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 considerable 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 uneventful 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.
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