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.