Popular Science Library
EDITOR-IN-CHIEF
GARRETT P. SERVISS
AUTHORS
WILLIAM J. MILLER · HIPPOLYTE GRUENER · A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER · ERNEST G. MARTIN
ARTHUR SELWYN-BROWN · ROBERT CHENAULT GIVLER
ERNEST INGERSOLL · WILFRED MASON BARTON
WILLIAM B. SCOTT · ERNEST J. STREUBEL
NORMAN TAYLOR · DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH
ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX
ILLUSTRATED

PHYSIOLOGY

The Science of the Body
BY
ERNEST G. MARTIN
Professor of Physiology, Stanford University

PREFACE

WHEN Alexander Pope wrote “The proper study of mankind is man,” he was thinking rather of man as a social being than as the possessor of an amazingly complex and interesting body. It is nevertheless true that to one who finds enjoyment in the study of intricate mechanisms or to one for whom that amazing sequence of events which we call life has appeal there is no more fascinating study than the study of the living body. That part of the study of the body which concerns itself primarily with activity and only secondarily with form and structure, makes up the science of Physiology. The way the body works is the central theme.

The practical value of Physiology to the general reader lies in the fact that it forms the basis of all sound rules of hygiene. Life is made up of bodily activities which may be carried on correctly or incorrectly. Carried on correctly they mean health, carried on incorrectly, unhealth. The world is flooded with health-preserving or health-restoring systems, urged upon the public, for the most part, by promoters in search of gain. Such of these as have merit are based on definite physiological principles, and anyone who has a common-sense working knowledge of his own body can order his life in accordance with them, at little or no expense. Moreover, a sound appreciation of Physiology drives home the truth that when the body is really out of order its restoration can be safely intrusted only to the properly trained physician: the man or woman who through years of painstaking study has won insight into the intricacies of the human mechanism and whose honest appreciation of the difficulties of his profession, and courageous sincerity in grappling with them, justify to the full the confidence in which he is held by his community.

Ernest G. Martin.

CONTENTS

LIST OF ILLUSTRATIONS

CHAPTER I
THE SIGNS OF LIFE

PHYSIOLOGY is the study of living things, so the first thing to be asked when we begin to think about physiology is how we are to know whether anything is alive or not. It is usually pretty easy to tell whether a dog or cat is alive or dead, although sometimes when a dog is stretched out on the road we have to look closely to tell whether he has already met his end or is merely courting it by sleeping in the public highway. There are in the world hosts of animals with which we are not familiar, and to tell whether these are alive or dead is often a puzzle. More than one picnicker has been thoroughly surprised by seeing what looked like a bit of dead twig begin to walk away, and recognized the walking stick. On the whole we will agree that the sign of life which we find most reliable is motion of some sort on the part of the living animal. If the stretched-out dog makes breathing movements, we pronounce him alive; if not, we decide that he is dead. It is because the walking stick moves off when disturbed that we know it is not a twig. But while motion is the thing we look for in living animals we know perfectly well that it would be foolish to assert that anything that moves is alive. When the wind is blowing the air may be full of dead leaves and butterflies, all moving, but only part, the butterflies, alive. Unless the motion is produced by the animal itself it is not good as a sign of life. So widespread among animals is the making of movements, either on their own account, or when disturbed, that we shall not often find ourselves mistaken if we decide that an object which remains quiet indefinitely is not a living animal. Now how about the other side of the question? Is anything that moves on its own account or when disturbed to be judged alive?

Suppose that the inhabitants of Mars have finally succeeded in perfecting a flying boat that can be hermetically sealed and shot across space from that planet to our earth. Suppose further, that the first exploring party has set forth on a voyage of discovery, and has reached a point high in our sky from which objects on the earth’s surface begin to be distinguishable. Of course the huge landmarks, cities, lakes, and rivers, have been in view for a long while, and now the explorers are on the lookout for signs of living things. They are watching, just as we would be, for moving objects. The first moving thing that they see will probably be a train, and we can imagine their speculations as to whether they are actually looking or not at an inhabitant of the earth. As their craft sinks toward the surface the make-up of the train becomes perceptible as also the fact that it runs on rails, showing that it is a mechanical contrivance and not a living being. As smaller objects come into view black, shiny specks are seen moving about. These show every appearance of life; they start and stop; pass each other without interference; in fact conduct themselves about as animals usually do. If their apparent great power has the effect of discouraging the exploring party, so that they give up further investigation and fly away to Mars, the inhabitants of that planet will always suppose the earth to be populated by automobiles. We know that automobiles are not alive, yet, as this little allegory shows, they behave enough like living beings to deceive distant observers. There must be some sign of life which will apply to an animal and not to an automobile; what is it? Evidently what the Martian explorers missed was the fact that the automobile does not really start or stop itself, or guide itself past obstructions. If it had been alive, it would have done these things of itself. It is not so much the power of motion, then, that proves that the thing is alive as the power of making motions that are under the control of the animal itself.

The sight of an automobile which is not alive behaving as though it were because it is under control of a driver who is alive may lead us to ask whether the animal that we know to be living is actually alive in all its parts, or is a dead mechanism of some sort which has somewhere within it a living controller, corresponding to the living driver of the car. The animals with which we are most familiar are ourselves; how is it with our own bodies? Are they alive in all their parts, or is the brain the only part of us which is living? When a patient goes under ether on the operating table, or even when he is sound asleep, the signs of life are not conspicuously present; the heart goes on beating, to be sure, but so does the engine of an automobile go on running when the driver is away, provided he has not shut it off. A favorite belief among the Hindus is that when they go into a trance the body actually becomes lifeless while the living spirit soars among the heights. How are we to decide whether the Hindus are right or not? Evidently we shall have to look deeper than we have thus far, and learn something of what is actually going on in the different parts of our bodies when we are asleep and when we are awake.

Nearly everyone learns in school the main facts about the construction of the body; that there is a bony skeleton which supports the softer parts; that motions are made by muscles; that sense organs inform us as to what is going on in the world around; that the brain is the seat of the mind; that heart, lungs, stomach, kidneys, and other organs contribute in various ways to our well-being. Not so many go into detail as to the make-up of these organs, or into the way in which they do their work. This is not a simple matter, for several reasons. The first is that the construction units are so tiny that they cannot be seen by the unaided eye, but must be studied under the high magnification of a first-class microscope. It is much harder to make out the manner of the working of tiny pieces of machinery than of those that are of convenient size. When the parts are as small as those that make up our bodies, the task of finding out how they operate is so difficult that even now, after years of study, there are many details about which we know very little.

The construction units have been named cells. In some tiny animals the whole body consists of but one cell; all higher animals, including ourselves, have millions of cells making up the body. Undoubtedly some cells are alive; our question is as to whether all of them are, or whether there are some that are alive and some that are not. There are parts of our bodies, and of the bodies of nearly all other kinds of animals, as well, that are certainly not alive. Examples are the hair, the nails, the enamel of the teeth, and the hard parts of the bones. Actual living stuff is very soft and liquid. It is too fragile to hold its own structure except in the very tiniest animals; those that are larger need some additional supporting framework. In a body the size of a man’s the supporting framework amounts to a very considerable percentage of the entire weight (25 per cent). Not only is there the large bony skeleton, but between and among the individual cells is a framework made up of fine fibers and sheets which hold the cells in place. This latter framework is called connective tissue; we run across it in the gristly parts of meat. It makes up the stringy mass that clings to the cutter of the meat grinder when beef is being ground for Hamburg steak. We shall consider later how all this supporting material is made and put in place. Just now we are interested in the cells, and in determining whether all of them are alive or not.

There are many different kinds of cells in the body; some are muscle cells, others nerve cells, still others gland cells, and so on. Careful study shows, however, that at bottom all cells are alike. All are composed of one kind of substance to which has been given the name of protoplasm, meaning first or primary flesh. It is because some, at least, of this protoplasm is alive that our bodies are alive, and our physical life consists of nothing more than the combined life of all the living protoplasm which our bodies contain. Is there any way by which to tell whether any particular mass of protoplasm is alive or not? In other words, what are the signs of life of protoplasm as contrasted with the signs of life of whole animals?

We shall scarcely expect it to be as simple a matter to tell whether the tiny mass of protoplasm that we call a cell is alive or not as to decide whether a dog is dead or alive. For one thing, our most useful test of life, namely motion, cannot always be applied to single cells. We have in our bodies a great many cells, those in the brain, that we know are alive if any part of us is, but aside from the exceedingly gradual shifts in position that take place during growth the brain cells never make any motions at all, so far as anyone has ever been able to find out. Of course in the body of any ordinary animal most of the cells are hidden from view beneath the skin, but there are enough small transparent animals whose internal parts can be watched through the microscope to let us say with certainty that some of the cells which we know to be alive do not move. Tests of life that can be applied to all kinds of cells will necessarily be difficult to use, and we shall have to take the word of experts as to whether they have found particular cells alive or not, but the principle on which the tests are based is simple enough so that we can examine it. To do this, it will be necessary to turn our attention for a little while to some of the very tiniest of all living animals, those whose whole bodies consist of but one cell.

When these tiny one-celled animals are watched through the microscope as they swim about it can be seen that in one important feature they behave just as we do ourselves; that is in their care not to neglect mealtime. To be sure, mealtime comes for them whenever they happen to hit against any tinier particle than themselves, which they can take in and digest. But for them, as for us, the taking of food from time to time is a necessity of life. Only a small part of the food thus taken in is added permanently to the bulk of the animal. In other words, the growth does not go on as fast as does the taking of food. Of course in ourselves, after we have reached full size, there is little or no increase in permanent bulk even though we do keep right on eating. Evidently in the tiny one-celled animal, and in us as well, food is constantly being used for something besides growth. It can be proved that this food is used for precisely the same purpose that gasoline is used in the automobile, namely to run the machine. In a very real sense every living thing is a machine, and will no more run without a supply of power than will any other machine. From the engineering standpoint animals can be classified along with automobiles and locomotives as “prime movers,” namely, as machines which develop their power within themselves. There are not many kinds of power which can be developed by prime movers. By far the commonest is that seen in locomotives and automobiles, namely the burning of some kind of fuel. We have always known that the locomotive operates by the burning of coal or oil in the fire box. A moment’s thought will show us, if we have not realized it before, that the explosion of the air-gas mixture in the automobile cylinders is also a burning. Every steam-driven power plant depends on burning fuel for its power. Evidently fuel materials contain abundant power, if it can be extracted, and burning is a good method for doing the extracting. The word “burning” is the common name for a chemical process known technically as “oxidation,” meaning the union of oxygen with the fuel. The air is one-fifth oxygen, so there is plenty available, and fuel will usually oxidize readily after it is properly started.

Not only do animals correspond with other machines in using fuel as their source of power; they correspond also in that the power is extracted through the process of oxidation. To be sure, the oxidation in animals is not accompanied by flame and smoke as it usually is in power plants, nor do any parts of the animal get as hot as does the furnace where fuel is ordinarily burned; but in spite of these differences the fundamental fact is the same, namely that the extraction of power is by means of oxidation. What this shows is that great heat, flame, and smoke are not necessary in oxidation, but only in the kinds of oxidation with which we are most familiar.

As soon as we have described one more feature of animal power development, we shall be ready to apply what has been said to the topic in hand, namely the signs of life in single cells. The point that remains to be made is that in living cells power development has to go on all the time whether the cell is active or not. This means that fuel is constantly being burned, and oxygen is constantly being taken in to do the burning. There has been, and still is, a great deal of debate as to how much the oxidation can be reduced in living cells without destroying life. It is evident that it can be cut down to a very low level indeed, for seeds remain alive for years without using up, or even noticeably depleting, the store of fuel material which they contain. Most botanists of the present time doubt the truth of the tale that grains of wheat have sprouted after being taken from the wrappings of mummies, where they had lain for thousands of years. Careful efforts have been made to preserve wheat under as favorable conditions as existed in the mummy wrappings, but in every case the power to sprout was lost within a comparatively few years. So far as experiment enables us to judge, the complete cessation of power development in cells, either of plants or animals, means their death.

Here we have our sign of life that is applicable to all kinds of cells wherever they are located, whether making up the whole of a microscopic animal or deeply imbedded in the body of a large animal which consists of millions of them. If power development is going on, the cell is alive; if no power is being developed, the cell is not alive. When this test is applied it is found that all the protoplasmic cell masses which are present in the body of a plant or animal are alive, and since such masses are everywhere throughout the body, life is present in all parts of it, and not confined to the brain or to any other single region. We might admit that the Hindus are correct in assuming that the spirit can sometimes soar away and leave the lifeless body behind, but we cannot accept the possibility that it can return and establish life within it again. When life is resumed after a trance, that fact is proof positive that life continued throughout the trance itself.

CHAPTER II
THE MAINTAINING OF LIFE

EQUAL in importance to being alive is the power to go on living; therefore, having described the signs of life, our next task is to consider how that life is maintained. When the primary fact of life was given as continuous power development, the foundation was laid for this topic, for life cannot fail to go on if continuous power development is maintained.

Power development in living animals as in locomotives depends on fuel and oxygen; evidently continuous supplies of these must be provided if life is to go on. The living animal differs from the locomotive in this: that while some one attends to supplying the locomotive with fuel, most living animals, except the very young, have to attend to providing themselves. There are exceptions to this rule. The tapeworms that inhabit the intestines of animals, and sometimes of men, live in a stream of food; they are put to no trouble to obtain it. The same is true of many kinds of parasites. Except for these, however, it holds true that animals must attend to their own wants. We shall now begin to see the utility of the most conspicuous sign of life spoken of in the first chapter, namely, motion, for food must be gotten where it is; only tapeworms and similar animals swim in it. All the rest, including ourselves, must go to where the food is. Even animals like oysters, that are anchored to the rocks, have to use motion in getting food. In their case the motion consists in setting up a current in the sea water into and through their bodies, from which current they sift out the tiny food particles which abound in the ocean.

If an animal happens to live in the ocean, where every drop furnishes its particle or two of foodstuff, and especially if the animal is small, or sluggish, like the oyster, almost any kind of motion will serve to bring the animal all the food it needs. The simplest of the one-celled animals, that must be watched through the microscope to see how they behave, blunder about aimlessly, and in the course of their blundering bump up against food particles often enough to keep themselves fed. If an animal happens to live where food is scarcer, or if it is big and active, and so must have large quantities of food, aimless blundering about will never get it enough to keep it alive. It must have some means of finding out where the food is. Since we ourselves come under the head of animals whose food needs are so large that we must locate food supplies, and not depend on happening onto them, we can identify in ourselves the means which are used for doing this. We all know that our sense organs, the eyes, ears, nose, and finger tips are what we depend on for telling us where food is to be found. The same is true of all animals that are able to hunt for food; they have some sort of sense organs to help in guiding them to where the food is.

The story of the machinery for finding food is not yet quite complete, for the muscles which actually make the movements by which the animal gets about are in one part of the body, while the sense organs which are to furnish the information by which the movements are guided are in quite a different part, and in animals as large as ourselves, some distance away. From our eyes to our leg muscles is quite a space, and it is evident that this space must be bridged somehow if our legs are to move in obedience to information which our eyes bring in. In ourselves and in almost all other animals this space is bridged by means of special machinery for the purpose. We are familiar with it under the name of the nervous system. We may not have been in the habit of thinking of the nervous system in just this way, but at bottom this is exactly what the nervous system does for us, namely, guides our muscles according to the information brought in by our sense organs. There is more to nervous activity than just this, but this is the starting point and groundwork for all the rest, as we shall try to show presently.

Continuous food supplies are the main necessity for continued life, but there are some other things that have to be looked out for in addition. The favorite food for large numbers of animals, and, indeed, in many cases the only food, consists of the bodies of other animals. All the flesh-eating sorts prey on other animals for their food. This places the other animals on the defensive, so that a large part of their activity consists in escaping the attacks of the beasts that wish to eat them. For most kinds of animals the greater part of their waking life is taken up with movements which serve either to get them food or to prevent them from becoming food for others. If we add to these the movements that are necessary to preserve the animals against other kinds of danger than the danger of being eaten, and those connected with the propagation and care of the young, we shall have about covered the list of what we may call the serious activities of animals, and of men as well. Many kinds are active besides in play. This is particularly true of young animals, although grown-ups, both among animals and men, find play both agreeable and beneficial when not overindulged.

