466 ANIMAL HEAT. [BOOK n. 



mechanism or to the various secreting nervous mechanisms, seems 

 in itself d 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 increased production of carbonic 

 acid, and an increased consumption of oxygen, since it is to these 

 products, rather than to the nitrogenous factors, on the peculiar- 

 ities 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 production 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, Pfliiger, and his pupils, as well as 

 other observers, have 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 dimin- 

 ishes 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 a cold-blooded animal behaves in this 

 respect like a mixture of dead substances in a chemist's retort : 

 heat promotes and cold retards chemical action in both cases. 

 Very different is the behaviour of a warm-blooded animal. In 

 this case, within a lower and a higher limit, cold increases and heat 

 diminishes the bodily metabolism, as shewn by the increased or 

 diminished consumption of oxygen and production of carbonic 

 acid as the temperature falls or rises. In these animals there is 

 obviously a mechanism of some kind, counteracting and indeed 

 overcoming those more direct effects, which alone obtain in cold- 

 blooded animals. And that this mechanism is of a nervous nature, 

 is indicated by the following facts. 



When an animal is poisoned by urari, the temperature falls 

 and the metabolism, measured by the consumption of oxygen and 



