a night light. 

Keep, so far as possible, the various bottles of medicine, spoons, glasses, and so on in an adjoining room, rather than to make a formidablearray of them on a bureau or table near the sick-bed. A few simple things, as an orange, a tiny bouquet, one or two playthings, or even a prettybook, may well take their place. 

The ideal bed is single, made of iron or brass, and provided with wovenwire springs and a hair mattress. Feather-beds are always objectionable inthe sick-room for many and obvious reasons. The proper making of asick-bed, with the forethought and skill demanded in certain diseases, isof great importance and an art learned only after long experience. Thesame principle obtains in all that concerns the lifting and the moving ofthe sick. 

Sick people take great comfort in the use of fresh linen and freshpillows. Two sets should be used, letting one be aired while the other isin use. In making changes the fresh linen should be thoroughly aired andwarmed and everything in readiness before the patient is disturbed. 

386. Rules for Sick-room. Do not deceive sick people. Tell what isproper or safe to be told, promptly and plainly. If a physician isemployed, carry out his orders to the very letter, as long as he visitsyou. Make on a slip of paper a note of his directions. Make a brief recordof exactly what to do, the precise time of giving medicines, etc. Thisshould always be done in serious cases, and by night watchers. Then thereis no guesswork. You have the record before you for easy reference. Allsuch things are valuable helps to the doctor. 

Whatever must be said in the sick-room, say it openly and aloud. How oftena sudden turn in bed, or a quick glance of inquiry, shows that whisperingis doing harm! If the patient is in his right mind, answer his questionsplainly and squarely. It may not be best to tell all the truth, butnothing is gained in trying to avoid a straightforward reply. 

Noises that are liable to disturb the patient, in other parts of the housethan the sick-room, should be avoided. Sounds of a startling character, especially those not easily explained, as the rattling or slamming ofdistant blinds and doors, are always irritating to the sick. 

Always attract the attention of a patient before addressing him, otherwisehe may be startled and a nervous spell be induced. The same hint appliesequally to leaning or sitting upon the sick-bed, or running againstfurniture in moving about the sick-room. 

387. Rest of Mind and Body. The great importance of rest for the sickis not so generally recognized as its value warrants. If it is worry andnot work that breaks down the mental and physical health of the well, howmuch more important is it that the minds and bodies of the sick should bekept at rest, free from worry and excitement! Hence the skilled nurse doesher best to aid in restoring the sick to a condition of health by securingfor her patient complete rest both of mind and body. To this end, sheskillfully removes all minor causes of alarm, irritation, or worry. Thereare numberless ways in which this may be done of which space does notallow even mention. Details apparently trifling, as noiseless shoes, quietness, wearing garments that do not rustle, use of small pillows ofdifferent sizes, and countless other small things that make up therefinement of modern nursing, play an important part in building up theimpaired tissues of the sick. 

388. Care of Infectious and Contagious Diseases. There are certaindiseases which are known to be infectious and can be communicated from oneperson to another, either by direct contact, through the medium of theatmosphere, or otherwise. 

Of the more prevalent infectious and contagious diseases are_scarlet fever, diphtheria, erysipelas, measles_, and _typhoid fever_. 

Considerations of health demand that a person suffering from any one ofthese diseases should be thoroughly isolated from all other members of thefamily. All that has been stated in regard to general nursing in previoussections of this chapter, applies, of course, to nursing infectious andcontagious diseases. In addition to these certain special directions mustbe always kept in mind. 

Upon the nurse, or the person having the immediate charge of the patient, rests the responsibility of preventing the spread of infectious diseases. The importance must be fully understood of carrying out in every detailthe measures calculated to check the spread or compass the destruction ofthe germs of disease. 

389. Hints on Nursing Infectious and Contagious Diseases. Strip theroom of superfluous rugs, carpets, furniture, etc. Isolate two rooms, ifpossible, and have these, if convenient, at the top of the house. Tacksheets, wet in some proper disinfectant, to the outer frame of thesick-room door. Boil these sheets every third day. In case of diseases towhich young folks are very susceptible, send the children away, ifpossible, to other houses where there are no children. 

Most scrupulous care should be taken in regard to cleanliness and neatnessin every detail. Old pieces of linen, cheese-cloth, paper napkins, shouldbe used wherever convenient or necessary and then at once burnt. Allsoiled clothing that cannot well be burnt should be put to soak at once indisinfectants, and afterward boiled apart from the family wash. Dishes andall utensils should be kept scrupulously clean by frequent boiling. Forthe bed and person old and worn articles of clothing that can be destroyedshould be worn so far as possible. 

During convalescence, or when ready to leave isolation, the patient shouldbe thoroughly bathed in water properly disinfected, the hair and nailsespecially being carefully treated. 

Many details of the after treatment depend upon the special disease, asthe rubbing of the body with carbolized vaseline after scarlet fever, thecare of the eyes after measles, and other particulars of which space doesnot admit mention here. 

Poisons and Their Antidotes. 

390. Poisons. A poison is a substance which, if taken into the systemin sufficient amounts, will cause serious trouble or death. Forconvenience poisons may be divided into two classes, irritants andnarcotics. 

The effects of irritant poisons are evident immediately after beingtaken. They burn and corrode the skin or membrane or other parts withwhich they come in contact. There are burning pains in the mouth, throat, stomach, and abdomen, with nausea and vomiting. A certain amount offaintness and shock is also present. 

With narcotic poisoning, the symptoms come on more slowly. After atime there is drowsiness, which gradually increases until there is aprofound sleep or stupor, from which the patient can be aroused only withgreat difficulty. There are some substances which possess both theirritant and narcotic properties and in which the symptoms are of a mixedcharacter. 

391. Treatment of Poisoning. An antidote is a substance which willeither combine with a poison to render it harmless, or which will have adirectly opposite effect upon the body, thus neutralizing the effect ofthe poison. Hence in treatment of poisoning the first thing to do, if youknow the special poison, is to give its antidote at once. 

If the poison is unknown, and there is any delay in obtaining theantidote, the first thing to do is to remove the poison from the stomach. Therefore cause vomiting as quickly as possible. This may be done by anemetic given as follows: Stir a tablespoonful of mustard or of commonsalt in a glass of warm water and make the patient swallow the whole. Itwill usually be vomited in a few moments. If mustard or salt is not athand, compel the patient to drink lukewarm water very freely untilvomiting occurs. 

Vomiting may be hastened by thrusting the forefinger down the throat. Twoteaspoonfuls of the syrup of ipecac, or a heaping teaspoonful of powderedipecac taken in a cup of warm water, make an efficient emetic, especiallyif followed with large amounts of warm water. 

It is to be remembered that in some poisons, as certain acids andalkalies, no emetic should be given. Again, for certain poisons (except incase of arsenic) causing local irritation, but which also affect thesystem at large, no emetic should be given. 

392. Reference Table of Common Poisons; Prominent Symptoms; Antidotes andTreatment. The common poisons with their leading symptoms, treatment, and antidotes, may be conveniently arranged for easy reference in the formof a table. 

It is to be remembered, of course, that a complete mastery of the table ofpoisons, as set forth on the two following pages, is really a physician'sbusiness. At the same time, no one of fair education should neglect tolearn a few of the essential things to do in accidental or intentionalpoisoning. 

 A Table of the More Common Poisons, With their prominent symptoms, antidotes, and treatment. 

 Poison Prominent Symptoms Antidotes and Treatment

 _Strong Acids:_

 Muriatic, Burning sensation in _No emetic_ Saleratus; Nitric, mouth, throat, and chalk; soap; plaster from Sulphuric (vitriol), stomach; blisters the wall; lime; magnesia; Oxalic. About mouth; vomiting; baking soda (3 or 4 great weakness teaspoonfuls in a glass of water). 

 _Alkalies_:

 Caustic potash and Burning sensation in _No emetic_ Olive oil soda, the parts; severe pain freely; lemon juice, vinegar; Ammonia, in stomach; vomiting; melted butter and vaseline; Lye, difficulty in thick cream. Pearlash, swallowing; cold skin; Saltpeter. Weak pulse. 

 _Arsenic:_

 Paris green, Intense pains in Vomit patient repeatedly, Rough on rats, stomach and bowels; give hydrated oxide of iron White arsenic, thirst; vomiting, with magnesia, usually kept Fowler's solution, perhaps with blood; by druggists for emergencies; Scheele's green. Cold and clammy skin. Follow with strong solution of common salt and water. 

 _Other Metallic Poisons_:

 Blue vitriol, Symptoms in general, Emetic with lead; none with Copperas, same as in arsenical copper and iron; white of Green vitriol, poisoning. With lead eggs in abundance with Sugar of lead, and mercury there may copper; with iron and lead Corrosive be a metallic taste in give epsom salts freely; sublimate, the mouth. Afterwards, oils, flour, and Bedbug poison. Water. _No emetic with mercury;_ raw eggs; milk, or flour, and water. 

 _Phosphorus from_

 Matches, rat Pain in the stomach; _Cause vomiting_. Poisons, etc. Vomiting; purging; Strong soapsuds; general collapse. Magnesia in water. Never give oils. 

 _Opium:_ Morphine, Sleepiness; dullness; _Cause vomiting_. Keep Laudanum, stupor; "pin-hole" patient awake by any means, Paregoric, pupils; slow especially by vigorous Dover's powder, breathing; profuse walking; give strong coffee Soothing syrups, sweat. Freely; dash cold water on Cholera and diarrhoea face and chest. Mixtures. 

 _Carbolic Acid:_ Creasote. Severe pain in abdomen; _No emetic. _ Milk or odor of carbolic acid, flour and water; white of mucous membrane in eggs. Around mouth white and benumbed; cold and clammy skin. 

 _Aconite:_ Wolfsbane Numbness everywhere, _Vomit patient freely. _ Monkshood great weakness; cold Stimulating drinks. Sweat. 

 _Belladonna_ Deadly Nightshade Eyes bright, with pupil _Vomit patient freely. _ Atropia enlarged; dry mouth and throat. 

 _Various Vegetable Poisons_ Wild parsley, Stupor, nausea, great _Cause brisk vomiting_. Indian tobacco, weakness and other Stimulating drinks. Toadstools, symptoms according to Tobacco plant, the poison. Hemlock, Berries of the mountain ash, Bitter sweet etc. 

393. Practical Points about Poisons. Poisons should never be kept inthe same place with medicines or other preparations used in the household. They should always be put in some secure place under lock and key. Neveruse internally or externally any part of the contents of any package orbottle unless its exact nature is known. If there is the least doubtabout the substance, do not assume the least risk, but destroy it atonce. Many times the unknown contents of some bottle or package hasbeen carelessly taken and found to be poison. 

Careless and stupid people often take, by mistake, with serious, and oftenfatal, results, poisonous doses of carbolic acid, bed-bug poison, horse-liniment, oxalic acid, and other poisons. A safe rule is to keep allbottles and boxes containing poisonous substances securely bottled orpacked, and carefully labeled with the word POISON plainly written inlarge letters across the label. Fasten the cork of a bottle containingpoison to the bottle itself with copper or iron wire twisted into a knotat the top. This is an effective means of preventing any mistakes, especially in the night. 

This subject of poisons assumes nowadays great importance, as it is acommon custom to keep about stables, workshops, bathrooms, and livingrooms generally a more or less formidable array of germicides, disinfectants, horse-liniments, insect-poisons, and other preparations ofa similar character. For the most part they contain poisonousingredients. 

Bacteria. 

394. Nature Of Bacteria. The word bacteria is the name applied tovery low forms of plant life of microscopic size. Thus, if hay be soakedin water for some time, and a few drops of the liquid are examined under ahigh power of the microscope, the water is found to be swarming withvarious forms of living vegetable organisms, or bacteria. Thesemicroscopic plants belong to the great fungus division, and consist ofmany varieties, which may be roughly divided into groups, according asthey are spherical, rod-like, spiral, or otherwise in shape. 

Each plant consists of a mass of protoplasm surrounded by anill-defined cell wall. The bacteria vary cably in size. Some of therod-shaped varieties are from 1/12, 000 to 1/8, 000 of an inch in length, andaverage about 1/50, 000 of an inch in diameter. It has been calculated thata space of one cubic millimeter would contain 250, 000, 000 of these minuteorganisms, and that they would not weigh more than a milligram. 

[Illustration: Fig. 168. --Examples of Micro-Organisms called Bacteria. (Drawn from photographs. )

 A, spheroidal bacteria (called _cocci_) in pairs; B, same kind of bacteria in chains; C, bacteria found in pus (grouped in masses like a bunch of grapes). [Bacteria in A, B, and C magnified about 1000 diameters]. D, bacteria found in pus (tendency to grow in the form of chains). [Magnified about 500 diameters. ]]

Bacteria are propagated in a very simple manner. The parent cell dividesinto two; these two into two others, and so on. The rapidity with whichthese organisms multiply under favorable conditions, makes them, in somecases, most dangerous enemies. It has been calculated that if all of theorganisms survived, one bacterium would lead to the production of severalbillions of others in twenty-four hours. 

395. The Struggle of Bacteria for Existence. Like all kinds of livingthings, many species of bacteria are destroyed if exposed to boiling wateror steam, but seem able to endure prolonged cold, far below thefreezing-point. Thus ice from ponds and rivers may contain numerous germswhich resume their activity when the ice is melted. Typhoid fever germshave been known to take an active and vigorous growth after they have beenkept for weeks exposed in ice to a temperature below zero. 

The bacteria of consumption (bacillus tuberculosis) may retain theirvitality for months, and then the dried expectoration of the invalids maybecome a source of danger to those who inhale air laden with suchimpurities (sec. 220 and Fig. 94). 

Like other living organisms, bacteria need warmth, moisture, and somechemical compound which answers for food, in order to maintain thephenomena of life. Some species grow only in contact with air, others needno more oxygen than they can obtain in the fluid or semi-fluid which theyinhabit. 

396. Importance of Bacteria in Nature. We might well ask why themyriads of bacteria do not devastate the earth with their marvelousrapidity of propagation. So indeed they might, were it not for the winds, rains, melting snow and ice which scatter them far and wide, and destroythem. 

Again, as in countless other species of living organisms, bacteria aresubject to the relentless law which allows only the fittest to survive. The bacteria of higher and more complex types devour those of a lowertype. Myriads perish in the digestive tract of man and other animals. Theexcreta of some species of bacteria act as poison to destroy otherspecies. 

It is true from the strictest scientific point of view that all livingthings literally return to the dust whence they came. While living theyborrow a few elementary substances and arrange them in new combinations, by aid of the energy given them by the sun, and after a time die and leavebehind all they had borrowed both of energy and matter. 

Countless myriads of bacteria are silently at work changing dead animaland vegetable matter into useful substances. In brief, bacteria preparefood for all the rest of the world. Were they all destroyed, life uponthe earth would be impossible, for the elements necessary to maintain itwould be embalmed in the bodies of the dead. 

397. Action of Bacteria. In certain well-known processes bacteriahave the power of bringing about decomposition of various kinds. Thus ahighly organized fungus, like the yeast plant, growing in the presence ofsugar, has the power of breaking down this complex body into simpler ones, _viz. _, alcohol and carbon dioxid. 

In the same way, various forms of bacteria have the power of breaking downcomplex bodies in their immediate neighborhood, the products dependingupon the substance, the kind of bacteria, and the conditions under whichthey act. Thus the _bacteria lactis_ act upon the milk sugar present inmilk, and convert it into lactic acid, thus bringing about the souring ofmilk. 

[Illustration: Fig. 169. --Examples of Pathogenic Bacteria. (Drawn fromphotographs. )

 A, spiral form of bacteria found in cholera (Magnified about 1000 diameters) B, rod-shaped bacteria (called _bacilli_) from a culture obtained in _anthrax_ or malignant fustule of the face. Diseased hides carry this micro-organism, and thus may occasion disease among those who handle hides and wool. (Magnified about 1000 diameters)]

Now, while most species of bacteria are harmless, some are the cause ofsickness and death when they gain admittance to the body under certainconditions. These disease-producing bacteria (known as _pathogenic_), whenestablished in the blood and tissues of the body, bring about importantchemical changes, depending upon the species of bacteria, and also producea particular form of disease. The production of certain diseases by theagency of bacteria has now been proved beyond all doubt. In yellow fever, erysipelas, diphtheria, typhoid fever, consumption and other diseases, theconnection has been definitely established. 

