THE COMMONWEALTH
OF CELLS

Some Popular Essays on Human Physiology

BY
H. G. F. SPURRELL, B.A. Oxon.

LONDON
BAILLIÈRE, TINDALL AND COX
8, HENRIETTA STREET, STRAND
1901

[All rights reserved]

To
MY ESTEEMED FRIEND AND TUTOR,
GUSTAV MANN, M.D., Etc.

PREFACE

Ever since the very beginning of my student days, when my contemporaries took to plying me with embarrassing demands for information upon all matters medical, I have been constantly impressed by the interest which the unscientific public take in the workings of their bodies and the material basis of their minds. It is this general display of interest among my friends that has emboldened me to add yet another book to the many already dealing with the subject. In using the word ‘unscientific’ I imply, of course, no reproach. I mean simply to denote those people who have specialized in some branch of knowledge other than those collectively known as Natural Science.

I usually find, when discussing physiology with such people, that they take more interest in general principles than in details, which they frequently find repellent, and that they frame their questions in an appallingly comprehensive manner.

My object throughout this little work has therefore been to present the fundamental principles of physiology in a brief, consecutive and readable form, for those who do not care to study the text-books. There is no lack of excellent books already, books illustrated by careful drawings quite gruesome in their accuracy, but they are almost all intended for “students,” and the casual reader, finding the organs divided up for exhaustive treatment, fails to form a conception of the body as an organic whole, and misses the very principles he is in search of, in the heap of details under which they are buried.

It may cause some surprise that, though in my efforts to be up-to-date I have in places outstripped the text-books, I quote no authorities. But a moment’s consideration will show that it would defeat the very object of a sketch like this to burden the text with an account of how my views were formed, while, on the other hand, the pioneers of science will forgive me. Their papers will be quoted in more durable works, and their names honoured long after this little book has been forgotten.

H. G. F. S.

Oxford, March, 1901.

CONTENTS

INTRODUCTION

The unscientific public is extremely prone, and not altogether without reason, to take medicine as a starting-point, and arrange all biological science around it. As it is, moreover, apt to gauge the interest and utility of every branch of this science from a practical point of view, and bestows most attention upon that which it imagines is of the greatest service to the doctor, I think a series of popular essays on physiology could not commence with more advantage, at any rate to physiology, than by briefly discussing, not with what it deals, for that is pretty generally known, but what is its relation to medicine. Further, as the doctor is more easily discussed than medicine, the physiologist will be more manageable for our immediate purpose than physiology in the abstract, so we will devote the first few pages to the question of how his labours benefit the patient.

Everyone knows the doctor, and everyone knows that physiology deals with the ‘functions of different organs of the body’; but the public rarely meet the physiologist, except in the fanciful caricatures of his enemies, which though frequently personal are rarely accurate. These rancorous libels, if anyone heeded them, would tend to raise doubts as to whether the physiologist was a good companion for the doctor, and if it were not as well for them to see as little of each other as possible.

The doctor, however, cannot move a step without the physiologist. His business is to correct the revolt of any organ from its allotted task, and repair the damage done by its deviation from the normal path. This he cannot possibly do if he does not know what that organ’s normal conditions are, and what they are it is the physiologist’s duty to tell him. A doctor, therefore, should be an enlightened physiologist, knowing how the body ought to work, and referring diseases to their real cause, such as the poisons formed by an invasion of bacteria or otherwise, or wrong feeding—that is to say, deficiency or excess of fuel for one of the body’s many engines. Medicine is still to a large extent rule of thumb. We don’t know to what many diseases are due, or why certain things relieve them, if any remedy is known; and until these questions are satisfactorily settled, it is vain to hope that disease as a whole can be successfully combated. It is no use knowing what will stop certain unpleasant symptoms if we do not know how to remove their real cause, and for this end the whole body and every individual component organ is being studied, that the process of life may be accurately understood; and the man who is doing this for his friend the doctor is the physiologist.

The physiologist has many enemies, a motley array of cranks held together by such noble bonds as general hatred of science and prejudiced ignorance masquerading as scepticism; but he can afford to ignore them, for the very good reason that people cannot get on without him. It is only on account of this that they are mentioned. People say, ‘The doctor is the person who requires a knowledge of physiology; he is the man who is most likely to study it successfully’—presumably by his mistakes—‘and not waste more time on it than is necessary,’ a point about which they are most solicitous. The doctor, however, prefers to trust the physiologist. If he did not, he would have very little time to do anything else. You might as well expect a tailor to make his own cloth before he makes a coat. He will doubtless be able to make better coats if the quality of the cloth supplied him is improved; but if in order to improve the finished article he lays down his scissors and applies his fingers to weaving, his business will be sure to suffer.

That physiology is a thing which can take up a man’s whole energies will, I think, be admitted by anyone who realizes how wide is its scope. The physiologist himself must specialize, for the subject is too vast for one man to undertake the whole. The body is composed of the same elements as the rest of the world, and their arrangement is very important, so he must be a proficient chemist. It is composed of solids, liquids and gases, and diffusion, filtration, leverage, are frequent processes, and every motion is accompanied by an electric manifestation, so that mechanics and physics must have been part of his training. He can scarcely study organs if he does not know their shape, so he should know some anatomy. And, lastly, as his business is to study life and all its attendant phenomena—and the basis of life is the cell—he must be a histologist. To be all these things is a great deal to require of one man; but though he may specialize for the advancement of a particular branch of his science, he must be au fait with the rest, as no vital function is dependent upon one alone of the factors enumerated. Hard work is required of him, though some people say he has done but little. What he has discovered is briefly, very briefly, set forth in the ensuing essays, with a hint or two at the knots he would like and is trying now to unravel.

THE
COMMONWEALTH OF CELLS.
Some Popular Essays on Human Physiology.

ESSAY I.
LIVING MATTER.

Physiology is the study of life, and the thing of all others which the physiologist would like to discover is what life really is. If this were fully known, all physiological processes could probably be deduced from it, and disease, which is an interference with one or other of them, could be scientifically treated. So far he has not got beyond describing the consequences of life, and his deductions carry him no further than this: life is a property of a substance, protoplasm, and protoplasm can only continue to exist in the form of a cell.

This definition may seem a little cryptic to some people, and very shocking to others. ‘Life,’ many people are accustomed to say, ‘means the presence of a soul, and is supernatural; and as to its being a question of chemical composition, that is absurd. My being made of cells, too, will not account for my thinking.’ But when people dogmatize thus about what they have not considered, they usually find themselves landed in difficulties. They go so fast: the most spiritual of men is so dependent upon matter and its properties that his soul will speedily quit his body if he is prevented from breathing. And the reason of this is, that if he cannot get air, the chemistry of the cells of which his body is made becomes altered; he no longer consists of protoplasm, therefore he no longer lives. Life, that it may exist in a material world, must have a material basis, and if that is interfered with it becomes extinct or quits the material plane; in any case, ceases to interest the physiologist as a physiologist. I do not think anyone need be shocked at this being recognised.

It is, of course, the ambition of the physiologist to make protoplasm, but so far he has got no further than making some of the complicated bodies into which protoplasm breaks up when it dies. A little while ago this bare possibility was loudly derided, but the advance in organic chemistry has been so great of late years, and so many complicated substances which once seemed as unobtainable as protoplasm itself have been made in the laboratory, that we now have hope of a precise knowledge of the chemistry of life some day, though that day may be yet very distant.

To give an account of life is to describe as far as we are able the nature of the living substance, protoplasm; and as protoplasm is a ‘structure of compounds,’ a word or two about chemical compounds may clear the ground for discussing it. If you were to take a compound, say a lump of sugar, and start breaking it up, you could hammer for a very long time and it would be still sugar; but if a tiny fairy with a minute hammer and chisel were to go on breaking up the grains, he would ultimately have molecules of sugar before him. Each molecule would consist of exactly the same number of atoms of carbon, hydrogen and oxygen, and if he divided it further by the removal of a single atom, it would no longer be sugar. He could hammer away at the atoms as hard as he liked, for they are incapable of further division.

There are seventy odd different kinds of atoms. When the molecules of a substance are composed of only one kind, it is said to be simple; when of several kinds, compound. Now, the difference between the various substances we see around us consists not only in what different kinds of atoms their molecules are composed of and their number, but in their arrangement. This arrangement may be in chains or rings, and the relative position which the different atoms occupy in the structure of a molecule makes all the difference in the world.

This difference of composition gives the difference of properties to compounds; so a compound must consist only of molecules which are all alike. If a substance is made up of molecules of different kinds, ununited by chemical bonds, and therefore capable of being mixed in any proportions, it is called, not a compound, but a mixture of compounds.

But just as atoms combine to form molecules, so the smaller molecules sometimes enter into combinations with one another to form new compounds having larger and more complex molecules. Such a compound is said to be composed of radicles, or groups of atoms, and on being decomposed can be broken up, first into simpler compounds, which can afterwards be further divided into their constituent elements.

Now, of all substances, protoplasm seems to consist of the largest number of components, and to have them arranged in the most complicated way known; though ‘known’ is really not the right word to use in this connection. The reason why we do not know what life is, is that we cannot find out in what way the constituent compounds in the protoplasmic structure are combined. Directly we try to analyze protoplasm, it dies; that is, it splits up into a number of simpler bodies and is altered beyond reconstruction.

These compounds into which protoplasm breaks up when it dies are themselves extremely complex; but though much careful study has been bestowed upon them, we cannot as yet say how they are put together to form the living substance. Protoplasm is too variable a body to be considered a single compound, while, on the other hand, the chemical relationships of its components must be too close to admit of its being called a mixture. Its chemical position is therefore unique, and we can only speak of it as a substance of unknown composition.

What, then, is it that makes protoplasm unmistakable and different to all other substances? The complexity of its structure is, after all, merely a matter of degree. The difference is not easily defined, but it roughly amounts to this: Protoplasm is always changing, yet always remains the same. Life is the change in the molecules.

