604 MOTION. 



portion to the amount of muscular work performed. It 

 has been found, however, both that the theory itself is 

 erroneous, and that the supposed facts on which it was 

 founded do not exist. 



It is true that in the action of muscles, as of all other 

 parts, there is a certain destruction of tissue or, in other 

 words, a certain l wear and tear/ which may be repre- 

 sented by a slight increase in the quantity of urea excreted : 

 but it is not the correlative expression (p. 9) or only source 

 of the power manifested. The increase in the amount of 

 urea which is excreted after muscular exertion is by no 

 means so great as was formerly supposed; indeed, it is 

 very slight. And as there is no reason to believe that the 

 waste of muscle-substance can be expressed, with unim- 

 portant exceptions, in any other way than by an increased 

 excretion of urea, it is evident that we must look 

 elsewhere than in destruction of muscle, for the source of 

 muscular action. For, it need scarcely be said, all force 

 manifested in the living body must be the correlative 

 expression of force previously latent in the food eaten or 

 the tissue formed ; and evidences of force expended in the 

 body must be found in the exereta. If, therefore, the 

 nitrogenous excreta, represented chiefly by urea, are not in 

 sufficient quantity to account for the work done, we must 

 look to the non-nitrogenous excreta as carbonic acid and 

 water, which, presumably, cannot be the expression of 

 wasted muscle- substance. 



The quantity of these non-nitrogenous excreta is 

 undoubtedly increased by active muscular efforts, and 

 to a considerable extent ; and whatever may be the 

 source of the water, the carbonic acid, at least, is the 

 result of chemical action in the system, and especially 

 of the combustion of non-nitrogenous food, although, 

 doubtless, of the carbon element in nitrogenous food also. 

 We are, therefore, driven to the conclusion, that the 

 substance of muscles is not wasted in proportion to< the 