Protective motions need to be even more accurately made than those whose purpose is the getting of food, for if the food is missed at one effort another trial may be more successful, but if an attempt to escape fails there will probably be no more chances to try. The sense organs and the nervous system are just as deeply concerned, therefore, in avoiding harm as in finding food materials, and it is as important for them to do their work well in the one case as in the other. When we think of the activities of animals, for whatever purpose they are carried on, we must think of them as made up of the combined actions of the muscles, the nerves, and the sense organs, and not of any of them working by themselves.

These parts of us that are so closely concerned in the maintaining of our life by getting us food and keeping us safe from harm make up, also, the only parts of us which really share in what we may call conscious living. When we come right down to it we could spare our other organs—heart, lungs, stomach, and the rest—and never miss them so far as adding anything to our happiness is concerned. In fact, the less these organs intrude themselves into our attention the better off we are; only when we are ailing do we begin to think about them. Of course, they are absolutely necessary to us, and we should die instantly if one of the more important of them were to stop working, but the part they play is not one which enters actively into our consciousness, as do the muscles, nerves, and sense organs.

Naturally, we will ask what all these other organs are for if they do not share in our conscious life. Why can we not get along with just those that we use for getting food, for avoiding harm, for play, and for the other activities which they carry on? The answer to this question is found in the fact emphasized above that continuous power development is necessary to continued life. By themselves the muscles, nerves, and sense organs cannot carry on power development; they require the aid of a great many other organs to do this. Just how these other organs work will be described later; at present it will be enough to recall that every muscle, every nerve, and every sense organ is actually made up of a great many of the tiny construction units—the cells about which we were talking a few pages back—and that every one of these cells must be developing power all the time if it is to go on living. In order to be able to do this they must, every one, be able to oxidize fuel continuously, and this means that they must receive constant supplies, both of the fuel itself and of the oxygen with which it combines. Some system of delivering these materials must be in operation, and in case the materials have to be prepared for use beforehand this must be provided for also. The heart, the lungs, the stomach, and the various other organs that are useful but not conspicuous, are concerned in these necessary jobs. In an automobile factory we have a similar situation; the men that stand at the machines actually make the parts that go into the finished automobiles, but unless other men are hard at work preparing the castings, and bringing them to where the machine operators can get them, not many automobiles will be turned out. So in the body, unless the various organs are carrying on their work of preparing and delivering materials to the muscles, the nerves, and the sense organs, these latter cannot perform their tasks of getting the food for the whole body and of securing the body against harm, nor can they carry on the pleasant, but not absolutely necessary, activities of play and recreation.

CHAPTER III
THE SOURCES OF FOOD

WE have talked about the necessity of power development in all living things, and have seen that power development depends on the oxidation of fuel. Of course, our fuel is the food that we eat. No substance is suitable for fuel unless it contains power which can be gotten out by oxidation, and unless, in addition, it is suited to the particular kind of oxidation that goes on in the body, and can be handled by the body. Wood is excellent fuel for some purposes, but as a food for man it has no value, even when ground fine and mixed with flour as was done in some European countries during the Great War, because wood cannot be handled by the body in the way in which a usable fuel must be. Although wood is not good food, closely related materials are, and in fact make up the bulk of it. All fuel food is either vegetable or animal. All animal food traces back finally to the vegetable world, and it is an interesting fact that we do not usually care to eat flesh that is more than one remove from the vegetable kingdom. Animals that are flesh eaters are not considered fit for food, except in the case of fish and birds, and the flesh that these eat is not commonly thought of as being such, since it consists mostly of the flesh of insects, frogs, and fish themselves.

The real sources of food, then, are in the vegetable world. Of the countless thousands of kinds of plants that exist a few dozen have proven to be of enough use for human food, or for food for the animals on which human beings feed, to justify us in taking the trouble to raise them on our farms and in our gardens. There must be something about these particular plants to make us prefer them. If we look into the reason for the preference we shall understand something of the qualities which make plants good for food in the first place. At the beginning of the chapter were set down the things which make a substance fit for food. These are: the ability to yield power by oxidation, and a composition suitable to be used by the body. The ability to yield power involves the possession of a store of it. Power, or energy, which means the same thing in our present use of the words, is never present anywhere except as the result of an earlier exhibition of power. It is not made out of nothing. The sun is a reservoir of energy on which the earth draws, and we do not know with any certainty from whence the sun got its power. The heated center of the earth itself is a reservoir of power on which we may draw at some time in the future, when cheaper sources are used up. Except for energy from these sources and for trifling amounts that may be brought in by meteorites, there is none on the earth’s surface that has not always been here. On the other hand, the earth is constantly losing energy into space. The amount that reaches us from the sun balances our losses into space, so that the total energy present holds fairly steady. The energy that comes to us from the sun is chiefly in the two forms of heat and light. In actual horsepower the heat far outweighs the light, but in importance to mankind one stands about on a par with the other, for while without the sun’s heat the earth would become so cold that we would all die, without its light there would be no food and we would all starve. This is another way of saying that the energy that plants store up, and that we get when we eat them, comes originally from the light of the sun. Plants, like animals, are made up of cells. Those with which we are familiar consist of a great many cells, of a good many different kinds. Some are in the roots, others in the stems, still others in the leaves; the blossoms, fruits, and seeds are made up, likewise, of cells. The cells near the surface of the leaves, and, in many kinds of plants, near the surface of the stems as well, contain a green substance known as chlorophyll. This substance enables the cells in which it is present, although we do not know just how, to manufacture sugar, utilizing the energy of the sunlight for the purpose.

Sugar is composed of three very common chemical elements, carbon, hydrogen, and oxygen. As we all know, hydrogen and oxygen in combination of two atoms of hydrogen to one of oxygen form water; the most familiar of chemical symbols is that expressing this combination, namely H{2}O. Carbon, which we know in an almost pure state in anthracite coal, and in even purer form in diamonds, forms a combination with oxygen known as carbon dioxide. This is a gas; it makes up a small fraction of the air. The amount in the air is increased whenever coal or any other carbon-containing material is burned, since carbon dioxide is the product of the oxidation of carbon. Except in the arid regions of the earth there is always some water in the soil a greater or less distance below the surface of the ground. Water and carbon dioxide between them contain all the elements of which sugar is composed. The chlorophyll-containing cells are the factory; the sunlight is the power; and the carbon dioxide and water are the raw materials. Sugar is the finished product, and wherever sunlight is falling on green plants, whether directly or through a layer of cloud, its manufacture is going on. Sugar will oxidize readily, and in so doing will yield abundant power. The energy which it contains was derived by transformation from the energy of the sunlight. With the exception of a few kinds of bacteria every living thing on the earth depends for its food, and so for its energy, either directly or indirectly on the sugar which green plants manufacture. Since sugar dissolves in water it cannot easily be held in storage, so by a simple chemical process the plant changes it to starch, and it is in this latter form that we get it, except in the case of a few plants, like sugar cane.

The green parts of plants are the only places where sugar is made. We eat a certain amount of green food in lettuce and asparagus and similar vegetables, but for the most part the sugar or starch we eat comes from parts of plants that are not green. There is evidently a transportation from the point of manufacture to points of storage. The means of transport is in the sap; since starch is not soluble in water, it must be changed back into sugar. This is done, and then, by the movement of sap the dissolved sugar is carried to the points of storage, roots in such vegetables as beets, underground stems in potatoes, above ground stems in sugar cane, fruits or seeds in orchard and grain crops. In such of these as are sweet, the sugar itself is held in storage; in most kinds it is changed back into starch. Where the storage is in the form of starch the vegetable ordinarily keeps better than when sugar is the substance on deposit.

A few kinds of plants—olives, peanuts, and cocoanuts, for example—convert the sugar into oil and store their surplus material in that form. The chemical elements in oils and fats are the same as in starch and sugar, although the proportions are not the same. Weight for weight oil has more than twice the energy value of sugar; in making a given amount of peanut oil the peanut vine used up more than twice the amount of starch or sugar; but since energy value is what counts rather than bulk the plant is just as well off, and perhaps better on account of the smaller bulk occupied by the stored material. One of the very interesting examples of oil storage is found in the very tiny plants, called diatoms, which abound in the water of the ocean. Each tiny diatom stores within itself an even more tiny drop of oil. Although by themselves single oil drops would make no impression, if enough of them could be brought into one place a respectable accumulation of oil would result. This is precisely what the geologists tell us has happened in past ages; the bodies of diatoms have accumulated through thousands of years, and finally the oil accumulations have been covered over with sediment of one kind or another. When we tap through the sediment we strike into the “oil sand,” which contains this residue of the diatoms, and an oil well results.

Since we depend for our food, and so for our life, on the sugar-making activities of green plants, it will be worth our while to think for a moment of the slowness with which the process goes on. The slice of bread which we may eat in a dozen bites represents the result of a season’s growth of several wheat plants, every one of which was absorbing the sun’s energy and laying up starch grains during every daylight minute throughout the growing season. From the standpoint of the plant which does the storing the material which serves us as food is the excess over the plant’s own daily needs. In most cases it would be utilized at the beginning of the next season’s growth before the plant had put out a leaf system, if the course of events were not disturbed to satisfy the needs of man.

In addition to starch, sugar, and fat there is another kind of food material manufactured by plants, known as protein. This substance is much more complex chemically than any of the others; it contains, in addition to the three chemical elements—carbon, oxygen, and hydrogen—that are found in them, the element nitrogen, and usually some phosphorus and sulphur. These materials are dissolved in the soil water in the form of simple chemical substances, and are taken up by the plant along with the water which enters the roots and flows as sap up to the leaves. The same cells of the plant that make sugar have the power to make protein, using as raw materials some of the sugar along with the substances brought in with the soil water. The energy for the manufacture of protein comes from the oxidation of some of the sugar or starch in the leaf. The finished protein has about the same energy value, weight for weight, as has the starch from which it was mainly derived.

When an animal eats a plant or part of one, he is eating for the sake of the sugar or sugar products which the plant has made. There is one sugar product that is useful as food for many animals, but not for man, except possibly to a very slight extent. That is the woody substance, cellulose, which is formed in plants mainly as a support to the delicate living protoplasm. Cotton fiber is nearly pure cellulose. Cellulose is very similar to starch chemically, and is an excellent fuel wherever it can be burned. The human digestive tract is unable to handle it in a manner to make it usable, although grazing animals do so quite efficiently. A good many plants make products that are either disagreeable in flavor or actually poisonous. Of course, in such cases the plants become useless as food unless a treatment can be devised that will remove the objectionable material or convert it into something harmless. The few dozen kinds of plants that we raise for food are those that are free from harmful substances and that yield large quantities of stored food materials, or in some cases that taste especially good, even though they may not have much food value. Tomatoes, lettuce, and the like, come in this latter class. The world has been pretty well ransacked for food grains, fruits, and herbs, but probably there are others yet to be found besides those we now have.

CHAPTER IV
THE USES OF FOOD

WE have had a good deal to say thus far about power development in living animals, and have talked about food in connection with its use as fuel for the purpose. While we are on the topic it may be as well to say something about other uses to which food is put in animals besides that of serving as fuel, and also something about what is done with the power that is developed by the burning of such food as is used for fuel. To begin with, it is evident that one use that is made of food is to build the body itself. The new-born infant usually weighs somewhere between 5 and 12 pounds. From birth until the body gets its growth there is an almost continuous gain in weight until a total which may range anywhere between 90 and 250 pounds is reached. Of course, every bit of this additional material came into the body in the form of food. The whole mass of the body divides itself, as has been said before, into living protoplasm and nonliving substance. We do not know accurately what proportion of the whole weight is made up by protoplasm; it has been estimated at about 60 per cent, but any estimate can be only very rough because about half of the nonliving substance consists of fat deposits which vary greatly in different people.

In any case, that part of the food which goes to make gain in weight is passed over to the living

cells. If we accept the rough estimate given above, about 60 per cent is then used for the actual manufacture of new protoplasm; the remainder is worked over by cells specially devoted for the purpose and put into place to serve as supporting structure, or to be held in reserve as fat. Living protoplasm is chemically a very complex mixture. In consistency it resembles a rather thin, transparent jelly; the thickness of the jelly depends on how much water it contains and this varies greatly in different kinds of protoplasm. The watery part of the protoplasm has dissolved in it several substances; among them may be mentioned ordinary table salt; also salts of potash and lime. Only tiny amounts of these are present, but it is a curious fact that without these tiny amounts of salts protoplasm cannot live. The chief solid substance in protoplasm is protein; this

material, which is one of the most complex substances known to chemistry, has certain peculiarities which seem to fit it specially to serve as the chemical basis of life. Evidently of all the foodstuffs protein is the most important for the manufacture of new protoplasm, in other words for growth. In the case of a tiny one-celled animal, whose body is made up of protoplasm, not much else would be needed, but any animal that has a bony skeleton has to build this up to keep pace with the growth of the soft parts of the body. For this purpose mineral substances are needed, chiefly lime salts.

In addition to these foods which are actually used for making new body substance it has recently been discovered that proper growth in the higher animals, including man, depends on the presence in the diet of certain dietary accessories, whose use is not at all understood, although there is no doubt of their importance. These materials, to which has been given the rather cumbersome name of “growth-promoting vitamines,” are found dissolved in certain food fats. Apparently they are insoluble in water and soluble in oil. Most animal fats appear to contain them in small amounts, while most vegetable fats do not. Milk and eggs, which are growth foods in an especial sense, are richer in these accessories than any other articles of the diet. The discovery of these facts has emphasized the importance of including animal fats in the diet of growing children, milk and eggs particularly. Since milk is also rich in the lime salts which are necessary for bone formation it forms the best single foodstuff for children that there is. When very young children have to be fed on cow’s milk, which differs somewhat in proportion from mother’s milk, it is often found necessary to feed the top milk diluted with water, instead of the whole milk. When this is done, lime water is usually used in part for diluting the milk, instead of all ordinary water. In this way the proportion of lime is brought up enough to insure that the child will get plenty of it.

In addition to the use of protein as a growth food it has another use which no other kind of foodstuff can share. This is also because protein is the foundation material of living protoplasm. We do not know a great deal about what goes on in living protoplasm to make up what we call the life processes, but we do know that these processes are of a chemical nature, and that in connection with them there is a steady wastage of protein. The protein that thus goes to waste is broken down into simpler chemical compounds which are expelled from the cells. Why this occurs we do not know, but since it does it is evident that unless the wastage is made good the time will presently come when so much protein will have been lost from the protoplasm that it can no longer exist as such and must die. As a matter of fact, one might go on a diet excessively rich in starchy foods and fats and still starve to death if there were no protein present. This use of protein is called cell maintenance to distinguish it from the other special use of protein in cell growth. Evidently, whatever may be missing from the diet, protein must not be left out. Fortunately most of our common foods contain protein. It is especially abundant in lean meat, in dried beans and peas, and in grain. Potatoes and most garden vegetables are deficient in protein, as are almost all common fruits. Bread and meat are our chief stand-bys as furnishers of protein.