If our definition of life seemed obscure, this sounds like a paradox; but perhaps the following fact may help to explain it: Under certain conditions some of the simpler compounds behave in a somewhat similar way. For instance, there is one which is so greedy of oxygen that it grabs it from whatever will readily give it up, and in order to do so is obliged to relinquish that which it has already got in its molecule to make room for that freshly acquired. Protoplasm is always behaving in this sort of way as long as it is protected from extremes of heat and cold, and from active chemicals which split up its molecules to form fresh compounds. Then it dies, or ceases to be protoplasm.

But the importance of this constant change lies in the fact that by continually breaking down its own molecules protoplasm obtains the energy to rebuild them out of non-living compounds of high potential energy, to modify its environment, and, in fact, to do the work of life.

It was said above that protoplasm only continued to exist in the form of a cell; therefore, what is a cell, and why its necessity?

We have seen that protoplasm has a very complicated structure, and that its normal condition is one of change. This being so, it obviously cannot exist in large masses, for if it did the change would be sure to be uneven in different parts from its very complexity; and the centre of the lump would either be starved or poisoned by the products of its own life. To avoid this, the mass is divided up into a vast number of small units each complete in itself, in communication more or less direct with its neighbours, and all equally accessible to fluids which both feed and cleanse them.

But there is another and still more important reason for such a division. The protoplasm is constantly discharging decomposition products, and needs to be repairing its waste by building in fresh compounds. The raw material around it requires dressing before it can be of use, and the building in is a difficult business. In each cell there is, therefore, a place set apart, where the protoplasm has peculiar capabilities, and it is here that this elaboration is carried out. This spot is called the nucleus. Thus it will be seen that the formation of an animal’s body by the aggregation of cells is a necessary and ingenious way of avoiding a difficulty.

To say, however, that an animal’s body consists of cells is to take an entirely wrong starting-point. A cell is complete in itself, and can live if properly fed, even though separated from its neighbours. Many whole animals consist of only one cell. A cell is, moreover, capable of growing and dividing, thus giving rise to two cells with two nuclei, and it is only because cells find that it pays better not to separate, but to form masses and specialize at different kinds of work, that we have large animals composed of millions of cells like ourselves.

Given a cell, it is necessary to keep that cell under favourable conditions. Otherwise the unstable protoplasm breaks up. It must have the elements necessary to keep up its cycle of changes in the proper form, which we may now call food, and many cells have to go and find this requisite. It must keep away from injurious influences, and it must race other cells for localities favourable to its growth and multiplication; in fact, the cell must work.

That a cell can, in virtue of its chemical affinities alone, move about, seeking favourable conditions, showing discrimination and doing work, seems incomprehensible. In the first place, how can it move? There is only one way: it must effect a redistribution of its substance, and contrive that those parts of the cell whose activity is applied to this end shall be so situated as to produce definite changes in its shape according to the cause which evokes them. Of the way in which different cells move we shall have a good deal more to say later.

Why protoplasm should be influenced to move still requires explanation. Yet the gap between protoplasm and other substances is really not so great, after all. Heat and magnetism cause movement in inanimate matter, and the response of protoplasm as exemplified by some of the minute unicellular animals is almost as mechanical. Some kinds which swim in water move to the positive pole of a galvanic battery, others to the negative, if the wires are dipped into the vessel containing them. Some move towards light, others away from it, with unvarying regularity. Temperature and chemical substances also cause a definite effect upon these micro-organisms. And all these movements are wholly involuntary, absolutely invariable, and, in fact, reactions evoked by fixed causes.

Nevertheless, it will be seen that protoplasm can only continue to exist in the form of a cell, since, unless thus organized, it can neither keep itself among favourable surroundings nor prepare fresh ingredients to make good its waste. If a cell be cut up into several pieces, these detached bits of protoplasm will live for a time; but death overtakes them as soon as they have used up their reserve material. When this is gone, they have to consume their own substance, a process which quickly proves fatal. Should a fragment contain a small part of the nucleus cut away with it, it will live a little longer; but it is only the piece which contains the nucleus more or less intact—in other words, the cell, damaged though it be—which can survive and recover from such mutilation.

The specialization of protoplasm to form a cell is perhaps its most remarkable peculiarity. Not only is protoplasm differentiated to form different structures, but it devotes the energy evolved in its ceaseless change to different purposes. The protoplasm of the motor organs of the cell expends itself wholly in producing the physical movements necessary to approach and capture food. When this has been passed into the cell, protoplasm of another variety works to refine and dissolve it, and then passes it on to the nucleus. The protoplasm of the nucleus, again, has different work to do. It devotes its energy to producing chemical changes in the raw material, and converting it into new compounds which the various parts of the cell can assimilate. Some of these it retains for its own needs; the rest it dispenses to the motor and other organs to repair their waste, and supply them with energy to obtain more food.

Thus do the different varieties of protoplasm which compose a cell supply one another’s needs, and enable one another to live; and thus does a cycle of chemical changes form the foundation upon which the whole fabric of life rests. But into details we cannot yet go, for our investigations of the material basis of life have not yet carried us beyond these general conclusions.

At present we know nothing definite about the first causes of life, and, though we have hopes, perhaps we never shall. Meanwhile we are observing, analyzing, and classifying the phenomena in which life is manifested, in the hope that at last light may break through upon our researches, and we may be able, if not to synthesise protoplasm in a test-tube, at least to demonstrate its workings in equations.

In the meantime, our actual knowledge of living matter can still be compressed into the words in which Professor Huxley summed it up years ago:

‘Carbon, hydrogen, oxygen, and nitrogen are all lifeless bodies. Of these, carbon and oxygen unite, in certain proportions and under certain conditions, to give rise to carbonic acid; hydrogen and oxygen produce water; nitrogen and hydrogen give rise to ammonia. These new compounds, like the elementary bodies of which they are composed, are lifeless. But when they are brought together under certain conditions they give rise to the still more complex body, protoplasm, and this protoplasm exhibits the phenomena of life.’

Until we have further knowledge of the changes which constitute these phenomena, physiology must remain descriptive rather than explanatory.

ESSAY II.
THE CHEMISTRY OF THE BODY.

I.

The cell is usually very minute—indeed, absolutely invisible without a microscope, though in some cases it is a fair size. The whole yolk of an egg is a single cell until its minute nucleus, a speck on one side, starts dividing and it becomes several. By the time the chick is ready to be hatched there are millions.

Usually, however, a cell is small—just as much protoplasm as its still more minute nucleus can keep going; though here, again, one must be guarded, for there may be several nuclei instead of only one. The protoplasm on the external surface and around the nucleus is specialized into a more or less definite membrane. To this outer envelope are attached fine fibrils, which join up to a small body within the cell, called the centrosome, and by the lengthening and shortening of these its shape can be altered. The contents are fluids; so if the containing membrane is loosened in any direction, they tend to bulge out and form an excrescence, and in this way the cell is enabled to throw out limbs and surround particles of food, and, by relaxing the fibrils in one direction and contracting them in others, to crawl whither its chemical, thermal, or physical affinities direct. ([See Diagram 1.])

Not particularly inspiring is the sight of life in its simplest form, but when a few millions of these cells group together and form one body, dividing the labour between them, the result is something stupendous. There are animals composed in this way, some of whose cells have developed their digestive capabilities to such an extent that they have almost lost all their others. These are carefully guarded in the interior of the body. Other cells in this same beast, receiving their food in a fluid form from these digestive specialists, secrete lime around them till a skeleton is built up. To the levers of this skeleton are attached bundles and strands of cells, which, if they can do nothing else, can lengthen and shorten and make it move. Yet, again, there are cells which have especial facilities for receiving, weighing, and transmitting chemical and physical promptings. These cells, again, lie in a protected corner of the interior, but they send out fine threads to one another and every part of the body, and control the whole.

The animal in which this beautiful system of division of labour has been carried to its greatest perfection has many and varied powers. He can in some cases even apply to the individuals of the species the principles of his own cellular economy, and thereby achieve not only the making of poetry and jokes, but the building of a Westminster Abbey, the construction of Maxim guns, and the enforcing of his economic refinements upon his less highly specialized neighbours.

We have now traced out a general idea of life. We have seen that its basis rests upon a chemical structure which, to maintain its identity, must be always changing. We have seen that to do this it must keep breaking down its substance, and giving off the products, and taking hold of extraneous materials, and building them in, not only to repair the loss, but in order to grow; and that to do that it has to be more or less modified in parts, in order that the main bulk may be brought within reach of its food, and be then able to convert it into the most useful form. And, lastly, we have seen that just as several specialized forms of plasm together make up a cell, so several kinds of cells, each with some peculiarity exaggerated, aggregate, and, supplying one another’s needs, compose a body.

Having now roughly sketched out the scheme upon which such a body works, we can go on to a more detailed examination of the division of the labour, and the way in which each department supplies, and is dependent upon, the others. If we were to do this thoroughly, it would take a great deal of time and space, for the physiology of a potato plant, though essentially the same, presents many differences from that of a horse; but the physiology of the great human interest is also that of the most complicated animal, namely, man, so it is on him that we shall focus our attention.

Protoplasm is more easily studied the more specialized is the animal it composes. When all the events of life are taking place in a speck of matter, invisible without a microscope, it is impossible to analyze the changes which it is working in its surroundings, or to infer those which are going on in itself. But when large numbers of cells are examined collectively, we can deal with what they take in and what they give out in sufficient bulk to arrive at a fairly accurate determination. The study is rendered still easier in an animal with extremely specialized organs, like man, in which food is nearly all taken in by the mouth, and thus kept quite distinct from what is eliminated; the latter, again, being mostly given off by the kidneys is kept equally distinct. Moreover, the intermediate changes being performed in different organs still further simplifies investigation of the vital process; for the physical effects are also more easily studied when exaggerated in a particular part of the animal. The electrical changes in a single cell might long have remained unsuspected had we not been able to observe those in a muscle with the galvanometer.

