CHAP, v.] NUTRITION. 805 



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 



Erobably be taken as under rather than over the true amount, 

 n any case they are to be regarded as furnishing nothing more 

 than a rough average, the exact amount varying according to the 

 size, the weight, and the condition of the individual, as well as 

 according to variations in circumstances. 



529. The Energy of Mechanical Work. We have already 

 in treating of muscle and elsewhere partly discussed this subject, 

 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 as 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 ( 63) 

 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 