Just as there are vitamines that are important for growth, so are there vitamines that are necessary for cell maintenance. Many years ago Dr. Sylvester Graham made himself prominent by arguing that the outer coats of wheat grains contain something that is needed in the diet, which is removed in the process of manufacturing white flour. He accordingly invented a form of flour, familiar to us all under his name, which includes some of the bran from the outer layers of the wheat. This idea, which originated with Dr. Graham, has since been substantiated, although not precisely as Graham had it. We know that there are necessary accessories to the diet, but we know, also, that they are much more widespread than Graham thought. They occur in so many kinds of foodstuffs that anyone who eats a mixed diet usually gets enough of them for his needs. The ill effects of their lack are most evident when the diet is restricted to a few kinds of food which happen not to contain them. A striking example of bodily injury directly due to the absence of these vitamines from the food is seen among Orientals whose diet is apt to be made up of rice plus small amounts of other substances. Of recent years the natives of Japan and China and the Philippines have suffered much from a disease of the nerves known as beriberi. Investigation has shown that this disease is due to the absence from the diet of needed vitamines, and dates from the time when rice-milling machinery was introduced. The old hand methods of milling rice were so imperfect that much of the hull was left clinging to the grains, but machinery polishes the rice clean of every trace of hull. The hulls of rice contain the accessory that is wanting from the polished grains. Wherever it has been possible to bring about the use of unpolished (brown) rice instead of the usual polished kind, beriberi has disappeared. Or the same result can be secured by adding small amounts of beans to the diet. It is probable, also, that the hulls of most grains, including wheat, contain some of the same, or a similar accessory, so to that extent Dr. Graham was right in emphasizing the importance of adding hulls to the flour. Quite recently it has been shown that raw foods are richer in these accessories than cooked, and that ordinary compressed yeast contains more of them than any other easily obtainable material. Many people are being benefited by taking part or all of a yeast cake daily in a glass of milk.

For growth, or the making of new protoplasm, and for maintenance, or the repair of protoplasmic wastage, then, we must eat protein-containing foods, also foods containing various kinds of salts, and foods containing the necessary vitamines. All these are to provide required materials; the actual substances built into the protoplasm. There remains the requirement of power, for both growth and maintenance represent chemical activity on the part of the cell, and this activity depends on power just as does any other activity. In saying this we are merely saying over again in different words what was set down at the very beginning of the book as the chief sign of life, namely, the necessity on the part of living cells of continuous power development. The use of food as a source of energy or power has been talked about already, but it is necessary to say something about the different sorts of power development that may go on in cells, and since we shall have to talk about this a good deal, right here is a good place to bring in for the first time a word that has come to be used whenever the matter of the chemical activities of living cells is being mentioned. The word is metabolism; when we speak of cell metabolism we mean the chemical processes that are going on in the cells. Hereafter, instead of saying power development, the word metabolism will be used as meaning practically the same thing.

First of all, in describing the various kinds of metabolism that cells may show, we have the metabolism of rest. By this we mean the power development that is going on when the cell is doing nothing more than keeping alive; neither growing nor showing any special activity. This is evidently the minimum amount that any cell can show, so it is often referred to as the basic metabolism. We know of at least two things that may change the amount of basic metabolism; the first of these is a change in temperature; when a cell is cold, its basic metabolism is less than when it is warm. There is a very simple chemical reason for this, namely, that chemical processes as a rule go on more slowly the lower the temperature. Since all metabolism consists of chemical processes, this rule applies not only to basic metabolism, but to all other kinds as well, and, as we shall see, explains why the lower animals show such marked differences in behavior in cold and warm weather. The second thing that influences the amount of basic metabolism is the percentage of water in the protoplasm of the cell. Highly organized animals, like ourselves, are destroyed if the cells lose more than a small fraction of their water, but there are many of the lower animals that can be dried until their bodies contain only a very little water and still live. This applies to microscopic forms that live in puddles and similar places; when the puddle dries up the animal dries up too, until all that is left of it is a tiny particle of highly concentrated protoplasm. But this tiny particle preserves all the original cells, or at least enough of them to make a fresh start, and a very sluggish metabolism goes on in each cell. Of course, the advantage of this is that the stored food materials will not be used up as rapidly as they would if metabolism went on at the usual rate, and so there is a better chance that the animal may survive until more water falls or drains into the puddle, or until the particle of dust which the animal has become may be blown by the wind where it will fall into another one. Whenever either of these things happens the protoplasm takes up water again and the former rate of metabolism is resumed. It is only by means of this reduction in rate of metabolism that many kinds of animals are able to persist, for in large parts of the globe there is a period of each year when conditions become so unfavorable that the usual rate of metabolism could not possibly be maintained.

Next in order to basic metabolism comes the metabolism of growth, by which we mean the energy necessary for the making of new protoplasm. Not a great deal is known about growth metabolism; in fact, about the only reason for believing that it requires any energy at all is that the metabolism of young animals, whenever it has been studied, has been found to be greater in proportion than that of animals that are fully grown. It is hard to account for this, unless the growth process itself, namely, the making of new protoplasm, requires energy. When we think of the extreme complexity of living protoplasm, we can easily believe that its formation involves the expenditure of energy, perhaps in considerable amounts.

The last kind of power development to be considered is the metabolism of special activity. Most kinds of cells, particularly in highly organized animals, have some special kind of work to do. For example, the muscle cells have the task of making the motions; the gland cells of manufacturing the secretions, and so on. These we speak of as the particular functions of the cells, and the metabolism by which they are performed as functional metabolism. In some of the lower animals one can scarcely tell where basic metabolism leaves off and functional begins. There is a small shrimp, about a half inch long, that is found quite commonly in small ponds. This little animal has several pairs of legs by which he swims about, and the strokes of these legs go on continuously, day and night, with almost no interruptions, at the rate of a hundred or more a minute, for days or even weeks. It looks as though this, and other animals, that are continuously on the move, were organized without any sharp line between basic and functional metabolism; their protoplasm liberates energy by the oxidation of food, and various things happen as the result; among them are the maintenance of the protoplasm and the making of motions. In the higher animals the distinction between basic and functional metabolism is sharp, and, necessarily so, for the well-being of any of the higher animals requires that he shall have pretty complete control over the activities of his protoplasm, and this he could not have if the functional metabolism were blended in with the basic. In other words, it is as important for bodily well-being that the cells be able to become inactive as that they be capable of activity.

CHAPTER V
BODY CELLS

A GOOD deal has been said thus far about living cells without anything at all having been said to tell what they look like, or how they are made up, beyond the statement that they consist of living protoplasm, which is of a jellylike consistency. To look at living cells through a microscope would almost surely be a disappointment at first, for protoplasm is so transparent that not much of its form can be seen on direct inspection. Fortunately for our knowledge of how cells are made up, protoplasm that has been properly killed and preserved takes stain very well, and different chemical substances in the protoplasm stain differently. Thus features that could not be made out at all in the living cells become clearly visible after killing and staining. The first thing that attracts the attention when cells thus prepared are studied is that every cell has somewhere within it, and usually near its middle, a spot which is more deeply stained than any other part of the cell. This indicates the presence of a substance or substances that take stain more readily than the mass of the protoplasm. This peculiarity led to the naming of the deeply staining portion of the protoplasm chromatin, referring to the ease of staining. The part of the cell which contains chromatin is called the nucleus. In many kinds of cells the nucleus can be made out by an expert observer without resorting to stains, although the details of structure cannot be seen in that way.

We now know that the nucleus, or rather the chromatin that it contains, plays a remarkable and interesting rôle in the life of the cell. To this we shall return presently. The remainder of the protoplasm, outside of the nucleus, shows the greatest possible variety of form, according to the kind of cell at which we happen to be looking. In some of the simpler types this part of the protoplasm seems to be merely a nearly uniform mass, perhaps with tiny particles scattered through it. In other types the protoplasm is drawn out into long slender threads, and these threads may have many branches; or the protoplasm may be distorted into a thin shell inclosing a mass of fat; or it may be subdivided into dense and thin portions with sharp lines of division between them. These various forms are related to the special functions which the cells have, and we shall learn more about them as we take up the different functions in order. On the whole, study of cell structure shows clearly that the protoplasm outside the nucleus carries on the greater part of the metabolism or power development, and is correspondingly important as the seat of the special functions shown by the cell. If it is a muscle cell, this is the part that does the moving; if a gland cell, this is the part that secretes. Nevertheless, the nucleus is a vital part of the cell. It has been definitely proven that a cell from which the nucleus is lost cannot survive more than a brief time. To gain some idea of the actual part played by the nucleus, we shall have to return to it in some detail.

Before undertaking a further description of the nucleus itself, we shall be helped to an understanding of its function if we trace briefly the history of the cells which make up our body. At the beginning, as we probably all know, we start life as a single cell. This cell, after a series of events which will be described in a later chapter, begins the process known as development. Development consists of a series of subdivisions of cell material. At first the single cell divides into two; each of these then divides, giving four. At the next stage eight are formed, then sixteen and so on, until finally the millions of cells that make up the body are produced, all derived from the original single cell. We know that in the adult body there are very many different kinds of cells. Since they are all derived from a single cell, these differences must have put in their appearance during the course of the various cell divisions. In fact, this happens all along; at definite points in the process the two cells that come from the subdivision of some particular one will not be alike. The special kinds of cells that are thus produced become the starting points for whole masses of similar cells in the fully developed body. In human beings, and probably in most other kinds of animals, the very first subdivision does not result in any difference between the cells. The proof of this is that sometimes, in fact fairly often, the two cells become separated. When this happens twinning results, and the twins are exactly alike, being known as “identical twins.” Not only are they alike in all other respects, but they are always of the same sex, a fact that has escaped the attention of some writers of fiction, who have made twins, identical in all other features, brother and sister, instead of both boys or both girls. Twins that are not identical come from different original cells that happened to start developing together. Such twins need have no more resemblance than any members of the same family, and may or may not be of the same sex.

In every cell division the first step consists in a division of the chromatin of the nucleus, which is followed by a division of the rest of the protoplasm. The process by which the chromatin is subdivided is so curious as to be worth a brief description. The

chromatin material is not a simple lump in the nucleus. It looks rather like a tiny string of beads thrown down carelessly, so as to become all mixed together. Each bead is a single bit of chromatin, and these bits are strung on a tiny thread. In an ordinary cell the beads are so mixed together that no order can be distinguished among them, but if a cell that is about to begin dividing is looked at it is found that the string has straightened itself out, and also that it has broken into pieces. The individual pieces are called chromosomes and their number is always the same for any one kind of animal or plant. There is a parasitic worm whose cells have only four chromosomes, and the number ranges from this up to as many as forty-eight in human beings. It may be that other species have even more, but they become so hard to count when there are as many as forty-eight that the number cannot be stated with certainty. So far as can be judged, the number of chromosomes has little to do with the complexity of the animal or plant, for some complex forms have few chromosomes, and some simple forms many.

At the same time as the chromatin is breaking up into chromosomes two tiny spots put in their appearance in the protoplasm of the cell on opposite sides of the nucleus, and tiny threads extend from one spot to the other through the nucleus. There are as many threads as there are chromosomes, the whole group making up a spindle-shaped figure. The chromosomes now become arranged at the middle of the spindle, and apparently each chromosome becomes fastened to a thread. Next each chromosome splits lengthwise through the middle and by what looks like a shortening of the threads the split halves are pulled apart and drawn to opposite tips of the spindle. The purpose of this elaborate scheme seems to be to insure an exactly equal division of the chromosomes between the cells, and the necessity of such an equal division will become clear when we learn something of what the chromatin is for. Meanwhile the description of cell division can be finished by saying that after the halves of the chromosomes are pulled apart the whole mass of protoplasm divides through the middle. As we stated above, sometimes the cells thus produced are alike and sometimes they are different, according to whether they are destined to become parts of similar or of different structures. In either case the chromatin material that goes into the two cells is exactly alike, so that if the cells themselves become different there must have developed a difference in the protoplasm at the two ends of the cell from which they came. Our bodies are made up of millions of cells, of a great many different kinds, but however different they may be the chromatin of each exactly duplicates that of every other one, or did when the cells were first formed; there is reason to believe that the chromatin may become changed during the lifetime of the cells, at least in some cases.

We may be interested in inquiring how long this process of cell division keeps up. Many children do not get through growing until they are twenty years old or more. Does cell division keep on during all this time? More than that; are there any cases of cell division that continue after full growth is reached? The answer to both these questions can be given in a brief paragraph. There are some tissues, particularly the outer layer of the skin, the connective tissues, the blood-corpuscle-forming tissues, and the reproductive tissues, in which cell division continues during all or most of life. The others finish at birth or shortly thereafter. We are born with the precise number of muscle cells with which we shall die, unless accident deprives us of some meanwhile; and if this happens no new ones will be formed to replace those that are lost. The same is true of gland cells. The last cell divisions among nerve cells are believed to occur within a few months after birth. As most of us have observed in our own cases, bodily injuries, if at all severe, are followed by the formation of scars. This means that connective tissue has grown in to fill the place of the cells destroyed by the injury, which cannot be replaced by cells of their own sort, since they have lost the power of cell division.

We have tried, in the above paragraphs, to get some idea of what living cells are like, and how they are derived, but have not attempted any detailed picture of particular kinds of cells. That will have to wait till we reach the story of the different kinds of bodily activity, when the cells that carry on each kind will have to be described more exactly. Something has also been told of the chromosomes, but the full account of them and their meaning is to be taken up in a later chapter, devoted to the matter of heredity and reproduction. In what remains of the present chapter we wish to talk about the conditions in which cells live so that we shall easily picture how they carry on their metabolism.

As an introduction to this topic a word may be said about the wide differences of complexity that are found in animals. They range from the simplest imaginable, a single cell with its nucleus and with protoplasm that appears almost uniform throughout, to a highly organized body like that of man, composed of millions of cells of many different kinds. Between these extremes almost every possible form is seen. The one-celled animals themselves show a wide range of complexity, and as soon as animals begin to be formed of numbers of cells grouped together the possibilities of complexity increase in proportion. One important difference between one-celled and many-celled animals needs to be emphasized; that is the matter of size. There are definite limits to the size that a single cell may attain; these limits are just over the boundary of naked eye vision. If animals are to attain larger sizes, they must necessarily be composed of many cells. The life of a single-celled animal presents no special problem, since it has only to take in through its outer layer from the surrounding water the various food materials and the oxygen which its metabolism requires, and to discharge into the same water any chemical products that may result from that same metabolism, and the question of whether it will live or die depends only on whether the water in which it happens to be contains sufficient materials and is otherwise suitable as a place to live. A many-celled animal, whose cells are arranged in not more than two layers, is in practically the same situation, for every cell has a frontage on the water and so can carry on interchanges of material directly; but the moment complexity reaches a stage where any cells are buried beneath other cells some special arrangement must be provided so that the buried cells can obtain the needed substances for their metabolism. The arrangement consists, in general, of furnishing what may be called an internal water frontage for the buried cells. In other words, complex animals have spaces all through their bodies, and these spaces are filled with fluid. There are no living tissues so dense that the cells of which they are composed are completely cut off from contact with body fluid. In thinking of our own bodies we should realize that this same arrangement applies; every one of our millions of living cells has contact with the fluid with which all the spaces of our bodies are filled, and it is from this fluid that the cells obtain the materials for their metabolism, and into this same fluid they discharge whatever substances their metabolism may produce.

The total amount of body fluid is not large, for the spaces among the cells are in most cases extremely tiny; it follows that with all the millions of cells absorbing food materials and oxygen from this fluid and discharging waste materials into it the time will soon come when no more food or oxygen will be left to be absorbed and there will be no more capacity for holding waste substances. If this state of affairs were actually to happen, metabolism would come to an end and death would be the result; evidently there must be some means of keeping the body fluids constantly renewed in respect to the things which the cells need for their metabolism, and constantly drained of the waste substances which the cells pour out. The way in which this renewal is accomplished is simple; part of the body fluid is separated off from the rest in a system of pipes, known to us as the blood vessels, and this part is kept in motion; at intervals along the system are stations at which the moving fluid can exchange substances with the fluid which actually comes in

contact with the cells; thus the stationary fluid can obtain from the moving fluid the materials which the cells, in turn, are constantly withdrawing from it, and can pass on to the moving fluid the products with which the cells are continuously charging it. All that is necessary to complete the successful operation of the system is to have additional stations at which the moving fluid can obtain supplies of food materials and of oxygen, and stations where it can get rid of the wastes which it accumulates from the stationary fluid, and there must be a pump by which the moving fluid is kept in motion. We are familiar with the moving fluid under the name of blood; the system of pipes in which it moves are the blood vessels; the pump which keeps it in motion is the heart; the various supply stations include the digestive organs, the lungs, and the kidneys. In later chapters the operation of all these stations will be described in detail. The present outline has been given to show in a general way how the problem of metabolism is handled in highly organized bodies in which the individual cells have no direct access to food or oxygen supplies.

