Removal of Waste.—The next step in this life process is one of difficulty. After the food and oxygen have reached the tissues it is seized by the living cell. The food material is now oxidized by the oxygen and its latent energy is liberated, and appears in the form of motion or heat or some other vital function. Herein is the really mysterious part of the life process; but for the present we will overlook the mystery of this action, and consider the results from a purely material standpoint.
In a steam engine the fundamental process by which the latent energy of the fuel is liberated is that of oxidation. The oxygen of the air unites with the chemical elements of the fuel, and breaks up that fuel into simple compounds—which may be chiefly considered as three—carbonic dioxide (CO2), water (H2O), and ash. The energy contained in the original compound can not be held by these simpler bodies, and it therefore escapes as heat. Just the same process, with of course difference in details, is found in the living machine. The food, after reaching the living cell, is united with the oxygen, and, so far as chemical results are concerned, the process is much the same as if it occurred outside the body. The food is broken into simpler compounds and the contained energy is liberated. The energy is, by the mechanism of the machine, changed into motion or nervous impulse, etc. The food is broken into simple compounds, which are chiefly carbonic dioxide, water, and ash; the ash being, however, quite different from the ash obtained from burning coal. Now the engine must have its chimney to remove the gases and vapours (the CO2 and H2O) and its ashpit for the ashes. In the same way the living machine has its excretory system for removing wastes. In the removal of the carbonic acid and water we have to do once more with the respiratory system, and the process is simply a repetition of the story of gas diffusion, chemical union, and osmosis. It is sufficient here to say that the process is just as simple and as easily explained as those already described. The elimination of these wastes is simply a problem of chemistry and mechanics.
In the removal of the ash, however, we have something more, for here again we are brought up against the vital action of the cell. This ash takes chiefly the form of a compound known as urea, which finds its way into the general circulatory system. From the blood it is finally removed by the kidneys. In the kidneys are a large number of bits of living matter (kidney cells), which have the power of seizing hold of the urea as the blood is flowing over them, and after thus taking it out of the blood they deposit it in a series of tubes which lead to the bladder and hence to the exterior. The bringing of this ash to the kidney cell is a mechanical matter, based simply upon the flow of the blood. The seizing of the urea by the kidney cell is a vital phenomenon which we must waive for the moment.
Up to this point in the analysis there has been no difficulty, and no one can fail to agree with the conclusions. The position we reach is as follows: So far as relates to the general problems of energy in the universe the body is a machine. It neither creates nor destroys energy, but simply transforms one form into another. In attempting to explain the action of the machine, we find that for the functions thus far considered (sometimes called the vegetative functions) the laws of chemistry and physics furnish adequate explanation.
We must now look a little further, and question some of the functions the mechanical nature of which is less obvious. The whole operation thus far described is under the control of the nervous system, which acts somewhat like the engineer of an engine. Can this phase of living activity be included within the conception of the body as a machine?
Nervous System.—When we come to try to apply mechanical principles to the nervous system, we meet with what seems at first to be no thoroughfare. While dealing with the grosser questions of chemical compounds, heat, and motion, there is little difficulty in applying natural laws to the explanation of living phenomena. But the problem with the nervous system is very different. It is only to-day that we are finding that the problem is open to study, to say nothing of solution. It is true that mental and other nervous phenomena have been studied for a long time, but this study has been simply the study of these phenomena by themselves without a thought of their correlation with other phenomena of nature. It is a matter of quite recent conception that nervous phenomena have any direct relation to the other realms of nature.
Our first question must be whether we can find any correlation between nervous energy and other types of energy. For our purpose it will be convenient to distinguish between the phenomena of simple nervous transmission and the phenomena of mental activity. The former are the simpler, and offer the greatest hope of solution. If we are to find any correlation between nervous energy and other physical energy, we must do so by finding some way of measuring nervous energy and comparing it with the latter. This has been very difficult, for we have no way of measuring a nervous impulse directly. In the larger experiments upon the income and outgo of the body, in the respiration apparatus mentioned above, nervous phenomena apparently leave no trace. So far as experiments have gone as yet, there is no evidence of an expenditure of extra physical energy when the nervous system is in action. This is not surprising, however, for this apparatus is entirely too coarse to measure such delicate factors.
That there is a correlation between nervous energy and physical energy is, however, pretty definitely proved by experiments along different lines. The first step in this direction was to find that a nervous stimulus can be measured at least indirectly. When the nerve is stimulated there passes from one end to the other an impulse, and the rapidity with which it travels can be accurately measured. When such an impulse reaches the brain it may give rise to a conscious sensation, and a somewhat definite estimation can be made of the amount of time required for this. The periods are very short, of course, but they are not instantaneous. The nervous impulse, can be studied in still other ways. We find that the impulse can be started by ordinary forms of energy. A mechanical shock, a chemical or an electrical shock will develop nervous energy. Now these are ordinary forms of physical energy, and if, when they are applied to a nerve, they give rise to a nervous stimulus, the inference is certainly a legitimate one that the nerve is simply a bit of machinery adapted to the conversion of certain kinds of physical energy into nervous energy. If this is the case, then it is necessary to regard nervous energy as correlated with other forms of energy.
Other facts point in the same direction. Not only can the nervous stimulus be developed by an electric shock, but the strength of the stimulus is within certain limits proportional to the strength of the shock which produces it. Again, not only is it found that an electrical shock can develop a nervous stimulus, but conversely a nervous stimulus develops electrical energy. In ordinary nerves, even when not active, slight electric currents can be detected. They are extremely slight, and require the most delicate instruments for their detection. Now when a nerve is stimulated these currents are immediately affected in such a way that under proper conditions they are increased in intensity. The increase is sufficient to make itself easily seen by the motion of a galvanometer. The motion of the galvanometer under these conditions gives a ready means of studying the character of the nervous impulse. By its use it can be determined that the nerve impulse travels along the nerve like a wave, and we can approximately determine the length and shape of the wave and its relative height at various points.
Now what is the significance of all these facts for our discussion? Together they point clearly to the conclusion that nervous energy is correlated with other forms of physical energy. Since the nervous stimulus is started by other forms of energy, and since it can, in turn, modify ordinary forms of energy, we can not avoid the conclusion that the nervous impulse is only a special form of energy developed within the nerve. It is a form of wave motion peculiar to the nerve substance, but correlated with and developed from other types of energy. This, of course, makes the nerve simply a bit of machinery.