The evil results these germs of disease produce vary greatly in kind andseverity. Thus the bacteria of Asiatic cholera and diphtheria may destroylife in a few hours, while those of consumption may take years to producea fatal result. Again, the bacteria may attack some particular organ, orgroup of organs, and produce mostly local symptoms. Thus in a boil thereis painful swelling due to the local effect of the bacteria, with slightgeneral disturbance. 

398. The Battle against Bacteria. When we reflect upon the terribleravages made by infectious diseases, and all their attendant evils forthese many years, we can the better appreciate the work done of late yearsby tireless scientists in their efforts to modify the activity ofdisease-producing bacteria. It is now possible to cultivate certainpathogenic bacteria, and by modifying the conditions under which they aregrown, to destroy their violence. 

In brief, science has taught us, within certain limitations, how tochange the virulent germs of a few diseases into harmless microbes. 

399. Alcoholic Fermentation and Bacteria. Men of the lowest, as wellas of the highest, type of civilization have always known that when thesugary juice of any fruit is left to itself for a time, at a moderatelywarm temperature, a change takes place under certain conditions, and theresult is a liquid which, when drank, produces a pronounced effect uponthe body. In brief, man has long known how to make for himself alcoholicbeverages, by means of which he may become intoxicated with theirpoisonous ingredients. 

Whether it is a degraded South Sea Islander making a crude intoxicant froma sugary plant, a Japanese preparing his favorite alcoholic beverage fromthe fermentation of rice by means of a fungus plant grown for the purpose, a farmer of this country making cider from fermenting apple juice, or aFrench expert manufacturing costly champagne by a complicated process, the outcome and the intent are one and the same. The essential thing isto produce an alcoholic beverage which will have a marked physiologicaleffect. This effect is poisonous, and is due solely to thealcoholic ingredient, without which man would have little or no usefor the otherwise harmless liquid. 

While the practical process of making some form of alcoholic beverage hasbeen understood for these many centuries, the real reason of thisremarkable change in a wholesome fruit juice was not known until revealedby recent progress in chemistry, and by the use of the microscope. We knownow that the change is due to fermentation, brought about from theinfluence, and by the action, of bacteria (sec. 125). 

In other words, fermentation is the result of the growth of low form ofvegetable life known as an organised ferment. The ferment, whether itbe the commonly used brewer's yeast, or any other species of alcoholicferment, has the power to decompose or break down a large part of thesugar present in the liquid into alcohol, which remains as a poison, and _carbon dioxid_, which escapes more or less completely. 

Thus man, ever prone to do evil, was once obliged, in his ignorance, tomake his alcoholic drinks in the crudest manner; but now he has forcedinto his service the latest discoveries in science, more especially inbacteriology, that he may manufacture more scientifically and moreeconomically alcoholic beverages of all sorts and kinds, and distributethem broadcast all over God's earth for the physical and moral ruin of thepeople. 

Disinfectants. 

400. Disinfectants, Antiseptics, and Deodorants. The worddisinfectant is synonymous with the term _bactericide_ or _germicide_. Adisinfectant is a substance which destroys infectious material. Anantiseptic is an agent which may hinder the growth, but does notdestroy the vitality, of bacteria. A deodorant is not necessarily adisinfectant, or even an antiseptic, but refers to a substance thatdestroys or masks offensive odors. 

401. Air and Water as Disinfectants. Nature has provided for ourprotection two most efficient means of disinfection, --pure air (sec. 218) and pure water (sec. 119). The air of crowded rooms containslarge quantities of bacteria, whereas in pure air there are comparativelyfew, especially after rain, which carries them to the earth. Livingmicro-organisms have never been detected in breezes coming from the sea, but in those blowing out from the shore large numbers may be found. 

In water tainted with organic matter putrefactive bacteria will flourish, whereas pure water is fatal to their existence. Surface water, because itcomes from that part of the soil where bacteria are most active, and wherethere is most organic matter, generally contains great quantities of theseorganisms. In the deeper strata of the soil there is practically nodecomposition of organic matter going on, hence, water taken from deepsources is comparatively free from bacteria. For this reason, deep wellwater is greatly to be preferred for drinking purposes to that fromsurface wells. 

402. Disinfectants. It is evident that air and water are not alwayssufficient to secure disinfection, and this must be accomplished by othermeans. The destruction of infected material by fire is, of course, a surebut costly means of disinfection. Dry heat, steam, and boiling water arevaluable disinfectants and do not injure most fabrics. These agents aregenerally used in combination with various chemical disinfectants. 

Certain chemical agents that are capable of destroying micro-organismsand their spores have come, of late years, into general use. A form ofmercury, called _corrosive sublimate_, is a most efficacious and powerfulgermicide, but is exceedingly poisonous and can be bought only underrestrictions. [54] _Carbolic acid, chloride of lime, permanganate ofpotash_, and various other preparations made from zinc, iron, andpetroleum, are the chemical disinfectants most commonly and successfullyused at the present time. There are also numerous varieties of commercialdisinfectants now in popular use, such as Platt's chlorides, bromo-chloral, sanitas, etc. , which have proved efficient germicides. 

Instructions for the Management of Contagious Diseases. 

The following instructions for the management of contagious diseases wereprepared for the National Board of Health by an able corps of scientistsand experienced physicians. 

403. Instructions for Disinfection. Disinfection is the destructionof the poisons of infectious and contagious diseases. Deodorizers, orsubstances which destroy smells, are not necessarily disinfectants, anddisinfectants do not necessarily have an odor. Disinfection cannotcompensate for want of cleanliness nor of ventilation. 

404. Disinfectants to be Employed. 1. Roll sulphur (brimstone); forfumigation. 

2. Sulphate of iron (copperas) dissolved in water in the proportion of oneand a half pounds to the gallon; for soil, sewers, etc. 

 [NOTE. A most useful little manual to consult in connection with this chapter is the _Hand-Book of Sanitary Information_, written by Roger S. Tracy, Sanitary Inspector of the New York City Health Department. Price, 50 cents. ]

3. Sulphate of zinc and common salt, dissolved together in water in theproportion of four ounces sulphate and two ounces salt to the gallon; forclothing, bed-linen, etc. 

405. How to Use Disinfectants. 1. _In the sick-room. _ The mostavailable agents are fresh air and cleanliness. The clothing, towels, bed-linen, etc. , should, on removal from the patient, and beforethey are taken from the room, be placed in a pail or tub of the zincsolution, boiling-hot, if possible. 

All discharges should either be received in vessels containing copperassolution, or, when this is impracticable, should be immediately coveredwith copperas solution. All vessels used about the patient should becleansed with the same solution. 

Unnecessary furniture, especially that which is stuffed, carpets, andhangings, should, when possible, be removed from the room at the outset;otherwise they should remain for subsequent fumigation and treatment. 

2. _Fumigation_. Fumigation with sulphur is the only practicable methodfor disinfecting the house. For this purpose, the rooms to be disinfectedmust be vacated. Heavy clothing, blankets, bedding, and other articleswhich cannot be treated with zinc solution, should be opened and exposedduring fumigation, as directed below. Close the rooms as tightly aspossible, place the sulphur in iron pans supported upon bricks placed inwashtubs containing a little water, set it on fire by hot coals or withthe aid of a spoonful of alcohol, and allow the room to remain closed fortwenty-four hours. For a room about ten feet square, at least two poundsof sulphur should be used; for larger rooms, proportionally increasedquantities. [55]

3. _Premises_. Cellars, yards, stables, gutters, privies, cesspools, water-closets, drains, sewers, etc. , should be frequently and liberallytreated with copperas solution. The copperas solution is easily preparedby hanging a basket containing about sixty pounds of copperas in a barrelof water. [56]

4. _Body and bed clothing, etc_. It is best to burn all articles whichhave been in contact with persons sick with contagious or infectiousdiseases. Articles too valuable to be destroyed should be treated asfollows:

_(a)_ Cotton, linen, flannels, blankets, etc. , should be treated with theboiling-hot zinc solution; introduce piece by piece, secure thoroughwetting, and boil for at least half an hour. 

_(b)_ Heavy woolen clothing, silks, furs, stuffed bed-covers, beds, andother articles which cannot be treated with the zinc solution, should behung in the room during fumigation, their surfaces thoroughly exposed andpockets turned inside out. Afterward they should be hung in the open air, beaten, and shaken. Pillows, beds, stuffed mattresses, upholsteredfurniture, etc. , should be cut open, the contents spread out andthoroughly fumigated. Carpets are best fumigated on the floor, but shouldafterward be removed to the open air and thoroughly beaten. 

Books for Collateral Study. Among the many works which may beconsulted with profit, the following are recommended as among those mostuseful: Parkes _Elements of Health_; Canfield's _Hygiene of theSick-Room;_ Coplin & Bevan's _Practical Hygiene;_ Lincoln's _SchoolHygiene_; Edward Smith's _Health_; McSherrys _Health; American HealthPrimers_ (12 little volumes, edited by Dr. Keen of Philadelphia);Reynold's _Primer of Health_; Corfield's _Health_; Appleton's _HealthPrimers;_ Clara S. Weeks' _Nursing_; Church's _Food_; Yeo's _Food in Healthand Disease;_ Hampton's _Nursing, its Principles and Practice_; Price's_Nurses and Nursing;_ Cullinworth's _Manual of Nursing_; Wise's _Text-Bookof Nursing_ (2 vols. ); and Humphrey's _Manual of Nursing_. 

Chapter XV. 

Experimental Work in Physiology. 

406. The Limitations of Experimental Work in Physiology in Schools. Unlike other branches of science taught in the schools from theexperimental point of view, the study of physiology has its limitations. The scope and range of such experiments is necessarily extremely limitedcompared with what may be done with the costly and elaborate apparatus ofthe medical laboratory. Again, the foundation of physiology rests uponsystematic and painstaking dissection of the dead human body and the loweranimals, which mode of study very properly is not permitted in ordinaryschool work. Experiments upon the living human body and the lower animals, now so generally depended upon in our medical and more advanced scientificschools, for obvious reasons can be performed only in a crude and quitesuperficial manner in secondary schools. 

Hence in the study of physiology in schools many things must be taken forgranted. The observation and experience of medical men, and theexperiments of the physiologist in his laboratory must be depended uponfor data which cannot be well obtained at first hand by young students. 

407. Value of Experiments in Physiology in Secondary Schools. Whilecircumstances and regard for certain proprieties of social life forbid theuse of a range of experiments, in anatomy and physiology, such as arepermitted in other branches of science in secondary schools, it by nomeans follows that we are shut out altogether from this most important andinteresting part of the study. However simple and crude the apparatus, theskillful and enthusiastic teacher has at his command a wide series ofmaterials which can be profitably utilized for experimental instruction. As every experienced teacher knows, pupils gain a far better knowledge, and keep up a livelier interest in any branch of science, if they see withtheir own eyes and do with their own hands that which serves to illuminateand illustrate the subject-matter. 

 [NOTE. For additional suggestions and practical helps on the subject of experimental work in physiology the reader is referred to Blaisdell's _How to Teach Physiology_, a handbook for teachers. A copy of this pamphlet will be sent postpaid to any address by the publishers of this book on receipt of ten cents. ]

The experimental method of instruction rivets the attention and arousesand keeps alive the interest of the young student; in fact, it is theonly true method of cultivating a scientific habit of study[57]. Thesubject-matter as set forth on the printed pages of this book should bemastered, of course, but at the same time the topics discussed should beilluminated and made more interesting and practical by a well-arrangedseries of experiments, a goodly show of specimens, and a certain amount ofmicroscopical work. 

408. The Question of Apparatus. The author well understands frompersonal experience the many practical difficulties in the way ofproviding a suitable amount of apparatus for classroom use. If there areample funds for this purpose, there need be no excuse or delay inproviding all that is necessary from dealers in apparatus in the largertowns, from the drug store, markets, and elsewhere. In schools where boththe funds and the time for such purposes are limited, the zeal andingenuity of teachers and students are often put to a severe test. Fortunately a very little money and a great deal of ingenuity and patiencewill do apparent wonders towards providing a working supply of apparatus. 

It will be noticed that many of the experiments in the preceding chaptersof this book can be performed with very simple, and often a crude andhome-made sort of apparatus. This plan has been rigidly followed by theauthor, first, because he fully realizes the limitations and restrictionsof the subject; and secondly, because he wishes to emphasize the fact thatexpensive and complicated apparatus is by no means necessary to illustratethe great principles of anatomy and physiology. 

409. Use of the Microscope. To do thorough and satisfactory work inphysiology in our higher schools a compound microscope is almostindispensable. Inasmuch as many of our best secondary schools are equippedwith one or more microscopes for use in other studies, notably botany, itis much less difficult than it was a few years ago to obtain thisimportant help for the classes in physiology. 

[Illustration: Fig. 170. --A Compound Microscope]

For elementary class work a moderate-priced, but well-made and strong, instrument should be provided. If the school does not own a microscope, the loan of an instrument should be obtained for at least a few weeks fromsome person in the neighborhood. 

The appearance of the various structures and tissues of the human body asrevealed by the microscope possesses a curious fascination for everyobserver, especially for young people. No one ever forgets the first lookat a drop of blood, or the circulation of blood in a frog's foot as shownby the microscope. 

 [NOTE. For detailed suggestions in regard to the manipulation and use of the microscope the student is referred to any of the standard works on the subject. The catalogues of scientific-instrument makers of our larger cities generally furnish a list of the requisite materials or handbooks which describe the use of the various microscopes of standard make. 

 The author is indebted to Bergen's _Elements of Botany_ for the following information concerning the different firms which deal in microscopes. "Several of the German makers furnish excellent instruments for use in such a course as that here outlined. The author is most familar with the Leitz microscopes, which are furnished by Wm. Krafft, 411 West 59th St. , New York city, or by the Franklin Educational Co. , 15 and 17 Harcourt St. , Boston. The Leitz Stand, No. IV. , can be furnished duty free (for schools only), with objectives 1, 3, and 5, eye-pieces I. And III. , for $24. 50. If several instruments are being provided, it would be well to have part of them equipped with objectives 3 and 7, and eye-pieces I. And III. 

 "The American manufacturers, Bausch & Lomb Optical Company, Rochester, N. Y. , and No. 130 Fulton St. , New York city, have this year produced a microscope of the Continental type which is especially designed to meet the requirements of the secondary schools for an instrument with rack and pinion coarse adjustment and serviceable fine adjustment, at a low price. They furnish this new stand, 'AAB, ' to schools and teachers at 'duty-free' rates, the prices being for the stand with two eye-pieces (any desired power), 2/3-inch and 1/4-inch objectives, $25. 60, or with 2-inch, 2/3-inch, and 1/4-inch objectives, and two eye-pieces, $29. 20. Stand 'A, ' the same stand as the 'AAB, ' without joint and with sliding tube coarse adjustment (as in the Leitz Stand IV. ), and with three eye-pieces and 2/3-inch and 1/4-inch objectives, is furnished for $20. 40. Stand 'A, ' with two eye-pieces, 2/3-inch and 1/6-inch objectives, $20. 40. "]

410. The Use of the Skeleton and Manikin. The study of the bones bythe help of a skeleton is almost a necessity. To this intent, schools of ahigher grade should be provided both with a skeleton and amanikin. If the former is not owned by the school, oftentimes a loanof one can be secured of some medical man in the vicinity. Separate boneswill also prove useful. In fact, there is no other way to study properlythe structure and use of the bones and joints than by the bonesthemselves. A good manikin is also equally serviceable, although not socommonly provided for schools on account of its cost. 

411. The Question of Vivisection and Dissection. There should be noquestion at all concerning vivisection. _In no shape or form shouldit be allowed in any grade of our schools. _ Nor is there any need of muchdissection in the grammar-school grades. A few simple dissections tobe performed with fresh beef-joints, tendons of turkey legs, and so on, will never engender cruel or brutal feelings toward living things. In thelower grades a discreet teacher will rarely advise his pupils to dissect adead cat, dog, frog, or any other animal. Instead of actual dissection, the pupils should examine specimens or certain parts previously dissectedby the teacher, --as the muscles and tendons of a sheep, the heart of anox, the eye of a codfish, and so on. Even under these restrictions theteacher should not use the knife or scissors before the class to open upany part of the specimen. In brief, avoid everything that can possiblyarouse any cruel or brutal feeling on the part of young students. 

In the higher schools, in normal and other training schools, differentconditions prevail. Never allow vivisection in any form whatever, eitherin school or at home. Under the most exact restrictions students in theseschools may be taught to make a few simple dissections. 

