80 SEVEN-YEAR SLEEPEES 



he has grown fat enough to supply bear's grease to all 

 Christendom. Then he hunts himself out a hollow tree or 

 rock-shelter, curls himself up quietly to sleep, and snores 

 away the whole livelong winter. During this period of 

 hibernation, the action of the heart is reduced to a minimum, 

 and the bear breathes but very slowly. Still, he does 

 breathe, and his heart does beat ; and in performing those 

 indispensable functions, all his store of accumulated fat is 

 gradually used up, so that he wakes in spring as thin as a 

 lath and as hungry as a hunter. The machine has been 

 working at very low pressure all the winter : but it has 

 been working for all that, and the continuity of its action 

 has never once for a moment been interrupted. This is the 

 central principle of all hibernation ; it consists essentially 

 of a very long and profound sleep, during which all muscular 

 motion, except that of the heart and lungs, is completely 

 suspended, while even these last are reduced to the very 

 smallest amount compatible with the final restoration of 

 full animal activity. 



Thus, even among warm-blooded animals like the bears 

 and dormice, hibernation actually occurs to a very con- 

 siderable degree ; but it is far more common and more 

 complete among cold-blooded creatures, whose bodies do 

 not need to be kept heated to the same degree, and with 

 whom, accordingly, hibernation becomes almost a complete 

 torpor, the breathing and the action of the heart being still 

 further reduced to very nearly zero. Mollusks in particular, 

 like oysters and mussels, lead very monotonous and un- 

 eventful lives, only varied as a rule by the welcome change 

 of being cut out of their shells and eaten alive ; and their 

 powers of living without food under adverse circumstances 

 are really very remarkable. Freshwater snails and mussels, 

 in cold weather, bury themselves in the mud of ponds or 

 rivers ; and land-snails hide themselves in the ground or 



