814 REGULATION OF PRODUCTION OF HEAT. [BOOK 11. 



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 528 as the average daily out- 

 put of heat, but the total setting free of energy and the total 

 production of heat is at the same time increased. And it need 

 hardly be said that the figures in question give only an average 

 estimate for a man of average build and weight, taking an average 

 amount of average food and doing an average amount of work. 



534. 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 



constant 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 



abnormally high or low temperatures of various diseases. Many 



(considerations have long led physiologists to suspect the existence 



of a nervous mechanism by which affnvnt 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. The existence in fact of a metabolic 



or therm ogenic nervous mechanism, comparable in many respects 



to the vaso-motor mechanism or to the various secreting nervous 



mechanisms, seems in itself <i priori probable. And we have 



experimental evidence that such a mechanism 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 

 temperature. 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 consumption 

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

 genous 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. Of these two, the produc- 

 tion of carbonic acid and the consumption of oxygen, the latter is 

 the more important and trustworthy measure of metabolism, 

 especially when , observations are made for short periods only at a 

 time ; for as we have seen in treating of respiration the exit of 

 carbonic acid is more closely dependent on the action of the 