Now, while the cells which make up the body of man differ very greatly owing to the different tasks which they have to perform in obtaining food and getting rid of refuse, they all require very much the same fuel to enable them to live, and having got it, they all treat it in very much the same way; therefore our first business is to consider what the body wants, and what it does with it. Afterwards we can try to find out how it gets it, and where.

The first and most indispensable requirement of protoplasm is water. The next is probably nitrogen, compounds of which seem to form the framework of the protoplasmic structure. The next is probably carbon, and the next free oxygen. The two last-mentioned combine with a release of energy. This happens in the grate when coal burns, and the result is heat. In the tissues of a body the result may be heat, growth, or movement, all three being present in the phenomenon of muscular activity. Finally there are mineral salts, the most important being sodium chloride, which is placed on the table at every civilized meal.

But though these elements are given here in order, their importance is really equal, for all are necessary. That is about as much as it is wise to say here. The chemistry of the living cells—their anabolism, or how fresh material is built into their structure; their katabolism, or how the same structure is broken down that work may be done; in fact, the general metabolism—is so complicated, and so little understood as yet, and requires so extensive a knowledge of chemistry to follow, that it is best left alone by people who do not want to go into it deeply. At best, such a discussion resolves itself into an exposition of different observers’ theories, with the reasons why they hold them—reasons based on laborious and technical studies. Pages might be written on the various theories, backed by pages more of chemical formulæ, to show why this view deserves deep consideration, while that, in spite of the obstinacy with which it is upheld, is absurd; but though such discussions take one nearest the secret of life, the general public is not unnaturally apt to stigmatize this side of physiology as dry. It is a matter which interests experts, not the casual reader.

Quite a different affair is the question of diet. That is everybody’s business, as the number of faddist societies and blatantly advertised ‘foods’ attest. And though the preparation of the food in the body up to the point where it merges into living matter and is lost sight of—in a word, ‘digestion’—is again a question of chemistry, it is one which may be approached without such an exhaustive knowledge of that science as the previous considerations would have required. It is, moreover, to judge from the way it is discussed, a topic of universal interest.

A casual glance at the animal kingdom will show that diet is a wide subject. A pigeon will eat peas; a tiger would not know what to do with the peas if he got them; while a monkey will eat almost anything he can lay hands on. A plant takes us still further afield, for it can use the atoms of substances with an extremely simple molecule—carbonic acid gas, for instance.

Our task, however, is simplified by our having only man to consider; and although most of the higher animals are so much alike that they might be considered in general and contrasted in detail, it is a great thing to get rid of the whole vegetable kingdom with bacteria and parasitic animals.

One of the first requisites for the maintenance of life, as was mentioned above, is nitrogen. Now, nitrogen is one of the commonest elements in the world, but it is the hardest to supply to the body. Four-fifths of the air is pure nitrogen, but pure nitrogen is useless as a food. We draw it into our lungs at every breath, and are none the better for it, for we breathe it out again unchanged; and if it were completely absent from the air we should not be so very much the worse. The Ancient Mariner exclaimed, ‘Water, water everywhere, and not a drop to drink’; a starving man might exclaim, ‘Nitrogen, nitrogen everywhere, and not an atom to assimilate.’

Animals have to get their nitrogen in the form of proteid, a substance whose molecule is composed of nitrogen, oxygen, hydrogen, carbon, etc., and might roughly be described as dead protoplasm. Plants on which animals feed, when they do not get their proteid by the simpler, though less moral, method of eating one another, are able to get their nitrogen in a simpler form; but with that we are not concerned.

The proteids are a group of substances which resemble protoplasm in the elements of which they are composed and in the complexity with which they are combined. The various proteids seem, however, to have a definite chemical composition, and therefore differ from protoplasm in being true compounds; moreover, if kept from bacteria they undergo no changes. One of the best forms of proteid for examination is white of egg; this, as is known, sets or coagulates when boiled, dissolves in water, from which it may be precipitated by boiling, and displays various other chemical properties common to all proteids. There is, however, a good deal of difference between the several varieties of proteids, and the more complex ones have to be converted into the simpler before they can be absorbed. Hence the necessity for digestion.

Now, as proteid resembles dead protoplasm, it might be supposed that a diet of proteid alone would be the most economical; but this is not so. If it were possible to live without work, i.e., without movement of any kind, it might be; but to do work, more carbon must be oxidized than the proteid molecule contains.

Carbon, the next item on our list, is familiar to everyone in the comparatively pure form of coal, charcoal, and the ‘lead’ of pencils. It is commonly used to burn—i.e., oxidize—that heat may be obtained to boil water and to work machinery. This is precisely what it is required to do in the body, where it is burnt by oxygen taken in by the lungs, that heat and energy may result. It is a commonplace that severe exercise causes laboured breathing, and the reason of this is that the carbon in the body is being oxidized, and the product, carbonic acid gas, has to be got rid of. The more work is being done, the more oxygen is required to burn carbon in the muscles. The more carbon is burnt, the more heat is evolved, and the more necessary it is that the blood should be cooled by drawing cool air into the lungs. Hence the more rapid breathing. The air normally breathed out is always warmer than that taken in, and always contains extra carbonic acid gas. After exercise the quantity is increased, and its increase on the normal amount given off can readily be demonstrated by analyzing samples of the air taken in and given out.

But carbon, like nitrogen, cannot be taken in in the crude form. No one would try to make a meal of charcoal. A certain amount is contained in the proteid molecule, enough, no doubt, to secure the basis of the protoplasmic structure; but unless one is prepared to eat an excessive quantity of proteid, a proceeding entailing waste and exhaustion of the digestive apparatus, the balance must be made up by eating carbohydrate.

The forms in which people are most familiar with carbohydrate are starch and sugar. Sugar is the better food, as it is so much more soluble than starch; and, in fact, starch is always turned into a kind of sugar before it is used by the body. The common cane-sugar, which everyone knows so well, is about the most useful food we have, owing to its purity, and therefore concentration, and its simplicity. A very small amount of digestion is necessary to convert it into the simplest of all carbohydrates, a substance easily stored, as glycogen, till wanted, which is present in muscle after a meal, and is used up when the muscle is active, being oxidized to carbonic acid gas, sarcolactic acid, and alcohol.

The importance of carbon in the diet is therefore obvious; and people who intend doing extra muscular work should take extra sugary food rather than extra proteid. A locomotive which is about to make a record run takes in more coal, not more engine-drivers, and our athletes now follow the same principle. We shall, however, have a good deal more to say about athletes presently.

There is yet another point to be considered in respect to carbon. Carbon need not be taken in the form of carbohydrate, the alternative being fats and oils. Fats and carbohydrates are both composed of the elements carbon, hydrogen, and oxygen, but the proportions in which they are joined are different. Fats are not such useful foods as carbohydrates, nor to most people so pleasant—compare a spoonful of olive-oil and a lump of sugar. But there is one important point to be urged in their favour: they yield twice as much heat as either proteids or carbohydrates; so their position among foods is assured.

The other chemical necessities of the body we need only mention here. Hydrogen is one of the components of proteid, carbohydrate, fat, and water; and if it does not enter in the last form, it—at any rate, most of it—leaves as such, being oxidized in the tissues. Sulphur and iron deserve honourable mention; common salt is required by the blood; lime and phosphates go to make bone; but important as they all are, they need not detain us further at present.

With regard to the amount of these elements which is required per day, and which is ascertained by collecting and weighing all that is given off, it is found that about ½ ounce of nitrogen and 10 ounces of carbon are necessary to an average man—i.e., weighing about 10 stone. The ½ ounce of nitrogen and about 2 ounces of the carbon are contained in 4 ounces of dry proteid, which leaves a balance of 8 ounces of carbon to be made up; and this is usually obtained by eating 4 ounces of fat and 18 ounces of carbohydrate.

Roughly speaking, these principles are contained in ¾ pound of ordinary butcher’s meat and 2 pounds of bread; but it would be well to defer considering diet for the present, until we have examined the apparatus by which the body extracts what it wants from the raw materials, and which of these offer it the least resistance.

II.

The way in which protoplasm gets its chemical requisites for growth is doubtless simply by absorbing them. Some of the lower structureless forms carry this to an absurd extreme, for when two individuals meet they fuse, and each no doubt claims to have eaten the other. As, moreover, the first thing which a cell does when it grows is to divide, the whole proceeding looks rather futile. But ready-made protoplasm of an assimilable shape is rare, and it is not often that a cell, unless it be a plant or a parasite, finds itself in a substance which can be handed straight to the nucleus without further elaboration. Usually the cell has to discharge from itself a reagent, which will develop the right chemical qualities in the matter it wants to absorb. This substance is known as an enzyme, or ferment. Ferments, however, are an expense to the cell, requiring a certain effort for their production; so, in order that they may be economized, they are, in the higher forms, poured over the food while it is in an enclosed cavity, or stomach. In the simplest animals, consisting of a single cell, the protoplasm simply flows round the particle of food, and it is ‘ingested’ with a drop of water. Into this ‘food vacuole’ the ferments are secreted, and when all that is useful has been dissolved out and absorbed, the bubble moves to the surface and bursts; or, to put it differently, the cell flows on its way, and the vacuole, with any shell or refuse it may contain, gets left behind. ([See Diagram 1.]) In other cells which are constant in shape there is an opening leading to the interior of the cell. Round this there are little projecting threads, which beat the water regularly. In some positions these threads enable the cell to swim, but here their duty is to cause a current and wash particles of food down the primitive throat into the interior, where, as in the preceding case, they become enclosed in a vacuole. ([See Diagram 2.])

Diagram 1.—The Amœba.

Diagram 2.—Paramœcium.

