HIBERNATION 335 



mammals there seems to be a suspension of the normal 

 " warm-bloodedness," or the power of retaining an almost 

 constant body-temperature. To appreciate this, a short 

 digression is necessary. The animal heat of the body is 

 mainly due to the chemical processes involved in the 

 contraction and life (apart from contraction) of muscle, 

 the secretion of glands, the oxidations in the blood, and 

 even in the work of the brain. There is necessarily a 

 similar production of heat in the cold-blooded animals, 

 but they have no physiological adjustment for con- 

 serving what they produce. They are thus dependent 

 on the temperature of the surrounding medium, towards 

 which they always approximate. In warm - blooded 

 animals, however, there is a somewhat intricate nervous 

 mechanism which adjusts income and expenditure of heat, 

 regulating both the heat-production and the heat-loss. 

 The passive muscles, for instance, can be incited if need 

 be to produce more heat ; or, on the other hand, a dilation 

 of blood-vessels and a flow of sweat may cause the animal 

 to lose more heat. And this automatic regulation is so 

 smooth in its working that the body-temperature of birds 

 and of mammals remains constant, year in year out, 

 except in peculiar cases, such as the fledgling in the 

 deserted nest which has not got its thermotaxic mechanism 

 established, or the fevering animal in which the mechanism 

 goes out of gear, or the birds among the snow which have 

 their regulating mechanism worn out by the hopeless 

 attempt to respond to the prolonged exposure, or, lastly, 

 the hibernating mammals. 



In the hibernating mammal, what happens is, that 

 as the outer world gets colder and colder, the heat-regulat- 

 ing mechanism ceases to act, and the creature is saved 

 from the collapse consequent on a certain defeat, by be- 

 coming temporarily cold-blooded (or poikilothermal). Its 



