Chap, v.] NUTRITION. 637 



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 dis- 

 tinct 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 conditions 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 follow- 

 ing 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 represented 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 respec- 

 tively, the total energy available from proteid metabolism dur- 

 ing 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 actuall}^ 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 muscular exercise at once and largely 

 increases the production of carbonic acid is beyond all doubt. 

 One hour's hard labour will increase fivefold the quantity of 

 carbonic acid given off within the hour. And in an experiment 