Moving a stage higher, we find animals consisting of several cells. Of these it is only natural to suppose that some have greater enzyme-forming powers than others.

Diagram 3.—Development of an Embryo: First Stage.

Diagram 4.—Formation of a Digestive Cavity.

Diagram 5.—Cross Section of a Developing Embryo.

A step higher in the animal scale, or a further advance in the development of the schematic embryo (depicted in [Diagrams 3 to 6]), and we find that these special digestive cells are losing their sturdier qualities and being placed in a position protected by cells which have specialized in another direction. This is shown in [Diagram 4], where the hollow ball of cells which resulted from the repeated division of one cell is represented in section. One side of the ball is pushed in, and now the beast consists of two layers of cells, an outer protecting and an inner digesting (Hydra and sea-anemone). Soon, however, it is found more convenient to have a tube for digesting food, for then different substances can be digested and absorbed in different parts; and the refuse, of which the animal can make no use, need not be brought back to the mouth to be got rid of.

This, however, requires a number of other changes in the structure of the animal, which are roughly shown in [Diagrams 5 and 6]. It is not to our purpose here to discuss the development of animals or an animal; but the figures are worth glancing at, as they show not only how certain of the cells are set apart for digesting food, but also that a large body consists really only of a mass of protoplasm, composing kindred cells of common origin.

Diagram 6.—Showing Development of an Embryo.

Now, for obvious reasons, the longer, within certain limits, this tube is the better. All sorts of different food-stuffs have to be acted upon in it, and some offer considerable resistance to digestion; and the further they have to travel in the tube, the more chance there is of their being successfully treated. Besides, different parts have different functions, and the longer the tube—again within necessary limits—the greater scope is there for division of labour, and consequent economy. The comparative length of the alimentary canal is not the same in all animals by any means. Carnivorous animals, like the cat, whose food is soft and easily digested, have a fairly short one. Herbivora, like the sheep, whose food is difficult to digest and mixed with much husk, which is wholly indigestible, have a comparatively very long one. Man, who is omnivorous, but eats less and more judiciously chosen food than either of the above classes, has one of medium length. But in all cases among the higher animals there is an attempt made to obviate the necessity of increasing the length of the animal by coiling the tube within the body. The annexed diagram ([7]) illustrates this principle. It shows a schematic animal whose digestive canal is much longer than itself.

Diagram 7.—Showing how the Digestive Canal is Lengthened.

Diagram 8.—Cross-section of the Digestive Tube.

The digestive canal has, however, another function. The cells which compose it have not only to secrete juices, to convert the food into a usable form; they have then to absorb it. The nearer a particle of food is to the wall of cells, the sooner it is reached by these juices, and the less chance there is of useful material being swept away and lost. In view of this fact, along certain tracts the digestive canal is folded inwards, and there are projections, which increase the number of cells to secrete and their opportunities of absorption. ([See Diagram 8.])

Diagram 9.—Showing how Glands arise.

Here again we have an illustration of a constantly recurring need, with a device for meeting it—increase of surface without increase of bulk. We met with it before in the cellular system; we shall meet with it again in glands, lungs, and brain, at least. The importance of a device for gaining this end is apparent when one remembers what the comparative value of surface and bulk is to an animal, and that, while surface increases by the square, bulk increases by the cube.

The principle is pressed to an extreme, together with the allied principle of division of labour, in glands. The object of these is to increase the number of secreting cells, and, as they are delicate, to keep them protected from contact with coarse particles of food. And, in order that nothing may interfere with their efficiency, they are absolved from the duty of absorbing. Hence tubes grow out from the cavity of the alimentary canal lined with the same cells, but, as no food ever enters, the cells which line them devote themselves entirely to pouring out digestive juices. Glands differ considerably in structure and in the liquids which they secrete. Some are very small; some, like the liver, very large. In some the tube is very short, in some long, coiled and branched, and sometimes the gland is connected with the surface by more or less of a duct. Some glands only secrete one enzyme, some several. In each, however, the principle is that shown in [Diagram 9], no matter how its structure is masked by the bloodvessels and supporting cells or connective tissue which envelop it.

After a meal, or, rather, when the process of digestion is over and the animal is beginning to think about its next, the gland cells start preparing their enzyme. There is great activity in the nucleus, and granules stream out from it towards the lumen of the gland in much the same way, to take a homely illustration, as bubbles in some effervescing drink form at the bottom of the tumbler and rise till the surface is covered with foam. At the right moment these granules are discharged, just as the bubbles on the surface of a liquid break at a slight jog. They are usually not the ferment or enzyme, but its precursor, a substance which only turns into the ferment when it gets outside the cells. The ferments, when formed, are very peculiar substances about which we should like the chemist to tell us more, though great advances have been made in our knowledge of them lately.

Among other peculiarities, one may mention that, though they will keep indefinitely if bottled, they are easily destroyed by too extreme a temperature or too acid or alkaline surroundings, that their composition is entirely unknown, and, strangest of all, that they do not become used up. A given amount of rennet will clot any amount of milk within reasonable limits, and yet remain rennet. The clergyman has been quoted as an illustration of the action of a ferment, and he makes a good one. He can make any number of suitable men and women into married couples, and yet his own state is unchanged.

III.

In man, the digestive process may be divided into three stages. They are arranged progressively, so that each clears the way for the next, and take place in the mouth, the stomach, and the upper part of the small intestine, the rest of the canal being mainly occupied in absorption.

Diagram 10.—General Scheme of the Alimentary Canal, with its Offshoots—Lungs and Glands.

By far the largest proportion of the food is carbohydrate, in some form, so one naturally expects the first stage of digestion will deal with the constituents which represent this class. This is the case. The food is taken into the mouth in small quantities and ground up with the teeth, during which process it is subjected to the action of the saliva. This fluid, which is the secretion of three pairs of glands, converts a large proportion of the carbohydrates, starch, cane-sugar, etc., into a very simple sugar which is absorbed directly it reaches the stomach.

One of the most sensational discoveries of the physiologist has been that the saliva leaving the gland does not contain the ferment necessary to effect this change until it has been subjected to the action of putrefactive bacteria. These, fortunately for us, it is pleasant to know, simply swarm in the mouth.

When the food is swallowed, it passes very rapidly down the first part of the alimentary canal, which is straight, and is then kept for some time in the stomach. The stomach differs from the rest of the canal in several particulars, among them the following: it is a large cavity, and is closed at each end by a valve to keep the food in until it has been thoroughly treated, and it deals with the whole mass of food taken at a meal at one time, and yet has no contrivances for increasing its surface.

Here the food is subjected to a most important and searching examination. Enclosed in this bag, it is thoroughly mixed with weak hydrochloric acid, secreted by numerous glands, and kept churning round and round by the muscular action of its walls, that the contents may be kept well mixed. The acid is just strong enough to kill protoplasm, and hence the putrefactive bacteria which were necessary in the mouth, but would be a very doubtful blessing in the interior of the body, are disposed of. Other things are also killed. Not only does the stomach execute intruding bacteria, but it also kills a good deal of our food. Fruit and salad consist largely of still living cells, and occasionally there is bigger game, e.g., oysters. One thing, however, the acid does not kill, and that is the cells lining the stomach, and it may as well be said here that the parts of the body exposed to ferments have the very necessary power of resisting them, so that a normal animal does not digest itself.

The stomach, however, is a kitchen as well as a slaughter-house. The gastric juice, or secretion of all the glands opening into it, contains, besides the acid, two important ferments, both of which act on proteids. Carbohydrates are absorbed, but not digested, in the stomach, as acid destroys saliva. One of the ferments is rennet, an article familiar to the culinary profession, which solidifies milk. The other acts on proteids generally, converting them ultimately into a very simple form, peptone, which is absorbed at once. How much of the proteid in the stomach is converted into peptone is not known, for the action of acid alone is sufficient to enable it to be absorbed. A solution of proteid, e.g., white of egg, is quite altered if made slightly acid; it no longer coagulates when boiled, but the change of the most practical interest is that, if injected into the veins, it seems to become part of the blood, while ordinary proteids act as poisons.

The peptonizing ferment, however, has one very important function: it digests the collagen of the connective tissue, the substance which becomes gelatin when boiled. The reason why this is so important is not only that nothing else in the body affects it, but that fat is enclosed in it, and if it were not thus set free would pass through the body unabsorbed.

The final stage is the digestion by the pancreatic juice. After the food has been exposed for some time to the gastric juice, it is allowed to escape a little at a time from the stomach, and continues its way along the alimentary tube. Before it has gone many inches it comes to the openings of two ducts, those of the liver and the pancreas, and immediately the acid stimulates them, and the glands pour out their secretion. That of the liver is largely excretion or refuse from the blood without direct action on the food, but it enables the pancreatic juice to do its work by making the food again alkaline, and stimulates the muscular coats of the intestine to force its contents along. That of the pancreas is the most important digestive fluid in the body, containing many ferments; it acts alike on proteids, carbohydrates, and fats—in fact, digests everything—so that the rest of the long tube is freed from any more laborious duty than absorbing them as they pass.

Note.—The digestive ferments are now prepared for examination by chopping up the gland and placing it in glycerine; this extracts the ferment and preserves it from the action of bacteria. The first experiments on digestion, however, constitute one of the romances of physiology. A Canadian named St. Martin got into trouble with Red Indians whilst in the United States, America, and was shot through the body. The surgeon who attended him was unable to make the wound close, and when it healed there remained an opening in the man’s body communicating directly with his stomach. The surgeon, Beaumont, saw possibilities in this, and, obtaining gastric juice from his patient, made those classical experiments which entitled him to a place among the fathers of physiology. Americans do well to be proud of Beaumont, for it cost him many sacrifices, and his patience and courage are above praise. Not only was he devoid of all but the crudest appliances out in the backwoods, but his subject proved intractable and mercenary. No sooner did he discover his value than he crossed the border, and refused to return except upon exorbitant payment. Even after this had been arranged, he repeated the performance whenever he thought fresh extortion possible. In spite of these difficulties, the investigations proved wonderfully accurate and complete.

