ceivable that during work the proteine is decomposed completely into carbonic acid, water, and urea, and that thus the latent energy which would otherwise be stored up in the fat is applied to the production of motion. If this were shown to be the case (and it seems not improbable that something similar to it actually takes place), it would become largely a question of nomenclature whether we should regard the proteine or the fat which is formed from it as the source of muscular power. For ourselves, we believe that the truth will eventually be found to lie between the two extreme views now advocated, and that muscular force will prove to have some such origin as that above indicated.
At the same time there are certain facts immediately to be considered which show that the process is by no means so simple as that just sketched.
If we turn from the study of the effects of muscular exertion to that of its conditions, we shall get much new light, and be helped to a more rational judgment of the theories as to its source. Presupposing the existence of a healthy and well-developed organism, we may specify four conditions as, from our point of view, the most important:
1. The facts of common experience appear to show unmistakably that a liberal supply of proteine in the food is one of the conditions of any sustained muscular exertion. This, however, does not necessitate the conclusion that the proteine is the source of the power exerted: its decomposition, as we have seen, goes on independently of muscular exertion, and may be regarded as simply one of the conditions of the healthy activity of the muscles.
2. The largely increased excretion of carbonic acid and water during work indicates a necessity for a liberal supply also of the nonnitrogenous constituents of food. At need, however, this demand may be supplied by the albuminoids of the latter, or perhaps by fat already formed in the body.
3. An essential condition of continued activity of the muscles is the constant removal from them by the circulation of the chemical products of their action. Certain of these products, notably lactic acid and acid potassium phosphate, if allowed to accumulate in the muscle, produce the sensation of weariness, and shortly incapacitate it for further action. If they be removed, either by the blood or by injection of a weak salt solution, the muscle is again capable of work; while, if they be injected into a fresh muscle, they produce the same effect as if naturally formed there. The same or similar processes go on in the muscle after death, and the rigor mortis is caused by the solidification of the jelly-like myosin, which is also one of the products of the action.
4. A most important condition of muscular activity is found in the capacity which the body has to store up oxygen in itself during sleep, to be used later in the waking hours. This capacity was discovered by Voit and Pettenkofer in experiments on men, and has been con-
