Physiology: The Science of the Body

by Ernest G. Martin

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

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

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

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

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

CHAPTER II
THE MAINTAINING OF LIFE

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

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

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

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