IV.

Of the absorption of the materials thus prepared it is not necessary to say much in a work of this compass, but the absorption of oxygen is too important to be passed over.

Oxygen is required by the body pure, and, as it is uncombined with anything in the air, it needs no digestion to free it. A special organ, however, is necessary to absorb it. This is the lung. The lungs originate, just like a gland, by a pouching of the alimentary canal near its origin, but differ from a gland in their cells being very much flattened, to offer a large surface to the air on one side and to the bloodvessels on the other. Incessantly during life air is being drawn into the lungs; that the cells to which it is there exposed may transfer its oxygen to the blood; and then, after the cells have also transferred the carbonic acid gas from the blood to the air, driven out again to be replaced by fresh.

(The mechanical means by which the lungs are filled and emptied come under another heading.)

V.

Food having been absorbed by cells set apart for the purpose, the next problem is, How is it distributed to those specialized for other work? The medium for this distribution is a liquid called lymph. All the spaces in the body are filled with lymph, all the organs bathed with it, every cell moistened with it; yet it is comparatively stagnant, and the food has to be conveyed from the walls of the alimentary canal to the lymph in the neighbourhood of the cell requiring nourishment by a more expeditious agent. This is done by the blood.

Diagram 11.—Principle of the System of Bloodvessels.

Diagram 12.—Principle of Double Circulatory System.

The blood is a fluid akin to the lymph, but confined in a system of tubes. Through these tubes it is driven at a considerable velocity, and in the course it takes passes within a reasonable distance of every cell in the body. As it passes the cells of the alimentary canal, they discharge the nutriment they have absorbed into it; as it passes through the other organs of the body, it discharges the requisite materials into the lymph bathing the actual cells: these are then able to help themselves.

The lymphatic system is very simple. Lymph is practically fluid which has exuded through the walls of the bloodvessels, and is like the plasma of the blood, a thin solution of proteids in water containing just enough salt to hold them in solution. From different parts of the body a series of tubes run towards the heart, going up with increase in size and decrease in number as they near it. Into these tubes the lymph is forced with every movement of the body. At a slow rate, but varying with the activity of the animal, it is forced to flow along these tubes, regurgitation being prevented by valves at intervals, until it reaches the place where the lymphatic vessels join a large vein, and it is poured back into the blood-stream, thus completing its cycle.

The blood is entirely confined in a closed system of tubes, along which it moves always in the same direction. The main principle of the system is that of a ring. One side of the ring is split into a vast number of fine tubes to give a large surface for absorption and discharge of food among the cells; the other side is a single tube, with an enlargement in which the blood from different parts is mixed (see [Diagram 11]). This enlargement, which is contractile and fitted with valves, rhythmically draws the blood in from one direction and pumps it out in another. (The mechanics of the process we shall study later.)

As a matter of fact, this system is twofold, as in [Diagram 12]. In passing through one-half of its course the blood absorbs oxygen in the lungs; in the other it yields oxygen to the tissues, and absorbs, whilst passing over the alimentary canal, proteid, carbohydrate, water, and salts, which are duly distributed to the other organs. Fat is absorbed by the lymph direct, but poured into the blood for distribution.

The blood which passes over the alimentary canal on its way back to the heart goes through the liver. In this gland it leaves the carbohydrate which it has taken up, and a large store is laid down there after a meal, to be doled out as it is wanted. Blood also passes through the liver from the spleen, where it has been, so to speak, overhauled for repairs.

Diagram 13.—Scheme of the Circulatory System.

Blood system on the right, lymph system on the left.

As the medium for chemical communication throughout the community of cells, the blood has another all-important and obvious function, viz., that of clearing away the waste products of life. Of these there is, of course, the same quantity as of new material introduced. Carbonic acid gas is discharged into the lungs, but all the nitrogen and most of the other elements in the new combinations which protoplasm has made them assume leave by the kidneys, plus a little water by the skin as sweat and a few items discharged into the last part of the alimentary canal amongst the unabsorbed portions of the food.

In their constituents, blood and lymph resemble one another, being both weak solutions of salts and proteid material; but the blood is distinguished from the lymph by the presence of innumerable extremely minute bodies, which give it its red colour. These corpuscles, to give them their proper name, are the vehicle by which oxygen is transported from the lungs to the tissues. They consist of an envelope of protoplasm filled with a red fluid (hæmoglobin), which combines loosely and easily with oxygen. In shape they are discoid, with a thickened rim and biconcave sides, another device for increasing surface and reducing bulk. ([See Diagram 14.])

Diagram 14.—A Red Blood Corpuscle.

With one more fact we may now conclude the chemical survey of the body. The blood has to pass through certain glands, or it becomes poisoned, and this quite apart from whether the gland secretes healthily or not.

Disease of the thyroid (a ductless gland in the Adam’s apple) causes goitre; of the suprarenal, Addison’s disease; of the pancreas, diabetes. Whether these organs secrete some substance into the blood which counteracts poisons formed in it, or whether they remove injurious elements from it, is not certain, but they are necessary to keep the great means of chemical communication in order.

Note.—The thyroid gland no longer secretes anything into the alimentary canal, and its duct disappears at an early age. If, however, it become diseased or is surgically removed, the distressing symptoms of goitre supervene. Such a patient may be completely cured by grafting a thyroid, excised from another animal, anywhere in his body. Doctors usually, however, give the patient extract of sheep’s thyroid either in pills or injections.

ESSAY III.
THE MECHANICS AND PHYSICS OF THE BODY.

I.

In the preceding essay we regarded protoplasm as a chemical factor in the universe.

We have seen how it is always changing, always taking in food, always giving off waste materials. We have seen, too, that it grows and that it does work, and that in a large mass the cells which compose it share the labour instead of each component cell performing all the vital functions. We have now to consider the work which protoplasm does—in a word, the mechanical effect of the chemical actions just described.

The simplest movement of protoplasm is to be seen by the aid of the microscope in certain vegetable cells, where granules seem always streaming about in different directions. A step higher, and we find this streaming movement converted into movements of the whole cell. In the simplest unicellular animals the fluid protoplasm is contained in a membrane, or denser bounding layer, to which are attached fine filaments springing from a minute body known as the centrosome. These centrosomes—for there are sometimes several in a cell—seem to control the mechanical department, just as the nucleus does the chemical. Along the fibrils at intervals are minute globules, and by watching the distance between them it is seen that the fibrils undergo changes in length, pulling in the membrane when they shorten, and letting the cell flow out in any direction when they relax. By adjusting these two movements to balance one another, the cell can move in any direction, surround and engulf particles of food, and assume a strange variety of shapes. ([See Diagram 1.])

Diagram 15.—Cell Division.

In some cells, probably in all, the centrosome presides over division. Cells, however, do not always divide in the same way. Some simply lengthen, the nucleus also lengthening inside, become constricted in the middle like a dumb-bell, and separate. ([See Diagram 15.])

Diagram 16.—Cell Division.

Others manage differently. In them the nucleus simply bursts, and turns its essential elements, a number—always a constant number—of coarse threads, adrift. Meanwhile, two centrosomes have moved to opposite ends of the cell, and there anchored themselves by fibrils; other fibrils springing from them become attached to the nuclear threads, and when all is ready pull them apart, equally divided, to their respective ends, where they re-form into two fresh nuclei. ([See Diagram 16.])

Unicellular animals, which are constant in shape and swim instead of flowing when they want to get anywhere, have at first sight nothing in common with those which do the latter. From their surfaces spring fringes of free protoplasmic threads, called cilia, from their fancied resemblance to eyelashes, which serve as motor organs, and beat the water like oars. ([See Diagram 2.]) Waves of movement, as they lash one after another, all in the same direction, seem to pass over the cell, and it is propelled through the water; while others, which are situated in the neighbourhood of the cell’s mouth, stir the water into eddies, and drive food particles into it.

Diagram 17.—Cilia of an Epithelial Cell.

These cilia are important, as they are adapted for many purposes in large animals. The cells which line the cavity at the back of the nose, the tubes of the lungs, and other parts of the body, have a few cilia on their free surface, and it is in them that the structure of these organs can best be made out. At the foot of each cilium is a minute globule, from which a fine fibril passes into the cell, and the fibrils, collectively forming a leash, are attached to its opposite end. ([See Diagram 17.]) It seems highly probable that the globule is a centrosome giving rise to two fibrils, one attached as described, the other passing up one side of the cilium, and fast to its apex. The result of this arrangement is that when the fibrils contract the cilium is bent over with a jerk to the side up which the fibril runs, and when they relax it slowly straightens itself. There is, therefore, no fundamental difference between this and the other mode of progression; both are dependent upon the centrosome.

Finally we have muscle cells. These are only found in a fairly complicated animal, since they are a product of the division of labour principle, and their sole business is movement. There are two varieties of muscle, but the principle is the same in both—a long thin cell, with fibrils traversing its length whose contraction causes the cell to shorten and thicken, thus reducing the distance between its two ends. At present the development of muscle and the way in which it ‘contracts,’ to use the word accepted in this case for describing a redistribution of bulk, are little understood, and there are accordingly many opinions; but I think careful study will eventually show that some modification of the centrosome, with its contractile fibrils, is responsible for the movement.

Diagram 18.—Muscle.

The two varieties are: the smooth, or involuntary, and the striped, or voluntary, muscle. Smooth muscle consists of spindle-shaped cells with one elongated nucleus. ([See Diagram 18, Fig. 1.]) It only contracts very slowly, and is not under control of the will; but it is very abundant in the body, since it effects practically all the movements of the alimentary canal and bloodvessels. Voluntary or striped muscle, so called from its appearance under the low power of a microscope, consists of long fibres, each containing many nuclei. ([See Diagram 18, Fig. 2.]) Its protoplasm is rich in hæmoglobin, and in it, under powerful microscopes, can be made out two kinds of fibrils: Rutherford’s fibrils, the complicated structure of which gives muscle its striped appearance; and Marshall’s fibrils, which are much finer and more difficult to see. The muscle of the heart, though not under control of the will, is striped; but it differs from ordinary striped muscle in being made up of small branched cells with only one nucleus.

