820 HIBERNATION. [BOOK n. 



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 

 adapted to undergo the change at the appointed season, and a 

 dormouse may fall into winter sleep at a temperature in winter 

 higher than that at which it awakes in spring. 



The phenomena of the hibernating mammal may be described 

 as those due to a lower rate of metabolism and hence to lowered 

 activity of the tissues in general. The heart beats very slowly, 

 and each beat is at best of but moderate strength ; and the breaths 

 are few, feeble and far between. Respiration and circulation are 

 thus going on, but go on so to speak at almost the slowest possible 

 rate consistent with the continuance of the working of the economy. 

 The breaths are, as we have said, few and far between, but they 

 suffice to carry to the tissues the small amount of oxygen which 

 these need and to cany off the small amount of carbonic acid 

 which they produce. So small is the respiration of the tissues that 

 in the depths of the winter sleep the venous blood is almost as 

 bright as the arterial, the colour of which is nearly normal. And 

 the^small amount of destructive katabolic changes which is going 

 on is shewn by a change in the respiratory quotient; oxygen is 



