844 UREA AND MUSCULAR WORK. [BOOK n. 



and largely increased by muscular exercise 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 

 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. By this of course is not meant 

 that an exclusively proteid diet cannot fully supply muscular 

 energy. For instance a dog may be kept for a long period on 

 a meat diet as free as possible from fat and glycogen and be made 

 to do a very large amount of work on that diet without losing 

 weight or in any way suffering. 



Animal Heat. 



530. The Sources and Distribution of Heat. We have already 

 seen that the conception of the non-nitrogenous portions 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 deposition of new material, in the 

 transformation of lifeless pabulum into living tissue, in the con- 

 structive metabolism, the anabolism of the body, and in the smaller 

 synthetic processes of which we spoke in dealing with urea 

 ( 489), heat is undoubtedly to a certain extent being absorbed and 

 rendered latent ; and the energy of the construction may be, in part at 

 least, supplied by the heat which is being generated by destructive 



