638 SOURCES OF HEAT. [BOOK n. 



directed to this point it was found that a man in 24 hours con- 

 sumed 954 grms. oxygen and produced 1284 grms. carbonic acid 

 when doing work, as against 708 grms. oxygen consumed and 

 911 grms. carbonic acid produced when remaining at rest, the 

 quantity of urea secreted being in the first case 37 grms., in the 

 second 37*2 grms. 



It is evident that the conclusions arrived at by the statistical 

 method entirely corroborate those gained by an examination of 

 muscle itself, viz. that during muscular contraction the explosive 

 decomposition which takes place bears chiefly, if not exclusively, 

 on the non-nitrogenous constituents of the muscle, and that it 

 is the non-nitrogenous products which alone escape from the 

 muscle and from the body, any nitrogenous products which 

 result being retained within the muscle, or at least within the 

 body. We must therefore reject the second as well as the first 

 division of the views under discussion; not only is the muscle 

 not fed exclusively on proteid material, but also its energy does 

 not arise from an exclusively proteid metabolism. 



Animal Heat. 



424. The Sources and Distribution of Heat. We have 

 already seen that the conception of the non-nitrogenous por- 

 tions of food being solely calorifacient or respiratory proves to 

 be unfounded when we attempt to trace the history of the food 

 on its way through the body. The same view is still more 

 strikingly shewn to be inadequate when we study the manner in 

 which the heat of the body is produced. We may indeed at once 

 affirm that the heat of the body is generated by the chemical 

 changes, which we may speak of generally as those of oxidation, 

 undergone not by any particular substances, but by the tissues 

 at large. Wherever metabolism is going on, or to be more 

 exact wherever destructive metabolism, katabolism, is going on, 

 heat is being set free. In growth and in repair, in the deposi- 

 tion of new material, in the transformation of lifeless pabulum 

 into living tissue, in the constructive metabolism, the anabolism 

 of the body, and in the smaller synthetic processes of which we 

 spoke in dealing with urea ( 387), heat is undoubtedly to a 

 certain extent being absorbed and rendered latent: the energy 

 of the construction may be, in part at least, supplied by the 

 heat present. But all this, and more than this, viz. the heat 

 present in a potential form in the substances themselves so 

 built up into the tissue, is lost to the tissue during its destruc- 

 tive metabolism; so that the whole metabolism, the whole cycle 

 of changes from the lifeless pabulum through the living tissue 

 back to the lifeless products of vital action, is eminently a 

 source of heat. 



