The mud-fish leaves a small canal or pipe open in his cell at
either end to admit the air for breathing, though he breathes (as I
shall proceed to explain) in a very slight degree during his aestivation;
whereas every proper toad-in-a-hole ought by all accounts to live
entirely without either feeding or breathing in any way. However, this
is a mere detail; and indeed, if toads-in-a-hole do really exist at all,
we must in all probability ultimately admit that they breathe to some
extent, though perhaps very slightly, during their long immurement.
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.
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