820 THE POPULAR SCIENCE MONTHLY. 



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 improb- 

 able that something similar to it actually takes place), it would become 

 largely a question of nomenclature whether we should regard the pro- 

 teine 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 indi- 

 cated. 



At the same time there are certain facts immediately to be consid- 

 ered 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 non- 

 nitrogenous 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 prod- 

 ucts 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 fur- 

 ther 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 Pettcnkofer in experiments ' on men, and has been con- 