Most teachers will find, however, even in schools of a higher grade, thatthe whole subject is fraught with many difficulties. It will not requiremuch oftentimes to provoke in a community a deal of unjust criticism. Ateacher's good sense and discretion are often put to a severe test. 

Additional Experiments. 

To the somewhat extended list of experiments as described in the precedingchapters a few more are herewith presented which may be used asopportunity allows to supplement those already given. 

 Experiment 193. _To examine white fibrous tissue. _ Snip off a very minute portion from the muscle of a rabbit, or any small animal recently dead. Tease the specimen with needles, mount in salt solution and examine under a high power. Note the course and characters of the fibers. 

 Experiment 194. _To examine elastic tissue. _ Tease out a small piece of ligament from a rabbit's leg in salt solution; mount in the same, and examine as before. Note the curled elastic fibers. 

 Experiment 195. _To examine areolar tissue. _ Gently tease apart some muscular fibers, noting that they are attached to each other by connective tissue. Remove a little of this tissue to a slide and examine as before. Examine the matrix with curled elastic fiber mixed with straight white fibers. 

 Experiment 196. _To examine adipose tissue. _ Take a bit of fat from the mesentery of a rabbit. Tease the specimen in salt solution and mount in the same. Note the fat cells lying in a vascular meshwork. 

 Experiment 197. _To examine connective tissues. _ Take a very small portion from one of the tendons of a rabbit, or any animal recently dead; place upon a glass slide with a drop of salt solution; tease it apart with needles, cover with thin glass and examine with microscope. The fine wavy filaments will be seen. Allow a drop of dilute acetic acid to run under the cover glass; the filaments will swell and become transparent. 

 Experiment 198. Tease out a small piece of ligament from the rabbit's leg in salt solution; mount in the same, and examine under a high power. Note the curled elastic fibers. 

 Experiment 199. _A crude experiment to represent the way in which a person's neck is broken. _ Bring the ends of the left thumb and the left second finger together in the form of a ring. Place a piece of a wooden toothpick across it from the middle of the finger to the middle of the thumb. Put the right forefinger of the other hand up through the front part to represent the odontoid process of the axis, and place some absorbent cotton through the other part to represent the spinal cord. Push backwards with the forefinger with just enough force to break the toothpick and drive its fragments on to the cotton. 

 Experiment 200. _To illustrate how the pulse-wave is transmitted along an artery. _ Use the same apparatus as in Experiment 106, p. 201. Take several thin, narrow strips of pine wood. Make little flags by fastening a small piece of tissue paper on one end of a wooden toothpick. Wedge the other end of the toothpick into one end of the strips of pine wood. Use these strips like levers by placing them across the long rubber tube at different points. Let each lever compress the tube a little by weighting one end of it with a blackboard eraser or book of convenient size. 

 As the pulse-wave passes along under the levers they will be successively raised, causing a slight movement of the tissue-paper flags. 

 Experiment 201. _The dissection of a sheep's heart. _ Get a sheep's heart with the lungs attached, as the position of the heart will be better understood. Let the lungs be laid upon a dish so that the heart is uppermost, with its apex turned toward the observer. 

 The line of fat which extends from the upper and left side of the heart downwards and across towards the right side, indicates the division between the right and left ventricles. 

 Examine the large vessels, and, by reference to the text and illustrations, make quite certain which are the _aorta_, the _pulmonary artery_, the _superior_ and _inferior venæ cavæ_, and the _pulmonary veins_. 

 Tie variously colored yarns to the vessels, so that they may be distinguished when separated from the surrounding parts. 

 Having separated the heart from the lungs, cut out a portion of the wall of the _right ventricle_ towards its lower part, so as to lay the cavity open. Gradually enlarge the opening until the _chordæ tendineæ_ and the flaps of the _tricuspid valve_ are seen. Continue to lay open the ventricle towards the pulmonary artery until the _semilunar valves_ come into view. 

 The pulmonary artery may now be opened from above so as to display the upper surfaces of the semilunar valves. Remove part of the wall of the right auricle, and examine the right auriculo-ventricular opening. 

 The heart may now be turned over, and the _left ventricle_ laid open in a similar manner. Notice that the mitral valve has only two flaps. The form of the valves is better seen if they are placed under water, and allowed to float out. Observe that the walls of the _left_ ventricle are much thicker than those of the _right_. 

 Open the left auricle, and notice the entrance of the _pulmonary veins_, and the passage into the ventricle. 

 The ventricular cavity should now be opened up as far as the aorta, and the semilunar valves examined. Cut open the aorta, and notice the form of the _semilunar valves_. 

 Experiment 202. _To show the circulation in a frog's foot_ (see Fig. 78, p. 192). In order to see the blood circulating in the membrane of a frog's foot it is necessary to firmly hold the frog. For this purpose obtain a piece of soft wood, about six inches long and three wide, and half an inch thick. At about two inches from one end of this, cut a hole three-quarters of an inch in diameter and cover it with a piece of glass, which should be let into the wood, so as to be level with the surface. Then tie up the frog in a wet cloth, leaving one of the hind legs outside. Next, fasten a piece of cotton to each of the two longest toes, but not too tightly, or the circulation will be stopped and you may hurt the frog. 

 Tie the frog upon the board in such a way that the foot will just come over the glass in the aperture. Pull carefully the pieces of cotton tied to the toes, so as to spread out the membrane between them over the glass. Fasten the threads by drawing them into notches cut in the sides of the board. The board should now be fixed by elastic bands, or by any other convenient means, upon the stage of the microscope, so as to bring the membrane of the foot under the object glass. 

 The flow of blood thus shown is indeed a wonderful sight, and never to be forgotten. The membrane should be occasionally moistened with water. 

 Care should be taken not to occasion any pain to the frog. 

 Experiment 203. _To illustrate the mechanics of respiration_[58] (see Experiment 122, p. 234). "In a large lamp-chimney, the top of which is closed by a tightly fitting perforated cork (A), is arranged a pair of rubber bags (C) which are attached to a Y connecting tube (B), to be had of any dealer in chemical apparatus or which can be made by a teacher having a bunsen burner and a little practice in the manipulation of glass (Fig. 171). From the center of the cork is attached a rubber band by means of a staple driven through the cork, the other end of which (D) is attached to the center of a disk of rubber (E) such as dentists use. This disk is held to the edge of the chimney by a wide elastic band (F). There is a string (G) also attached to the center of the rubber disk by means of which the diaphragm may be lowered. 

 [Illustration: Fig. 171. ]

 Such is a description of the essentials of the model. The difficulties encountered in its construction are few and easily overcome. In the first place, the cork must be air-tight, and it is best made so by pouring a little melted paraffin over it, care being taken not to close the tube. The rubber bags were taken from toy balloon-whistles. 

 In the construction of the diaphragm, it is to be remembered that it also must be air-tight, and in order to resemble the human diaphragm, it must have a conical appearance when at rest. In order to avoid making any holes in the rubber, the two attachments (one of the rubber band, and the other of the string) were made in this wise: the rubber was stretched over a button having an eye, then under the button was placed a smaller ring from an old umbrella; to this ring was attached the rubber band, and to the eye of the button was fastened the operating string. When not in use the diaphragm should be taken off to relieve the strain on the rubber band. "

 Experiment 204. _To illustrate the action of the intercostal muscles_ (see sec. 210). The action of the intercostal muscles is not at first easy to understand; but it will be readily comprehended by reference to a model such as that represented in Fig. 172. This maybe easily made by the student himself with four laths of wood, fastened together at the corners, A, B, C, D, with pins or small screws, so as to be movable. At the points E, F, G, H, pins are placed, to which elastic bands may be attached (A). B D represents the vertebral column; A C, the sternum; and A B and C D, the ribs. The elastic band F G represents the _external_ intercostal muscles, and E H, the _internal_ intercostals. 

 [Illustration: Fig. 172. ]

 If now the elastic band E H be removed, the remaining band, F G, will tend to bring the two points to which it is attached, nearer together, and the result will be that the bars A B and C D will be drawn upwards (B), that is, in the same direction as the ribs in the act of _inspiration_. When the elastic band E H is allowed to exert its force, the opposite effect will be produced (C); in this case representing the position of the ribs in an act of _expiration_. 

 Experiment 205. Pin a round piece of bright red paper (large as a dinner-plate) to a white wall, with a single pin. Fasten a long piece of thread to it, so it can be pulled down in a moment. Gaze steadily at the red paper. Have it removed while looking at it intently, and a greenish spot takes its place. 

 Experiment 206. Lay on different parts of the skin a small, square piece of paper with a small central hole in it. Let the person close his eyes, while another person gently touches the uncovered piece of skin with cotton wool, or brings near it a hot body. In each case ask the observed person to distinguish between them. He will always succeed on the volar side of the hand, but occasionally fail on the dorsal surface of the hand, the extensor surface of the arm, and very frequently on the skin of the back. 

 Experiment 207. _Wheatstone's fluttering hearts_. Make a drawing of a red heart on a bright blue ground. In a dark room lighted by a candle hold the picture below the level of the eyes and give it a gentle to-and-fro motion. On continuing to look at the heart it will appear to move or flutter over the blue background. 

 Experiment 208. At a distance of six inches from the eyes hold a veil or thin gauze in front of some printed matter placed at a distance of about two feet. Close one eye, and with the other we soon see either the letters distinctly or the fine threads of the veil, but we cannot see both equally distinct at the same time. The eye, therefore, can form a distinct image of a near or distant object, but not of both at the same time; hence the necessity for accommodation. 

 Experiment 209. Place a person in front of a bright light opposite a window, and let him look at the light; or place one's self opposite a well-illuminated mirror. Close one eye with the hand and observe the diameter of the other pupil. Then suddenly remove the hand from the closed eye: light falls upon it; at the same time the pupil of the other eye contracts. 

 Experiment 210. _To illustrate the blind spot. Marriott's experiment_. On a white card make a cross and a large dot, either black or colored. Hold the card vertically about ten inches from the right eye, the left being closed. Look steadily at the cross with the right eye, when both the cross and the circle will be seen. Gradually approach the card toward the eye, keeping the axis of vision fixed on the cross. At a certain distance the circle will disappear, i. E. , when its image falls on the entrance of the optic nerve. On bringing the card nearer, the circle reappears, the cross, of course, being visible all the time (see Experiment 180, p. 355). 

 Experiment 211. _To map out the field of vision_. A crude method is to place the person with his back to a window, ask him to close one eye, stand in front of him about two feet distant, hold up the forefingers of both hands in front of and in the plane of your own face. Ask the person to look steadily at your nose, and as he does so observe to what extent the fingers can be separated horizontally, vertically, and in oblique directions before they disappear from his field of vision. 

 Experiment 212. _To illustrate imperfect judgment of distance_. Close one eye and hold the left forefinger vertically in front of the other eye, at arm's length, and try to strike it with the right forefinger. 

 On the first trial one will probably fall short of the mark, and fail to touch it. Close one eye, and rapidly try to dip a pen into an inkstand, or put a finger into the mouth of a bottle placed at a convenient distance. In both cases one will not succeed at first. 

 In these cases one loses the impressions produced by the convergence of the optic axes, which are important factors in judging of distance. 

 Experiment 213. Hold a pencil vertically about twelve inches from the nose, fix it with both eyes, close the left eye, and then hold the right index finger vertically, so as to cover the lower part of the pencil. With a sudden move, try to strike the pencil with the finger. In every case one misses the pencil and sweeps to the right of it. 

 Experiment 214. _To illustrate imperfect judgment of direction_. As the retina is spherical, a line beyond a certain length when looked at always shows an appreciable curvature. 

 Hold a straight edge just below the level of the eyes. Its upper margin shows a slight concavity. 

Surface Anatomy and Landmarks. 

In all of our leading medical colleges the students are carefully andthoroughly drilled on a study of certain persons selected as models. Theobject is to master by observation and manipulation the details of what isknown as surface anatomy and landmarks. Now while detailed work of thiskind is not necessary in secondary schools, yet a limited amount of studyalong these lines is deeply interesting and profitable. The habit oflooking at the living body with anatomical eyes and with eyes at ourfingers' ends, during the course in physiology, cannot be too highlyestimated. 

In elementary work it is only fair to state that many points of surfaceanatomy and many of the landmarks cannot always be defined or located withprecision. A great deal in this direction can, however, be done in higherschools with ingenuity, patience, and a due regard for the feelings of allconcerned. Students should be taught to examine their own bodies for thispurpose. Two friends may thus work together, each serving as a "model" tothe other. 

To the following syllabus may be added such other similar exercises asingenuity may suggest or time permit. 

Syllabus. 

I. Bony Landmarks. 

1. The _occipital protuberance_ can be distinctly felt at the back ofthe head. This is always the thickest part (often three-quarters of aninch or more) of the skull-cap, and is more prominent in some than inothers. The thinnest part is over the temples, where it may be almost asthin as parchment. 

2. The working of the _condyle of the lower jaw_ vertically and fromside to side can be distinctly felt and seen in front of the ear. When themouth is opened wide, the condyle advances out of the glenoid cavity, andreturns to its socket when the mouth is shut. In front of the ear, liesthe zygoma, one of the most marked and important landmarks to the touch, and in lean persons to the eye. 

3. The sliding movement of the _scapula_ on the chest can be properlyunderstood only on the living subject. It can move not only upwards anddownwards, as in shrugging the shoulders, backwards and forwards, as inthrowing back the shoulders, but it has a rotary movement round a movablecenter. This rotation is seen while the arm is being raised from thehorizontal to the vertical position, and is effected by the cooperation ofthe trapezius with the serratus magnus muscles. 

4. The _patella_, or knee-pan, the _two condyles of the tibia_, the_tubercle on the tibia_ for the attachment of the ligament of the patella, and the _head of the fibula_ are the chief bony landmarks of the knee. Thehead of the fibula lies at the outer and back part of the tibia. Inextension of the knee, the patella is nearly all above the condyles. Theinner border of the patella is thicker and more prominent than the outer, which slopes down toward its condyle. 

5. The short, front edge of the _tibia_, called the "shin, " and thebroad, flat, subcutaneous surface of the bone can be felt all the waydown. The inner edge can be felt, but not so plainly. 

6. The head of the _fibula_ is a good landmark on the outer side ofthe leg, about one inch below the top of the tibia. Note that it is placedwell back, and that it forms no part of the knee joint, and takes no sharein supporting the weight. The shaft of the fibula arches backwards and isburied deep among the muscles, except at the lower fourth, which can bedistinctly felt. 

7. The _malleoli_ form the great landmarks of the ankle. The outermalleolus descends lower than the inner. The inner malleolus advances moreto the front and does not descend so low as the outer. 

8. The line of the _clavicle_, or collar bone, and the projection ofthe joint at either end of it can always be felt. Its direction is notperfectly horizontal, but slightly inclined downwards. We can distinctlyfeel the _spine_ of the scapula and its highest point, the _acromion_. 

9. Projecting beyond the acromion (the arm hanging by the side), wecan feel, through the fibers of the _deltoid_, the upper part of thehumerus. It distinctly moves under the hand when the arm is rotated. It isnot the head of the bone which is felt, but its prominences (thetuberosities). The greater, externally; the lesser in front. 

10. The _tuberosities of the humerus_ form the convexity of theshoulder. When the arm is raised, the convexity disappears, --there is aslight depression in its place. The head of the bone can be felt bypressing the fingers high up in the axilla. 

11. The _humerus_ ends at the elbow in two bony prominences (internaland external condyles). The internal is more prominent. We can always feelthe _olecranon_. Between this bony projection of the ulna and the internalcondyle is a deep depression along which runs the ulna nerve (commonlycalled the "funny" or "crazy" bone). 

12. Turn the hand over with the palm upwards, and the edge of the_ulna_ can be felt from the olecranon to the prominent knob (styloidprocess) at the wrist. Turn the forearm over with the palm down, and thehead of the ulna can be plainly felt and seen projecting at the back ofthe wrist. 

13. The upper half of the _radius_ cannot be felt because it is socovered by muscles; the lower half is more accessible to the touch. 

14. The three rows of projections called the "knuckles" are formed bythe proximal bones of the several joints. Thus the first row is formed bythe ends of the metacarpals, the second by the ends of the firstphalanges, and the third by the ends of the second phalanges. That is, inall cases the line of the joints is a little in advance of the knucklesand nearer the ends of the fingers. 

II. Muscular Landmarks. 

1. The position of the _sterno-mastoid_ muscle as an important andinteresting landmark of the neck has already been described (p. 70). 

