CHAP, v.] NUTRITION. 853 



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 animal with wrappings of cotton- 

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

 augments and exposure to lower temperatures diminishes meta- 

 bolism; 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 spinal bulb. 

 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 

 the skin leads to a decrease, of the muscular metabolism. Further, 

 the centre of this thermotaxic reflex mechanism appears to be 

 placed somewhere in the nervous system above the spinal cord. 

 When urari is given, the reflex chain is broken at its muscular 

 end; when the spinal cord is divided the break is nearer the 

 centre. Whether we should conclude that the working of this 

 reflex mechanism is of such a kind that cold to the skin excites 

 the centre to a heat-producing activity, or of such a kind that 

 warmth to the skin inhibits a previously existing automatic 

 activity of the centre, may be left for the present undetermined. 



We may add. that the muscular metabolism which thus helps 

 to regulate temperature need not involve visible muscular con- 



