852 REGULATION OF PRODUCTION OF HEAT. [BOOK n. 



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 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. The existence in fact of a metabolic 

 or thermogenic nervous mechanism, comparable in many respects 

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

 mechanisms, seems in itself a 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 

 respiratory mechanism than is the income of oxygen, and carbonic 

 acid can be retained in loose combination and so temporarily stored 

 up by various constituents of the body. 



Taking then the consumption of oxygen, and though with 

 less confidence the production of carbonic acid, as a measure of 

 metabolic activity and so of heat-production, it has been shewn 

 that a marked contrast in this respect exists between cold-blooded 

 and warm-blooded animals exposed to changes of temperature. In 

 the cold-blooded animal, cold diminishes and heat increases the 

 metabolic activity of the body; as the temperature to which the 

 animal is subjected rises or falls, so the consumption of oxygen and 

 production of carbonic acid is increased or lessened. The body of 