CHAPTER VI
THE SUPPORTING FRAMEWORK

SINCE protoplasm is so very soft and fragile it must be supported in all animals and plants except the very tiniest. The nature of the supporting framework has a great deal to do with both the form and the working of the body, so it is desirable that we become familiar with it before trying to go further in the examination of the living protoplasm itself.

A large heavy body like that of man requires an arrangement for support that shall meet several conditions. In the first place there must be strength and stiffness, combined with flexibility, so that the body as a whole shall be firm, yet not rigid. The weight, also, must be kept as small as possible. Then every single cell, and every grouping of cells that we call an organ, must be supported in its place securely but without hindering the free performance of its function. Not only must the protoplasm be held in place, but on account of its fragility it has also to be protected against injury; the vital parts require more careful protection than those that are less immediately essential to life. Finally, bodily motions of all sorts depend on the framework to give purchase to the muscles, which are the actual organs of motion, and so to make their movements effective. For support, for protection, and for motion, then, the framework is important.

The material that does the real supporting is not, of course, alive, for living protoplasm lacks the necessary qualities needed here. It is manufactured and put in place, however, by living cells. They do this by withdrawing the special materials needed from the body fluid which surrounds them; in large part what they get from the fluid is not the finished substance but material from which the living cells make the finished substance. It is then passed outside their bodies and deposited in the surrounding space. Of course this is a gradual process. Bit by bit the structure, bone, cartilage, or connective tissue, as the case may be, is built up by the combined activities of many cells.

Of the three kinds of supporting material mentioned above, bone is the most familiar. No description of its appearance is necessary, for everyone has seen it as it appears in meat animals and in poultry, and it looks precisely the same in man. There are several things about bone, however, that are worth describing. One is the arrangement by which the very hard, compact material is deposited in large masses without cutting off the cells which are doing the depositing from their contact with the body fluid, and so destroying them and bringing their work to an end. The way this is managed can be made out by examination of the figure, showing the structure of bone. At the beginning the bone cells are lying near one of the tiny blood vessels known as capillaries, which are the exchange stations for material between blood and the stationary part of the body fluid. Thus these cells are favorably located for obtaining materials from which bone can be constructed. As they proceed with the formation of bone they always leave tiny passages open between themselves and the blood capillary. Finally the capillary may become completely surrounded by bone, but all along it will be left the passages through which fluid can make its way from the blood to where the cells are imprisoned within the bony walls of their own construction. The metabolism of bone cells is not on a very active scale; the amount of bone substance that a single bone cell has to produce in a day is only a fraction of the amount of saliva, for instance, that a single cell of the salivary gland turns out in the same time; so the bone cell can manage even though its supply of material has to come to it through a few very tiny passages in the bone.

Another interesting feature of bone is the ease with which it can be remodeled. We are apt to think of bone as permanent, after it has once been formed, but as a matter of fact bone is about as subject to change as any of the softer tissues. This is because there are in and around the bones, in addition to the bone-forming cells, a great many cells of different appearance which may be named bone-destroying cells. These latter have the ability to dissolve out the hard material which the bone-forming cells have deposited. Good examples of their work are seen in the hollows of the long bones. We know, of course, that the bones in a child’s leg are so much smaller than those in the leg of an adult that they could almost be fitted into the hollows of the latter. Evidently the bone substance has been moved bodily outward during the course of growth. As the bone-forming cells add material to the outer surface of the bone, the bone-destroying cells dissolve it away from the inner surface. The same thing happens all over the body. A child’s face grows by an increase in size of the bones. Again the inner surfaces are dissolved away. Apparently one condition which makes the bone-destroying cells active is constant pressure. A good example of this is seen in what is known as a gumboil. If a tooth becomes ulcerated, gas and pus are formed at its root, and cannot escape since this is completely surrounded by bone. They accordingly press upon the surrounding bone, and also upon the sensitive tissues, resulting in extreme pain. The pressure upon the bone starts the bone-destroying cells into great activity and in the course of a few days they will dissolve a hole right through the bone, allowing the gas and pus to escape to the outside, and relieving the pain.

Of recent years school authorities have had much to say about the importance of adjusting school seats and desks so that they shall be at the proper height for the particular children that are to occupy them. This is because if the feet hang clear of the ground for hours at a time, as they will if the seat is too high, or if the body must be screwed around to enable the child to work at his desk, as happens when the desk is too low, there is real danger that some of the bones may become misshapen. Most of the stoop shoulders and many of the crooked backs that we see are the result of the habitual taking of wrong postures. Children, and adults as well, should form habits of standing and sitting so straight that none of the bones are put under a pressure that may tend to distort them.

After the teeth are lost the bony sockets in which they lie are dissolved away, making the jaws much shallower than formerly, a fact that accounts for the shortening of the distance between chin and nose in aged people. An important result of this dissolving away of bone by the bone-destroying cells is that the bones are kept as light as possible, without undue sacrifice of strength.

A second kind of supporting material is cartilage. This is both softer and more flexible than bone. It is found in places where flexibility is more important than great strength, as in the ears, the parts of the nose just below the bridge, the Adam’s apple and wind pipe. The chief difference in make-up between bone and cartilage is that while in bone about three-fourths of the nonliving substance consists of lime salts, in cartilage there is almost none of this material, organic substances making up the entire mass. There are no living cells in the body that are more poorly located with respect to obtaining supplies from the body fluids than the cartilage cells, for as these deposit the cartilage around themselves they leave no definite passages through which fluid may pass; the material incloses the cells completely. Although cartilage looks as though it were altogether nonporous, there must be some degree of sponginess present, since the cells do succeed in getting the materials on which their life depends. Cartilage seems to be a more primitive kind of supporting substance than bone. This is shown by the fact that it makes up the entire skeleton in the lowest fishes, and also by the fact that in the higher animals, including man, the bony skeleton starts, in large part, as cartilage. In the parts in which this happens a mass of cartilage is deposited in the place which is later to be occupied by bone. Then at certain points the cartilage begins to be dissolved away by cartilage-destroying cells, which are precisely like bone-destroying cells, and the bone-forming cells come in and build up the real bone as fast as the cartilage is removed. This process of replacing cartilage by bone is practically completed at birth, except in the long bones of legs and arms. These bones, which will about double in length during the growth of the body to adult size, as well as the other bones, which grow to some extent, retain plates of cartilage near each end during all the growth period, and the increase in length is obtained by a continuous formation of new cartilage, which is continuously replaced by bone.

The third kind of supporting material is connective tissue. This is composed of tiny threads or fibers, some of which are inelastic, others are very elastic. The inelastic fibers are found in places where a flexible, but unyielding support is required; the elastic fibers are located where elasticity is particularly important. Either kind of fiber may be grouped into sheets, or into loose networks, or into stout cords. A good example of inelastic connective tissue in sheet form is in the mesentery which holds the organs of the abdominal cavity in place. Just under the skin, anchoring it loosely to the underlying muscles, is connective tissue in network form. The tendons by which most of the muscles are attached to the bones upon which they pull are made up of inelastic connective tissue in the form of cords. The best example of elastic connective tissue is in the large arteries, which are just as elastic as best quality rubber tubing. Another good example is the large and strong elastic cord which passes along the back of the neck in cattle and sheep, and helps to support the weight of the head. Connective tissue fibers are deposited by living connective tissue cells. Since connective tissue is of open and relatively loose construction, there is no problem presented in supplying the cells with material. The meshes among the fibers are filled with fluid, and this fluid has ready connection, in turn, with the blood. Use is made of the abundance of body fluid in the connective tissue spaces whenever a subcutaneous injection is given, for what is done is to inject the desired material into the fluid which fills the spaces in the connective tissue just beneath the skin, trusting that it will pass from there to the blood, which it does rather gradually, and so is distributed about the whole body.

While we are on the topic of the supporting framework, something must be said about the grouping of the bones into what we know as the skeleton. Of course it is evident that the effectiveness of the bony part of the framework depends almost altogether on the way in which the individual bones are grouped together. If the whole skeleton were

composed of one great bone, or of different bones anchored solidly together, the body would be perfectly rigid; since motion is necessary to life, flexible connections between some of the bones are absolutely essential. Our movements are actually made by means of muscles, but nearly all of them become effective through the motions of bones to which the muscles are fastened. The bones are often very irregular in shape; careful study shows that the irregularities are due either to provision for the contact of one bone with another in the joints, a contact that must allow in most cases for motion of one on the other, or to provision of places to which muscles can be fastened in such a way as to make their pull effective. It is, of course, out of the question for us to examine the skeleton bone by bone. Figures are given of a number of typical bones: all that we can do in addition is to mention some of the interesting features of the skeleton.

The skeleton of the head is called the skull; its chief features are the brain case, the eye sockets, and the parts about the nose and mouth. The brain case is made up of eight bones firmly joined together by saw-tooth margins to make up a roughly spherical box which holds the brain, and protects this delicate and vitally important organ from all injury except the most severe. There are a number of small openings out from the brain case through which nerves pass, and one large opening below and at the back through which the spinal cord merges into the brain. The bones which make up the sides of the brain case are much thickened just behind the ears. A hollow extends from each ear into the bone, and within this hollow, securely protected from harm, is the actual organ of hearing. There are extensions of the hollow backward which are not occupied by any organs, and which communicate with the cavity of the ear. These sometimes become infected from the ear, causing the condition known as mastoiditis. Not only is this condition excruciatingly painful, but on account of the thin layer of bone which separates it from the brain itself it is highly dangerous. For this reason any ear trouble should be carefully watched lest it develop into mastoid trouble.

Of the bones that make up the eye sockets not much need be said, except that they have a great deal to do with determining the shape of the upper part of the face and so the appearance. There are bones within the nostrils that are very irregular in outline. Their effect is to increase greatly the surface over which the air that is breathed must pass, enabling it to become both warm and moist before entering the lungs. The jaw bones serve as receptacles for the teeth; the lower jaw, which is the only movable bone of the head, except for the tiny bones within the ears, has also the duty of operating as a mill in reducing the food to suitable form for swallowing. To aid in this function the lower jaw is hinged to the rest of the skull in such a way that it not only opens and closes but can slide forward and back or from side to side. All these motions are used in chewing. There are twenty-two bones altogether in the skull, not counting the three tiny ones in each ear which will be described later.

The body consists of trunk and limbs, and each part has its skeleton. The skeleton of the trunk consists of the spinal column, the rib cage, the shoulder girdle, and the hip girdle. The skeletons

of the limbs are all according to a single plan to be described in a moment. The spinal column is a remarkable example of strength combined with flexibility and elasticity. It is made up of thirty-three bones or vertebræ; each of these has a disk-shaped part known as the body, and these disks are placed in line as shown in the figure. Between each disk and its neighbor is an elastic pad composed of a mixture of cartilage and elastic connective tissue. There is a small amount of give in each pad and this, taken along the whole length of the spinal column, is enough to give it the great flexibility which it enjoys. During the day the weight of the body packs these pads down hard, so that it is said that a man may be as much as an inch shorter at night than in the morning. Behind the disk each vertebra has an arch of bone, and beyond and beside this arch most of them have projections. All the arches together make up a bony canal which contains and protects the spinal cord. The projections serve for the attachment of the ribs and the back muscles by which the bending motions of the body are made.

The rib cage includes the breastbone and twelve pairs of ribs. It serves two purposes: to protect the heart and lungs from injury; and to take part in the movements of breathing. The latter function involves some degree of motion of the rib cage. All the ribs are attached behind to the vertebral column in a fashion that permits of a little motion up and down. All except the last two are fastened in front, seven pairs to the breastbone, three pairs each to the rib above it. In breathing the breastbone and ribs are moved up and down by muscles attached to them.

The shoulder girdle is made up of the collar bones and shoulder blades. Each collar bone is fastened at its inner end to the upper edge of the breastbone; this is the only direct contact the shoulder girdle has with any other part of the skeleton of the trunk; at the point where collar bone and shoulder blade meet, there is a shallow cup into which the head of the skeleton of the arm fits. The arrangement is favorable to great freedom of movement of the arm. Not only is the shoulder joint very flexible owing to the shallowness of the cup into which the arm bone fits, but the shoulder blade itself is capable of a considerable range of movement. This is because it is imbedded in and held in place by muscles. If one watches a person with bare shoulders while he raises his arms, it will be seen that the shoulder blades do not move much while the arms are being lifted to the horizontal position, but as that point is passed they begin to swing outward rapidly, so that when the arms are high above the head the shoulder blades are in a quite different position from that which they have when the arms are down.

The hip girdle consists of five bones of the vertebral column welded firmly together to make up what is called the sacrum, and two other large bones known as the innominate bones, each of which, in turn, is made up of three bones tightly fused together. The innominate bones are firmly joined to the sacrum at the back and they meet in front, also in a firm joint. The hip girdle or pelvis is rigid, suiting it to bear the strains that come upon it on account of its position at the junction of the legs with the trunk. At the outer side of each innominate bone is a cup, much deeper than the corresponding cup of the shoulder girdle, and into this fits the head of the skeleton of the leg. The arrangement is a typical ball and socket, and has been much copied in machinery where a flexible joint is required. In a good many people the union of the innominate bones to the sacrum is not so firm but that it yields somewhat when strains are put on it. Ordinary strains in these cases produce severe backache. Heavy strains may cause an excessively painful as well as disabling dislocation. In either case medical attention is needed.

Each arm can be subdivided into upper arm, forearm, wrist, and hand. The skeleton of the upper arm is a single long bone. The forearm has two bones, one of which is hinged at the elbow to the bone of the upper arm in a way to limit the movement to the single back and forth swing of which the elbow is capable. The other bone of the forearm can be rolled over the one which is fast at the elbow; this is what happens whenever the hand is changed from the palm up to the palm down position. There are eight bones in the wrist; these are irregular in shape, and are so grouped as to permit of a very wide range of movement. The bones of the hand and fingers make up five rows numbering four bones each for the fingers and three for the thumb. The joints are all practically simple hinges except for the one where the thumb joins the wrist, which is a much more flexible joint; flexible enough, in fact, to allow the thumb to be brought opposite any of the fingers. No animal except man enjoys this degree of flexibility in the thumb, so no animal equals man in the nicety of the grasp, particularly of small tools. When we recall how constantly we take advantage of this property of our hands we can realize how greatly our superiority over the lower animals has been aided by this rather slight structural difference between our hands and theirs.

The leg subdivides along the same lines as the arm into upper leg, or thigh, lower leg, or shin, and foot. The order of bones is, on the whole, the same; one in the thigh; two in the lower leg. Instead of a flexible wrist the corresponding bones of the foot are grouped into a less flexible, but much stronger, heel and upper instep. Two of the bones of this group are fused together into one, reducing the total number from eight to seven. The bones of the lower instep and toes correspond in number and arrangement to those of the hand and fingers, but the great toe does not have superior flexibility as does the thumb. There is one bone in the leg, the knee cap or patella, that does not correspond to any bone in the arm, although it does correspond to a part of a bone, namely, the projection, at the elbow, of the long bone of the forearm. A feature of the skeleton of the foot that is worth a word is the arching of the instep. This undoubtedly adds greatly to the ease of walking. The natural position for the foot is, of course, with both the heel and the ball of the foot on the ground. For some reason it has become the universal custom among civilized people to raise the heel off the ground by adding a heel to the shoe. This does not seem to make much difference as long as the heel is not too high. In fact soldiers wearing properly fitted heel shoes can march as far and fast as can be expected. Excessively high heels throw the weight too much on the ball of the foot, thus doing away with the benefits that come from the arching of the instep. The effect on the gait is very apparent in any one who walks in high-heeled shoes. The foot itself does not appear to be greatly harmed by the wearing of high heels provided the shoes are otherwise well fitting. Whether the heels are high or low, the fit of the shoe is of utmost importance to the preservation of the feet. Crowding the feet into shoes that are too small in any direction is a fruitful means of bringing on foot trouble. Wearing shoes that are loose enough to allow the foot to turn over inside the shoe is nearly as bad. If the shoes are properly fitted in the beginning and then the heels are kept squared up, so that the feet will always stand straight on the ground, there will usually be little trouble with fallen arches or other foot disturbances.