The way in which the three elements of striped muscle contribute to a contraction is practically unknown, and the subject of much dispute. In fact, one could hardly wish for a better soil for theories, and some which grow in it are very wonderful indeed. We have reason for supposing that there are two contractile substances—one which gives a sharp twitch, the other a slow, hard pull; and on the whole there seems good reason to believe that Rutherford’s fibrils give the sudden movements, while Marshall’s give the more forcible ones; and that the ordinary protoplasm of the cell is restricted to the duty of nourishing the fibrils.

Diagram 19.—Striped Muscle Fibre, more highly magnified than in [Diagram 18].

The muscle cells are modified from among those of the bud forming the middle layers of the embryo. ([See Diagram 5.]) Other cells of this bud form connective tissue, by, so to speak, spinning long fibres of the substance called collagen, which turns to gelatin when boiled. ([See Diagram 20.]) This connective tissue permeates the whole body, affording a firm foundation for the many layers of cells which form the skin and the single layer of digestive cells; supporting the other organs throughout, and keeping the different parts of the body in their places, in doing which, however, it is assisted by other fibres which are not collagenous, but elastic. It also forms tracts which become lymph and blood vessels.

In parts of the animal which require special support it forms solid rods, the collagen combining with calcium salts to form a clear, hard substance—cartilage. At one period in the development of an animal or animals we find the only solid support is cartilage, but cartilage is not sufficiently rigid for a very large beast, especially on land, so is only used for outlying parts, the main framework being bone.

Diagram 20.—A Connective-tissue Cell giving rise to Long Collagenous Fibres.

Bone is formed very much as if Nature were rectifying a mistake. When a rod of cartilage is unequal to its work it is eaten hollow, and fresh connective-tissue cells immigrate and fill up the cavity, eventually laying down a fine network of cells in its place, the meshes of which are filled with inorganic calcium salts, chiefly phosphate of lime. Nature then benefits by experience, and the last bones to be formed are not preceded by any makeshift cartilage, but built up straight away in ordinary connective tissue.

This brings us back again to muscle, for the object of nearly all the voluntary muscle is to cause movement among the bones. For this purpose the muscle cells or fibres are arranged parallel to one another, and bound up together by connective tissue, the whole bundle being known as ‘a muscle.’ The two ends of a muscle are attached to two bones by connective tissue, which sometimes forms a short cord, or tendon. Then, when the muscle contracts, the two places of its attachment are pulled towards one another, and something has to move. But before saying more about the way in which the bones are jointed and muscles attached—in fact, what movements are possible in the human body—it would be as well here to describe the chief properties of muscle and the way in which they are studied.

II.

Diagram 21.—Apparatus for recording a Muscular Contraction.

The way in which voluntary muscle is studied is very simple. A frog is killed by thrusting a probe into the brain and down the spinal cord, and a muscle is then dissected out and attached to a piece of apparatus ([see Diagram 21]) in such a way that on its contracting it raises a lever, and draws a line on a moving surface. The rate at which the surface is moving is ascertained, so that the nature of the curve, which is a graphic record of the contraction, can be analyzed. ([See Diagram 22.]) For instance, when an electric shock is used to make the muscle contract, we find that a slight shock causes a small contraction, as shown by a low curve, while a stronger one, up to a certain point, causes an increase.

Diagram 22.—Graphic Record of a Response to a Single Stimulus applied at A.

Lower line = tuning-fork records of ⅟₁₀₀″.

But having described how muscle is studied, it is only necessary to state a few facts concerning it; to discuss muscle, fully describing the experiments by which its more obscure properties have been elucidated, and the devices by which causes of error have been eliminated, would fill volumes.

Diagram 23.—Contractions with Two Stimuli at Different Intervals of Time.

Muscle is thrown into a state of contraction by an impulse reaching it from a nerve, but it contracts quite as readily if excited directly by a mechanical or electrical shock. A second shock causes a second contraction, or, if the muscle is still in a state of contraction owing to the first, causes it to contract still more. ([See Diagram 23.]) If a number of stimuli are applied to a muscle in such rapid succession that the effect of the preceding one has not passed off by the time the next arrives, it will contract as far as possible, and remain contracted—a state known as tetanus. ([See Diagram 24.]) A muscle is therefore kept in a state of contraction by a continuous nervous effort, not arranged and then left contracted.

Diagram 24.—Tetanus.

Diagram 25.—Fatigue Curves.

Fast drum: a, point of stimulation. Every tenth contraction recorded.

Diagram 26.—Effect of Fatigue on Muscular Contraction.

Slow drum. Every contraction recorded.

Various conditions alter the character of a muscular response. With repeated stimuli at short intervals a muscle fatigues, and each contraction becomes smaller in extent and longer in duration. ([See Diagrams 25 and 26.]) If the muscle has to lift a load it has a certain check on its contraction, and its relaxation time is shortened. Temperature also affects muscular contraction, moderate increase causing a sharper, and moderate cooling a slower, rise and fall of the lever on stimulation. ([See Diagram 27.]) Lastly, we have drugs which exert an influence, but the only one of these which it is necessary to mention here is veratria, which makes the slowly contracting fibrils continue their activity after the quick ones have subsided. ([See Diagram 28.])

Diagram 27.—Effect of Temperature.

Diagram 28.—Veratria Curve.

Finally, there are the electrical changes in muscle. These, again, may be passed over briefly, since they are not easily understood or described. To put the facts in a nutshell, the part of a muscle which is in activity is negative to all other parts. Thus, if a muscle be dissected out and cut across, the activity at the seat of the injury, while it lasts, causes a current to pass through a galvanometer from uninjured parts to the wounded. ([See Diagram 29.]) Again, if a muscle be dissected out without injury, connected at two points with a galvanometer, and then stimulated at one end, as the wave of contraction passes along it, first one, then the other, contact becomes negative. ([See Diagram 30.]) S, Stimulating electrodes; N, contraction which marks the wave of excitation passing along the muscle; G, galvanometer which shows that the seat of activity (N) is negative to the rest of the muscle.

Diagram 29.—Injury Current: Cross-section of Muscle Negative to Rest.

Diagram 30.—Action Current.

In passing, it may be mentioned that, as the heart is a muscle slung obliquely across the body, and waves of contraction are continually passing down its long axis, the whole body is affected by continual electrical changes. By very delicate instruments it can be demonstrated that with each beat the two hands alternately become electrically positive and negative to each other.

Whilst dealing with the electrical phenomena of muscle, it may be as well to state that nerve fibres, which are studied with very much the same apparatus, show the same electrical changes, the point of injury or of the greatest activity being negative to all the rest. Single cells are less easily investigated, but in glands it is possible to show that the same rule holds.

Undoubtedly the most curious fact about the generation of electricity by protoplasm is that, by a modification of muscle and nerve, which causes them to lose their ordinary properties, they are converted into a special organ for giving electric shocks. Armed with powerful batteries of this description, an otherwise rather helpless class of fish are enabled to defend themselves from their enemies, and deal unexpected death to their more agile prey.

Having now run over a few of the physical properties of protoplasm, we may pass on to a brief investigation of the movements we find in the body of man.

III.

In describing the movements of the body, we shall have to treat them as several and distinct, as indeed they are; but the fact should not be lost sight of that they cannot really be isolated: one idea embraces the whole. Two kinds of movement may, however, be distinguished in the vital functions: movement of the actual cells, such as muscles; and movement of non-protoplasmic elements acted upon by the cells—e.g., lymph.

There is a parallel to this in the chemical side of life, where we find some phenomena peculiar to the living elements, and others, like digestion, going on in the living body, but outside the cells.

Taking the movements in the natural order—that is, proceeding from the simpler to the more complex—the first to be considered is undoubtedly that of the leucocytes, or general scavengers of the tissues. The body consists, so far as we have defined its anatomy, of three layers of cells, and its shape is that of a tube with hollow walls. ([See Diagram 6.]) Within the cavity of the body are various organs, such as the muscles, which are formed from the middle layer; and its space is largely reduced by glands, lungs, and other ramifications of the inner layer which forms the alimentary canal.

These organs hang more or less freely in the body cavity, slung to its walls by enveloping sheets of connective tissue, the whole being bathed in lymph. Now, in such an arrangement the products of wear and tear must accumulate. Cells here and there die for various reasons, and pieces of cells become detached even in adult animals. The interior of a bone is always being eaten away to decrease its weight, or in order that it may be replaced by fresh bone of a closer texture, and in young animals and embryos there are many structures which, useful for a time, have eventually to be removed; as an instance, we may quote the tadpole’s tail. In fact, if the tissues were left to themselves, the body would soon be choked with débris, and to avoid this it is supplied with an army of scavengers, the leucocytes.

The leucocytes are detached cells which owe their origin to the middle layer. In size they are, of course, very small, quite invisible to the naked eye. In appearance they resemble unicellular organisms of the amœba type, which we have had occasion to mention several times already ([Essay II., Section II.]; [Essay III., Section I.], [Diagram 1]). They are of several different varieties, some being larger and more active than others; but they all wander about in the lymph and blood like independent animals, creeping in and out between the cells of the organs, and devouring any foreign matter they come across. They sometimes multiply, like independent animals, by division, especially in the presence of inflammation, or when they have much work to do, and a rapid increase in their numbers is needed; and they have been induced to live, and feed, and multiply, outside the body (in which case they must be considered to have become independent organisms), thanks to the careful attentions of the experimenter.