2. If the left arm be raised to a vertical position and dropped to ahorizontal, somewhat vigorously, the tapering ends of the _pectoralismajor_ and the tendons of the _biceps_ and _deltoid_ may be felt bypressing the parts in the axilla between the fingers and thumb of theright hand. 

3. The appearance of the _biceps_ as a landmark of the arm hasalready been described (p. 70). The action of its antagonist, the_triceps_, may be studied in the same manner. 

4. The _sartorius_ is one of the fleshy landmarks of the thigh, asthe biceps is of the arm, and the sterno-cleido-mastoid of the neck. Itsdirection and borders may be easily traced by raising the leg, --a movementwhich puts the muscle in action. 

5. If the model be directed to stand on tiptoe, both of the largemuscles of the calf, the _gastrocnemius_ and _soleus_, can bedistinguished. 

6. Direct the model, while sitting upright, to cross one leg over theother, using his utmost strength. The great muscles of the inner thigh arefully contracted. Note the force required to pull the legs to the ordinaryposition. 

7. With the model lying in a horizontal position with both legsfirmly held together, note the force required to pull the feet apart whilethe great muscles of the thigh are fully contracted. 

8. In forcible and resisted flexion of the wrist two tendons come upin relief. On the outer side of one we feel the pulse at the wrist, theradial artery here lying close to the radius. 

9. On the outer side of the wrist we can distinctly see and feel whenin action, the three extensor tendons of the thumbs. Between two of themis a deep depression at the base of the thumb, which the French call the"anatomical tobacco box. "

10. The relative position of the several extensor tendons on the backof the wrist and fingers as they play in their grooves over the back ofthe radius and ulna can be distinctly traced when the several muscles areput in action. 

11. There are several strong tendons to be seen and felt about theankle. Behind is the _tendo Achillis_. It forms a high relief with ashallow depression on each side of it. Behind both the inner and outerankle several tendons can be felt. Over the front of the ankle, when themuscles are in action, we can see and feel several tendons. They start uplike cords when the action is resisted. They are kept in their properrelative position by strong pulleys formed by the annular ligament. Mostof these tendons can be best seen by stand a model on one foot, _i. E. _ inunstable equilibrium. 

III. Landmarks of the Heart. 

To have a general idea of the form and position of the _heart_, map itsoutline with colored pencils or crayon on the chest wall itself, or onsome piece of clean, white cloth, tightly pinned over the clothing. Apattern of the heart may be cut out of pasteboard, painted red, or paperedwith red paper, and pinned in position outside the clothing. The apex ofthe heart is at a point about two inches below the left nipple and oneinch to its sternal side. This point will be between the fifth and sixthribs, and can generally be determined by feeling the apex beat. 

IV. Landmarks of a Few Arteries. 

The pulsation of the _temporal_ artery can be felt in front of the ear, between the zygoma and the ear. The _facial_ artery can be distinctly feltas it passes over the upper jaw at the front edge of the masseter muscle. The pulse of a sleeping child can often be counted at the anteriorfontanelle by the eye alone. 

About one inch above the clavicle, near the outer border of thesterno-mastoid, we can feel the pulsation of the great _subclavian_artery. At the back of the knee the _popliteal_ artery can be feltbeating. The _dorsal_ artery of the foot can be felt beating on a linefrom the middle of the ankle to the interval between the first and secondmetatarsal bones. 

When the arm is raised to a right angle with the body, the _axillary_artery can be plainly felt beating in the axilla. Extend the arm with palmupwards and the _brachial_ artery can be felt close to the inner side ofthe biceps. The position of the _radial_ artery is described in Experiment102. 

Glossary. 

Abdomen (Lat. _abdo_, _abdere_, to conceal). The largest cavity of thebody, containing the liver, stomach, intestines, and other organs. 

Abductor (Lat. _abduco_, to draw from). A muscle which draws a limb fromthe middle line of the body, or a finger or toe from the middle line ofthe foot or hand. 

Absorbents (Lat. _absorbere_, to suck up). The vessels which take part inthe process of absorption. Absorption. The process of sucking upnutritive or waste matters by the blood-vessels or lymphatics. 

Accommodation of the Eye. The alteration in the shape of the crystallinelens, which accommodates, or adjusts, the eye for near or remote vision. 

Acetabulum (Lat. _acetabulum_, a small vinegar-cup). The cup-shaped cavityof the innominate bone for receiving the head of the femur. 

Acid (Lat. _acidus_, from _acere_, to be sour). A substance usually sour, sharp, or biting to the taste. 

Acromion (Gr. Akron the tip, and omos, the shoulder). The part of thescapula forming the tip of the shoulder. 

Adam's Apple. An angular projection of cartilage in the front of the neck. It may be particularly prominent in men. 

Adductor (Lat. _adduco_, to draw to). A muscle which draws towards themiddle line of the body, or of the hand or foot. 

Adenoid (Gr. Aden, a gland). Tissue resembling gland tissue. 

Afferent (Lat. _ad_, to, and _fero_, to convey). Vessels or nervescarrying the contents or impulses from the periphery to the center. 

Albumen, or Albumin (Lat. _albus_, white). An animal substance resemblingthe white of an egg. 

Albuminuria. A combination of the words "albumin" and "urine. " Presence of_albumen_ in the _urine_. 

Aliment (Lat. _alo_, to nourish). That which affords nourishment; food. 

Alimentary (Lat. _alimentum_, food). Pertaining to _aliment_, or food. 

Alimentary Canal (Lat. _alimentum_). The tube in which the food isdigested or prepared for reception into the blood. 

Alkali (Arabic _al kali_, the soda plant). A name given to certainsubstances, such as soda, potash, and the like, which have the power ofcombining with acids. 

Alveolar (Lat. _alveolus_, a little hollow). Pertaining to the alveoli, the _cavities_ for the reception of the teeth. 

Amoeba (Gr. Ameibo, to change). A single-celled, protoplasmic organism, which is constantly changing its form by protrusions and withdrawals ofits substance. 

Amoeboid. Like an _amoeba_. 

Ampulla (Lat. _ampulla_, a wine-flask). The dilated part of thesemicircular canals of the internal ear. 

Anabolism (Gr. Anaballo, to throw or build up). The process by means ofwhich simpler elements are _built up_ into more complex. 

Anæsthetics (Gr. An, without, and aisthesia, feeling). Those medicinalagents which prevent the feeling of pain, such as chloroform, ether, laughing-gas, etc. 

Anastomosis (Gr. Ana, by, and stoma, a mouth). The intercommunication ofvessels. 

Anatomy (Gr. Anatemno, to cut up). The science which describes thestructure of living things. The word literally means dissection. 

Antiseptic (Lat. _anti_, against, and _sepsis_, poison). Opposing orcounter-acting putrefaction. 

Antrum (Lat. _antrum_, a cave). The cavity in the upper jaw. 

Aorta (Gr. Aorte, from aeipo, to raise up). The great artery that _risesup_ from the left ventricle of the heart. 

Aponeurosis (Gr. Apo, from, and neuron, a nerve). A fibrous membranousexpansion of a tendon; the nerves and tendons were formerly thought to beidentical structures, both appearing as white cords. 

Apoplexy (Gr. Apoplechia, a sudden stroke). The escape of blood from aruptured blood-vessel into the substance of the brain. 

Apparatus. A number of organs of various sizes and structures workingtogether for some special object. 

Appendages (Lat. _ad_ and _pendeo_, to hang from). Something connectedwith a part. 

Aqueous Humor (Lat. _aqua_, water). The watery fluid occupying the spacebetween the cornea and crystalline lens of the eye. 

Arachnoid Membrane (Gr. Arachne, a spider, and eidos, like). The thincovering of the brain and spinal cord, between the dura mater and the piamater. 

Arbor Vitæ. Literally, "the tree of life"; a name given to the peculiarappearance presented by a section of the cerebellum. 

Areolar (Lat. _areola_, a small space, dim. Of _area_). A term applied toa connective tissue containing _small spaces_. 

Artery (Gr. Aer, air, and tereo, to contain). A vessel by which blood iscarried away from the heart. It was supposed by the ancients to containonly air, hence the name. 

Articulation (Lat. _articulo_, to form a joint). The more or less movableunion of bones, etc. ; a joint. 

Arytenoid Cartilages (Gr. Arytaina, a ladle). Two small cartilages of thelarynx, resembling the mouth of a pitcher. 

Asphyxia (Gr. A, without, and sphixis, the pulse). Literally, "withoutpulse. " Condition caused by non-oxygenation of the blood. 

Assimilation (Lat. _ad_, to, and _similis_, like). The conversion of foodinto living tissue. 

Asthma (Gr. Asthma, a gasping). Spasmodic affection of the bronchial tubesin which free respiration is interfered with, owing to their diminishedcaliber. 

Astigmatism (Gr. A, without, and stigma, a point). Irregular refraction ofthe eye, producing a blurred image. 

Atrophy (Gr. A, without, and trophe, nourishment). Wasting of a part fromlack of nutrition. 

Auditory Nerve (Lat. _audio_, to hear). The special nerve of hearing. 

Auricle (Lat. _auricula_, a little ear). A cavity of the heart. 

Azygos (Gr. A, without, and zugos, a yoke). Without fellow; not paired. 

Bacteria (bakterion, a staff). A microscopic, vegetable organism; certainspecies are active agents in fermentation, while others appear to be thecause of infectious diseases. 

Bactericide (_Bacterium_ and Lat. _caedere_, to kill). Same as_germicide_. 

Bile. The gall, or peculiar secretion of the liver; a viscid, yellowishfluid, and very bitter to the taste. 

Biology (Gr. Bios, life, and logos, discourse). The science which treatsof living bodies. 

Bladder (Saxon _bleddra_, a bladder, a goblet). A bag, or sac, serving asa receptacle of some secreted fluid, as the _gall bladder_, etc. Thereceptacle of the urine in man and other animals. 

Bright's Disease. A group of diseases of the kidney, first described byDr. Bright, an English physician. 

Bronchi (Gr. Brogchos, windpipe). The first two divisions, or branches, ofthe trachea; one enters each lung. 

Bronchial Tubes. The smaller branches of the trachea within the substanceof the lungs terminating in the air cells. 

Bronchitis. Inflammation of the larger bronchial tubes; a "cold" affectingthe air passages. 

Bunion. An enlargement and inflammation of the first joint of the greattoe. 

Bursa. A pouch; a membranous sac interposed between parts which aresubject to movement, one on the other, to allow them to glide smoothly. 

Callus (Lat. _calleo_, to be thick-skinned). Any excessive hardness of theskin caused by friction or pressure. 

Canal (Lat. _canalis_, a canal). A tube or passage. 

Capillary (Lat. _capillus_, hair). The smallest blood-vessels, so calledbecause they are so minute. 

Capsule (Lat. _capsula_, a little chest). A membranous bag enclosing apart. 

Carbon Dioxid, often called _carbonic acid_. The gas which is present inthe air breathed out from the lungs; a waste product of the animal kingdomand a food of the vegetable kingdom. 

Cardiac (Gr. Kardia, the heart). The cardiac orifice of the stomach is theupper one, and is near the heart; hence its name. 

Carnivorous (Lat. _caro_, flesh, and _voro_, to devour). Subsisting uponflesh. 

Carron Oil. A mixture of equal parts of linseed oil and lime-water, socalled because first used at the Carron Iron Works in Scotland. 

Cartilage. A tough but flexible material forming a part of the joints, airpassages, nostrils, ear; gristle, etc. 

Caruncle (Lat. _caro_, flesh). The small, red, conical-shaped body at theinner angle of the eye, consisting of a cluster of follicles. 

Casein (Lat. _caseus_, cheese). The albuminoid substance of milk; it formsthe basis of cheese. 

Catarrh. An inflammation of a mucous membrane, usually attended with anincreased secretion of mucus. The word is often limited to _nasal_catarrh. 

Cauda Equina (Lat. , horse's tail). The collection of large nervesdescending from the lower end of the spinal cord. 

Cell (Lat. _cella_, a storeroom). The name of the tiny miscroscopicelements, which, with slender threads or fibers, make up most of the body;they were once believed to be little hollow chambers; hence the name. 

Cement. The substance which forms the outer part of the fang of a tooth. 

Cerebellum (dim. For _cerebrum_, the brain). The little brain, situatedbeneath the posterior third of the cerebrum. 

Cerebrum. The brain proper, occupying the upper portion of the skull. 

Ceruminous (Lat. _cerumen_, ear wax). A term applied to the glandssecreting cerumen, or _ear wax_. 

Chloral. A powerful drug and narcotic poison used to produce sleep. 

Chloroform. A narcotic poison generally used by inhalation; of extensiveuse in surgical operations. It produces anæsthesia. 

Chondrin (Gr. Chondros, cartilage). A kind of gelatine obtained by boiling_cartilage_. 

Chordæ Tendineæ. Tendinous cords. 

Choroid (Gr. Chorion, skin, and eidos, form). The middle coat of theeyeball. 

Chyle (Gr. Chulos, juice). The milk-like fluid formed by the digestion offatty articles of food in the intestines. 

Chyme (Gr. Chumos, juice). The pulpy liquid formed by digestion in thestomach. 

Cilia (pl. Of _cilium_, an eyelash). Minute hair-like processes found uponthe cells of the air passages and other parts. 

Ciliary Muscle. A small muscle of the eye which assists in accommodation. 

Circumvallate (Lat. _circum_, around, and _vallum_, a rampart). Surroundedby a rampart, as are certain papillæ of the tongue. 

Coagulation (Lat. _coagulo_, to curdle). Applied to the process by whichthe blood clots or solidifies. 

Cochlea (Lat. _cochlea_, a snail shell). The spiral cavity of the internalear. 

Columnæ Carneæ. Fleshy projections in the ventricles of the heart. 

Commissure (Lat. _con_, together, and _mitto_, _missum_, to put). Ajoining or uniting together. 

Compress. A pad or bandage applied directly to an injury to compress it. 

Concha (Gr. Kogche, a mussel shell). The shell-shaped portion of theexternal ear. 

Congestion (Lat. _con_, together, and _gero_, to bring). Abnormalgathering of blood in any part of the body. 

Conjunctiva (Lat. _con_, together, and _jungo_, to join). A thin layer ofmucous membrane which lines the eyelids and covers the front of theeyeball, thus joining the latter to the lids. 

Connective Tissue. The network which connects the minute parts of most ofthe structures of the body. 

Constipation (Lat. _con_, together, and _stipo_, to crowd close). Costiveness. 

Consumption (Lat. _consumo_, to consume). A disease of the lungs, attendedwith fever and cough, and causing a decay of the bodily powers. Themedical name is _phthisis_. 

Contagion (Lat. _con_, with, and _tango_ or _tago_, to touch). Thecommunication of disease by contact, or by the inhalation of the effluviaof a sick person. 

Contractility (Lat. _con_, together, and _traho_, to draw). The propertyof a muscle which enables it to contract, or draw its extremities closertogether. 

Convolutions (Lat. _con_, together, and _volvo_, to roll). The tortuousfoldings of the external surface of the brain. 

Convulsion (Lat. _convello_, to pull together). A more or less violentagitation of the limbs or body. 

Coördination. The manner in which several different organs of the body arebrought into such relations with one another that their functions areperformed in harmony. 

Coracoid (Gr. Koraxi, a crow, eidos, form). Shaped like a crow's beak. Cornea (Lat. _cornu_, a horn). The transparent horn-like substance whichcovers a part of the front of the eyeball. 

Coronary (Lat. _corona_, a crown). A term applied to vessels and nerveswhich encircle parts, as the _coronary_ arteries of the heart. 

Coronoid (Gr. Koro#x3CE;ne, a crow). Like a crow's beak; thus the_coronoid_ process of the ulna. 

Cricoid (Gr. Krikos, a ring, and eidos, form). A cartilage of the larynxresembling a seal ring in shape. 

Crystalline Lens (Lat. _crystallum_, a crystal). One of the humors of theeye; a double-convex body situated in the front part of the eyeball. 

Cumulative. A term applied to the violent action from drugs whichsupervenes after the taking of several doses with little or no effect. 

Cuticle (Lat. Dim. Of _cutis_, the skin). Scarf skin; the epidermis. 

Cutis (Gr. Sky#1FE6;tos, a skin or hide). The true skin, also called the_dermis_. 

Decussation (Lat. _decusso_, _decussatum_, to cross). The _crossing_ orrunning of one portion athwart another. 

Degeneration (Lat. _degenerare_, to grow worse, to deteriorate). A changein the structure of any organ which makes it less fit to perform its duty. 