The bones are fastened together at the movable joints by stout sheets or bands of connective tissue known as ligaments. These hold them in place very securely and as additional support the muscles which surround every joint help to prevent the bones from slipping out of place. At nearly all the joints of the body the combined action of ligaments and muscles is sufficient to guarantee the joint against dislocation; the shoulder joint, and to a less extent the hip joint, is more likely to suffer this accident. The reason is that in obtaining flexibility of movement security of attachment is somewhat lessened. If the ligaments at the shoulder were tight enough to prevent the joint from ever becoming dislocated they would bind it to a serious degree. Most of the ligaments are of inelastic connective tissue, but those that fasten the separate vertebræ of the spinal column together are elastic, allowing of the bending in every direction which makes our backs as flexible as they are. The only movable joints which are bound by other means than ligaments are the connections of the ribs with the breastbone. These are of cartilage, but the movement here is so slight that the cartilage yields enough to permit it.

This completes our account of the bony skeleton. We shall finish the description of the supporting framework by a word about what may be called the connective tissue skeleton. The bony skeleton serves to support the body as a whole and to permit the muscles to do their work; the individual organs and the cells which make them up are held in place by sheets and bands of connective tissue. These are coarse and strong when their purpose is to support a large and heavy organ like the stomach; they become finer and finer as the parts to be supported become smaller, and when the individual cells are reached the connective tissue which surrounds them is almost inconceivably delicate. So completely does connective tissue permeate the whole body that it has been said that if everything else could be dissolved away, leaving only this tissue in place, there would still remain a model of the body, complete to the last detail.

CHAPTER VII
MOTION

OUR account of the body has now reached the point where we can take up in detail the special activities of the different kinds of cells. The first to be considered is motion, both because it is the familiar sign of life, as pointed out in the first chapter, and because it has so much to do with everything that enters into life. There are probably no animals that live out their entire lives without making any active motions, although some parasites, like the tapeworm, are stationary most of the time. There are a number of different ways in which movements are brought about. The very simplest animals, which consist of nothing but a bit of protoplasm, move by causing the protoplasm to flow bodily in one direction or another, a projection of part of the protoplasm being balanced by withdrawal of an equal part on the opposite side, and the whole mass progresses in the direction of the first projection. Next beyond this simplest means comes motion by tiny threads of protoplasm which project beyond the surface of the cell and whip back and forth. The stroke of these threads or cilia, as they are called, is stronger in one direction than in the other, so the effect of their beating is to propel the cell of which they are part in one direction through the water; or if they are on a surface which is stationary they set up a current in the water itself. This latter is the means by which oysters and similar animals which are anchored to the rocks get their food supplies. In some one-celled animals there are only one or two large cilia at one end; these beat back and forth, propelling the animal much as a fish swims.

The commonest, as well as the most effective, means of making motions is by cells specially developed for that purpose. These are called muscle cells, and every highly organized animal depends on them for most if not all of the motions which take place in its body. In muscle cells the functional metabolism takes the form of forcible changes in shape of the cells by which bodily motions are brought about. A muscle cell might be described as a mechanical device for transforming the chemical energy of burning fuel into the energy of motion. We have something comparable in the automobile cylinder, where the energy obtained from the explosive burning of the air-gas mixture drives the piston and so propels the car. Of course the two devices are not even remotely alike in the actual way in which they operate; their resemblance is purely the general one of converting one type of energy (chemical) into another type (motion).

There are three kinds of muscle cells in our bodies. The simplest are those that are found in the wall of the stomach and intestines and other internal organs that are capable of movements; the next kind is found only in the heart; the third, and most complex, makes up the bulk of our muscular tissue; it is the muscle that moves the bones. The first kind, because it shows no particular markings when examined through the microscope, is usually called smooth muscle; the second kind is known as heart muscle; the third kind, because it moves the skeleton, is named skeletal muscle. We shall devote most of our attention to this third kind of muscle, because it is a much more efficient machine than the others, and also because it has to do with our general bodily movements instead of with motions of internal organs.

A single skeletal muscle cell is an exceedingly slender fiber, much smaller than the finest thread; it may also be very short, not more than a twenty-fifth of an inch long, or it may be as much as an inch long. A muscle is made up of many of these fibers grouped side by side in bundles, and also, if the muscle is long, placed end to end. The fibers are held in place and fastened together by connective tissue. Lean meat consists of thousands of these muscle cells with their connective tissue fastenings. In coarse meat there is relatively more connective tissue and less actual muscle tissue than in the finer grades. In every muscle the connective tissue is loose enough to allow body fluid to penetrate among the muscle cells. Blood vessels are also distributed through the mass of the muscles between and among the cells; thus their nutrition is provided for.

Although not all muscle cells are exactly equal in power, on the whole the force that muscle can show is the force of one cell multiplied by the number of cells that can join in the pull. A strong muscle must have many cells side by side; in other words it must be thick. Also, the distance through which muscles can make movements depends on their length, so a muscle that has to pull for a considerable way must be long, and since single muscle cells are short there will have to be a good many cells end to end to make the whole muscle long enough for its task. The actual make-up and arrangement of muscles in the body depends in part, therefore, on the thickness and length needed for the particular work to be done, and in part on the architecture of the part of the body where the muscle is located. For example, the strongest muscle in the body is that by which one rises on the toes. This muscle operates by pulling upward at the back of the heel. If it were located right at the ankle, where it would have to be if attached directly to the place where its force is exerted, the resulting clumsiness can easily be imagined. By shifting it up to the middle of the lower leg room is found for the large mass of muscle needed for the work. The connection with the heel is made by means of a long and very strong tendon, known as the tendon of Achilles, because that was the part Achilles’ mother failed to immerse when she was dipping the infant in the river Styx to make him invulnerable. Other equally good examples are the muscles for operating the fingers. If placed in the hands, the latter would be too bulky and clumsy for any kind of efficient use. By placing them up in the forearms out of the way, and connecting them with the fingers by long tendons, delicacy is secured for the hands.

The muscles of the arms and legs are arranged in groups about the joints, and these groups always include opposing sets. Thus if the joint is a simple hinge, as at the elbow, where the only motion possible is back and forth, there will be one muscle to bend the joint and another opposing muscle to straighten it out again. The first is known as a flexor; the second as an extensor. In the arm the biceps, on the upper surface, is the flexor and the triceps, on the under side, the extensor. Joints that permit of motion in several directions have correspondingly more opposing sets of muscles acting upon them. The same scheme applies to the trunk, but since in the trunk instead of a few very movable joints we have the whole row of slightly movable vertebræ, the grouping of muscles is more complicated. Not all the skeletal muscles work about joints. The tongue, the muscles of the lips and about the eyes, those along the front of the abdomen, and some others are attached to bones only at one end or not at all, and do their work by pulling upon one another.

In earlier paragraphs we have seen that the movements made by muscles represent their functional metabolism, and also that the actions of whole muscles are merely the sum of the actions of the individual cells. Our present task is to see how muscles act; in other words to examine their functional metabolism. One feature that must be in mind from the very beginning is that the functional metabolism of muscle cells is under control; they do not go off at random, but only when started. This is more or less true of the functional metabolism of all the cells in highly organized animals. The agency that starts them off is named a stimulus. To picture how stimuli act we shall have to think for a moment of the state of affairs in cells at rest. As we have tried to make clear, cells at rest are not stagnating; a more or less active basic metabolism goes on within them all the time. This metabolism is of such a sort that it does not disturb the balance existing within the cell. The various chemical processes go on, using up material and producing wastes, but without arousing the additional chemical processes of functional metabolism. Meanwhile the substances that are required for this latter are present in the cell, so that when the disturbance that we call a stimulus comes along there is an increase in the total amount of metabolism, the extra chemical processes being those which perform the special function of the cell. In the case of muscle cells the stimulus ordinarily reaches them by way of the nervous system, although electric shocks, sharp blows, some irritating chemicals, and perhaps one or two other kinds of disturbance can act as stimuli. The effect of the stimulus is to start certain chemical processes; these in turn bring about the forcible shortening which is the thing that happens in active muscle. In skeletal muscle the shortening may be very rapid; the muscle can contract and relax again more quickly than the eye can follow. This is true at the temperature of our bodies. In cold-blooded animals, like fish or frogs, muscles become sluggish when they are cold. We see here one of the advantages we enjoy in having bodies that stay at the same temperature the year around; if our bodies cooled off in cold weather as do those of frogs, we should have to do as they do, become inactive whenever the weather becomes cold. As each muscle cell shortens it pulls upon the connective tissue that surrounds it; this communicates with the connective tissue of other cells, and all the connective tissue within the mass of the muscle fastens to the very stout sheets or cords of the same at the ends which are called tendons, by which the muscles are attached to the bones. Thus, although the pull of any single cell is so feeble as to be scarcely measurable, when hundreds or thousands of them pull all at once the effect may be very powerful.

We are familiar with the very wide range of effort that our muscles can show. They may contract with utmost delicacy, as when we hold a humming bird’s egg in our fingers, or they may pull with a force, in our largest muscles, of several hundred pounds. Of course this possibility of variation is of great advantage in our use of our muscles. It depends upon the very large number of individual fibers of which even our smallest muscles are made up. Whenever any single fiber contracts, it pulls to its full extent; if only a few become active, the pull of the whole muscle will be slight; as more come into action, more force will be exerted; the muscle will show its utmost power when all the fibers are contracting at once. We are conscious of greater mental effort when we make a powerful muscular contraction. This can be explained as due to the greater nervous discharge required to excite all the muscle fibers at once.

One feature of muscular action calls for an additional word. This is the temporary loss of power, resulting from too long-continued use, which is called fatigue. We know that a well-constructed machine can operate day in and day out without having to stop to rest; why cannot our muscles do the same? Evidently the necessity of resting cuts down the possibilities of life more than any other one thing; our real life is only two-thirds as long as it counts up in years because we have to spend one-third of the time in sleep. Of course muscular fatigue is not the only kind; there is nervous fatigue, as well, about which something will be said later. The activity of our muscles is based on functional metabolism; it follows, therefore, that fatigue is also due to metabolism. We can think of two ways in which metabolism might cause fatigue; the first of these is by using up the materials which furnish energy; clearly no cells can go on working after they have exhausted their supplies of fuel. The second results from the fact that metabolism produces waste products. It is a familiar fact of chemistry that when the substances formed in chemical processes are not removed they interfere with the processes themselves. In active muscles very rapid metabolism is going on and large quantities of waste substance are being formed; these have to be discharged from the cells into the surrounding fluid, and removed from there in turn by the blood. We can easily imagine that this might not take place as fast as necessary to keep the cells from becoming more or less clogged; in fact this clogging is exactly what happens, so that muscles begin to show fatigue some time before their supplies of fuel material are used up.

One familiar fact of muscular fatigue is that soreness, which indicates that fatigue has really

been present in large amount, occurs much more often when we use our muscles in ways to which we are not accustomed than when they are exercised according to habit. It is the experience of every one who does manual labor that when he gets a new job, one that calls for different use of the muscles than he has been in the habit of, his muscles are very sore until he is “broken in.” After that, although he does as much or even more work than at first, he no longer becomes sore. This is explained as being due to two things. First, whenever we make an unaccustomed movement we overstimulate our muscles; that is, we call more fibers into action than are necessary to do the job; as the motion becomes familiar we cut down the action to that which just meets the demand. Thus there is a great deal more metabolism than necessary when unfamiliar motions are being made. Then, secondly, there is a spot in every muscle cell, just at the point where the nerve makes its connection with the muscle, that is more easily fatigued than any other part of the muscle cell. This spot, by becoming fatigued first, tends to cause metabolism to stop in time to prevent the rest of the cell from being seriously fatigued. Only when we are so much interested in what we are doing that we pay no attention to the fatigue of this safety spot, or when necessity keeps us at work after we would quit if we had our own way, do we push the metabolism so far that muscular soreness results. Other types of fatigue, including feelings of exhaustion, are due to effects on the nervous system, and will be described when we have that system before us.

Before we leave the subject of the skeletal muscles it will be interesting to say a word about the different kinds of motions that they bring about. We have already seen that they work by pulling at the joints, and we have no intention of enlarging on that topic. What we want to do here is to group the bodily motions into a few classes, regardless of what joints are actually moved. First, and most important, comes locomotion; by that we mean any motions that move the body from one place to another. Under that head we have walking, running, swimming, jumping; in birds, flying. Next in order comes grasping; this includes all motions by which we take hold of anything. We can realize the importance of this group of movements when we think that our fore limbs are specifically grasping organs, while in the great majority of animals they are organs of locomotion along with the hind limbs. Originally grasping had to do, undoubtedly, with the taking of food and not much else. In civilized man we have in addition the use of all kinds of tools from the coarsest to the finest. In most four-legged animals the chief organ of grasping is the mouth. We still use our mouths to some extent as grasping organs, and could probably learn to make even more use of them in that direction if forced to it. Chewing and swallowing make up a group of movements concerned primarily with the handling of food after it has been grasped. Not much need be said about them. Of small extent but great importance are the motions connected with sense perception; these include chiefly the motions of the body, neck, and eyes in vision; we are constantly turning to look at something; in such animals as horses movements of the ears help greatly in hearing; and both man and animals make sniffing motions to increase the keenness of smelling. There is a group of motions devoted to voice production (including breathing). In man the vocal cords, tongue, and muscles of the cheeks are the chief muscles that have to do with the voice, not including the muscles of breathing, which, of course, are essential. The interesting things about the vocal cords are the excessive fineness of their operation, enabling expert singers to produce tones that vary by only a few vibrations a second, and the amazing exactitude of the control that the nerves have over them, so that good singers can set them at the tension needed for producing a particular tone with absolute certainty. The tongue is not a single muscle, but a mass of several muscles working one upon the other. It plays a part both in voice production and in the chewing of food. As an organ of voice production it helps by changing the shape of the mouth cavity. Speech depends very largely on this, since not the tension of the vocal cords but the shape of the mouth and throat determines the making of letters and syllables.

In addition to these familiar uses of the muscles there is a use which is just as important but about which we are apt to think less. This is their use in connection with posture, the taking and holding of particular bodily positions. Posture is unlike other muscular activities in several things. In the first place there is a steady, but rather feeble, tension which can be held without marked fatigue for long periods; all other forms of muscular contraction become severely fatiguing rather quickly if held steadily. In the second place the nervous control of posture seems to be different in some respects from our ordinary control of our muscles. Finally there is some doubt as to whether the contractions of the muscles themselves are the same. Measurements of the functional metabolism of posture show that it is much less than would be expected if the muscular action were of the ordinary type. This, of course, explains why posture is less fatiguing than other forms of activity.

The other two kinds of muscle, heart muscle and smooth muscle, must have a word of description. Heart muscle contracts quickly and powerfully, as does skeletal. It differs from skeletal in not depending on nervous stimulation to make it contract; the heart can be cut clean out of the body and will go on beating for a short time; in cold-blooded animals, like frogs or turtles, for a long time. This could not be true if the heart muscle had to be aroused to activity by nerves. Besides being automatic, heart muscle shows the peculiarity that whenever it contracts all the fibers join. We do not have a varying strength of pull shown by heart muscle as we do in skeletal. As we shall see, it would be a serious disadvantage rather than an advantage if heart muscle were to be like skeletal in this respect.

