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.