THE ENERGY OF THE BODY. 495 



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

 total income is expended as mechanical labor, 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 

 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. 



443. The energy of mechanical ivork. 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 nitro- 

 genous egesta arose solely from the functional metabolism of these tissues, 

 while the non-nitrogenous food was used with equal exclusiveuess for respira- 

 tory 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 exam- 

 ined, 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 

 labor. 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 labor ; 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 metab- 

 olism, 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 labor, especially by severe labor. 



