CHAP, v.] NUTRITION. 815 



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 

 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 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 animals with wrappings of cotton 

 wool. In such a urarized animal, exposure to higher temperatures 

 augments and exposure to lower temperatures diminishes metabol- 

 ism ; the urarized warm-blooded animal in fact behaves like a cold- 

 blooded animal. Similar but perhaps not such striking or so 

 constant results are gained by division of the medulla oblongata. 

 After this operation the temperature of the body sinks, and the 

 fall, though partly due to increased loss of heat by the skin, caused 

 by the dilated condition of the cutaneous vessels, is also accom- 

 panied by diminished metabolism and is therefore in part due to 

 diminished production of heat. And when an animal is in this 

 condition, exposure to higher temperatures increases and exposure 

 to lower temperatures diminishes the bodily metabolism. We can 

 best explain these results by supposing that, under normal con- 

 ditions, 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 