Smooth muscle has the duty of operating the internal organs. For this function no great strength is required; the motions do not have to be powerful. Nor is rapid motion important. Smooth muscle does not have to be so highly developed, then, as is skeletal. It is sluggish and rather feeble in its actions. There are, however, two points of superiority about smooth muscle, which fit in well with its special task. The first of these is its freedom from fatigue. There are in the body numerous smooth muscle masses that are in contraction practically all the time. This would be impossible if fatigue were to develop. These masses make up what are called the sphincters, rings of muscle surrounding openings like that from the esophagus to the stomach or from the stomach to the small intestine. It is the duty of these sphincters to hold the openings closed all the time except occasionally when they open for just an instant to let material through. The second point about smooth muscle which fits it for its work is that it is capable of stretching out greatly or contracting sharply without much difference in the force with which it is pulling. For example, at the beginning of a meal the walls of the stomach are drawn up, so that the food that is swallowed enters a small space. With the progress of the meal the stomach enlarges, so that at the end it has a much greater bulk than at the beginning. But the actual pressure of the stomach upon its contents is about the same as at the beginning. If the stomach were an ordinary elastic bag this could not happen; the walls would have to stretch as the stomach filled, and the stretching would mean greater pressure. Since the stomach walls are of smooth muscle they adjust themselves to the progress of the meal. It is important to note that there is a limit to this possibility of adjustment. If one is so greedy as to keep on stuffing after the stomach has reached its full size, stretching does occur, and if this is repeated it may lead to a diseased condition known as “dilated stomach,” which will cause much digestive trouble.

CHAPTER VIII
SENSATION—INTERNAL AND CONTACT SENSES

WE have talked a good deal about muscles and the different sorts of activities they can perform. We have also mentioned the fact that the skeletal muscles are under accurate nervous control. Our next task is to investigate the control of this nervous control; in other words to find out just what it is that causes the nerves to stimulate the muscles so that they shall perform as skillfully and usefully as they do. In Chapter II we saw that our bodily movements are adjusted to our needs through the sense organs. These bring information of the situation and we act accordingly. We may group the kinds of information which the sense organs furnish under three heads; first, what is going on inside our bodies; second, what is happening at the surface of the body, and third, what is happening at a distance from us. The senses which bring the first kind of information are called the internal senses; the second group are the contact senses; and the third are the distance senses. We need to remember that the primary purpose of the senses is to guide our muscles, and that our muscles are to find food for us, to keep us from bodily harm, and to assist in the perpetuation of life by propagating and caring for the young. By keeping these facts in mind we shall have no difficulty in understanding the way in which the various senses do their work.

Pain, hunger, and thirst are the internal senses with which we are most familiar. Pain is evidently a protective sense. It is never aroused unless something is amiss; for that reason pain should never be neglected. Of course, in the majority of cases the pain is due to some simple disturbance which can be located, and if no permanent harm is to follow, or if no relief is possible, the heroic bearing of the pain is meritorious; but thousands of women, thinking mistakenly that to complain of suffering is a sign of weakness, or hoping to spare loved ones distress, bear in secret or make light of pains that are the signs of insidious disease, curable if taken in hand early enough, but sure to cause acutest suffering and untimely death if allowed to go on unchecked. Unfortunately our most dangerous internal enemies, the organisms of infectious disease, do not give warning of their attack by causing pain until the disease itself is so far advanced that there is no escaping it. In this respect pain falls short of being efficient as a means of warning us against impending injury.

Hunger and thirst are the stimuli which drive us to the taking of food and water. It is interesting to think that of all the living things that roam the earth only men have discovered the connection between the taking of food and the avoidance of starvation; all other animals are impelled to nourish themselves wholly through the operation of these senses. There are two distinct phases to hunger. The first is appetite, and this by itself seems not to be a sense in the strict meaning of the word, but rather a memory of agreeable experiences associated with the taking of food. In man appetite is often sufficient by itself to lead to eating, as is proved by the frequency with which food is eaten between meals when there cannot possibly be any genuine hunger, but probably in animals it acts to arouse genuine hunger, rather than to cause eating by itself. Genuine hunger is a sense as definite as any other. It is aroused by spasmodic contractions of the stomach. These contractions cannot occur except when the wall of the stomach is in a certain state of tension. Various things can influence the coming on of this degree of tension in the stomach, and so the possibility of hunger. Appetite itself probably does this very effectively. Habit seems also to have something to do with it. Hunger is usually felt just as mealtime draws near, and it is often much keener at noon or night than before breakfast, although the stomach has been longer empty at breakfast than at any other meal. A curious fact about hunger is that it may disappear completely after a few days of complete starvation. Contrary to the popular idea that hunger becomes more and more acute as starvation continues, the testimony of practically all persons who have starved for more than a few days is that all sensations of hunger, as well as all strong longings for food, subside and do not return. This is especially true if the body is kept quiet and if the mind is diverted, so that recollection of meals particularly enjoyed shall not come up.

Thirst is due to actual drying of the throat. When the cells lining that region become deficient in moisture the sense is aroused. The drying may occur from without or from within. When it occurs from without, as in sleeping with the mouth open, relief can usually be obtained by merely swallowing saliva copiously. The same treatment helps for the moment when the lack of moisture is due to deficiency in the amount in the body, but in this latter case no permanent relief can be had except by the taking of water. When the amount in the body falls below the proper level no comfort can be had until the loss has been made good. An interesting thing about thirst is that it is the only sense which is said never to be lost or seriously impaired by disease.

In addition to these familiar internal senses we have some that are less well known. They are for the purpose of what may be described as the routine guidance of the muscles. The act of walking, as we well know, is made up of a series of muscular movements which are both accurately timed and accurately graded. We obtain startling realization of this when we come to the bottom step on our way down stairs without noticing that we have arrived there. This timing and grading are done for us by our bodies without our having to attend to it. The amount of labor that is saved is shown by walking upon railroad ties. These are irregularly spaced, and on that account it is necessary for us to pay attention to every step. There is no comparison between the fatigue of this kind of walking and ordinary progress along a smooth path. The senses that keep track of the position of the body and of individual muscles are known respectively as the equilibrium sense and the muscle-and-joint sense. The equilibrium sense has as its organ a part of the internal ear. Deeply imbedded in the bone is a series of chambers and canals lined with a delicate membrane and filled with liquid. The canals, which are three in number in each ear, are semicircular in shape, and accordingly have been named the semicircular canals. One of them is horizontal; the other two are vertical, and the two vertical canals lie at right angles to one another. This arrangement makes it inevitable that any movement of the head, in any direction whatsoever, will register differently on the canal system than any other movement, which is exactly what is required to make the apparatus efficient as an organ by which motions of the body are kept track of and guided. Along with the semicircular canals is a structure known as the vestibule which registers the position of the head, and so indirectly of the body, when no movements are being made. We are not ordinarily conscious of the working of these senses; they carry on their guidance of muscular movement without our attention. We can, however, pay attention to what they show if we wish. For example, one who is swimming under water is never in doubt as to whether his head is turned up or down, even though his eyes may be shut. His knowledge of position in such a place is obtained from his equilibrium organ, even though he may not be aware of the fact. Sometimes the organ becomes diseased. The results, so far as the victim is concerned, are highly distressing. He usually has to stay in bed because he cannot balance himself well enough to get about.

The organs for muscle-and-joint sense consist of tiny spindles distributed around the joints and embedded within the mass of the muscles. They are arranged so as to be affected by every motion of a joint or every contraction of a muscle. They register not only the fact of motion but also the extent. There is a disease, commonly known as locomotor ataxia, in which the muscle-and-joint sense is impaired or lost, particularly in the legs. The result is that walking becomes difficult and unsteady, and usually impossible when the eyes are shut or the room is dark. This is because the victim learns to make his sight serve instead of his muscle-and-joint sense for guiding his muscular movements, and when this also is withdrawn all knowledge of where his legs are or what they are doing fails, and the only course is to fall down or lie down as quickly as possible.

We have some additional bodily sensations, such as nausea, repletion, fatigue, ill feeling or malaise, which guide our conduct more or less, and are not very different in consciousness from hunger or thirst. So far as is known there are no sense organs by which these sensations are aroused. They are not strictly senses, therefore. We do not know enough about how they originate to say anything more about them.

The contact senses are touch, warmth, cold, and taste. Pain that comes as the result of bodily injury might also be classified as a contact sense, since its cause is something that comes in direct contact with the body from outside, but it differs from internal pain only in its source and not at all in the sensations it arouses, so there is no need of describing it over again. The sense of touch is the fundamental sense; the very lowest animals, even those that have no specially developed sense organs, and few organs of any kind, react to the contact of objects with their bodies just as the highest animals react to the sense of touch. When no other information is available, that of simple contact guides the animal in its securing of food and its avoidance of harm. In accordance with this primitive character of the touch sense, the psychologists tell us that we interpret the information from our more highly developed sense organs, sight particularly, in terms of the feel of objects. When we look at anything our judgment of it actually consists in an idea of how it would feel if we were to take hold of it. Our touch organs consist of tiny spots scattered all over the surface of the body. They are much closer together on some parts than on others. The total number is estimated at a half million or more. A good way to test their sensitiveness is by pressing down on different parts of the skin with fine hairs. When this is done it is found that the most sensitive regions—the tip of the tongue, for instance—are fifty or sixty times as sensitive as the dullest regions, like the small of the back. To obtain sensations of touch it is necessary that there be unaffected points alongside those that are affected. If all are acted on alike, there will be no more sensation than if none is acted upon. This can be shown by dipping the hand into quicksilver. The very heavy liquid presses on all the touch points hard enough to affect them, but since it presses on all alike nothing at all can be felt except along the line where the hand enters the quicksilver where the pressure is strongly marked. It is this feature of the touch sense that makes the wearing of clothing bearable. If we had to feel the contact of the clothes constantly we should presently find them so trying that we could no longer endure them. We do feel rough places and are often seriously annoyed by them, so we can judge what would be the effect if the whole surface were felt as plainly.

Closely related to touch is the sensation of tickling or itching. Curious facts about this sensation are the violence of the feeling that may be aroused by very delicate irritation, drawing a thread along the corner of the nose, for example; the persistence of the feeling beyond the actual irritation; and the effectiveness of scratching as a means of alleviating the condition. Almost nothing is known in explanation of any of these peculiarities.

In addition to organs of touch the skin contains two kinds of organs for perceiving differences of temperature. The first of these detects warmth; the second cold. It is by means of these senses that we judge whether the place where we are is of a suitable temperature in which to remain; whether we should be quiet or active; whether special provisions, like changes in the clothing, are necessary. In the case of both senses the temperature of the skin is the comparison point. We judge that an object is warm or cold according as its temperature is above or below that of the skin which touches it. The ears are usually a few degrees cooler than the hands; thus it is possible for one and the same object to feel cold to the hands and warm to the ears. The two kinds of temperature organs are side by side in the skin, although there are many more “cold” spots than “warmth” spots. Very warm objects affect both kinds, and then we get the sensation that we call “hot,” as distinguished from merely warm. The cold spots are a little nearer the surface of the skin than are the warmth spots; for this reason a hot bath may feel cold at the very instant of stepping into it, although the sensation changes to hot almost at once. We need to remember that our sensations of warmth or cold depend altogether on the state of the skin, and tell us nothing at all about whether our bodies as a whole are warm or cold. Because the blood is always warm a flushed skin always feels warm, and to produce flushing by means of alcohol has long been used as a means of making the body feel warm and comfortable. This may be a serious mistake in cold weather, for to drive the blood to the surface then may mean that the body as a whole will cool off to the point of actual injury. It is better to feel cold and conserve the body’s heat than to feel warm and waste it.

The last of the contact senses is that of taste. This is found only on the tongue. Scattered about on that organ are many tiny sense organs known as taste buds. These are usually in little hollows, so they cannot be affected unless liquids which can enter the hollows are on the surface of the tongue. If the tongue is wiped dry and then dry sugar is sprinkled on it, no sweet taste will develop so long as the dryness continues. The purpose of taste is evidently to give final information about the food after it has passed the inspection of the other senses and has been inserted into the mouth, but before it is swallowed. In the higher animals there has been a subdivision of this sense into two. The other is the sense of smell. In large part smell is a distance sense, and will be treated when we are talking about the distance senses. Smell has monopolized most of the properties of food-judging, so there is left for taste proper only four kinds of perception. These are sweet, sour, salty and bitter. We have, apparently, four kinds of taste buds, one for each of these kinds of taste. All the other sensations that we call taste are flavors, and are really smells. Of the four tastes sour and bitter would probably be called warning and sweet and salty recommending. Only by practice do we come to care for bitter foods, and children usually object just as strongly to those that are sour. Tropical savages, for whom salt is a rarity, esteem it much more highly than sugar, which they can usually get in abundance.

In concluding this chapter we need to remember that the contact senses make up the court of last resort; by the time anything comes close enough to the body to act upon any of them it is so close that the effect in guiding the muscles must be immediate; there is no time for deliberation; whatever the muscles are going to do in response to information thus obtained must be done at once. Later we shall see how this affects our whole bodily make-up.

CHAPTER IX
SENSATION—DISTANCE SENSES

THE three senses that give us information of what is happening beyond the surface of our bodies are smell, hearing, and sight. Since smell is closely related to taste, which was talked about in the last chapter, we shall take it up first. Smell is like taste in that it is aroused by chemical substances, but to be smelled these must be in gaseous form, not dissolved in water, as for taste. The organ for smell is in the upper part of the nasal chamber. There are really two of them, one in each nostril. They are made up simply of little patches of mucous membrane, as the membrane that lines the nose is called, in which are many of the particular kind of cells that are affected by odors. An interesting thing about these patches is that they are not in the part of the nostrils through which the main current of air sweeps in breathing, but in a little pocket off this main channel. If air containing an odorous substance is breathed in or out, a little of it works its way into the side pocket and is smelled. If we wish to get more of the odor we do it by sniffing, which is changing the shape of the nostrils to throw the air current more directly against the smell organs.

These organs are amazingly sensitive. It is hard to appreciate the minuteness of the amounts of material that can be smelled. Especially is this true of those animals that have a really keen sense of

smell. The amount of substance that rubs off from a rabbit’s feet onto the ground at each step cannot be much to begin with; yet this continues for hours to give off gas into the air, and a dog coming along at any time meanwhile will get enough of the gas into his nostrils to smell it. Fortunately for our comfort the sense of smell fatigues very rapidly. An odor that is excessively disagreeable at first presently no longer troubles us. If it were not for this, it would be almost, if not quite, impossible to obtain laborers in those industries where the odor is necessarily bad. There is, however, a source of danger in this quick subsidence of smell perception. About our only method of judging offhand as to the ventilation of a room is by the smell, and this fails as a guide when we have been in the room for a time. Persons coming in from outside are often struck by the bad state of the air in rooms whose occupants are not conscious that there is anything amiss. Because of this the ventilation of schoolrooms usually is not, and never should be, left merely to the judgment of the teacher, but definite rules are laid down as to opening of ventilators or windows.

When the gas that is smelled is part of an inward air current we recognize it as coming from the outside and call it an odor; when it is part of an outward current we call it a flavor. On account of the rapid fatigue of the sense of smell we are unconscious of the smell of our own breath, but can get fresh smells from within, and these come, practically always, from materials that have just been taken into the mouth. In comparison with taste flavor furnishes great variety of perception. As persons become connoisseurs in food their enjoyment depends more and more on flavor and less and less on taste. The sensation from spices is a combination of flavor with irritation of the tongue that is partly pain and partly touch. The sense of smell has evidently a two-fold use; it makes us aware that there is food in the vicinity, or sometimes that disagreeable things are near at hand; and it shares with taste the duty of enabling us to judge of the food as it is being eaten. Agreeable tastes, flavors, and odors add much to the enjoyment of life. Within reasonable limits it is well to cultivate this kind of enjoyment, for while there is no doubt that it can be overdone, as in the excessive lengths to which the decadent Romans went to gratify their taste and smell, neither is there any doubt that bodily health in general, and the bodily function of digestion in particular, benefit definitely from the kind of enjoyment that savory food and delightful odors bring. It is the duty of those charged with the responsibility of preparing and serving food to take pains that full advantage is taken of the possibilities present in what food is to be prepared. This does not mean expensive food; what it does call for is skillful handling of all food, whether cheap or costly.

