The Story of the Living Machine

The four properties of irritability, contractibility, assimilation, and reproduction, belong to these vital units—the cells, and it is these properties which we are trying to trace to their source as a foundation of vital activity. We may first ask whether we have any facts which indicate that any special parts of the cell are associated with any of these fundamental activities. The first fact that stands out clearly is that the nucleus is connected most intimately with the process of reproduction and especially with heredity. This has long been believed, but has now been clearly demonstrated by the experiments of cutting into fragments the cell bodies of unicellular animals. As already noticed, those pieces which possess a nucleus are able to continue their life and reproduce themselves, while those without a nucleus are incapable of reproduction. With greater force still is the fact shown by the process of fertilization of the egg. The egg is very large and the male reproductive cell is very small, and the amount of material which the offspring derives from its mother is very great compared with that which it derives from its father. But the child inherits equally from father and mother, and hence we must find the hereditary traits handed down in some element which the offspring obtains equally from father and mother. As we have seen (Figs. 34-44), the only element which answers this demand is the nucleus, and more particularly the chromosomes of the nucleus. Clearly enough, then, we must look upon the nucleus as the special agent in reproduction of cells. Again, we have apparently conclusive evidence that the nucleus controls that part of the assimilative process which we have spoken of as the constructive processes. The metabolic processes of life are both constructive and destructive. By the former, the material taken into the cell in the form of food is built up into cell tissue, such as linin, microsomes, etc., and, by the latter, these products are to a greater or less extent broken to pieces again to liberate their energy, and thus give rise to the activities of the cell. If the destructive processes were to go on alone the organism might continue to manifest its life activities for a time until it had exhausted the products stored up in its body for such purposes, but it would die from the lack of more material for destruction. Life is not complete without both processes. Now, in the life of the cell we may apparently attribute the destructive processes to the cell substance and the constructive processes to the nucleus. In a cell which has been cut into fragments those pieces without a nucleus continue to show the ordinary activities of life for a time, but they do not live very long (Fig. 25). The fragment is unable to assimilate its food sufficiently to build up more material. So long as it still retains within itself a sufficiency of already formed tissue for its destructive metabolism, it can continue to move around actively and behave like a complete cell, but eventually it dies from starvation. On the other hand, those fragments which retain a piece of the nucleus, even though they have only a small portion of the cell substance, feed, assimilate, and grow; in other words, they carry on not only the destructive but also the constructive changes. Plainly, this means that the nucleus controls the constructive processes, although it does not necessarily mean that the cell substance has no share in these constructive processes. Without the nucleus the cell is unable to perform those processes, while it is able to carry on the destructive processes readily enough. The nucleus controls, though it may not entirely carry on, the constructive metabolism. It is equally clear that the cell substance is the seat of most of the destructive processes which constitute vital action. The cell substance is irritable, and is endowed with the power of contractility. Cell fragments without nucleii are sensitive enough, and can move around as readily as normal cells. Moreover, the various fibres which surround the centrosomes in cell division and whose contractions and expansions, as we have seen, pull the chromosomes apart in cell division, are parts of the cell substance. All of these are the results of destructive metabolism, and we must, therefore, conclude that destructive processes are seated in the cell substance. The centrosome is too problematical as yet for much comment. It appears to be a piece of the machinery for bringing about cell division, but beyond this it is not safe to make any statements. In brief, then, the cell body is a machine for carrying on destructive chemical changes, and liberating from the compounds thus broken to pieces their inclosed energy, which is at once converted into motion or heat or some other form of active energy. This chemical destruction is, however, possible only after the chemical compounds have become a part of the cell. The cell, therefore, possesses a nucleus which has the power of enabling it to assimilate its food—that is, to convert it into its own substance. The nucleus further contains a marvellous material—chromatin—which in someway exercises a controlling influence in its life and is handed down from one generation to another by continuous descent. Lastly, the cell has the centrosome, which brings about cell division in such a manner that this chromatin material is divided equally among the subsequent descendants, and thus insures that the daughter cells shall all be equivalent to each other and to the mother cell. We must therefore look upon the organic cell as a little engine with admirably adapted parts. Within this engine chemical activity is excited. The fuel supplied to the engine is combined by chemical forces with the oxygen of the air. The vigour of the oxidation is partly dependent upon temperature, just as it is