20 PHYSIOLOGY CHAP. 



which, as we have already seen, all the important functions are 

 actively carried on, though in a less intense form than under 

 ordinary normal conditions. 



Even among the mammals, which are generally termed warm- 

 blooded, there are many species which, when placed in a cold 

 environment, do not maintain their usual temperature, but 

 become as cold as reptiles, amphibians, and fish, and can bear this 

 lowered temperature without injury ; this low bodily temperature, 

 which would prove fatal to most of the higher animals, only 

 produces a more or less profound state of torpor. The term 

 hibernating is applied to such animals, because regularly every 

 winter they fall into a deep lethargic sleep or torpor. 



The torpor of hibernation differs in degree. It is most pro- 

 found in the small animals, such as the bat, hedgehog, marmot, 

 dormouse, and hamster. Brown bears fall into a less sound sleep 

 during the winter, but white polar bears, which are carnivorous, 

 keep awake even in winter. 



As a rule the hibernating mammals are represented by a group 

 of animals which live on insects, fruits, or other similar foods in 

 a country in which these foods are not procurable in winter. 

 They therefore fast in winter, and fall into a state of torpor, curling 

 themselves up in sheltered spots, and reducing all their functions, 

 especially the combustion of material, to the lowest possible 

 limit, so that they can live for a long time by consuming the fat 

 which they accumulated during the favourable season. During 

 torpor, breathing is suspended or very slow, one or two respirations 

 in five minutes ; the internal temperature falls to about 4 C. At 

 long intervals they awake, come out to discharge their excreta, 

 then return to their hiding-places and fall asleep again. Valentin 

 noticed the interesting fact that the temperature rises when they 

 awake. 



It does not appear unlikely that the hibernating mammals 

 have been evolved by a gradual process of adaptation to an 

 environment which did not afford them food in winter. The 

 animals which are unable to endure a reduction of respiration 

 sufficient to enable them to live without food till the spring, either 

 emigrate or die. The only ones to survive are those endowed 

 with this capacity, which becomes greater and greater in their 

 descendants. 1 



Man under certain special conditions does not appear to be 

 wholly devoid of this peculiar capacity of hibernating animals, 

 the power to reduce his metabolic activities to such a degree that 

 he becomes lethargic and remains in a fasting condition for a 

 marvellously long time. This is said to be the case with the 



1 Another view is that hibernating mammals have retained in greater measure 

 those characteristics of their cold-blooded ancestors which are found in all young 

 mammals and birds. [Note by Editor.] 