Apart from their duties of devouring the inside layers of bones and clearing away dead tissue, they are supposed by some to assist in the absorption of food by creeping between the cells lining the alimentary canal, and, after throwing out arms to engulf particles of food, returning with their spoils into the body. Perhaps, however, the most interesting, or at any rate most romantic, of their many and important functions are what may be called their emergency duties. Frequently people, especially those who live in smoky towns, draw into their lungs particles of dust and soot, which if left adhering to the walls of the air cavities would cause dangerous irritation. As if by magic a leucocyte will discover the presence of such a nuisance, and, crawling between the cells forming the wall of the lung, in which, by the way, it is outside the body proper, will engulf it and carry it away with him. This exploit, however, pales beside the warfare which goes on in the body between leucocytes and invading bacteria. A bacterium thrives in the blood or lymph, since it finds itself in a warm alkaline fluid containing complex organic substances, by breaking down which it can easily obtain energy. Unfortunately, the products of such a process are frequently virulent poisons, the effect of which upon neighbouring cells produces the distressing symptoms which we associate with disease. No sooner, however, has the bacterium begun to generate poisons, than leucocytes, influenced by chemical attraction ([Essay I.]), swarm upon it. First come leucocytes of a small kind, full of zymogen granules, which crowd round the bacterium till they have covered it. After a time they creep away, leaving it dead. They are now in an exhausted condition, and no longer contain granules, having doubtless discharged them as a destructive ferment upon their enemy. Then a leucocyte of another kind moves to the attack, or, rather, to clear up the remains, for he is a large, non-granular, active fellow, and eats up the dead bacterium by the simple process of engulfing him whole. ([See Diagram 31.])

A natural question arising out of the study of leucocytes is, What becomes of them? Particles of soot and similar refuse can hardly be considered nutritious, or even digestible, food, and one is rather drawn to the conclusion that the leucocyte performs its functions for the good of the body at large, not of itself, and that when its work is done it must die. Many leucocytes, probably, loaded with unconsidered and undesirable trifles, cast themselves into the alimentary canal, and are got rid of with the useless portions of the food; but they do not always have the luck or energy to get to a natural outlet. An unpleasantly familiar phenomenon is the boil. Here we have some irritating substance under the skin setting up inflammation, and leucocytes swarm up to remove the cause of the trouble. Before, however, this is done, many have perished in the fray, and they have collected in numbers to the formation of what is commonly known as pus, or matter. Their dispersal into the body is now neither easy nor desirable, and the surgeon usually lets them escape from the surface by a touch of the lancet.

Diagram 31.

A, Eosinophile leucocyte; B, bacterium; C, leucocytes killing bacterium with their enzyme; D, leucocytes leaving bacterium dead; E, hyaline leucocyte devouring dead bacterium.

Such, then, is very briefly the story of the leucocyte, neglecting such problems as the differences between those found in the blood, called white corpuscles to distinguish them from the red corpuscles, with which they have no sort of connection; those found in the lymph, called lymphocytes to distinguish them from those found in the blood; those caught in the act of devouring bone, called osteoclasts; and those found with bacteria inside them, therefore known as phagocytes; and without speculating on how long an individual lives, and whether the different varieties differ in origin or are merely at progressive stages of development. The study of leucocytes is one of the most fascinating in physiology, but we have many other things calling for our attention, and we have said enough about the part they play in the life of the body to justify our passing on to consider another essential movement.

IV.

Next in natural order for consideration come the movements of the alimentary canal.

So far we have considered this structure as a chemical laboratory, a tube consisting of a single layer of cells which secrete ferments into the lumen, where digestion takes place, and then absorb the products, and we have not yet accounted for the food travelling along the tube, without which its functions, as described in the earlier part of the book, could not be performed. That the passage of the food is not due to gravitation is obvious from the many directions of the tube’s coils—not to quote the old instance of a horse drinking, in which case the liquid first travels upwards. One must therefore conclude in favour of some muscular method of propulsion.

We have so far described the alimentary canal as a single layer of cells, but it must be obvious that these soft secreting portions of the tube are not capable of vigorous movement. The canal proper is surrounded by a tough sheath of connective tissue which prevents its being overdistended or ruptured, and, by means of a layer—or, rather, two layers—of non-striped muscle which it contains, produces the movements which result in the passage of its contents along the tube. These two layers lie well to the outside of the connective-tissue sheath. The fibres of the inner layer are arranged circularly, so as to form rings round the tube; those of the outer have a longitudinal direction, running, therefore, parallel with its long axis. When the former contract, the diameter of the tube is reduced, while contraction of the latter has the effect of enlarging it. ([See Diagram 32.])

The movements of the intestine are what is known as peristaltic. Contraction of the muscle fibres is not simultaneous in all parts, but passes in waves along it. Just in front of the food the longitudinal fibres contract, and thus offer less resistance, while just behind the circular fibres reduce the size of the tube, and so get up a pressure. The result of a number of successive waves of contraction passing down the alimentary canal is that the food is propelled along it.

Diagram 32.—To illustrate the Passage of Food along the Intestine.

The arrangement of the muscle varies in places to suit special needs. Where the tube suddenly enlarges to form the stomach, and where the stomach suddenly narrows to the intestine, there are two strong rings of muscle, whose constricting influence converts the enlargement into a closed chamber during gastric digestion; while the coats which actually clothe it here run obliquely, and their activity causes the contents to be slowly churned about inside.

Thus it will be seen that it is not only the voluntary muscles which give the alimentary system its opportunities; without these unobtrusive non-striped cells we should toil for our bread and swallow it in vain.

V.

Our next step, after having surveyed the principle of movement by which the chemical necessities of the body are exposed to its absorbing surface, must be to see how the fluid which transports them is made to pass along the tubes containing it. We have already had occasion to describe how these blood and lymph vessels ramify through all the organs, when we were dealing with the chemical influence of the blood and lymph.

The tubes through which the lymph is brought back to the blood-stream have thin walls, and no muscle of their own. They are subjected, however, to a constantly varying pressure by the movements of the limbs and trunk, and as, owing to valves inside them, the lymph can only escape in one direction, there is a constant flow towards the junction with the bloodvessels.

The bloodvessels are quite different. A far more certain and expeditious current is necessary—hence the steady circulation through a system of closed tubes.

In order to understand this passage of the blood, it is necessary to keep in mind the great principle with which hydrostatics supplies us, viz., that a liquid always flows from a region of high to a region of lower pressure. The problem of the vascular system is, therefore: How can the pressure within a ring of tube be so arranged as to maintain a regular flow always in the same direction?

Let us begin with the structure of the system. The tube through which the blood first passes on leaving the heart is composed of four distinct and essential elements: A lining of endothelial cells, which we need not discuss at length; a main substance of tough white fibrous connective tissue; elastic fibres and muscle fibres, the two last arranged in the substance of the connective tissue. All these parts are present in the main arteries which leave the heart, but in the fine meshwork of capillaries to which the arteries give rise by repeated branchings there is nothing left of the outer coats, only the lining of endothelial cells separating the blood from the organ traversed. In the veins which these capillaries unite to form, the connective-tissue sheath reappears, and also some muscle; but the elastic coat is quite absent. The heart is really a double coil of the tube ([see Diagram 12]), in which the muscular coat is predominant, and is divided into four chambers by the valves, which insure the blood flowing in the right direction when it contracts. ([See Diagram 33.])

Diagram 33.—Scheme of Circulation

The way in which these structures work is as follows: Two of the chambers of the heart (the auricles) receive blood from the veins, and when full suddenly contract, driving their contents into the other two chambers (the ventricles). The blood does not run back into the veins, although the pressure in them is very low and there are no valves to prevent it, because there is still less pressure in the ventricles, and also because the veins enter the auricles obliquely, and the tendency of the increasing pressure is to close their orifices. Having discharged the blood into the ventricles, the auricles relax, and the pressure within being a minus quantity, they are speedily filled with blood from the veins, blood not being able to return after entering the ventricles, as valves close automatically to prevent it.

Stimulated by the blood distending them, the ventricles then contract simultaneously like the auricles, only with much greater force: for the right ventricle has to drive the blood all through the vessels pervading the lungs back to the left auricle; whilst the left ventricle, which is proportionately stronger than the right, has to send its contents to the furthest extremities of the body. They then relax, in order that conditions of their internal pressure may favour another inflow from the auricles, return of blood from the arteries being, as in the preceding case, prevented by valves.

The pressure in the arteries during life is always fairly high; indeed, the ventricles have to get up a considerable force before the valves leading from them will open. The result of this is not only that the blood is driven along them with a rush, but also that they are slightly distended at each beat; and so, owing to the elasticity of their walls, the blood continues to flow forwards even between the beats of the heart. The rest of the journey is quite simple; the pressure in the capillaries is lower than in the arteries, and the pressure in the veins lower than in the capillaries, and lower in the veins, too, as they approach the heart, till, where they join the auricle, it is actually minus, and the blood has no other course open to it but to return to the auricle. It looks as though accidents might happen in the veins owing to there being so low a pressure there to direct the current, but this is prevented by the presence of valves at intervals, to stop any return.

The rate at which the blood travels is another point which has an important bearing on the nutrition. It does its work—i.e., gives out nutriment and picks up refuse—whilst flowing through the capillaries; so here one finds that it moves slowly. On the other hand, the sooner it reaches them the better, so it races fast through the arteries. Finally, its return to the heart need not be delayed, so it is quickened up again through the veins. The principle by which this variation in the rate of flow is obtained is simple and inevitable. If a tube through which liquid is flowing is not the same size all the way along, the liquid will be found to flow faster in the narrow parts than in the wider ones. Now, in branching, the arteries do not keep becoming smaller in regular proportion, and the result is that the capillaries have collectively a diameter five hundred times larger than the aorta; hence the blood flows through them only one-five-hundredth of the pace at which it leaves the heart. But in uniting again to form the veins their cross-section is reduced once more, so that that of the large veins near the heart is only two and a half times larger than that of the aorta, and hence a flow only two and a half times slower results.

