CHAP, v.] NUTRITION. 843 



that when the muscle itself is examined, we find no proof of 

 nitrogenous waste, but, on the other hand, clear evidence of the 

 production of non-nitrogenous bodies, such as carbonic acid. And 

 when we ask the question, Does muscular exercise proportionately 

 increase the urea given off by the body as a whole ? for this, 

 according to the theory in question, it certainly ought to do, the 

 evidence we can obtain, though somewhat varying, gives on the 

 whole a .decidedly negative answer. 



In the majority of observations no marked change at all in the 

 amount was met with ; indeed in some cases there was a distinct 

 decrease, followed by an increase on the following days. Some 

 observers however found a very marked increase, and this was 

 especially the case when the subject under observation took a 

 large amount of food and performed very severe labour. On the 

 whole the various results obtained by different observers justify 

 the conclusion that exercise by itself, even when severe, does not 

 necessarily increase the amount of urea excreted, but that con- 

 ditions may obtain in which such an increase undeniably occurs. 

 We may draw the further conclusion that experiments of this 

 kind do not supply the right method for determining the point 

 at issue. It must be remembered that it is not the muscles 

 alone which feel the influence of the labour; the circulation and 

 indeed the whole body are affected by it. If we suppose a large 

 part or even only some part of the urea to come from other 

 than muscular metabolism, from changes in the hepatic cells for 

 instance, we should expect that these changes, and with them 

 the amount of urea discharged, would be influenced by labour, 

 especially by severe labour. 



In no case has a direct relation between the amount of labour 

 and amount of urea been observed. More than this, the following 

 experience lands us in an absurdity if we suppose the whole 

 energy of muscular work to arise from proteid metabolism. Two 

 observers performed a certain amount of work (an ascent of a 

 mountain) on a non-nitrogenous diet, and estimated the amount 

 of urea passed during the period. Assuming the urea to represent 

 the oxidation of so much proteid matter, which oxidation repre- 

 sented in turn so much energy set free, they found that whereas 

 the actual work done amounted to 129'026 and 148 ! 656 kilogram.- 

 kilometers, for each observer respectively, the total energy avail- 

 able from proteid metabolism during the period was in the case 

 of the first 68*69, and of the second 68'376 kilogram.-kilometers. 

 That is to say, the energy set free by the proteid metabolism of 

 the muscles engaged in the work was far less than the amount 

 necessary to accomplish the work actually done, to say nothing of 

 its having to provide as well for the movements of respiration and 

 circulation. Their muscular energy therefore must have had other 

 sources than proteid metabolism. 



That on the contrary the production of carbonic acid is at once 



542 