Deglutition (Lat. _deglutire_, to swallow). The process of swallowing. 

Deltoid. Having a triangular shape; resembling the Greek letter D(_delta_). 

Dentine (Lat. _dens_, _dentis_, a tooth). The hard substance which formsthe greater part of a tooth; ivory. 

Deodorizer. An agent which corrects any foul or unwholesome odor. 

Dextrin. A soluble substance obtained from starch. 

Diabetes Mellitus (Gr. Dia, through, baino, to go, and me#x3AD;li, honey). Excessive flow of sugar-containing urine. 

Diaphragm (Gr. Diaphrasso, to divide by a partition). A large, thin musclewhich separates the cavity of the chest from the abdomen. 

Diastole (Gr. Diastello, to dilate). The _dilatation_ of the heart. 

Dietetics. That part of medicine which relates to diet, or food. 

Diffusion of Gases. The power of gases to become intimately mingled. 

Diplöe (Gr. Diploo, to double, to fold). The osseous tissue between thetables of the skull. 

Dipsomania (Gr. Dipsa, thirst, and mania, madness). An insatiable desirefor intoxicants. Disinfectants. Agents used to destroy the germs orparticles of living matter that are believed to be the causes ofinfection. 

Dislocation (Lat. _dislocare_, to put out of place). An injury to a jointin which the bones are displaced or forced out of their sockets. 

Dissection (Lat. _dis_, apart, and _seco_, to cut). The cutting up of ananimal in order to learn its structure. 

Distal (Lat. _dis_, apart, and _sto_, to stand). Away from the center. 

Duct (Lat. _duco_, to lead). A narrow tube. 

Duodenum (Lat. _duodeni_, twelve). The first division of the smallintestines, about twelve fingers' breadth long. 

Dyspepsia (Gr. -dys, ill, and peptein, to digest). A condition of thealimentary canal in which it digests imperfectly. Indigestion. 

Dyspnoea (Gr. Dys, difficult, and pneo, to breathe). Difficult breathing. 

Efferent (Lat. _effero_, to carry out). _Bearing_ or _carrying outwards_, as from the center to the periphery. 

Effluvia (Lat. _effluo_, to flow out). Exhalations or vapors coming fromthe body, and from decaying animal or vegetable substances. 

Element. One of the simplest parts of which anything consists. 

Elimination (Lat. _e_, out of, and _limen, liminis_, a threshold). The actof _expelling_ waste matters. Signifies, literally, "to throw out ofdoors. "

Emetic (Gr. Emeo, to vomit). A medicine which causes vomiting. 

Emulsion (Lat. _emulgere_, to milk). Oil in a finely divided state, suspended in water. 

Enamel (Fr. _émail_). Dense material covering the crown of a tooth. 

Endolymph (Gr. Endon, within, and Lat. _lympha_, water). The fluid in themembranous labyrinth of the ear. 

Endosmosis (Gr. Endon, within, and o#x1F60;theo, to push). The currentfrom without _inwards_ when diffusion of fluids takes place through amembrane. 

Epidemic (Gr. Epi, upon, and demos, the people). An extensively prevalentdisease. 

Epiglottis (Gr. Epi, upon, and glottis, the entrance to the windpipe). Aleaf-shaped piece of cartilage which covers the top of the larynx duringthe act of swallowing. 

Epilepsy (Gr. Epilepsis, a seizure). A nervous disease accompanied by fitsin which consciousness is lost; the falling sickness. 

Ether (Gr. Aither, the pure, upper air). A narcotic poison. Used as ananæsthetic in surgical operations. 

Eustachian (from an Italian anatomist named Eustachi). The tube whichleads from the throat to the middle ear, or tympanum. Excretion (Lat. _excerno_, to separate). The separation from the blood of the wastematters of the body; also the materials excreted. 

Exosmosis (Gr. Exio, without, and atheo, to push). The current from within_outwards_ when diffusion of fluids takes place through a membrane. 

Expiration (Lat. _expiro_, to breathe out). The act of forcing air out ofthe lungs. 

Extension (Lat. _ex_, out, and _tendo_, to stretch). The act of restoringa limb, etc. , to its natural position after it has been flexed or bent;the opposite of _flexion_. 

Fauces. The part of the mouth which opens into the pharynx. 

Fenestra (Lat. ). Literally, "a window. " Fenestra ovalis and fenestrarotunda, the oval and the round window; two apertures in the bone betweenthe tympanic cavity and the labyrinth of the ear. 

Ferment. That which causes fermentation, as yeast. 

Fermentation (Lat. _fermentum_, boiling). The process of undergoing aneffervescent change, as by the action of yeast; in a wider sense, thechange of organized substances into new compounds by the action of aferment. It differs in kind according to the nature of the ferment. 

Fiber (Lat. _fibra_, a filament). One of the tiny threads of which manyparts of the body are composed. 

Fibrilla. A little fiber; one of the longitudinal threads into which astriped muscular fiber can be divided. 

Fibrin (Lat. _fibra_, a fiber). An albuminoid substance contained in theflesh of animals, and also produced by the coagulation of blood. 

Flexion (Lat. _flecto_, to bend). The act of bending a limb, etc. 

Follicle (Lat. Dim. Of _follis_, a money bag). A little pouch ordepression. 

Fomentation (Lat. _foveo_, to keep warm). The application of any warm, medicinal substance to the body, by which the vessels are relaxed. 

Foramen. A hole, or aperture. 

Frontal Sinus. A blind or closed cavity in the bones of the skull justover the eyebrows. 

Fumigation (Lat. _fumigo_, to perfume a place). The use of certain fumesto counteract contagious effluvia. 

Function (Lat. _functio_, a doing). The special duty of any organ. 

Ganglion (Gr. Ganglin, a knot). A knot-like swelling in a nerve; a smallernerve center. 

Gastric (Gr. Gaster, stomach). Pertaining to the stomach. 

Gelatine (Lat. _gelo_, to congeal). An animal substance which dissolves inhot water and forms a jelly on cooling. Germ (Lat. _germen_, a sprout, bud). Disease germ; a name applied to certain tiny bacterial organismswhich have been demonstrated to be the cause of disease. 

Germicide (_Germ_, and Lat. _caedere_, to kill). Any agent which has adestructive action upon living germs, especially _bacteria_. 

Gland (Lat. _glans_, an acorn). An organ consisting of follicles andducts, with numerous blood-vessels interwoven. 

Glottis (Gr. Glotta, the tongue). The narrow opening between the vocalcords. 

Glucose. A kind of sugar found in fruits, also known as grape sugar. 

Gluten. The glutinous albuminoid ingredient of cereals. 

Glycogen. Literally, "producing glucose. " Animal starch found in liver, which may be changed into glucose. 

Gram. Unit of metric system, 15. 43 grains troy. 

Groin. The lower part of the abdomen, just above each thigh. 

Gustatory (Lat. _gusto_, _gustatum_, to taste). Belonging to the sense of_taste_. 

Gymnastics (Gr. Gumnaxio, to exercise). The practice of athleticexercises. 

Hæmoglobin (Gr. Haima, blood, and Lat. _globus_, a globe or globule). Acomplex substance which forms the principal coloring constituent of thered corpuscles of the blood. 

Hemispheres (Gr. Hemi, half, and sphaira, a sphere). Half a sphere, thelateral halves of the cerebrum, or brain proper. 

Hemorrhage (Gr. Haima, blood, and hregnumi, to burst). Bleeding, or theloss of blood. 

Hepatic (Gr. He#x1F27;par, the liver). Pertaining to the liver. 

Herbivorous (Lat. _herba_, an herb, and _voro_, to devour). Applied toanimals that subsist upon vegetable food. 

Heredity. The predisposition or tendency derived from one's ancestors todefinite physiological actions. 

Hiccough. A convulsive motion of some of the muscles used in breathing, accompanied by a shutting of the glottis. 

Hilum, sometimes written Hilus. A small fissure, notch, or depression. Aterm applied to the concave part of the kidney. 

Homogeneous (Gr. Ho#1F41;mos, the same, and genos, kind). Of the _samekind_ or quality throughout; uniform in nature, --the reverse ofheterogeneous. 

Humor. The transparent contents of the eyeball. 

Hyaline (Gr. Hyalos, glass). Glass-like, resembling glass in transparency. 

Hydrogen. An elementary gaseous substance, which, in combination withoxygen, produces water. 

Hydrophobia (Gr. Hydor, water, and phobeomai, to fear). A disease causedby the bite of a rabid dog or other animal. 

Hygiene (Gr. Hygieia health). The art of preserving health and preventingdisease. 

Hyoid (Gr. Letter u, and eidos, form, resemblance). The bone at the rootof the tongue, shaped like the Greek letter u. 

Hypermetropia (Gr. Hyper over, beyond, metron, measure, and ops, the eye). Far-sightedness. 

Hypertrophy (Gr. Hyper, over, and trophe, nourishment). Excessive growth;thickening or enlargement of any part or organ. 

Incisor (Lat. _incido_, to cut). Applied to the four front teeth of bothjaws, which have sharp, cutting edges. 

Incus. An anvil; the name of one of the bones of the middle ear. 

Indian Hemp. The common name of _Cannabis Indica_, an intoxicating drugknown as _hasheesh_ and by other names in Eastern countries. 

Inferior Vena Cava. The chief vein of the lower part of the body. 

Inflammation (Lat. Prefix _in_ and _flammo_, to flame). A redness orswelling of any part of the body with heat and pain. 

Insalivation (Lat. _in_ and _saliva_, the fluid of the mouth). Themingling of the saliva with the food during the act of chewing. 

Inspiration (Lat. _inspiro, spiratum_, to breathe in). The act of drawingin the breath. 

Intestine (Lat. _intus_, within). The part of the alimentary canal whichis continuous with the lower end of the stomach; also called the bowels. 

Iris (Lat. _iris_, the rainbow). The thin, muscular ring which liesbetween the cornea and crystalline lens, giving the eye its special color. 

Jaundice (Fr. _jaunisse_, yellow). A disorder in which the skin and eyesassume a yellowish tint. 

Katabolism (Gr. Kataballo, to throw down). The process by means of whichthe more complex elements are rendered more simple and less complex. Theopposite of _anabolism_. 

Labyrinth. The internal ear, so named from its many windings. 

Lacrymal Apparatus (Lat. _lacryma_, a tear). The organs for forming andcarrying away the tears. 

Lacteals (Lat. _lac, lactis_, milk). The absorbent vessels of the smallintestines. 

Laryngoscope (Gr. Larugxi, larynx, and skopeo, to behold). An instrumentconsisting of a mirror held in the throat, and a reflector to throw lighton it, by which the interior of the larynx is brought into view. 

Larynx. The cartilaginous tube situated at the top of the windpipe. 

Lens. Literally, a lentil; a piece of transparent glass or othersubstance so shaped as either to converge or disperse the rays of light. 

Ligament (Lat. _ligo_, to bind). A strong, fibrous material binding bonesor other solid parts together. 

Ligature (Lat. _ligo_, to bind). A thread of some material used in tying acut or injured artery. 

Lobe. A round, projecting part of an organ, as of the liver, lungs, orbrain. 

Lymph (Lat. _lympha_, pure water). The watery fluid conveyed by thelymphatic vessels. 

Lymphatic Vessels. A system of absorbent vessels. 

Malleus. Literally, the mallet; one of the small bones of the middle ear. 

Marrow. The soft, fatty substance contained in the cavities of bones. 

Mastication (Lat. _mastico_, to chew). The act of cutting and grinding thefood to pieces by means of the teeth. 

Meatus (Lat. _meo_, _meatum_, to pass). A _passage_ or canal. 

Medulla Oblongata. The "oblong marrow"; that portion of the brain whichlies upon the basilar process of the occipital bone. 

Meibomian. A term applied to the small glands between the conjunctiva andtarsal cartilages, discovered by _Meibomius_. 

Membrana Tympani. Literally, the membrane of the drum; a delicatepartition separating the outer from the middle ear; it is sometimespopularly called "the drum of the ear. "

Membrane. A thin layer of tissue serving to cover some part of the body. 

Mesentery (Gr. Mesos, middle, and enteron, the intestine). A duplicatureof the peritoneum covering the small _intestine_, which occupies the_middle_ or center of the abdominal cavity. 

Metabolism (Gr. Metabole, change). The _changes_ taking place in cells, whereby they become more complex and contain more force, or less complexand contain less force. The former is constructive metabolism, or_anabolism_; the latter, destructive metabolism, or _katabolism_. 

Microbe (Gr. Mikros, little, and bios, life). A microscopic organism, particularly applied to bacteria. 

Microscope (Gr. Mikro#3CC;s, small, and skopeo, to look at). An opticalinstrument which assists in the examination of minute objects. 

Molar (Lat. _mola_, a mill). The name applied to the three back teeth ateach side of the jaw; the grinders, or mill-like teeth. 

Molecule (dim. Of Lat. _moles_, a mass). The smallest quantity into whichthe mass of any substance can physically be divided. A molecule may bechemically separated into two or more atoms. 

Morphology (Gr. Morphe, form, and logos, discourse). The study of the lawsof form or structure in living beings. 

Motor (Lat. _moveo_, _motum_, to move). The name of the nerves whichconduct to the muscles the stimulus which causes them to contract. 

Mucous Membrane. The thin layer of tissue which covers those internalcavities or passages which communicate with the external air. 

Mucus. The glairy fluid secreted by mucous membranes. 

Myopia (Gr. Myo, to shut, and o#x1F64;ps, the eye). A defect of visiondependent upon an eyeball that is too long, rendering distant objectsindistinct; _near sight_. 

Myosin (Gr. Mos, muscle). Chief proteid substance of muscle. 

Narcotic (Gr. Narkao, to benumb). A medicine which, in poisonous doses, produces stupor, convulsions, and sometimes death. 

Nerve Cell. A minute round and ashen-gray cell found in the brain andother nervous centers. 

Nerve Fiber. An exceedingly slender thread of nervous tissue. 

Nicotine. The poisonous and stupefying oil extracted from tobacco. 

Nostril (Anglo-Saxon _nosu_, nose, and _thyrl_, a hole). One of the twoouter openings of the nose. 

Nucleolus (dim. Of _nucleus_). A little nucleus. 

Nucleus (Lat. _nux_, a nut). A central part of any body, or that aboutwhich matter is collected. In anatomy, a cell within a cell. 

Nutrition (Lat. _nutrio_, to nourish). The processes by which thenourishment of the body is accomplished. 

Odontoid (Gr. O#x1F40;doys, a tooth, eids, shape). The name of the bonypeg of the second vertebra, around which the first turns. 

OEsophagus. Literally, that which carries food. The tube leading from thethroat to the stomach; the gullet. 

Olecranon (Gr. Olene, the elbow, and kranion, the top of the head). Acurved eminence at the upper and back part of the ulna. 

Olfactory (Lat. _olfacio_, to smell). Pertaining to the sense of smell. 

Optic (Gr. O#1F40;pteyo, to see). Pertaining to the sense of sight. 

Orbit (Lat. _orbis_, a circle). The bony socket or cavity in which theeyeball is situated. 

Organ (Lat. _organum_, an instrument or implement). A portion of the bodyhaving some special function or duty. 

Osmosis (Gr. Osmos, impulsion). Diffusion of liquids through membranes. 

Ossa Innominata, pl. Of Os Innominatum (Lat. ). "Unnamed bones. " Theirregular bones of the pelvis, unnamed on account of their non-resemblanceto any known object. 

Otoconia (Gr. Oys, an ear, and konia, dust). Minute crystals of lime inthe vestibule of the ear; also known as _otoliths_. 

Palate (Lat. _palatum_, the palate). The roof of the mouth, consisting ofthe hard and soft palate. 

Palpitation (Lat. _palpitatio_, a frequent or throbbing motion). A violentand irregular beating of the heart. 

Papilla. The small elevations found on the skin and mucous membranes. 

Paralysis (Gr. Paralyo, to loosen; also, to disable). Loss of function, especially of motion or feeling. Palsy. 

Parasite. A plant or animal that grows or lives on another. 

Pelvis. Literally, a basin. The bony cavity at the lower part of thetrunk. 

Pepsin (Gr. Pepto, to digest). The active principle of the gastric juice. 

Pericardium (Gr. Peri, about, and kardia, heart). The sac enclosing theheart. 

Periosteum (Gr. Peri, around, osteon, a bone). A delicate fibrous membranewhich invests the bones. 