The pace of the blood-stream must depend, obviously, on the pressure of the blood in the arteries. This pressure is altered either by changing the rapidity of the heart-beat or the diameter of the arteries, which are capable of considerable variation owing to their muscular coat. The regulation of the blood-pressure is managed by the nervous system, so does not belong here, and we may leave it after mentioning one or two facts. High pressure is due to a large quantity of blood being in the arteries, and this may be due either to the rapidity with which it is injected by the heart or to the reduced capacity of the bloodvessels themselves. High pressure, due to the latter cause, throws a great strain upon the heart, owing to the hard work it has in pumping blood into the arteries; with a low pressure the heart beats feebly, having less resistance to overcome.

Blood-pressure can be raised by stimulating the muscular coat and reducing the capacity of the bloodvessels, and lowered by causing the heart to beat more slowly or by removing blood from the body. This latter operation was a favourite way with doctors of the old school; but as our knowledge of physiology, and with it our control over the vital functions, increases, such crude and heroic remedies are able to be replaced by others which are less dangerous.

VI.

Comparisons are rightly regarded as objectionable, so it would hardly be safe to say that the group of movements whose primary object is filling the lungs, and which we must study next, is the most important in the body, especially when we have just been speaking of the circulation, which, however, would be of but little use if the blood could not be oxidized; but we can at least say that its importance cannot be overrated, so far-reaching are its effects.

The lungs are, as we have described them above, a pair of delicate membranous sacs connected by a tube, the trachea, with the alimentary canal, from which they originally budded out. They are subdivided, though how we need not describe in detail, into a vast number of small compartments, so as to give the maximum surface in the space accorded them, and the whole somewhat resembles a cluster of grapes, the stalks being the branches of the trachea. The membranous parts are pervaded by an elastic network, enveloping the compartments in such a way that it would reduce them permanently to the resemblance of a bunch of raisins rather than grapes, were it not that they are enclosed in an airtight box—the thorax—from the walls of which they cannot shrink without causing a vacuum. Owing, however, to the latter arrangement and the trachea being open to the external world, they are always more or less distended with air.

The thorax, which they thus must always exactly fill, is a conical-shaped box, its walls being the ribs, and its floor a sheet of muscle known as the diaphragm. It contains, besides the lungs, only the heart and large bloodvessels. The problem, therefore, of drawing air into the lungs and (after the gaseous interchange described in [Essay II., Section IV.], has taken place) of expelling it again, becomes solely a matter of increasing and decreasing the capacity of the thorax. ([See Diagram 34.]) This can be done in two ways: the diameter through the ribs can be increased, or the diaphragm can be pulled down, increasing its depth. Actually, both these methods come into play together. [Diagram 35] will probably give a better idea of how this is done than could easily be conveyed by a verbal description. An attempt is here made to show the action of the ribs and the diaphragm—first, of each separately, then of the two combined. The elasticity of the lungs themselves is sufficient to drive out the tidal air if the diaphragm and the muscles of the ribs are relaxed, though in hard breathing a muscular movement may depress the ribs and a contraction of the abdominal muscles force up the diaphragm.

Diagram 34.—Model (adapted from Rutherford) for showing how the Lungs are filled with Air by altering the Size of the Thorax.

But though the primary object of raising the ribs and depressing the diaphragm may be to fill the lungs, its secondary influence upon the trunk as a whole is hardly less important. The effect upon the circulation is profound. The compartments of the lungs are enveloped in innumerable capillary bloodvessels, and, as these lie around and between them in the cavity of the thorax, they must, when breath is drawn in, be subjected to a negative pressure before the lung itself, and be the first to experience a positive pressure when the air is expelled. Here, again, a diagram is the best explanation. ([See Diagram 36.])

The pulmonary vessels, moreover, are not the only ones influenced. The reader who attentively examined [Diagram 13] must have been struck by the peculiarities of the circulation through the spleen, intestine and liver, and the obstacles which this repeated breaking up into fine vessels must offer to the flow of blood, as described in [Section V. of this essay].

Diagram 35.—Showing how the Capacity of the Thorax is increased by raising the Ribs and depressing the Diaphragm.

Diagram 36.—Model for showing Effect of Movements of the Thorax on the Pulmonary Circulation.

The liver forms the crux of the situation. ([See Diagram 37.]) A vein carrying blood from the intestine and spleen is broken up into fine capillaries to pass through that organ, and the pressure in this vein is extremely low. How is a sufficiently rapid flow of blood to be maintained? The answer to this riddle is best given by [Diagram 38], which shows how, by the contraction of the diaphragm at each breath, the large veins entering the heart are subjected to a negative pressure which draws blood out of the liver, while, simultaneously, that organ is squeezed and the blood it contains forced out. Obviously this natural pump influences not only the flow of blood, but also that of the lymph, and what was said about the hepatic vessels also holds good for the thoracic duct, up which the lymph, rich with fat absorbed from the intestine, passes to be emptied into the large veins near the heart. So, though vigour in the action of the diaphragm is more favourable to health than necessary to life, deep breathing is an essential factor in the well-being of the body.

Diagram 37.—A Diagrammatic View of the Circulation through the Organs upon which the Diaphragm presses when it descends.

Diagram 38.—Illustrating the Influence of the Diaphragm upon the Circulation through the Viscera.

VII.

All the movements as yet described are absolutely necessary to the continuation of life; they are, moreover, independent of the efforts of the will. But there remains yet another kind of movement without which the body, left to itself, would die. This is the movement of the limbs—organs by which the body is able to move from one place to another, to capture its food and convey it, viâ the mouth, to its stomach—in a word, to satisfy its chemical and physical needs.

To understand how the limbs work requires a knowledge of their anatomy, for which we have not time or space here; but the principle throughout is that of a system of levers, the bones, worked by the voluntary muscles. Here, as before, a diagram will probably be found to convey more than could ever be expressed in words. ([See Diagram 39.])

The diagram represents very roughly, but it is hoped very plainly, the main principle of the elbow-joint; but for an exact knowledge of the mechanism of the joint, and the comparative strain upon, and therefore strength of, each muscle, the reader must consult some work on anatomy. He will there find, if he goes on to read the description of the hand, what a wonderful precision, complexity, and amount of movement can be obtained by variations of this simple device.

Diagram 39.—Diagram of the Arm.

1, Lifting; 2, pressing.

Here, however, we must leave the study of the manner and object of the bodily movements, and proceed to investigate the far more intricate question of how they originate and are controlled.

ESSAY IV.
THE NERVOUS SYSTEM.

I.

Now comes the final problem. Protoplasm forms a structure always changing, always making good its waste by chemical action upon raw material, always capturing raw material or in search of it, always, when it exists in large quantities, and the labour is therefore divided between many cells, economically apportioning the work and the spoils. How is it that all the actions, chemical as well as physical, of a vast number of cells composing a large body are, no matter how complicated, always harmonious, and always with purpose directed to the advantage of the whole animal?

In the first essay in this book we discussed the phenomenon of life, and described briefly the chemical and physical peculiarities of protoplasm. These in the two succeeding essays we have gone into more fully; but there is one characteristic of that interesting substance which yet remains for us to examine in specialized cells, viz., its extreme readiness to respond to changes in its environment.

In [Essay I.] we saw that chemical agents, light, heat, electricity, etc.—had a definite effect upon protoplasm, and that, though they might influence different kinds in different ways, the effect was nevertheless invariable; in a word, the response of protoplasm to circumstances is automatic. But the most remarkable thing about this is that the response is not confined to the protoplasm actually affected, but is transmitted to that nearest to the part stimulated, and again passed on to that beyond, so that a wave of excitation passes through the whole mass, not stopping till it has reached the extreme confines of the cell. It may even pass beyond these and set up activity in neighbouring cells. The power of conductivity once grasped, it may easily be seen that certain cells, by specializing in this direction and adapting their shape to the needs of the body, might by throwing out long threads to reach distant parts set up an organic system of telegraphy.

The organs developed for the control of the body owe their origin to the outer layer. ([See Diagram 5.]) This was only to be expected. In [the second essay], in which we treated of the chemistry of the body, we, of course, touched upon all three layers from which the body is built up; but the one which chiefly occupied our attention was the innermost layer, which is so admirably arranged as a chemical laboratory. In [the third essay] we dealt chiefly with the middle layer, which both by its position and its bulk might have been guessed to be the foundation of most of the motor organs. Now that we have come to the organs of perception and transmission of impressions, it is only natural to expect that they should be specialized from the cells already in contact with the external world, and which, since they form the envelope of the animal, must allow all such stimuli as reach the subjacent motor layer to pass through them.

Hitherto we have not dealt at great length with the development of the organs whose functions we have been describing, either from the point of view of the embryologist or the evolutionist. Nor have we spent much time upon their gross anatomy. With the nervous system we must proceed rather differently; for to understand how its higher functions can be performed they must be traced from their origin step by step, while their complexity is largely vested in the structure of special organs.

The way in which the nervous system was evolved is shown in [Diagram 5]. Originally, no doubt, the cells of the outer layer, when the latter was in its simplest form—that is to say, only one cell thick, not several, as it is in our skin—would, when influenced in any way directly call forth the activity of the motor cells lying beneath them. ([See Diagram 40, Fig. 1.]) In [Fig. 2], however, we see one cell of the outer layer becoming specialized. It has thrown out a process above the surface of the skin the more readily to catch impressions, and has sent another down into the body the better to distribute them. [Diagram 41, Fig. 1] shows the nerve cell at a further stage. The principle is the same, but the cell is removed to a safer place. In [Fig. 2] it is not exposed to the outside world at all, but by receiving its impulses second-hand from several cells the same work is done with greater economy and uniformity. Some of the special sense organs are still developed in this way.

Diagram 40.—Showing Origin of a Nerve Cell.