Hitherto we have, as it were, been looking at a steam-engine
with the fires out, and nothing in the boiler; but the body of the
living animal is a beautifully-formed active machine, and every part
has its different work to do in the working of that machine, which is
what we call its life. The Horse, if you see him after his day's work
is done, is cropping the grass in the fields, as it may be, or munching
the oats in his stable. What is he doing? His jaws are working as a
mill--and a very complex mill too--grinding the corn, or crushing the
grass to a pulp. As soon as that operation has taken place, the food
is passed down to the stomach, and there it is mixed with the chemical
fluid called the gastric juice, a substance which has the peculiar
property of making soluble and dissolving out the nutritious matter in
the grass, and leaving behind those parts which are not nutritious; so
that you have, first, the mill, then a sort of chemical digester; and
then the food, thus partially dissolved, is carried back by the
muscular contractions of the intestines into the hinder parts of the
body, while the soluble portions are taken up into the blood.
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