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And this leads us on to consider what in reality hibernation is.
Everybody knows nowadays, I suppose, that there is a very close analogy
between an animal and a steam-engine. Food is the fuel that makes the
animal engine go; and this food acts almost exactly as coal does in the
artificial machine. But coal alone will not drive an engine; a free
draught of open air is also required in order to produce combustion.
Just in like manner the food we eat cannot be utilised to drive our
muscles and other organs unless it is supplied with oxygen from the air
to burn it slowly inside our bodies. This oxygen is taken into the
system, in all higher animals, by means of lungs or gills. Now, when we
are working at all hard, we require a great deal of oxygen, as most of
us have familiarly discovered (especially if we are somewhat stout) in
the act of climbing hills or running to catch a train. But when we are
doing very little work indeed, as in our sleeping hours, during which
muscular movement is suspended, and only the general organic life
continues, we breathe much more slowly and at longer intervals. However,
there is this important difference (generally speaking) between an
animal and a steam-engine. You can let the engine run short of coals and
come to a dead standstill, without impairing its future possibilities of
similar motion; you have only to get fresh coals, after weeks or months
of inaction, and light up a fresh fire, when your engine will
immediately begin to work again, exactly the same as before. But if an
animal organism once fairly runs down, either from want of food or any
other cause–in short, if it dies–it very seldom comes to life again.

I say ‘very seldom’ on purpose, because there are a few cases among the
extreme lower animals where a water-haunting creature can be taken out
of the water and can be thoroughly dried and desiccated, or even kept
for an apparently unlimited period wrapped up in paper or on the slide
of a microscope; and yet, the moment a drop of water is placed on top of
it, it begins to move and live again exactly as before. This sort of
thorough-going suspended animation is the kind we ought to expect from
any well-constituted and proper-minded toad-in-a-hole. Whether anything
like it ever really occurs in the higher ranks of animal life, however,
is a different question; but there can be no doubt that to some slight
extent a body to all intents and purposes quite dead (physically
speaking) by long immersion in water–a drowned man, for example–may
really be resuscitated by heat and stimulants, applied immediately,
provided no part of the working organism has been seriously injured or
decomposed. Such people may be said to be pro tem. functionally,
though not structurally, dead. The heart has practically ceased to beat,
the lungs have ceased to breathe, and physical life in the body is
temporarily extinct. The fire, in short, has gone out. But if only it
can be lighted again before any serious change in the system takes
place, all may still go on precisely as of old.

Many animals, however, find it convenient to assume a state of less
complete suspended animation during certain special periods of the year,
according to the circumstances of their peculiar climate and mode of
life. Among the very highest animals, the most familiar example of this
sort of semi-torpidity is to be found among the bears and the dormice.
The common European brown bear is a carnivore by descent, who has become
a vegetarian in practice, though whether from conscientious scruples or
mere practical considerations of expediency, does not appear. He feeds
chiefly on roots, berries, fruits, vegetables, and honey, all of which
he finds it comparatively difficult to procure during winter weather.
Accordingly, as everyone knows, he eats immoderately in the summer
season, till 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.