In what sense, then, the blood is hot and in what sense fluid, and
how far it partakes of the opposite properties, has now been fairly
explained. Now since everything that grows must take nourishment,
and nutriment in all cases consists of fluid and solid substances, and
since it is by the force of heat that these are concocted and changed,
it follows that all living things, animals and plants alike, must on
this account, if on no other, have a natural source of heat. This
natural heat, moreover, must belong to many parts, seeing that the
organs by which the various elaborations of the food are effected
are many in number. For first of all there is the mouth and the
parts inside the mouth, on which the first share in the duty clearly
devolves, in such animals at least as live on food which requires
disintegration. The mouth, however, does not actually concoct the
food, but merely facilitates concoction; for the subdivision of the
food into small bits facilitates the action of heat upon it. After the
mouth come the upper and the lower abdominal cavities, and here it
is that concoction is effected by the aid of natural heat. Again, just
as there is a channel for the admission of the unconcocted food into
the stomach, namely the mouth, and in some animals the so-called
oesophagus, which is continuous with the mouth and reaches to the
stomach, so must there also be other and more numerous channels by
which the concocted food or nutriment shall pass out of the stomach
and intestines into the body at large, and to which these cavities
shall serve as a kind of manger.
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