In introducing the subject of hearing we shall have to say a few words about that which is heard, namely sound. Any object that has any degree of elasticity at all is apt, if struck or rubbed or otherwise set suddenly in motion, to start vibrating back and forth; the vibrations will nearly always be regular, and will occur at a rate that is the same for that particular object whenever it vibrates. The rate depends on the size, the character, and the degree of stretch of the object. Air is to all intents and purposes perfectly elastic; it is set vibrating by any object that is vibrating in it, but since it has no particular size nor degree of stretch it takes the vibration rate of the object that started it going in the first place. The vibrations once started in air spread in all directions, just as waves spread from a stone thrown into a pond, and when these air waves strike upon another object that is free to vibrate they will set it going at the same rate. The human hearing apparatus is a device which is set in vibration by air waves, and the result is called sound. The ear is limited in its ability to respond to vibrations; they must be neither too fast nor too slow; if slower than 16 a second, most people will fail to hear them, and the same is true if they are more rapid than about 40,000 a second. Between these limits vibrations that strike upon the ear are heard as sounds.

Differences in vibration rate between one sound and another can be recognized by the ear; the difference is a matter of pitch. By the pitch of a tone we mean the vibration rate which it has. More rapid vibrations give tones of higher pitch; a slow rate means a low pitch. Middle C on the piano has a rate of either 256 or 261 a second according to the system used by the tuner. The human voice has an extreme range starting with the lowest note that the bass voice can compass with a rate of about 80 vibrations a second, to the highest note that famous sopranos can attain at about 1,400 a second. There is a record of a singer who could achieve a tone with a rate of 2,100 a second, but this has not been duplicated so far as is known. Of course no single voice can cover more than a fraction of this range. Most men produce all their tones at rates of between 90 and 500 a second, and women between 200 and 800 a second. Not every different vibration rate is heard as a tone of different pitch; the ear is not sensitive enough for that. The interval between one note and the next includes several vibrations, more the higher one goes in the scale. A perfectly true tone has exactly the rate called for; a departure of one or two vibrations a second may not be noticed, but if the error is greater the singer is sharping or flatting his tone, according as he is above or below the true rate. A note that has just double the rate of another one is said to be its octave. For convenience the interval of the octave has been split up into twelve tones, and all our music is constructed on that basis.

It is evident that the ear must be a very complicated organ; not only must it perceive differences in pitch, as just indicated, but differences in loudness must also register differently. More than that, the ear has to be able to deal with sounds made up of a great many tones coming into it all at once. When we listen to an orchestra or band, the waves that strike our ears represent the commotion set up in the air by all the instruments together. It is a remarkable fact that in this case, instead of getting a meaningless jumble, we actually get a blend of tones from which, if we are sufficiently musical, we can pick out the individual elements.

The ear consists, in the first place, of a vibrator that will respond accurately to any vibration rate or combination of vibration rates within its range, and secondly of a sensitive apparatus that is acted upon by the vibrator. The vibrator must respond freely to feeble impulses, and, what is of prime importance, to any vibration rate as readily as to any other. Almost all elastic bodies have a preferred vibration rate; that is, they will respond better to some rates than to others. About the only exception to this rule

is in the case of membranes that are not tightly stretched. A stretched membrane, like a drumhead, has its own vibration rate, but one that is not on the stretch is able to vibrate at almost any rate. This fact is taken advantage of in the telephone and the phonograph, both of which depend on being able to vibrate at various rates almost equally well. In the ear, also, an unstretched membrane is the vibrator. We are familiar with it as the eardrum. It is located at the bottom of the ear canal, but cannot be seen by looking therein, because of a slight curve at about the middle. When the ear specialist wants to examine the eardrum he thrusts a small metal tube into the canal. This straightens it out enough to bring the drum into view.

The eardrum does not act directly upon the sensitive hearing apparatus, but its vibrations are transmitted across a space known as the middle ear. The necessity for this space is found in the fact that atmospheric pressure is not constant, but changes frequently from day to day, besides falling off as one ascends higher above sea level. The free action of the eardrum depends on its not being stretched; if there were no means of readjustment it might be properly set for one air pressure, but greater pressures would bulge it inward, putting it on the stretch, and so cutting down its ability to respond to a wide range of tones. The middle ear, which is the space behind the drum, connects with the outside air by a tube leading from it to the back of the throat, which latter communicates freely with the air through the nose, as well as through the mouth whenever it is open. The tube is known as the Eustachian tube. Its walls are ordinarily collapsed, so it is not an open passage, but every time one swallows the tube is pulled open, thus allowing differences in air pressure on the two sides of the eardrum to equalize. Whenever one ascends a high hill quickly, as by train or automobile, or even in going to the top of a high building by elevator, the difference in air pressure behind and in front of the eardrum can be felt. The sensation is disagreeable, and there is definite impairment of hearing. Repeated swallowing gives relief.

The vibrations of the eardrum are transmitted across the space of the middle ear by a chain of three tiny bones; these are very irregular in shape, and are attached to one another in such a way that every movement of the drum is followed exactly as to time and direction, but with reduced size and increased power. The hearing apparatus, which is part of the internal ear, but not the same part as makes up the equilibrium organ, contains liquid which is set moving by the last of the chain of bones, and this liquid acts upon the actual sensitive cells which make up the sense organ proper. These latter are arranged in an exceedingly complicated fashion. There are various theories as to the precise manner in which the vibrations of the liquid in the inner ear arouse the sensitive cells. It is thought that different cells respond to tones of different pitch, but exactly how this is accomplished is not known.

Deafness may result from failure of the sensitive inner ear to respond, or from poor transmission of vibrations across the middle ear by the chain of bones, or by interference with the freedom of action of the eardrum, or by preventing the air waves from striking upon the drum. Injury to the inner ear is rare, because of its secure position within the bone. A common form of deafness is the result of hardening of the connections between the ear bones, so that the chain no longer follows well the vibrations of the drum. This usually begins to come on during the twenties or thirties, and causes almost complete deafness by the age of fifty. In most if not all cases it is hereditary. Interference with the action of the eardrum may be due to the partial destruction of the drum itself. Scarlet fever and measles are particularly likely to leave the drum in a delicate condition, and any strain upon it then may rupture it beyond repair. Continuous closing of the Eustachian tube, by preventing equalization of air pressure on the two sides of the drum, causes partial deafness. Inflammation accompanying a cold may cause this, or the growth of adenoids in the back of the throat. Adenoids will be described later; here they are mentioned only because they may press upon and close the Eustachian tubes. There is danger from the closure of the tubes by inflammation, because the inflammation may creep up to the middle ear and cause serious trouble, both there and in the mastoid region adjoining. Earache in children should be carefully watched, since it usually means that the middle ear has been invaded by the same inflammatory condition that is present in the throat in colds, and may do serious damage to the delicate structures there. Often in children, and sometimes in adults, the hearing is impaired by the accumulation of wax in the ear canal. This wax is a sticky secretion that serves to catch particles entering the ear canal and to prevent them from striking against the drum. Unless the ear canals are washed out frequently with hot water the wax accumulates and hardens into a plug which closes the ear canal, shutting off faint sounds. The wax dissolves in hot water, but not in cold, so its accumulation is to be prevented by taking pains that hot water actually gets to the bottom of the ear canal once in a while. Digging the wax out with hard instruments should be done only by an expert with greatest caution, lest the drum be injured.

The last of the senses to be described is sight; this is the one of which we make the most use ordinarily, and curiously enough is the only one that we can turn on and off. Loud noises or penetrating smells must be endured, but by shutting our eyes tightly we can escape sight whenever we want to. Altogether there are three kinds of information which the sense of sight brings to us; the first is the knowledge simply of light and darkness; the second is the knowledge of the shape and size of objects; the third is the knowledge of color. Nearly all animals seem to have some power of distinguishing between light and shade. Even the one-celled kinds, that have no eyes or anything corresponding to eyes, behave in a way that proves them to have this power. A lot of them can be put in a dish of water that is well lighted on one side and in shade on the other and in a few minutes all of them will be found to have traveled to one side or the other according as they happen to be a light-seeking kind or a dark-seeking species. As we go higher up the animal scale we find that parts of the body show this same power of distinguishing light from darkness. In the case of the common angleworm, or earthworm as it is more properly called, the front end has the power but the rear end has not. In very highly organized animals only the special organs known as eyes possess the power; all the rest of the body has lost it. The next feature of sight, the ability to perceive the shape and size of objects, requires a special apparatus, the eye, so animals that lack eyes cannot perceive objects, although they may be able to tell light from darkness.

In order to see an object it is necessary that a pattern or image of it be thrown on a sensitive surface; this surface registers the details of the pattern, and so the object is seen. What we have to do here is to find out how these patterns or images are formed in our eyes. In the first place we must realize that every visible object has rays of light going off from every part of its surface in every direction. So-called self-luminous objects, like lamps or the sun, produce the light within themselves; all others merely reflect light that falls on them from some source. Whenever light falls on any object, unless it is a perfect mirror, part is absorbed by the object and the rest is reflected; that which is reflected presently strikes against another object and is again in part absorbed and in part reflected; as this process is repeated over and over again the light becomes so broken up that rays of it are traveling in every direction from every point on every object. Any spot so protected that no rays can strike it evidently cannot reflect any out again, and such a spot will be absolutely dark.

For the formation of an image of any object all that is necessary is that some of the rays of light from every point on the object be caused to fall in exactly corresponding positions on the image. The simplest possible means of doing this takes advantage of the fact that rays of light travel in straight lines. If we inclose an incandescent bulb in a tight box with a round hole in one side of it, every spot on the incandescent filament will be giving off light in every direction, but all the light will be cut off by the box except that which happens to have the direction which takes it out through the hole. From every incandescent spot, then, there will be a beam of light in the form of a cone escaping from the box through the hole. The tip of the cone will be the incandescent spot; the slope of its sides will depend upon the size of the hole. If a screen is placed in front of the hole, all these cones of light will strike on it, and it will be illuminated in a pattern which is made up of all the cones from all the incandescent spots which make up the filament, but these will overlap so much that one cannot be told from another. Now if the hole in

the box is made small enough, a pinhole, in fact, and if the screen is placed close to the hole, the cones of light from the different incandescent spots become so narrow that when they strike the screen they overlap scarcely at all, and what we get is a tiny spot of light on the screen corresponding to every incandescent spot on the filament and straight in line with it through the hole. Here we have exactly what we have been talking about, namely a pattern or image of an object. The image will be upside down, because those rays from the top of the bulb that strike the tiny hole will be below on the outside, and those from the bottom will be above. The same thing can be worked exactly in reverse; we can place a box with a pinhole in it in front of any object and get an image inside the box on the back; by placing a photographic plate or film there an excellent picture can be taken. There is just one reason why this scheme is not used in all cameras; that is that unless the object is very brightly illuminated indeed the amount of light that passes through the pinhole is not enough to affect the plate or film except on long exposure. Perfect pictures can be taken with a pinhole camera wherever long exposures are possible, or wherever the object shines brightly enough. This difficulty is gotten around in the ordinary camera by gathering up all the light in a wide cone from each spot on the object and condensing it again on the plate or film. The image is formed just as before, but now each spot on the image includes, not only the beam of light that comes in a straight line from the corresponding spot on the object, but in addition that in a wide cone surrounding the straight beam. It is naturally brighter the wider the cone; which explains why in poor light we open the diaphragm of the camera wider than when the light is good; a brightly illuminated object will pass enough light through a narrow opening, but a dimmer object must have as wide an opening as possible in order that enough may get through.

The method of condensing the light in a spreading cone so that it shall come back to a point again is by means of a lens; not only is this true of cameras, but also of the eye; in fact everything that has been said thus far about cameras applies perfectly to the eye. There is one thing about the way in which light is brought to a point by a lens that makes the formation of images by this method troublesome in comparison with their formation in pinhole cameras. That is that the cone of light which strikes the lens is condensed as an opposite cone on the other side, and since the formation of an image requires that every point of the object shall be reproduced as a point in the image, there is only one place where the image can come, which is where the tips of the cones of light are. This place is spoken of as the focus. Unless the screen or film is exactly there the image will be made up of overlapping circles of light instead of points, and so will be blurred. In a pinhole camera the cones are so small that they cannot overlap, so there is no one place where the image is better than elsewhere; in other words, there is no necessity of focusing. The chief reason that focusing gives trouble is that the farther away from the lens the object is the closer to the lens will be its image; hence if the field of view consists of several objects at different distances they will not all focus at the same level; the distant objects will focus near the lens; the near objects farther back. In practice this trouble is met by using a thick lens, which has a very short focus to begin with, so that a considerable range of distances will be covered without serious blurring, and for finer work by adjusting the distance between the lens and the back of the camera, where the film or plate is, so that there shall be a good focus of the particular object that is desired.

In the eye the clear part that projects between the lids, and is called the cornea, is the important lens. Just behind is the arrangement that corresponds to the diaphragm of the camera; this is the colored part with a round hole in the center; it is called the iris and the round hole is the pupil. Behind the iris, and resting right against it, is the secondary lens of the eye, known as the crystalline lens. The eye as a whole is a globe just under an inch in diameter; at the back of it, straight behind the lenses and pupil, is the sensitive surface upon which images are formed. This is the retina; it extends pretty well around to the sides, but the part we use most in seeing is the small portion straight in line with the pupil. The cornea by itself is a lens whose focus is

longer than the length of the eyeball, and the crystalline lens by itself also has a long focus, but the two in combination give a focus that just corresponds with the length of the eyeball, so that the images of all objects at a distance of eighteen feet or more fall sharply on the retina. Since near objects focus farther away from the lens than far objects, the effect of this is to make objects nearer than eighteen feet out of focus. For them a longer eyeball would be needed, and it would have to become longer and longer the nearer the object was brought to the eye. We all know that when we look at near objects we make an adjustment in the eyes. This is known as accommodation; for a long time it was supposed that accommodation was actually secured by lengthening or shortening the eyeball to bring the focus right, but we now know that the eyeball does not change in shape when we accommodate. The same result is secured by another means, namely, by letting the crystalline lens bulge out and become thicker. It was stated incidentally a few pages back that thick lenses have shorter focuses than thin. So when the crystalline lens thickens it shortens the focus of the eye, and throws the image forward. This will locate the image of near objects on the retina, instead of behind it, as in the unaccommodated eye. The crystalline lens is not stiff like glass, but rather like a thick jelly; it is in a transparent bag, or capsule, which is fastened to the inside of the eyeball all around, and the pull on the capsule stretches it out pretty flat, making the lens thin. There are tiny muscles inside the eyeball, known as the ciliary muscles. These are so fastened that when they contract they pull the eyeball forward and inward, loosening the tension on the capsule of the lens. This then bulges out, taking up all the slack. When we learn, as babies, to accommodate for near objects we find out just how much the capsule must be loosened to give the proper adjustment for any distance. As people get along in years the crystalline lens often loses its elasticity so that it does not bulge when the capsule is loosened. After this happens vision for near objects is no longer clear. Relief is obtained by wearing reading glasses. These are merely glass lenses worn in front of the eyes selected so that their focus fits in with that of the lenses of the eye itself to bring the image of objects at reading distance sharply on the retina. It is usually necessary to replace these glasses from time to time as the crystalline lens becomes stiffer and stiffer and ordinary accommodation fails more and more.

There is a disease known as cataract in which the crystalline lenses become cloudy and finally completely opaque. Of course this means blindness, since the passage of light to the retina is interfered with. Relief is obtained by the simple expedient of removing the opaque lenses bodily. This is possible merely because the crystalline lens is not the chief lens of the eye. To be sure the cornea by itself will not focus on the retina, but a glass lens can be placed in front of it which will add itself to the cornea and the combined lenses will. There is no possibility of accommodation in a case like this, so the patient has to be furnished with bifocal lenses; the main part gives clear distance vision; the lower section gives clear vision at the reading distance. The patient has to get along with blurred vision in the regions between.

