502 NUTRITION. 



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 in- 

 crease 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 vasomotor mechan- 

 ism 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, main- 

 tains 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, there should be 

 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 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. 



Taking 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 shown 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 in- 

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

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

 tion 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 chem- 

 ist's retort ; heat promotes and cold retards chemical action in both cases. 

 Very different is the behavior of a warm-blooded animal. In this case, within 

 a lower and a higher limit, cold increases and heat diminishes the bodily 

 metabolism, as shown 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, the more direct effects which alone obtain in cold- 

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

 cated by the following facts : 



When a warrn-blooded animal is poisoned byurari,the temperature falls 

 and the metabolism, measured by the consumption of oxygen and the pro- 

 duction of carbonic acid, sinks also ; and that the latter is the cause, not the 

 effect, of the former is shown by the fact that the metabolism continues ta 

 fall though loss of heat be prevented by surrounding the animals with wrap- 

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

 peratures augments and exposure to lower temperatures diminishes metab- 

 olism; 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 tem- 

 perature 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 accompanied 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 tempera- 



