CHAP, v.] NUTRITION. 645 



are always going on; in an ordinary active body a considerable 

 quantity of heat must be thus generated. Hence the more 

 active the body the greater the production of heat. As we 

 stated before, in a contraction the proportion of the energy set 

 free to do work to that set free as heat appears to vary under 

 different circumstances ; and the increase of heat due to labour 

 probably varies in a corresponding way. The details of this 

 relation have yet to be worked out, but we may at least conclude 

 that, when a man pushes his daily labour beyond the 150,000 

 k.m., the additional energy thus leaving his body as work done 

 is not taken out of the 850,000 k.m. given in 422 as the aver- 

 age daily output of heat, but the total setting free of energy 

 and the total production of heat is at the same time in- 

 creased. 



428. The production of heat thus determined by these 

 several influences, some of which are themselves regulated by 

 the nervous system, is further regulated in a remarkable manner. 

 For it is not solely by variations in the loss of heat that the con- 

 stant temperature of the warm-blooded animal is maintained. 

 Variations in the amount of heat actually generated in the body 

 constitute an important factor not only in the maintenance of 

 the normal temperature, but also in the production of the abnor- 

 mally high or low temperatures of various diseases. Many con- 

 siderations have long led physiologists to suspect the existence 

 of a nervous mechanism by which afferent impulses arising in 

 the skin or elsewhere might through the central nervous system 

 originate efferent impulses whose effect would be to increase or 

 to diminish the metabolism of the muscles or other organs, and 

 thus to increase or diminish the amount of heat generated for 

 the time being in the body. And we have experimental evi- 

 dence that such metabolic or thermogenic nervous mechanism, 

 comparable in many respects to the vaso-motor mechanism or 

 to the various secreting nervous mechanisms, does really 

 exist. 



The warm-blooded animal is distinguished from the cold- 

 blooded animal by the fact that when it is exposed to cold or 

 heat, it does not like the latter become colder or hotter, as the 

 case may be, but, within certain limits, maintains its normal tem- 

 perature. If the maintenance of the temperature of the warm- 

 blooded animal during exposure to cold is assisted by an increased 

 production of heat and is not due simply to a diminished loss, we 

 ought to find evidence of an increased metabolism during that 

 exposure. We ought to find under these circumstances an in- 

 creased production of carbonic acid, and an increased consump- 

 tion of oxygen, since it is to these products, rather than to the 

 nitrogenous factors, on the peculiarities of which as uncertain 

 signs of metabolism we have already insisted, we must look for 

 indications of the rise or fall of metabolic activity. 