Not all eyes are exactly the right size so that distant objects shall focus sharply on the retina. In fact a large proportion of them are either too long or too short. It is clear that in an eyeball that is too long the image of distant objects will fall in front of the retina, but near objects that happen to be at just the right distance will focus exactly on the retina. The distance at which this happens depends, of course, on how much too long the eyeball is. Persons that have unduly long eyeballs are, therefore, nearsighted. The condition is called myopia. The correction for it consists in the use of lenses in front of the eyes that instead of shortening the focus shall lengthen it. Concave lenses, namely, those that are thick at the edge and thin in the middle, will do this, and these are the kind that are worn by near-sighted people.

When the eyeball is too short the image of distant objects falls behind the retina, and of course that of near objects tends to fall farther back yet. Since by

accommodation the focus can be thrown forward, most persons with short eyeballs can see distant objects clearly by accommodating for them, and near objects that are not too near by extreme accommodation. For this reason hyperopia, as this condition is called, is usually not discovered until the person begins to feel the strain of the constant accommodation that is necessary whenever the eyes are open. Eyestrain usually shows itself in headaches; in fact, so large a proportion of headaches come from this cause that anyone who suffers from them at all frequently should have his eyes examined by a competent oculist. Relief for hyperopia is by means of glasses that shorten the focus and thus bring the image of distant objects forward to where the retina is in the short eyeball.

There is one other defect of vision that is so common as to call for a word; this is astigmatism. It is the condition in which the cornea is not curved equally in all directions; the vertical curvature may be greater or less than the horizontal. The effect is that points on objects do not focus sharply as points in the image, but as little elliptical spots. If one adjusts the accommodation so that the top and bottom edges of objects are sharp the sides will be blurred and vice versa. Usually the blurring is not great enough to be noticeable, but only enough to make the person unconsciously dissatisfied with the accommodation. He, therefore, constantly tries to improve it by changing the tension of his ciliary muscles, and so brings on eyestrain. The correction for this condition is glasses that are not equally curved in all directions, but so selected that their less curvatures shall fit in with the greater curvatures of the cornea. In fact, glasses that are curved only in one direction are usually used, this curvature being just enough to bring up the total curvature in that direction to equal the other curvatures of the cornea. It ought not be necessary to give warning that only competent persons should be allowed to examine and prescribe for the eyes. Great skill is needed to determine accurately just how far from correct the eyeball is, and unless this is known there is no means except guesswork by which to decide on a prescription.

The third feature of vision is the perception of color. Color is to light what pitch is to sound; that is, it depends on the vibration rate of the light waves. Light, as already explained, is one of the forms in which energy reaches the earth from the sun. Heat is another form. Both are portions of a great energy stream to which we give the name of radiant energy. This, as it comes from the sun, is made up of a mixture of vibrations having almost every imaginable rate, except that the slowest are many times faster than the highest pitched sound. At a certain rate, and one that for these vibrations ranks as slow, the energy is what we know as heat, and over a considerable range it continues to be called heat; more rapid vibrations, and they are so rapid that they have to be expressed in trillions per second, cause the effect on our retinas that we call light. The slowest that we can see give the sensation of red; the most rapid the sensation of violet; the other colors of the rainbow, which in order after red are orange, yellow, green and blue, are vibration rates between those that give red and those that give violet. It will be noticed that only six colors are given for the rainbow instead of seven. This is because there is not enough real difference between blue and indigo to justify making them separate colors. The distinction was made at the time when it was supposed that there was something specially wonderful about the number seven, which made it necessary that every important feature in nature should show that number. We now know of no reason why seven should have virtue over any other number. When all the vibration rates are mixed together, as they are in the sunlight, the sensation is white. There are other mixtures of colors that give white also, but they are not exactly equivalent to the white of sunlight, as is proven when one tries to match colors under artificial light. A great deal of labor has been devoted to the attempt to get an artificial light that shall be practically equivalent to sunlight, and only lately have good results been obtained. When no light enters the eye the sensation is of black, and it is worth while to note that so far as our sensations are concerned black is as much a color as white or any other, notwithstanding the fact that no light falls on the retina when the black sensation is being felt. Contrary to ordinary belief, blind persons who are blind because their eyes have been destroyed do not see black all the time; they simply have no sensations at all from the eyes. On the other hand, persons blind because of cataract do see black, because in them the retinas are still present, but no light falls on them.

The perception of color is very complicated, and not at all well understood. Persons who do not have the same color perception as most of us are called color-blind, and by learning some things about color-blindness we shall best get an idea of color perception itself. About four men in one hundred have defective color sense; the proportion in women is only about a tenth as great. By far the commonest type is one in which neither red nor green is seen correctly, but both are seen as neutral tints, and in many cases look so much alike that the person cannot tell one from the other. The practical importance of knowing whether or not this defect is present is seen when we think that red and green lights are used more than any other colors in signaling, so that railroad men and others who work by signals must have normal color sense. It has been discovered that even people that have normal color vision are color-blind in the margins of the retina. This can easily be demonstrated by bringing a red or green disk slowly around in front of the eye of a person who, meanwhile, keeps looking straight ahead. He will see the disk out of the corner of his eye some time before he can tell what color it is. In fact, if a red or green disk is used, he usually will not be certain as to the color until it is almost straight in front. Blue or yellow disks can be told with certainty much farther out, but even these colors are not perceived clear to the edge of the field of vision. These experiments show that the retina becomes more and more highly developed as an organ for perceiving color as we get closer and closer to the center; at the extreme edges there is no color sense at all, but only the primitive ability to tell light from darkness; closer in blue and yellow are distinguished and not red and green; only in the central part are all colors clearly seen.

In addition to the kinds of information which have been described thus far, that the distance senses bring in, there is another kind, fully as important as any in our actual use of our senses; that is information as to the “direction from us” of the object or objects which are arousing the sense. We can get this through all the distance senses, but much more perfectly in the case of sight than in the others. We locate the direction of objects that we smell by turning the head this way and that, sniffing meanwhile, and noting the position in which the odor is caught most clearly. Animals with a keen sense of smell, like dogs, can locate directions very accurately by this means. In the case of hearing the method is to turn the head until the sound is equally loud in both ears. We would expect that a person who was hard of hearing in one ear would never be able to locate sounds by this method; but, as a matter of fact, such persons unconsciously allow for the difference in hearing in the two ears, and so can judge the direction of sounds about as well as any of us. Animals, like horses or rabbits, that have very movable outer ears, undoubtedly can locate sounds much more accurately than we can. Our outer ears are of almost no use in hearing; persons who have had the misfortune to lose them hear practically as well as anyone.

We locate directions with the sense of sight with perfect accuracy, because unless the image of the object we are looking at falls on the center of the retina it is not seen clearly. The only way to make the image fall just there is to look directly at the object. The muscle sense in the eye muscles is extremely delicate, so that if the eyes are rolled at all in looking at anything we know it and can judge, also, how much they are turned from the straight position. In this way we are able to tell exactly the direction from us of any object we can see.

We can judge the distance of a near object very accurately by noting the degree to which the two eyes have to be turned in in order to see it clearly with both. We are quite unconscious of this means of making the judgment; all we know is that we can tell. It is easy to prove that it depends on the two eyes by closing one and trying to make movements that depend on accurate knowledge of distance. A good example is threading a needle sideways. With both eyes open this can be done fairly easily, but with one shut it cannot be done at all, except by chance. Objects so far away that the eyes are not turned in perceptibly in looking at them are judged as to distance wholly on the basis of their size. It is clear that the actual size of any image on the retina will depend in part on how large the object is, and in part on how far away it is. If an object that we know to be large casts a small image on the retina, we conclude that it is far away. It follows that unless there are some familiar objects in view, judgments of distance are not at all trustworthy. A good illustration is in looking up a bare hillside, and trying to estimate the distance to the top. If, while this estimation is being made, a man or horse suddenly comes into view at the top, the man or horse will nearly always appear unexpectedly large, showing that the top of the hill is not actually as far away as it was judged to be.

The possession of two eyes instead of one is an advantage to us in another way, in addition to helping in the estimation of near distances. This is in making objects appear solid, or in other words, in helping the estimation of depth. When we look about us we have no difficulty in realizing that some objects are near and others far, and that the objects themselves have some parts that are nearer to us than other parts. A great many things assist us in this realization. First and foremost comes that which is known in art as perspective, namely the tendency of distant objects or distant parts of objects to appear smaller than those that are near. This can best be illustrated in the case of parallel lines extending away from the eye, as when one stands on a straight railroad track and looks along it. Although we know perfectly well that the rails are the same distance apart all along, if we were to believe our eyesight implicitly we should think that they came gradually together. It is on account of this matter of perspective that drawings of solid objects must show supposedly parallel sides nearer together at the far end than at the near. Besides perspective there are the shadows to be taken into account. Only solid objects cast shadows, so if we see a shadow apparently cast by any object we naturally conclude that it is solid. Both perspective and shadows can be and are used by artists in making drawings and paintings look real. In fact, they have almost no other means of doing this. As we all know, even the cleverest paintings do not give an impression of depth equal to that which comes from actually looking at solid objects. This is because of help we get from the two eyes in the latter case. The reason for the difference is that when we look at a solid object with both eyes the view we get with one eye is not exactly the same as with the other; we see a little farther around on the left side with the left eye, and on the right side with the right eye. The combined view with the two eyes gives us an impression of solidity that cannot possibly be had when the view with the two eyes is exactly the same, as when we look at a picture. The only way in which the impression of depth can really be gotten in a picture is by using the familiar method of the stereoscope, where two pictures are taken simultaneously by two cameras, placed a little farther apart than the two eyes; and then the two are looked at together through a special pair of prisms.

By means of the three distance senses, smell, hearing, and sight, we are informed pretty completely as to what is around us. All three give an idea as to the direction from us of objects; although sight does this better than either of the others. Sight, also, lets us know accurately as to the distance away of objects, provided they are fairly near. Smell and hearing, as well as sight, may give us some idea as to the distance of far objects, but only when we are dealing with familiar sensations. A very faint smell, or faint sound, means a distant object provided we know enough about the source to know that if the object were near the sensations would be keener. Our judgment of distant objects by sight is better than this, but not by any means perfect. When we contrast the distance senses with the contact senses we see at once that the great advantage coming from the possession of distance senses is in the time that is permitted for action. In the chapter on contact senses we emphasized the fact that the response to them must be immediate; there is no time to pick and choose. When we can learn of the presence of objects before they reach us, and something of their direction and distance as well, we can usually take time to select the most fitting course of action. We do not have to jump into a mud puddle to escape an automobile if we see it soon enough. We shall learn in a later chapter how this opportunity of choice is wrapped up with our development into highly intelligent animals.

CHAPTER X
THE NERVOUS SYSTEM AND SIMPLE NERVOUS ACTIONS

IN the second chapter we saw that to make our muscles act in accordance with the information brought in by the sense organs some means of communication between them is necessary; we saw, also, that this means consists of the nervous system. Now that we have learned something about both muscles and sense organs we are ready to look into the way in which communication between them is carried on. First of all, we must realize that living protoplasm does this. The nerve cells are alive and have their basic metabolism just as do all other living cells. They also have their functional metabolism; but in them, instead of taking the form of forcible motion, as in muscles, or of the manufacture of special materials, as in gland cells, it takes the form of the transmission of a disturbance from one part of the cell to another. An interesting and important fact about this transmission of disturbances is that the actual amount of functional metabolism required by it is very small. Only by the most careful measurements has it been shown that nerve cells that are functioning have a greater metabolism than those that are at rest. For a long time it was thought that a nerve cell acted very much like a telephone or a telegraph wire, transmitting some kind of a disturbance which was set up in it, but not having any active part itself in the process. We now know that the special activity of nerve cells is a form of functional metabolism, just as is the special activity of muscle cells or gland cells.

The nerve cells have to make communication between sense organs and muscles, and, as we have already seen, these are often quite a distance apart. It is necessary, therefore, that the nerve cells be long enough to reach over these distances. As a matter of fact, it is not necessary for single cells to have this great length, because it is possible for them to be arranged end to end, making a path of living protoplasm consisting not of one cell but of a chain of them. If we look at a nerve cell under the microscope we see that it is made up of a little central mass of protoplasm to which has been given the name of “cell body.” From this cell body extends a tiny thread of living protoplasm. This thread is called the axon. It is so very slender that it cannot be seen except under a powerful microscope, and yet in our own bodies and the bodies of all large animals many of these are three feet or more in length without a break. This tiny protoplasmic thread, the axon, was formed originally by growing out from the cell body. As it grew it became surrounded by a sheath, which probably gives it strength and decreases the danger of its being broken. Another thing which helps to keep the axons from being injured is that they are always in bundles. Instead of one of these very slender axons lying all by itself, it will be bound up with several hundred others; the arrangement is similar to that in a telephone cable, where a great many single wires are bound together in the large and very strong cable. The living protoplasm of the nerve cell has a gray color, so that wherever this shows we have what is commonly called gray matter. We saw a moment ago that every axon is inclosed in a sheath. Some of these sheaths are transparent, so that the gray color can be seen underneath, but most of them have a layer of white material, which makes them look white instead of gray. The bundles of axons corresponding to the telephone cables make up what we call the nerves. Nearly all nerves are white in color because of the white material in the sheaths.

The sense organs, as we have seen in Chapters VIII and IX, are some of them inside the body, others spread over the surface of the skin, and the rest in the special sense organs, like the eyes or the ears. A very complex organ, like the eye or the ear, has thousands of axons leading from it. In the case of the eye these are grouped into a large nerve leading away from it at the back, which is called the optic nerve. There is a similar large nerve leading from the ear. When any sense organ is acted upon, as when light falls in the eye or sound on the ear, it starts a disturbance in some or all of the axons leading away from it.

As we have said over and over, the purpose of the nervous system is to arouse the muscles to activity, and to guide them in that activity. We must ask next, then, how the nerves are distributed to the muscles. If we dissect the body of any animal or bird, we can find nerves passing to various parts. For example, a large nerve goes down each leg; this nerve subdivides here and there. Since we know that the nerve consists of a great many axons bundled together, we will realize that this subdivision is not a real branching, but simply a passing of some of the axons away from the main trunk along the smaller stem. Some of these smaller stems can be traced to endings in the skin; these contain the axons connecting with sense organs. Others lead directly into muscles. Some of these axons may also connect with sense organs, since, as we have already seen, every muscle has embedded in it the organs of muscle sense, but in addition any nerve that leads to a muscle contains a great many axons which pass directly to the muscle fibers. These are the axons by which the muscles are aroused to activity. It is a general rule of the nervous system that no nerve cell extends without a break from any sense organ to any muscle fiber. The axon which communicates with the sense organ belongs to one nerve cell; the axon which connects with the muscle fiber belongs to a different nerve cell. The first is called a sensory nerve cell, the second a motor nerve cell. Some idea of the appearance of these cells can be gotten from the figures on page 126.

It will be seen that the cell body of the sensory cell appears to be off on a little side branch. As a matter of fact, the branch is double, so that when a nervous disturbance passing along from the sense organ comes to the beginning of this branch it can pass up to the cell body and then out from the cell body along the second part of the branch, and so along the other part of the axon. This part of the axon is seen in the figure to have several branches; these are really branches of protoplasm and not separate axons coming off, as in the case of the nerve trunk. The use of these branches we shall see in a moment. Also at the tips of each branch there is a tiny feathering. We shall explain this presently. Let us look first at the figure of the motor nerve cell. This has a cell body and long axon, and, besides these, has a great many short protoplasmic branches sticking out in all directions from the cell body. Since a nervous disturbance to get from a sense organ to a muscle has to pass over a sensory nerve cell, and also over a motor nerve cell, evidently there will have to be some point at which it leaves the sensory cell and gets into the motor. This is accomplished by having the tiny feathering at the tip of the sensory cell interwoven with the fine processes projecting from the body of the motor cell. This arrangement we may call a nerve junction. In the whole body there are, of course, millions of these nerve junctions.