CHAP, v.] NUTRITION. 807 



muscle itself, viz. that during muscular contraction the explosive 

 decomposition which takes place hears chiefly, it' 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. 



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 : 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 destructive 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. 



Of all the tissues of the body the muscles, not only from their 

 bulk, forming as they do so large a portion of the whole frame, but 

 also from the characters of their metabolism, must be regarded as 

 the chief sources of heat. 



In treating ( 65) of the thermal changes in muscle we have 

 seen that in the total energy expended in a muscular contraction, 

 the ratio of that which appears as heat to that which appears 

 as external work is variable. If we take a proportion which is 