Peristaltic Movements (Gr. Peri, round, and stello, to send). The slow, wave-like movements of the stomach and intestines. 

Peritoneum (Gr. Periteino, to stretch around). The investing membrane ofthe stomach, intestines, and other abdominal organs. 

Perspiration (Lat. _perspiro_, to breathe through). The sweat. 

Petrous (Gr. Petra, a rock). The name of the hard portion of the temporalbone, in which are situated the drum of the ear and labyrinth. 

Phalanges (Gr. Phalagxi, a body of soldiers closely arranged in ranks andfiles). The bones of the fingers and toes. 

Pharynx (Gr. Pharmgxi, the throat). The cavity between the back of themouth and the gullet. 

Physiology (Gr. Physis, nature, and logos, a discourse). The science ofthe functions of living, organized beings. 

Pia Mater (Lat. ). Literally, the tender mother; the innermost of the threecoverings of the brain. It is thin and delicate; hence the name. 

Pinna (Lat. A feather or wing). External cartilaginous flap of the ear. 

Plasma (Gr. Plasso, to mould). Anything formed or moulded. The liquid partof the blood. 

Pleura (Gr. Pleura, the side, also a rib). A membrane covering the lung, and lining the chest. 

Pleurisy. An inflammation affecting the pleura. Pneumogastric (Gr. Pneymon, the lungs, and gaster, the stomach). The chief nerve ofrespiration; also called the _vagus_, or wandering nerve. 

Pneumonia. An inflammation affecting the air cells of the lungs. 

Poison (Fr. _poison_). Any substance, which, when applied externally, ortaken into the stomach or the blood, works such a change in the animaleconomy as to produce disease or death. 

Pons Varolii. Bridge of Varolius. The white fibers which form a _bridge_connecting the different parts of the brain, first described by_Varolius_. 

Popliteal (Lat. _poples_, _poplitis_, the ham, the back part of the knee). The space _behind the knee joint_ is called the _popliteal_ space. 

Portal Vein (Lat. _porta_, a gate). The venous trunk formed by the veinscoming from the intestines. It carries the blood to the liver. 

Presbyopia (Gr. Presbus, old, and ops, the eye). A defect of theaccommodation of the eye, caused by the hardening of the crystalline lens;the "far sight" of adults and aged persons. 

Process (Lat. _procedo_, _processus_, to proceed, to go forth). Anyprojection from a surface; also, a method of performance; a procedure. 

Pronation (Lat. _pronus_, inclined forwards). The turning of the hand withthe palm downwards. 

Pronator. The group of muscles which turn the hand palm downwards. 

Proteids (Gr. Protos, first, and eidos, form). A general term for thealbuminoid constitutents of the body. 

Protoplasm (Gr. Pro#x1FF6;tos, first, and plasso, to form). A_first-formed_ organized substance; primitive organic cell matter. 

Pterygoid (Gr. Pteron, a wing, and eidos, form, resemblance). Wing-like. 

Ptomaine (Gr. Ptoma, a dead body). One of a class of animal bases oralkaloids formed in the putrefaction of various kinds of albuminousmatter. 

Ptyalin (Gr. Sialon, saliva). A ferment principle in _saliva_, havingpower to convert starch into sugar. 

Pulse (Lat. _pello, pulsum_, to beat). The throbbing of an artery againstthe finger, occasioned by the contraction of the heart. Commonly felt atthe _wrist_. 

Pupil (Lat. _pupilla_). The central, round opening in the iris, throughwhich light passes into the interior of the eye. 

Pylorus (Gr. Pulouros, a gatekeeper). The lower opening of the stomach, atthe beginning of the small intestine. 

Reflex (Lat. _reflexus_, turned back). The name given to involuntarymovements produced by an excitation traveling along a sensory nerve to acenter, where it is turned back or reflected along motor nerves. 

Renal (Lat. _ren_, _renis_, the kidney). Pertaining to the _kidneys_. 

Respiration (Lat. _respiro_, to breathe frequently). The function ofbreathing, comprising two acts, --_inspiration_, or breathing in, and_expiration_, or breathing out. 

Retina (Lat. _rete_, a net). The innermost of the three tunics, or coats, of the eyeball, being an expansion of the optic nerve. 

Rima Glottidis (Lat. _rima_, a chink or cleft). The _opening_ of theglottis. 

Saccharine (Lat. _saccharum_, sugar). The group of food substances whichembraces the different varieties of sugar, starch, and gum. 

Saliva. The moisture, or fluids, of the mouth, secreted by the salivaryglands; the spittle. 

Sarcolemma (Gr. Sarxi, flesh, and lemma, a husk). The membrane whichsurrounds the contractile substance of a striped muscular fiber. 

Sclerotic (Gr. Skleros, hard). The tough, fibrous, outer coat of theeyeball. 

Scurvy. Scorbutus, --a disease of the general system, having prominent skinsymptoms. 

Sebaceous (Lat. _sebum_, fat). Resembling fat; the name of the oilysecretion by which the skin is kept flexible and soft. 

Secretion (Lat. _secerno_, _secretum_, to separate). The process ofseparating from the blood some essential, important fluid; which fluid isalso called a _secretion_. 

Semicircular Canals. Three canals in the internal ear. 

Sensation. The perception of an external impression by the nervous system. 

Serum. The clear, watery fluid which separates from the clot of the blood. 

Spasm (Gr. Spasmos, convulsion). A sudden, violent, and involuntarycontraction of one or more muscles. 

Special Sense. A sense by which we receive particular sensations, such asthose of sight, hearing, taste, and smell. 

Sputum, pi. Sputa (Lat. _spuo_, _sputum_, to _spit_). The matter which iscoughed up from the air passages. 

Stapes. Literally, a stirrup; one of the small bones of the middle ear. 

Stimulant (Lat. _stimulo_, to prick or goad on). An agent which causes anincrease of vital activity in the body or in any of its parts. 

Striated (Lat. _strio_, to furnish with channels). Marked with fine lines. 

Styptics (Gr. Stuptikos astringent). Substances used to produce acontraction or shrinking of living tissues. 

Subclavian Vein (Lat. _sub_, under, and _clavis_, a key). The great veinbringing back the blood from the arm and side of the head; so calledbecause it is situated underneath the _clavicle_, or collar bone. 

Superior Vena Cava (Lat. , upper hollow vein). The great vein of the upperpart of the body. 

Suture (Lat. _sutura_, a seam). The union of certain bones of the skullby the interlocking of jagged edges. 

Sympathetic System of Nerves. A double chain of nervous ganglia, situatedchiefly in front of, and on each side of, the spinal column. 

Symptom (Gr. Syn, with, and pipto, to fall). A sign or token of disease. 

Synovial (Gr. Syn, with, and oon, an egg). The liquid which lubricates thejoints; joint-oil. It resembles the white of a raw egg. 

System. A number of different organs, of similar structures, distributedthroughout the body and performing similar functions. 

Systemic. Belonging to the system, or body, as a whole. 

Systole (Gr. Sustello, to contract). The contraction of the heart, bywhich the blood is expelled from that organ. 

Tactile (Lat. _tactus_, touch). Relating to the sense of touch. 

Tartar. A hard crust which forms on the teeth, and is composed of salivarymucus, animal matter, and a compound of lime. 

Temporal (Lat. _tempus_, time, and _tempora_, the temples). Pertaining tothe temples; so called because the hair begins to turn white with age inthat portion of the scalp. 

Tendon (Lat. _tendo_, to stretch). The white, fibrous cord, or band, bywhich a muscle is attached to a bone; a sinew. 

Tetanus (Gr. Teino, to stretch). A disease marked by persistentcontractions of all or some of the voluntary muscles; those of the jaw aresometimes solely affected; the disorder is then termed lockjaw. 

Thorax (Gr. Thoraxi, a breast-plate). The upper cavity of the trunk of thebody, containing the lungs, heart, etc. ; the chest. 

Thyroid (Gr. Thureos, a shield, and ei#x313;dos, form). The largest of thecartilages of the larynx: its projection in front is called "Adam'sApple. "

Tissue. Any substance or texture in the body formed of various elements, such as cells, fibers, blood-vessels, etc. , interwoven with each other. 

Tobacco (Indian _tabaco_, the tube, or pipe, in which the Indians smokedthe plant). A plant used for smoking and chewing, and in snuff. 

Trachea (Gr. Trachys, rough). The windpipe. 

Tragus (Gr. Tragos, a goat). The eminence in front of the opening of theear; sometimes hairy, like a goat's beard. 

Transfusion (Lat. _transfundo_, to pour from one vessel to another). Theoperation of injecting blood taken from one person into the veins ofanother. 

Trichina Spiralis. (A twisted hair). A minute species of parasite, orworm, which infests the flesh of the hog: may be introduced into the humansystem by eating pork not thoroughly cooked. 

Trochanter (Gr. Trochao, to turn, to revolve). Name given to twoprojections on the upper extremities of the femur, which give attachmentto the _rotator_ muscles of the thigh. 

Trypsin. The ferment principle in pancreatic juice, which converts proteidmaterial into peptones. 

Tubercle (Lat. _tuber_, a bunch). A pimple, swelling, or tumor. A morbidproduct occurring in certain lung diseases. 

Tuberosity (Lat. _tuber, tuberis_, a swelling). A protuberance. 

Turbinated (Lat. _turbinatus_, from _turbo, turbinis_, a top). Formed likea _top_; a name given to the bones in the outer wall of the nasal fossæ. 

Tympanum (Gr. _tympanon_, a drum). The cavity of the middle ear, resembling a drum in being closed by two membranes. 

Umbilicus (Lat. , the navel. ) A round cicatrix or scar in the median lineof the abdomen. 

Urea (Lat. _urina_, urine). Chief solid constitutent of _urine_. Nitrogenous product of tissue decomposition. 

Ureter (Gr. _oureo_, to pass urine). The tube through which the _urine_ isconveyed from the kidneys to the bladder. 

Uvula (Lat. _uva_, a grape). The small, pendulous body attached to theback part of the palate. 

Vaccine Virus (Lat. _vacca_, a cow, and _virus_, poison). The materialderived from heifers for the purpose of vaccination, --the great preventiveof smallpox. 

Valvulae Conniventes. A name given to transverse folds of the mucousmembrane in the small intestine. 

Varicose (Lat. _varix_, a dilated vein). A distended or enlarged vein. 

Vascular (Lat. _vasculum_, a little vessel). Pertaining to or possessingblood or lymph vessels. 

Vaso-motor (Lat. _vas_, a vessel, and _moveo, motum_, to move). Causing_motion_ to the _vessels_. Vaso-motor nerves cause contraction andrelaxation of the blood-vessels. 

Venæ Cavæ, pl. Of Vena Cava. "Hollow veins. " A name given to the two greatveins of the body which meet at the right auricle of the heart

Venous (Lat. _vena_, a vein). Pertaining to, or contained within, a vein. 

Ventilation. The introduction of fresh air into a room or building in sucha manner as to keep the air within it in a pure condition. 

Ventral (Lat. _venter, ventris_, the belly). Belonging to the abdominal orbelly cavity. 

Ventricles of the Heart. The two largest cavities of the heart. 

Vermiform (Lat. _vermis_, a worm, and _forma_, form). Worm-shaped. 

Vertebral Column (Lat. _vertebra_, a joint). The backbone; also called thespinal column and spine. 

Vestibule. A portion of the internal ear, communicating with thesemicircular canals and the cochlea, so called from its fanciedresemblance to the vestibule, or porch, of a house. 

Villi (Lat. _villus_, shaggy hair). Minute, thread-like projections uponthe internal surface of the small intestine, giving it a velvetyappearance. 

Virus (Lat. , poison). Foul matter of an ulcer; poison. 

Vital Knot. A part of the medulla oblongata, the destruction of whichcauses instant death. 

Vitreous (Lat. _vitrum_, glass). Having the appearance of glass; appliedto the humor occupying the largest part of the cavity of the eyeball. 

Vivisection (Lat. _vivus_, alive, and _seco_, to cut). The practice ofoperating upon living animals, for the purpose of studying somephysiological process. 

Vocal Cords. Two elastic bands or ridges situated in the larynx; theessential parts of the organ of voice. 

Zygoma (Gr. Zugos, a yoke). The arch formed by the malar bone and thezygomatic process of the temporal bone. 

Index. 

Absorption from mouth and stomach by the intestinesAccident and emergenciesAchilles, Tendon ofAir, made impure by breathing Foul, effect of, on healthAlcohol, Effect of, on bones Effect of, on muscles Effect of, on muscular tissue Effect of, on physical culture Nature of Effects of, on human system and digestion Effect of, on the stomach and the gastric juice Final results on digestion Effects of, on the liver Fatty degeneration due to Effect of, on the circulation Effect of, on the heart Effect of, on the blood-vessels Effect of, on the lungs Other results of, on lungs Effect of, on disease Effect of, on kidneysAlcohol as cause of Bright's disease and the brain How, injures the brain Why brain suffers from the enemy of brain work Other physical results of Diseases produced by Mental and moral ruin by Evil results of, inherited Effect of, on taste Effect of, on the eye Effect of, on throat and voiceAlcoholic beveragesAlcoholic fermentation and BacteriaAnabolism definedAnatomy definedAntidotes for poisonsAntisepticsApparatus, Question ofArm, UpperArteriesAstigmatismAsphyxiaAtlas and axisAtmosphere, how made impure

Bacteria, Nature ofBacteria, Struggle for existence of Importance of, in Nature Action of Battle againstBaths and bathingBathing, Rules and precautionsBicyclingBileBiology definedBladderBleeding, from stomach from lungs from nose How to stopBlood, Circulation of Physical properties of corpuscles Coagulation of General plan of circulationBlood-vessels, Nervous control of connected with heart Effect of alcohol on Injuries toBodies, living, Characters ofBody, General plan ofBone, Chemical composition of Physical properties of Microscopic structure ofBones, uses of, The Kinds of in infancy and childhood positions at school in after life Broken broken, Treatment for Effect of alcohol on Effect of tobacco onBreathing, Movements ofBreathing, Mechanism of Varieties of Nervous control of change in the air Air, made impure byBrain, as a whole Membranes of as a reflex center Effects of alcohol onBrain center, Functions of, in perception of impressionsBright's disease caused by alcoholBronchial tubesBurns or scalds

CapillariesCarbohydratesCarpusCartilage Hyaline White fibro- Yellow fibro- Thyroid Arytenoid CricoidCells and the human organism Kinds of Vital properties of Epithelial NerveCerebrumCerebellumChemical compounds in the bodyChloralChyleChymeCilia of air passagesCirculation General plan of Portal Pulmonic Systemic Effect of alcohol onClavicleCleanliness, Necessity forClothing, Use of Material used for Suggestions for use of Effects of tight-fitting Miscellaneous hints on use of Catching, on fireCoagulation of bloodCocaine, ether, and chloroformCochlea of earCocoaCoffeeColonColor-blindnessComplemental airCompounds, Chemical OrganicCondimentsConjunctivaConnective tissueConsonantsContagious diseasesContraction, Object ofContusions and bruisesConvulsionsCookingCoughingCorneaCorpuscles, Blood Red ColorlessCorti, Organ ofCranial NervesCranium, Bones ofCryingCrystalline lensCuticleCutis vera, or true skin

Degeneration, Fatty, due to alcoholDeglutition, or swallowingDeodorantsDiet, Important articles of Effect of occupation on Too generous Effect of climate onDigestion, Purpose of General plan of in small intestines in large intestines Effect of alcohol onDisease, Effect of alcoholics uponDiseases, infectious and contagious, Management of Care of Hints on nursingDisinfectants Air and water as How to useDislocationsDogs, mad, Bites ofDrowning, Apparent Methods of treating Sylvester method Marshall Hall methodDuct, Hepatic Cystic Common bile Thoracic NasalDuodenumDura mater

Ear, External Middle Bones of the Internal Practical hints on care of Foreign bodies inEating, Practical points aboutEggs as foodElements, Chemical, in the bodyEpidermis, or cuticleEpiglottisEpithelium Squamous Columnar Glandular CiliatedEpithelial tissues, Functions ofErect positionEthmoid boneEustachian tubeExcretionExercise, Physical Importance of Effect of, on muscles Effect of, on important organs Effect of, on personal appearance Effect of excessive Amount of, required Time for Physical, in school Practical points about Effect of alcohol and tobacco onExperiments, Limitations of Value ofEye Inner structure of Compared to camera Refractive media of Movements of Foreign bodies in Practical hints on care of Effect of alcohol on Effect of tobacco onEyeball, Coats ofEyelids and eyebrowsEyesight in schools