in any other oxidation process, and is of course dependent upon the presence of fuel to be oxidized, and air to furnish the oxygen. Unless the fuel is supplied and the air has free access to it, the machine stops, the cell dies. The energy liberated in this machine is converted into motion or some other form. We do not indeed understand the construction of the machine well enough to explain the exact mechanism by which this conversion takes place, but that there is such a mechanism can not be doubted, and the structure of the cell is certainly complex enough to give plenty of room for it. The irritability of the cell is easily understood; for, since it is made of very unstable chemical compounds, any slight disturbance or stimulation on one part will tend to upset its chemical stability and produce reaction; and this is what is meant by irritability. Or, again, we may look upon the cell as a little chemical laboratory, where chemical changes are constantly occurring. These changes we do not indeed understand, but they are undoubtedly chemical changes. The result is that some compounds are pulled to pieces and part of the fragments liberated or excreted, while other parts are retained and built into other more complex compounds. The compounds thus manufactured are retained in the cell body, and it grows in bulk. This continues until the cell becomes too big, and then it divides. If a machine is broken it ceases to carry on its proper duties, and if the parts are badly broken it is ruined. So with the cell. If it is broken by any means, mechanical, thermal, or otherwise, it ceases to run—we say it dies. It has within itself great power of repairing injury, and therefore it does not cease to act until the injury is so great as to be beyond repair. Thus it only stops its motion when the machinery has become so badly injured as to be beyond hope of repair, and hence the cell, after once ceasing its action, can never resume it again. There are, of course, other functions of living things besides the few simple ones which we have considered. But these are the fundamental ones; and if we can reduce them to an intelligible explanation, we may feel that we have really grasped the essence of life. If we understand how the cell can move and grow and reproduce itself, we may rest assured that the other phenomena of life follow as a natural consequence. If, therefore, we have obtained an understanding of these fundamental vital phenomena, we have accomplished our object of comprehending the life phenomena in our chemical and mechanical laws. But have we thus reduced these fundamental phenomena to an intelligible explanation? It must be acknowledged that we have not. We have reduced them to the action of chemical forces acting in a machine. But the machine itself is unintelligible. The organic cell is no more intelligible to us than is the body as a whole. The chemical understanding which we thought we had a few years ago in protoplasm has failed us, and nothing has taken its place We have no conception of what may be the primitive life substance. All we can say is that this most marvellous of all natural phenomena occurs only within that peculiar piece of machinery which we call the cell, and that it is the result of the action of physical forces in that machine. How the machine acts, or even the structure of the machine, we are as far from understanding as we were fifty years ago. The solution has retreated before us even faster than we have advanced toward it. Summary.—We may now notice in a brief summary the position which we have reached. In our attempt to explain the living organism on the principle of the machine, we are very successful so far as secondary problems are concerned. Digestion, circulation, respiration, and motion are readily solved upon chemical and mechanical principles. Even the phenomena of the nervous system are, in a measure, capable of comprehension within a mechanical formula, leaving out of account the purely mental phenomena which certainly have not been touched by the investigation. All of these phenomena are reducible to a few simple fundamental activities, and these fundamental activities we find manifested by simple bits of living matter unincumbered by the complicated machinery of organisms. With the few fundamental properties of these bits of organic matter we can construct the complicated life of the higher organism. When we come, however, to study these simple bits of matter, they prove to be anything but simple bits of matter. They, too, are pieces of complicated mechanism whose action we do not even hope to understand. That their action is dependent upon their machinery is evident enough from the simple description of cell activity which we have noticed. That these fundamental vital properties are to be explained as the result of chemical and mechanical forces acting through this machinery, can not be doubted. But how this occurs or what constitutes the guiding force which corresponds to the engineer of the machine, we do not know. Thus our mechanical explanation of the living machine lacks a foundation. We can understand tolerably well the building of the superstructure, but the foundation stones upon which that structure is built are unintelligible to us. The running of the living machine is thus only in part understood. The living organism is a machine or, it is better to say, it is a series of machines one within the other. As a whole it is a machine, and its parts are separate machines. Each part is further made up of still smaller machines until we reach the realm of the microscope. Here still we find the same story. Even the parts formerly called units, prove to be machines, and when we recognize the complexity of these cells and their marvellous activities, we are ready to believe that we may find still further machines within. And thus vital activity is reduced to a complex of