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

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

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

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

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

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

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

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

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

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

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