6±6 REGULATION OF PRODUCTION OF HEAT. [Book ii. 



Taking then the consumption of oxygen, and 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 ani- 

 mals exposed to changes of temperature. In the cold-blooded 

 animal, cold diminishes and heat increases the metabolic activ- 

 ity of the body ; as the temperature to which the animal is 

 subjected rises or falls, so the consumption of oxygen and pro- 

 duction 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 overcom- 

 ing 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 a warm-blooded animal is poisoned by urari, the tem- 

 perature falls and the metabolism, measured by the consumption 

 of oxygen and the production of carbonic acid, sinks also; and 

 that the latter is the cause, not the effect, of the former is 

 shewn by the fact that the metabolism continues to fall though 

 loss of heat be prevented by surrounding the animal with wrap- 

 pings of cotton wool. In such a urarized animal, exposure to 

 higher temperatures augments and exposure to lower tempera- 

 tures diminishes metabolism; the urarized warm-blooded ani- 

 mal in fact behaves like a cold-blooded animal. Similar but 

 perhaps not such striking or so constant results are gained by 

 division of the spinal bulb. After this operation the tempera- 

 ture of the body sinks, and the fall, though partly due to 

 increased loss of heat by the skin, caused by the dilated condi- 

 tion of the cutaneous vessels, is also accompanied by diminished 

 metabolism and is therefore in part due to diminished produc- 

 tion of heat. And when an animal is in this condition, expo- 

 sure to higher temperatures increases and exposure to lower 

 temperatures diminishes the bodily metabolism. We can best 

 explain these results by supposing that, under normal condi- 

 tions, the muscles, which as we have seen contribute so largely 

 to the total heat of the body, are placed, by means of their 

 motor nerves and the central nervous system, in some special 

 connection with the skin, so that a lowering of the temperature 

 of the skin leads to an increase, while a heightening of the 

 temperature of the skin leads to a decrease, of the muscular 

 metabolism. Further, the centre of this thermotaxic reflex 



