858 HIBERNATION. [BOOK n. 



register this or a higher temperature while remaining alive, to say 

 nothing of shewing no tokens of distress, entails the supposition 

 that such a body can differ from its fellows in its absolutely 

 fundamental qualities, and yet make no other sign. 



539. Effects of Great Cold. The effects of a too great lower- 

 ing of the temperature of the body, which is generally the result 

 of too great external cold and rarely if ever arises from internal 

 causes lowering the metabolism and thus the production of heat, 

 are in their origin the reverse of those of a too high temperature. 

 The metabolism of the tissues is lowered ; and not only are the 

 katabolic changes which lead to the setting free of energy thus 

 affected, but the anabolic changes also share in the depression. 

 The " living substance" falls to pieces less readily, but is also 

 made up less readily; and could this slackening of metabolism 

 be carried on in the several tissues at a rate proportionate to 

 the rate at which each tissue lives, life might thus be brought 

 to a peaceful end by gradual arrest of the life of each part 

 of the whole body. And indeed in some cases, where the lowering 

 of the temperature takes place gradually, something like this 

 does occur even in warm-blooded animals. The diminished meta- 

 bolism tells first and chiefly on the central nervous system, 

 especially on the brain and more particularly on those parts 

 of that organ which are concerned in consciousness. The intrinsic 

 lowering of the cerebral metabolism is further assisted by a slowing 

 of the heart-beat and of the breath, drowsiness is succeeded by a 

 condition very like to, if not identical with, that known as sleep, 

 which we shall study later on, but by a sleep which insensibly passes 

 into the sleep of death. In some cases, however, especially those 

 in which the lowering of the temperature is sudden and rapid, 

 disorders of the nervous system intervene, and convulsions like 

 those of asphyxia are produced. 



540. Hibernation. In the majority of warm-blooded animals 

 the conditions thus induced by cold are rapidly fatal, and moreover 

 in their progress very soon reach a stage from which recovery 

 becomes impossible. In the case of some few animals, scattered 

 members of several groups of mammalia, a similar depression of 

 metabolism by cold is of yearly occurrence, taking place regularly 

 as the external temperature falls in winter, and being thrown off 

 regularly as the external temperature rises in spring. Such 

 animals are spoken of as hibernating animals. 



We are not able at present to explain why these animals behave 

 in this way. It is obvious that for some reason they lack that 

 power of reaction against external cold which, as we have seen, is 

 one of the characteristics of the warm-blooded animal, but we 

 cannot state what is the difference in their economy which leads to 

 this lack. The 'winter sleep' is undoubtedly due to the cold of 

 winter, and may in some cases at all events be induced by cold 

 produced artificially in summer; but the system is predisposed and 