Face Bones of theFaintingFats and oilsFemurFibrinFibulaFish as foodFood and drinkFood, why we need it Absorption of, by the blood Quantity of, as affected by age Kinds of, requiredFoods, Classification of Nitrogenous Proteid Saline or mineral Vegetable Proteid vegetable Non-proteid vegetable Non-proteid animal Table ofFood materials, Table of Composition ofFootFoul air, Effect of, on healthFrontal boneFrost bitesFruits as food

Gall bladderGarden vegetablesGastric glandsGastric juice, Effect of alcohol onGlands Mesenteric Lymphatic Ductless Thyroid Thymus Suprarenal LacrymalGlottis

Hair Structure ofHair and nails, Care ofHall, Marshall, method for apparent drowningHandHaversian canalsHead and spine, how joinedHead, Bones ofHearing, Sense of Mechanism of Effect of tobacco onHeart Valves of General plan of blood-vessels connected with Rhythmic action of Impulse and sounds of Nervous control of Effect of alcohol on Effect of tobacco onHeat, Animal Sources ofHiccoughHip bonesHistology definedHumerusHygiene definedHyoid boneHypermetropia

IleumInjured, Prompt aid toInsalivationIntestine, Small Coats of small LargeIntoxicants, Physical results ofIris of the eye

JejunumJoints Imperfect Perfect Hinge Ball-and-socket Pivot

Katabolism definedKidneys Structure of Function of Action if, how modified Effect of alcohol onKidneys and skin

Lacrymal apparatus glandLactealsLandmarks, Bony Muscular heart arteriesLarynxLaughingLens, CrystallineLevers in the bodyLife, The process ofLigamentsLimbs, Upper LowerLiver Minute structure of Blood supply of Functions of Effect of alcohol onLungs Minute structure of Capacity of Effect of alcohol on Bleeding fromLymphLymphatics

Mad dogs, Bites ofMalar boneMasticationMaxillary, Superior InferiorMeals, Hints aboutMeats as foodMedulla oblongataMembrane, Synovial Serous ArachnoidMembranes, BrainMesenteryMetabolism definedMetacarpal bonesMetatarsal bonesMicroscope, Use ofMilkMineral foodsMorphology definedMotion in animalsMouthMovement, Mechanism ofMuscles, Kinds of voluntary, Structure of involuntary, Structure of Arrangement of Important Effect of alcohol on Effect of tobacco on Review analysis of Rest forMuscular tissue, Effect of alcohol on Changes in Properties of activity contraction fatigue senseMyopia

Nails Care ofNasal bonesNerve cells fibers cells and fibers, Function ofNerves, Cranial Spinal Motor Sensory spinal, Functions ofNervous system, General view of compared to telegraph system Divisions of Effect of alcohol on Effect of tobacco onNitrogenous foods. Non-proteid vegetable foods animal foodsNose, Bleeding from Foreign bodies in

Occipital boneOEsophagusOpium Poisonous effects of In patent medicines Victim of the, habitOrganic compoundsOutdoor gamesOxidation

Pain, Sense ofPalate bonesPancreasPancreatic juiceParietal bonesPatellaPepsinPericardiumPeriosteumPeritoneumPhalangesPharynx and oesophagusPhysical exercisePhysical education in schoolPhysical exercises in schoolPhysiology defined Study of what it should teach Main problems of, briefly stated. Physiological knowledge, Value ofPia materPneumogastric nervePoisonsPoisons, Table of Antidotes for Practical points aboutPoisoning, Treatment ofPortal circulationPortal veinPresbyopiaPressure, Where to applyProteidsProteid vegetable foodsProtoplasmPulmonary artery veinsPulmonary infectionPulsePupil of the eye

RadiusReceptaculum chyliRectumReflex centers in the brainReflex action, Importance ofRenal secretionResidual airRespiration, Nature and object of Nervous control of Effect of, on the blood Effect of, on the air Modified movements of Effect of alcohol on Effect of tobacco on artificial, Methods ofRest, for the muscles Need of Benefits of The Sabbath, a day of of mind and bodyRetinaRibs and sternum

Saline or mineral foodsSalivaSalt as foodSalts, Inorganic, in the bodyScalds or burnsScapulaSchool, Physical education in Positions atSchool and physical educationSecretionSemicircular canalsSensations, GeneralSensation, Conditions ofSense, Organs ofSense organ, The essentials ofSerous membranesSick-room, Arrangement of Ventilation of Hints for Rules forSighingSight, Sense ofSkating, swimming, and rowingSkeleton Review analysis ofSkeleton and manikin, Use ofSkin, The regulating temperature Action of, how modified Absorbent powers of and the kidneysSkull Sutures ofSleep, a periodical rest Effect of, on bodily functions Amount of, required Practical rules aboutSmell Sense ofSneezingSnoringSobbingSpecial sensesSpeechSphenoid boneSpinal columnSpinal cord Structure of Functions of conductor of impulses as a reflex centerSpinal nerves Functions ofSpleenSprains and dislocationsStammeringStarches and sugarsSternumStomach Coats of Digestion in Effect of alcohol on Bleeding fromStrabismusStutteringSunstrokeSupplemental airSuprarenal capsulesSutures of skullSweat glandsSweat, Nature ofSylvester method for apparent drowningSympathetic system Functions ofSynovial membrane sheaths and sacs

Taste, Organ of Sense ofTaste, Physiological conditions of Modifications of the sense Effect of alcohol on Effect of tobacco onTeaTear gland and tear passagesTearsTechnical terms definedTeeth Development of Structure of Proper care of Hints about savingTemperature, Regulation of bodily Skin as a regulator of Voluntary regulation of Sense ofTemporal bonesTendon of AchillesTendonsThighThoracic ductThroat Care of Effect of alcohol on Effect of tobacco on Foreign bodies inThymus glandThyroid glandTibiaTidal airTissue, White fibrous Connective Yellow elastic Areolar Adipose Adenoid MuscularTissues, EpithelialTissues, epithelial, Varieties of Functions of ConnectiveTobacco, Effect of, on bones Effect of, on muscles Effect of, on physical culture Effect of, on digestion Effect of, on the heart Effect of, on the lungs Effect of, on the nervous system Effect of, on the mind Effect of, on the character Effect of, on taste Effect of, on hearing Effect of, on throat and voiceTouch, Organ of Sense ofTracheaTrunk, Bones ofTympanum, Cavity of

UlnaUrine

Valve, MitralValves of the heartValves, Tricuspid SemilunarVegetable foodsVeinsVentilation Conditions of efficient of sick-roomVestibule of earVermiform appendixVision, Common defects of Effect of tobacco onVivisection and dissectionVocal cordsVoice, Mechanism of Factors in the production of Care of Effect of alcohol on Effect of tobacco onVowel sounds

Walking, jumping, and runningWaste and repairWaste material, Nature ofWaste products, Elimination ofWater as foodWhisperingWounds, Incised and lacerated

Yawning

Footnotes:

[1] The Value of Physiological Knowledge. "If any one doubts theimportance of an acquaintance with the fundamental principles ofphysiology as a means to complete living, let him look around and see howmany men and women he can find in middle life, or later, who arethoroughly well. Occasionally only do we meet with an example of vigoroushealth continued to old age; hourly do we meet with examples of acutedisorder, chronic ailment, general debility, premature decrepitude. Scarcely is there one to whom you put the question, who has not, in thecourse of his life, brought upon himself illness from which a littleknowledge would have saved him. Here is a case of heart disease consequenton a rheumatic fever that followed a reckless exposure. There is a case ofeyes spoiled for life by overstudy. 

"Not to dwell on the natural pain, the gloom, and the waste of time andmoney thus entailed, only consider how greatly ill health hinders thedischarge of all duties, --makes business often impossible, and always moredifficult; produces irritability fatal to the right management ofchildren, puts the functions of citizenship out of the question, and makesamusement a bore. Is it not clear that the physical sins--partly ourancestors' and partly our own--which produce this ill health deduct morefrom complete living than anything else, and to a great extent make life afailure and a burden, instead of a benefaction and a pleasure?"--HerbertSpencer. 

[2] The word protoplasm must not be misunderstood to mean a substance of adefinite chemical nature, or of an invariable morphological structure; itis applied to any part of a cell which shows the properties of life, andis therefore only a convenient abbreviation for the phrase "mass of livingmatter. "

[3] "Did we possess some optic aid which should overcome the grossness ofour vision, so that we might watch the dance of atoms in the doubleprocess of making and unmaking in the living body, we should see thecommonplace, lifeless things which are brought by the blood, and which wecall food, caught up into and made part of the molecular whorls of theliving muscle, linked together for a while in the intricate figures of thedance of life, giving and taking energy as they dance, and then we shouldsee how, loosing hands, they slipped back into the blood as dead, inert, used-up matter. "--Michael Foster, Professor of Physiology in theUniversity of Cambridge, England. 

[4] "Our material frame is composed of innumerable atoms, and eachseparate and individual atom has its birth, life, and death, and then itsremoval from the 'place of the living. ' Thus there is going on acontinuous process of decay and death among the individual atoms whichmake up each tissue. Each tissue preserves its vitality for a limitedspace only, is then separated from the tissue of which it has formed apart, and is resolved into its inorganic elements, to be in due courseeliminated from the body by the organs of excretion. "--Maclaren's_Physical Education_. 

[5] The periosteum is often of great practical importance to the surgeon. Instances are on record where bones have been removed, leaving theperiosteum, within which the entire bone has grown again. The importanceof this remarkable tissue is still farther illustrated by experiments uponthe transplantation of this membrane in the different tissues of livinganimals, which has been followed by the formation of bone in thesesituations. Some years ago a famous surgeon in New York removed the wholelower jawbone from a young woman, leaving the periosteum and evenretaining in position the teeth by a special apparatus. The entire jawbonegrew again, and the teeth resumed their original places as it grew. 

[6] The mechanism of this remarkable effect is clearly shown by anexperiment which the late Dr. Oliver Wendell Holmes used to take delightin performing in his anatomical lectures at the Harvard Medical College. He had a strong iron bar made into a ring of some eight inches indiameter, with a space left between the ends just large enough to befilled by an English walnut. The ring was then dropped to the floor so asto strike on the convexity just opposite to the walnut, which invariablywas broken to pieces. 

[7] For the treatment of accidents and emergencies which may occur withreference to the bones, see Chapter XIII. 

[8] "Besides the danger connected with the use of alcoholic drinks whichis common to them with other narcotic poisons, alcohol retards the growthof young cells and prevents their proper development. Now, the bodies ofall animals are made up largely of cells, . .. And the cells being theliving part of the animal, it is especially important that they should notbe injured or badly nourished while they are growing. So that alcohol inall its forms is particularly injurious to young persons, as it retardstheir growth, and stunts both body and mind. This is the theory of Dr. Lionel S. Beale, a celebrated microscopist and thinker, and is quitegenerally accepted. "--Dr. Roger S. Tracy, of the New York Board of Health. 

[9] "In its action on the system nicotine is one of the most powerfulpoisons known. A drop of it in a concentrated form was found sufficient tokill a dog, and small birds perished at the approach of a tube containingit. "--Wood's _Materia Medica_. 

"Tobacco appears to chiefly affect the heart and brain, and I havetherefore placed it among cerebral and cardiac poisons. "--Taylor's_Treatise on Poisons_. 

[10] "Certain events occur in the brain; these give rise to other events, to changes which travel along certain bundles of fibers called nerves, andso reach certain muscles. Arrived at the muscles, these changes in thenerves, which physiologists call nervous impulses, induce changes in themuscles, by virtue of which these shorten contract, bring their endstogether, and so, working upon bony levers, bend the arm or hand, or liftthe weight. "--Professor Michael Foster. 

[11] The synovial membranes are almost identical in structure with serousmembranes (page 176), but the secretion is thicker and more like thewhite of egg. 

[12] "Smoking among students or men training for contests is a mistake. Itnot only affects the wind, but relaxes the nerves in a way to make themless vigorous for the coming contest. It shows its results at once, andwhen the athlete is trying to do his best to win he will do well to avoidit. " Joseph Hamblen Sears, Harvard Coach, and Ex-Captain of the HarvardFootball Team, Article in _In Sickness and in Health_. 

[13] "There is no profession, there is no calling or occupation in whichmen can be engaged, there is no position in life, no state in which a mancan be placed, in which a fairly developed frame will not be valuable tohim; there are many of these, even the most purely and highlyintellectual, in which it is essential to success--essential simply as ameans, material, but none the less imperative, to enable the mind to doits work. Year by year, almost day by day, we see men (and women) falterand fail in the midst of their labors; . .. And all for want of a littlebodily stamina--a little bodily power and bodily capacity for theendurance of fatigue, or protracted unrest, or anxiety, orgrief. "--Maclaren's _Physical Education_. 

[14] "One half the struggle of physical training has been won when a boycan be induced to take a genuine interest in his bodily condition, --towant to remedy its defects, and to pride himself on the purity of hisskin, the firmness of his muscles, and the uprightness of his figure. Whether the young man chooses afterwards to use the gymnasium, to run, torow, to play ball, or to saw wood, for the purpose of improving hisphysical condition, matters little, provided he accomplishes thatobject. "--Dr. D. A. Sargent, Director of the Hemenway Gymnasium at HarvardUniversity. 

[15] "It is _health_ rather than _strength_ that is the great requirementof modern men at modern occupations; it is not the power to travel greatdistances, carry great burdens, lift great weights, or overcome greatmaterial obstructions; it is simply that condition of body, and thatamount of vital capacity, which shall enable each man in his place topursue his calling, and work on in his working life, with the greatestamount of comfort to himself and usefulness to his fellowmen. "--Maclaren's_Physical Education_. 

[16] To this classification may be added what are called albuminoids, agroup of bodies resembling proteids, but having in some respects adifferent nutritive value. Gelatine, such as is found in soups or tablegelatine is a familiar example of the albuminoids. They are not found toany important extent in our raw foods, and do not therefore usually appearin the analyses of the composition of foods. The albuminoids closelyresemble the proteids, but cannot be used like them to build upprotoplasm. 

[17] The amount of water in various tissues of the body is given by thefollowing table in parts of 1000:

 Solids. Liquids. Enamel, 2 Blood, 791 Dentine, 100 Bile, 864 Bone, 486 Blood plasma, 901 Fat, 299 Chyle, 928 Cartilage, 550 Lymph, 958 Liver, 693 Serum, 959 Skin, 720 Gastric juice, 973 Brain, 750 Tears, 982 Muscle, 757 Saliva, 995 Spleen, 758 Sweat, 995 Kidney, 827 Vitreous humor, 987

[18] The work of some kinds of moulds may be apparent to the eye, as inthe growths that form on old leather and stale bread and cheese. That ofothers goes on unseen, as when acids are formed in stewed fruits. Concerning the work of the different kinds of moulds. Troussart says:"_Mucor mucedo_ devours our preserves; _Ascophora mucedo_ turns our breadmouldy; _Molinia_ is nourished at the expense of our fruits; _Mucorherbarium_ destroys the herbarium of the botanist; and _Choetoniumchartatum_ develops itself on paper, on the insides of books and on theirbindings, when they come in contact with a damp wall. "--Troussart's_Microbes, Ferments, and Moulds_. 

[19] "The physiological wear of the organism is constantly being repairedby the blood; but in order to keep the great nutritive fluid from becomingimpoverished, the matters which it is constantly losing must be suppliedfrom some source out of the body, and this necessitates the ingestion ofarticles which are known as food. "--Flint's _Text-book of HumanPhysiology_. 

[20] Glands. Glands are organs of various shapes and sizes, whosespecial work it is to separate materials from the blood for further use inthe body, the products being known as secretion and excretion. The means by which secretion and excretion are effected are, however, identical. The essential parts of a gland consist of a basement membrane, on one side of which are found actively growing cells, on the other is theblood current, flowing in exceedingly thin-walled vessels known as thecapillaries. The cells are able to select from the blood whatever materialthey require and which they elaborate into the particular secretion. InFig. 47 is illustrated, diagrammatically, the structure of a few typicalsecreting glands. The continuous line represents the basement membrane. The dotted line represents the position of the cells on one side of thebasement membrane. The irregular lines show the position of theblood-vessels. 

[21] Tablets and other material for Fehling and additional tests for sugarcan be purchased at a drug store. The practical details of these and othertests which assume some knowledge of chemistry, should be learned fromsome manual on the subject. 

