636 UREA AND MUSCULAR WORK. [BOOK n. 



Various attempts have been made to ascertain the amount 

 of heat given out by the body in an indirect manner, as for 

 instance by calculating the heat given out by the oxidation of 

 the food. As trustworthy as any is the plan of simply sub- 

 tracting the normal daily mechanical expenditure from the 

 normal daily income. Thus, 150,000 k.-m. subtracted from 

 one million k.-m. gives 850,000 k.-m. as the daily expenditure 

 in the form of heat ; i.e. between one-fifth and one-sixth of the 

 total income is expended as mechanical labour, the remaining 

 four-fifths or five-sixths leaving the body in the form of heat. 

 The results given by direct calorimetric observations and by 

 other calculations give somewhat higher figures than these ; 

 and indeed these may probably be taken as under rather than 

 over the true amount. In any case they are to be regarded as 

 furnishing hardly more than a rough average estimate for a 

 man of average build and weight, taking an average amount 

 of average food and doing an average amount of work. 



423. The Energy of Mechanical Work. We have already 

 in treating of muscle and elsewhere partly discussed this sub- 

 ject, but may here say the rest that has to be said. 



The older writers, even after it had been proved that the 

 animal body was constructive so far as the formation of fat was 

 concerned, still held to the distinction between nitrogenous or 

 plastic and non-nitrogenous or respiratory food. Put broadly, 

 this view was that all the nitrogenous food went to build up the 

 proteid tissues, the muscular flesh and the like, and that the 

 nitrogenous egesta arose solely from the functional metabolism 

 of these tissues, while the non-nitrogenous food was used with 

 equal exclusiveness for respiratory or calorific purposes, being 

 either directly oxidized in the blood or, if present in excess, 

 stored up as fatty tissue. According to this view the two classes 

 of income corresponded exactly to the two forms of expenditure. 

 We have already urged several objections against this view. 

 We have seen that in the blood itself very little oxidation takes 

 place, that it is the active tissue, and not the passive blood- 

 plasma, which is the seat of oxidation. We have further seen 

 that proteid food may undoubtedly be, in the above sense, 

 respiratory and incidentally give rise to the storing-up of fat. 

 One division of the view is thereby overthrown. We have now 

 to inquire whether the other division holds good, whether 

 muscle and the other proteid tissues are fed exclusively on the 

 proteid material of food, and whether muscular energy comes 

 exclusively from the metabolism of the proteid constituents of 

 muscle. We have already seen ( 60) 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 





