eeding
rapidly, or the animal is undergoing much exertion and expenditure of
material, an increased quantity of food is invariably required. On the
other hand, where no new substance is forming, and where, from bodily
inactivity, little loss is sustained, a comparatively small supply will
suffice. In endowing animals with the sense of _appetite_, including the
sensation of hunger and thirst, the Creator has effectually provided
against any inconvenience which might otherwise exist, and given to them
a guide in relation to both the quality and quantity of food needful for
them, and the times of partaking of it, with that beneficence which
distinguishes all his works. He has not only provided an effectual
safeguard in the sensations of hunger and thirst, but he has attached to
their regulated indulgence a degree of pleasure which never fails to
insure attention to their demands, and which, in highly-civilized
communities, is apt to lead to excessive gratification. Their end is
manifestly to proclaim that nourishment is required for the support of
the system. When the body is very actively exercised, and a good deal of
waste is effected by perspiration and exhalation from the lungs, the
appetite becomes keener, and more urgent for immediate gratification;
and if it is indulged, we eat with a relish unknown on other occasions,
and afterward experience a sensation of internal comfort pervading the
frame, as if every individual part of the body were imbued with a
feeling of contentment and satisfaction; the very opposite of the
restless discomfort and depression which come upon us, and extend over
the whole system, when appetite is disappointed. There is, in short, an
obvious and active sympathy between the condition and bearing of the
stomach, and those of every part of the animal frame; in virtue of
which, hunger is felt very keenly when the general system stands in
urgent need of repair, and very moderately when no waste has been
suffered.

We have seen that _waste_ is every where attendant upon _action_, and
that the object of nutrition is to repair waste and admit of growth. We
come now to consider the _Process of Digestion_.

All articles used for food necessarily undergo several changes before
they are fitted to constitute a part of the body. In the process of
digestion, four different changes should be noticed. More might be
specified.

1. MASTICATION.--The first step in the preparation of food for imparting
nouri