On The Parts Of Animals   

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. For plants get their food from the
earth by means of their roots; and this food is already elaborated
when taken in, which is the reason why plants produce no excrement,
the earth and its heat serving them in the stead of a stomach. But
animals, with scarcely an exception, and conspicuously all such as are
capable of locomotion, are provided with a stomachal sac, which is as
it were an internal substitute for the earth. They must therefore have
some instrument which shall correspond to the roots of plants, with
which they may absorb their food from this sac, so that the proper end
of the successive stages of concoction may at last be attained. The
mouth then, its duty done, passes over the food to the stomach, and
there must necessarily be something to receive it in turn from this.
This something is furnished by the bloodvessels, which run throughout
the whole extent of the mesentery from its lowest part right up to the
stomach. A description of these will be found in the treatises on
Anatomy and Natural History. Now as there is a receptacle for the
entire matter taken as food, and also a receptacle for its excremental
residue, and again a third receptacle, namely the vessels, which serve
as such for the blood, it is plain that this blood must be the final
nutritive material in such animals as have it; while in bloodless
animals the same is the case with the fluid which represents the
blood. This explains why the blood diminishes in quantity when no food
is taken, and increases when much is consumed, and also why it becomes
healthy and unhealthy according as the food is of the one or the other
character. These facts, then, and others of a like kind, make it plain
that the purpose of the blood in sanguineous animals is to subserve
the nutrition of the body. They also explain why no more sensation is
produced by touching the blood than by touching one of the excretions
or the food, whereas when the flesh is touched sensation is produced.
For the blood is not continuous nor united by growth with the flesh,
but simply lies loose in its receptacle, that is in the heart and
vessels. The manner in which the parts grow at the expense of the
blood, and indeed the whole question of nutrition, will find a more
suitable place for exposition in the treatise on Generation, and in
other writings. For our present purpose all that need be said is that
the blood exists for the sake of nutrition, that is the nutrition of
the parts; and with this much let us therefore content ourselves.
Part 4
What are called fibres are found in the blood of some animals but not
of all. There are none, for instance, in the blood of deer and of
roes; and for this reason the blood of such animals as these never
coagulates. For one part of the blood consists mainly of water and
therefore does not coagulate, this process occurring only in the other
and earthy constituent, that is to say in the fibres, while the fluid
part is evaporating.
Some at any rate of the animals with watery blood have a keener
intellect than those whose blood is of an earthier nature. This is due
not to the coldness of their blood, but rather to its thinness and
purity; neither of which qualities belongs to the earthy matter. For
the thinner and purer its fluid is, the more easily affected is an
animal's sensibility. Thus it is that some bloodless animals,
notwithstanding their want of blood, are yet more intelligent than
some among the sanguineous kinds. Such for instance, as already said,
is the case with the bee and the tribe of ants, and whatever other
animals there may be of a like nature. At the same time too great an