[22] The Peritoneum. The intestines do not lie in a loose mass in theabdominal cavity. Lining the walls of this cavity, just as in a generalway, a paper lines the walls of a room, is a delicate serous membrane, called the peritoneum. It envelops, in a greater or less degree, allthe viscera in the cavity and forms folds by which they are connected witheach other, or are attached to the posterior wall. Its arrangement istherefore very complicated. When the peritoneum comes in contact with thelarge intestine, it passes over it just as the paper of a room would passover a gas pipe which ran along the surface of the wall, and in passingover it binds it down to the wall of the cavity. The small intestines aresuspended from the back wall of the cavity by a double fold of theperitoneum, called the mesentery. The bowels are also protected fromexternal cold by several folds of this membrane loaded with fat. This isknown as the _great omentum_. 

The peritoneum, when in health, secretes only enough fluid to keep itssurface lubricated so that the bowels may move freely and smoothly on eachother and on the other viscera. In disease this fluid may increase inamount, and the abdominal cavity may become greatly distended. This isknown as _ascites_ or dropsy. 

[23] The human bile when fresh is generally of a bright golden red, sometimes of a greenish yellow color. It becomes quite green when kept, and is alkaline in reaction. When it has been omited it is distinctlyyellow, because of its action on the gastric juice. The bile contains agreat deal of coloring matter, and its chief ingiedients are two salts ofsoda, sodium taurocholate and glycocholate. 

[24] Nansen emphasizes this point in his recently published work, _Farthest North_. 

[25] We should make it a point not to omit a meal unless forced to do so. Children, and even adults, often have the habit of going to school or towork in a hurry, without eating any breakfast. There is almost sure to bea fainting, or "all-gone" feeling at the stomach before another mealtime. This habit is injurious, and sure to produce pernicious results. 

[26] The teeth of children should be often examined by the dentist, especially from the beginning of the second dentition, at about the sixthyear, until growth is completed. In infancy the mother should make it apart of her daily care of the child to secure perfect cleanliness of theteeth. The child thus trained will not, when old enough to rinse the mouthproperly or to use the brush, feel comfortable after a meal until theteeth have been cleansed. The habit thus formed is almost sure to becontinued through life. 

[27] "If the amount of alcohol be increased, or the repetition becomefrequent, some part of it undergoes acid fermentation in the stomach, andacid eructations or vomitings occur. With these phenomena are associatedcatarrh of the stomach and liver with its characteristic symptoms, --lossof appetite, feeble digestion, sallowness, mental depression, andheadache. "--James C. Wilson, Professor in the Jefferson Medical College, Philadelphia. 

"Man has recourse to alcohol, not for the minute quantity of energy whichmay be supplied by itself, but for its powerful influence on thedistribution of the energy furnished by other things. That influence is avery complex one. "--Professor Michael Foster. 

[28] "When constantly irritated by the direct action of alcoholic drinks, the stomach gradually undergoes lasting structural changes. Its vesselsremain dilated and congested, its connective tissue becomes excessive, itspower of secreting gastric juice diminishes, and its mucous secretionsabnormally abundant. "--H. Newell Martin, late Professor of Physiology inJohns Hopkins University. 

"Chemical experiments have demonstrated that the action of alcohol on thedigestive fluids is to destroy its active principle, the pepsin, thusconfirming the observations of physiologists that its use gives ride tothe most serious disorders of the stomach and the most malignantaberrations of the entire economy. "--Professor E. C. Youmans, author ofstandard scientific works. 

"The structural changes induced by habitual use of alcohol and the actionof this agent on the pepsin, seriously impair the digestive power. Henceit is, that those who are habitual consumers of alcoholic fluids sufferfrom disorders o digestion. "--Robert Bartholow, recently Professor ofMateria Medica in the University of Pennsylvania. 

"Alcohol in any appreciable quantity diminishes the solvent power of thegastric fluid so as to interfere with the process of digestion instead ofaiding it. "--Professor W. B. Carpenter, the eminent English physiologist. 

[29] "Cirrhosis of the liver is notoriously frequent among drunkards, andis in fact almost, though not absolutely, confined to them. "--Robert T. Edes, formerly Professor of Materia Medica in Harvard Medical College. 

"Alcohol acts on the liver by producing enlargement of that organ, and afat deposit, or 'hob-nailed' liver mentioned by the Englishwriters. "--Professor W. B. Carpenter. 

[30] Preparation of Artificial Gastric Juice. _(a)_ Take part of thecardiac end of the pig's stomach, which has been previously opened andwashed rapidly in cold water, and spread it, mucous surface upwards, onthe convex surface of an inverted capsule. Scrape the mucous surfacefirmly with the back of a knife blade, and rub up the scrapings in amortar with fine sand. Add water, and rub up the whole vigorously for sometime, and filter. The filtrate is an artificial gastric juice. 

_(b)_ From the cardiac end of a pig's stomach detach the mucous membranein shreds, dry them between folds of blotting-paper, place them in abottle, and cover them with strong glycerine for several days. Theglycerine dissolves the pepsin, and on filtering, a glycerine extract withhigh digestive properties is obtained. 

These artificial juices, when added to hydrochloric acid of the properstrength, have high digestive powers. 

Instead of _(a)_ or _(b)_ use the artificial pepsin prepared for themarket by the wholesale manufacturers of such goods. 

[31] The cause of the clotting of blood is not yet fully understood. Although the process has been thoroughly investigated we have not yet asatisfactory explanation why the circulating blood does not clot inhealthy blood-vessels. The ablest physiologists of our day do not, asformerly, regard the process as a so-called vital, but a purely chemicalone. 

[32] Serous Membranes. --The serous membranes form shut sacs, of whichone portion is applied to the walls of the cavity which it lines; theother is reflected over the surface of the organ or organs contained inthe cavity. The sac is completely closed, so that no communication existsbetween the serous cavity and the parts in its neighborhood. The variousserous membranes are the _pleura_ which envelops the lungs; the_pericardium_ which surrounds the heart; the _peritoneum_ which investsthe viscera of the abdomen, and the _arachnoid_ in the spinal canal andcranial cavity. In health the serous membranes secrete only sufficientfluid to lubricate and keep soft and smooth the opposing surfaces. 

[33] A correct idea may be formed of the arrangement of the pericardiumaround the heart by recalling how a boy puts on and wears his toboggancap. The pericardium encloses the heart exactly as this cap covers theboy's head. 

[34] "Alcohol taken in small and single doses, acts almost exclusively onthe brain and the blood-vessels of the brain, whereas taken in large andrepeated doses its chief effects are always nervous effects. The firsteffects of alcohol on the function of inhibition are to paralyze thecontrolling nerves, so that the blood-centers are dilated, and more bloodis let into the brain. In consequence of this flushing of the brain, itsnerve centers are asked to do more work. "--Dr. T. S. Clouston, MedicalSuperintendent of the Royal Asylum, Edinburgh. 

"Alcoholic drinks prevent the natural changes going on in the blood, andobstruct the nutritive and reparative functions. "--Professor E. L. Youmans, well-known scientist and author of _Class Book of Chemistry_. 

[35] The word "cell" is not used in this connection in its technicalsignification of a histological unit of the body (sec. 12), but merely inits primary sense of a small cavity

[36] "The student must guard himself against the idea that arterial bloodcontains no carbonic acid, and venous blood no oxygen. In passing throughthe lungs venous blood loses only a part of its carbonic acid; andarterial blood, in passing through the tissues, loses only a part of itsoxygen. In blood, however venous, there is in health always some oxygen;and in even the brightest arterial blood there is actually more carbonicacid than oxygen. "--T. H. Huxley. 

[37] "Consumption is a disease which can be taken from others, and is notsimply caused by colds. A cold may make it easier to take the disease. Itis usually caused by germs which enter the body with the air breathed. Thematter which consumptives cough or spit up contains these germs in greatnumbers--frequently millions are discharged in a single day. This matterspit upon the floor, wall, or elsewhere is apt to dry, become pulverized, and float in the air as dust. The dust contains the germs, and thus theyenter the body with the air breathed. The breath of a consumptive does notcontain the germs and will not produce the disease. A well person catchesthe disease from a consumptive only by in some way taking in the mattercoughed up by the consumptive. "--Extract from a circular issued by theBoard of Health of New York City. 

[38] "The lungs from the congested state of their vessels produced byalcohol are more subject to the influence of cold, the result beingfrequent attacks of bronchitis. It has been recognized of late years thatthere is a peculiar form of consumption of the lungs which is very rapidlyfatal and found only in alcohol drinkers. "--Professor H. Newell Martin. 

[39] "The relation to Bright's Disease is not so clearly made out as isassumed by some writers, though I must confess to myself sharing thepopular belief that alcohol is one among its most importantfactors. "--Robert T. Edes, M. D. 

[40] Thus the fibers which pass out from the sacral plexus in the loins, and extend by means of the great sciatic nerve and its branches to theends of the toes, may be more than a yard long. 

[41] Remarkable instances are cited to illustrate the imperative demandfor sleep. Gunner boys have been known to fall asleep during the height ofa naval battle, owing to the fatigue occasioned by the arduous labor incarrying ammunition for the gunner. A case is reported of a captain of aBritish frigate who fell asleep and remained so for two hours beside oneof the largest guns of his vessel, the gun being served vigorously all thetime. Whole companies of men have been known to sleep while on the marchduring an arduous campaign. Cavalrymen and frontiersmen have slept soundlyin the saddle during the exhausting campaigns against the Indians. 

[42] According to the Annual Report of New York State Reformatory, for1896, drunkenness among the inmates can be clearly traced to no less than38 per cent of the fathers and mothers only. 

Drunkenness among the parents of 38 per cent of the prisoners in areformatory of this kind is a high and a serious percentage. It shows thatthe demoralizing influence of drink is apt to destroy the future of thechild as well as the character of the parent. 

"There is a marked tendency in nature to transmit all diseased conditions. Thus the children of consumptive parents are apt to be consumptive. But, of all agents, alcohol is the most potent in establishing a heredity thatexhibits itself in the destruction of mind and body. There is not only apropensity transmitted, but an actual disease of the nervous system. "--Dr. Willard Parker. 

[43] "It is very certain that many infants annually perish from thissingle cause. "--Reese's _Manual of Toxicology_. 

[44] If an eye removed from its socket be stripped posteriorly of thesclerotic coat, an inverted image or the field of view will be seen on theretina; but if the lens or other part of the refractive media be removed, the image will become blurred or disappear altogether. 

[45] This change in the convexity of the lens is only a slight one, as thedifference in the focal point between rays from an object twenty feetdistant and one four inches distant is only one-tenth of an inch. Whilethis muscular action is taking place, the pupil contracts and the eyeballsconverge by the action of the internal rectus muscles. These three actsare due to the third nerve (the motor oculi). This is necessary in orderthat each part should he imprinted on the same portion of the retina, otherwise there would be double vision. 

[46] The Germans have a quaint proverb that one should never rub his eyesexcept with his elbows

[47] "The deleterious effect of tobacco upon eyesight is an acknowledgedfact. The Belgian government instituted an investigation into the cause ofthe prevalence of color-blindness. The unanimous verdict of the expertsmaking the examination was that the use of tobacco was one of theprincipal causes of this defect of vision. 

"The dimness of sight caused by alcohol or tobacco has long beenclinically recognized, although not until recently accurately understood. The main facts can now be stated with much assurance, since thepublication of an article by Uhthoff which leaves little more to be said. He examined one thousand patients who were detained in hospital because ofalcoholic excess, and out of these found a total of eye diseases of aboutthirty per cent. 

"Commonly both eyes are affected, and the progress of the disease is slow, both in culmination and in recovery. .. . Treatment demands entireabstinence. "--Henry D. Noyes, Professor of Otology in the BellevueHospital Medical College, New York. 

[48] "The student who will take a little trouble in noticing the ears ofthe persons whom he meets from day to day will be greatly interested andsurprised to see how much the auricle varies. It may be a thick and clumsyear or a beautifully delicate one; long and narrow or short and broad, mayhave a neatly formed and distinct lobule, or one that is heavy, ungainly, and united to the cheek so as hardly to form a separate part of theauricle, may hug the head closely or flare outward so as to form almosttwo wings to the head. In art, and especially in medallion portraits, inwhich the ear is a marked (because central) feature, the auricle is ofgreat importance"--William W. Keen, M. D. , editor of Gray's _Anatomy_

[49] The organ of Corti is a very complicated structure which it isneedless to describe in this connection. It consists essentially ofmodified ephithelial cells floated upon the auditory epithelium, orbasilar membrane, of the cochlea. There is a series of fibers, each madeof two parts sloped against each other like the rafters of a roof. It isestimated that there are no less than 3000 of these arches in the humanear, placed side by side in a continuous series along the whole length ofthe basilar membrane. Resting on these arches are numbers of conicalepithelial cells, from the free surface of which bundles of stiff hairs(cilia) project. The fact that these hair-cells are connected with thefibers of the cochlear division of the auditory nerve suggests that theymust play an important part in auditory sensation. 

[50] The voices of boys "break, " or "change, " because of the sudden growthor enlargement of the larynx, and consequent increase in length of thevocal cords, at from fourteen to sixteen years of age. No such enlargementtakes place in the larynxes of girls: therefore their voices undergo nosuch sudden change. 

[51] This experiment and several others in this book, are taken fromProfessor Bowditch's little book called _Hints for Teachers ofPhysiology_, a work which should be mastered by every teacher ofphysiology in higher schools. 

[52] The teacher or student who is disposed to study the subject morethoroughly and in more detail than is possible in a class text-book, willfind all that is needed in the following excellent books, which arereadily obtained by purchase, or may be found in the public libraries oflarger towns: Dulles' _Accidents and Emergencies;_ Pilcher's _First Aid inIllness and Injury_; Doty's _Prompt Aid to the Injured;_ and Johnston's"Surgical Injuries and Surgical Diseases, " a special article inRoosevelt's _In Sickness and in Health_. 

[53] "A tourniquet is a bandage, handkerchief, or strap of webbing, intothe middle of which a stone, a potato, a small block of wood, or any hard, smooth body is tied. The band is tied loosely about the limb, the hardbody is held over the artery to be constricted, and a stick is insertedbeneath the band on the opposite side of the limb and used to twist theband in such a way that the limb is tightly constricted thereby, and thehard body thus made to compress the artery (Fig. 160). 

"The entire circumference of the limb may be constricted by any sort ofelastic band or rubber tube, or any other strong elastic material passedaround the limb several times on a stretch, drawn tight and tied in aknot. In this way, bleeding may be stopped at once from the largestarteries. The longer and softer the tube the better. It requires no skilland but little knowledge of anatomy to apply it efficiently. " Alexander B. Johnson, Surgeon to Roosevelt Hospital, New York City. 

[54] Corrosive sublimate is probably the most powerful disinfectant known. A solution of one part in 2000 will destroy microscopic organisms. Twoteaspoonfuls of this substance will make a solution strong enough to killall disease germs. 

[55] The burning of sulphur produces sulphurous acid, which is anirrespirable gas. The person who lights the sulphur must, therefore, immediately leave the room, and after the lapse of the proper time, musthold his breath as he enters the room to open the windows and let out thegas. After fumigation, plastered walls should be white-washed, thewoodwork well scrubbed with carbolic soap, and painted portions repainted. 

[56] Put copperas in a pail of water, in such quantity that some mayconstantly remain undissolved at the bottom. This makes a saturatedsolution. To every privy or water-closet, allow one pint of the solutionfor every four persons when cholera is about. To keep privies from beingoffensive, pour one pint into each seat, night and morning. 

[57] "While physiology is one of the biological sciences, it should beclearly recognized that it is not, like botany or zoology, a science ofobservation and description; but rather, like physics or chemistry, ascience of experiment. While the amount of experimental instruction (notinvolving vivisection or experiment otherwise unsuitable) that may withpropriety be given in the high school is neither small nor unimportant, the limitations to such experimental teaching, both as to kind and as toamount, are plainly indicated. 

"The obvious limitations to experimental work in physiology in the highschool, already referred to, make it necessary for the student to acquiremuch of the desired knowledge from the text-book only. Nevertheless, muchmay be done by a thoughtful and ingenious teacher to make such knowledgereal, by the aid of suitable practical exercises anddemonstrations. "--_Report of the Committee of Ten on Secondary SchoolStudies_. 

[58] This ingenious and excellent experiment is taken from the _New YorkSchool Journal_ for May, 1897, for which paper it was prepared by CharlesD. Nason, of Philadelphia.