On The Parts Of Animals   

central source, and since it is not the liver which is such, it
follows of necessity that it is the heart which is the source of the
blood, as also the primary organ in other respects. For the definitive
characteristic of an animal is the possession of sensation; and the
first sensory part is that which first has blood; that is to say is
the heart, which is the source of blood and the first of the parts to
contain it.
The apex of the heart is pointed and more solid than the rest of the
organ. It lies against the breast, and entirely in the anterior part
of the body, in order to prevent that region from getting chilled. For
in all animals there is comparatively little flesh over the breast,
whereas there is a more abundant covering of that substance on the
posterior surface, so that the heat has in the back a sufficient
amount of protection. In all animals but man the heart is placed in
the centre of the pectoral region; but in man it inclines a little
towards the left, so that it may counterbalance the chilliness of that
side. For the left side is colder in man, as compared with the right,
than in any other animal. It has been stated in an earlier treatise
that even in fishes the heart holds the same position as in other
animals; and the reason has been given why it appears not to do so.
The apex of the heart, it is true, is in them turned towards the head,
but this in fishes is the front aspect, for it is the direction in
which their motion occurs.
The heart again is abundantly supplied with sinews, as might
reasonably be expected. For the motions of the body commence from the
heart, and are brought about by traction and relaxation. The heart
therefore, which, as already said,' as it were a living creature
inside its possessor, requires some such subservient and strengthening
parts.
In no animals does the heart contain a bone, certainly in none of
those that we have ourselves inspected, with the exception of the
horse and a certain kind of ox. In these exceptional cases the heart,
owing to its large bulk, is provided with a bone as a support; just as
the bones serve as supports for the body generally.
In animals of great size the heart has three cavities; in smaller
animals it has two; and in all has at least one, for, as already
stated, there must be some place in the heart to serve as a receptacle
for the first blood; which, as has been mentioned more than once, is
formed in this organ. But inasmuch as the main blood-vessels are two
in number, namely the so-called great vessel and the aorta, each of
which is the origin of other vessels; inasmuch, moreover, as these two
vessels present differences, hereafter to be discussed, when compared
with each other, it is of advantage that they also shall themselves
have distinct origins. This advantage will be obtained if each side
have its own blood, and the blood of one side be kept separate from
that of the other. For this reason the heart, whenever it is possible,
has two receptacles. And this possibility exists in the case of large
animals, for in them the heart, as the body generally, is of large
size. Again it is still better that there shall be three cavities, so
that the middle and odd one may serve as a centre common to both
sides. But this requires the heart to be of greater magnitude, so that
it is only in the largest hearts that there are three cavities.
Of these three cavities it is the right that has the most abundant and
the hottest blood, and this explains why the limbs also on the right
side of the body are warmer than those on the left. The left cavity
has the least blood of all, and the coldest; while in the middle
cavity the blood, as regards quantity and heat, is intermediate to the
other two, being however of purer quality than either. For it behoves
the supreme part to be as tranquil as possible, and this tranquillity
can be ensured by the blood being pure, and of moderate amount and
warmth.
In the heart of animals there is also a kind of joint-like division,
something like the sutures of the skull. This is not, however,
attributable to the heart being formed by the union of several parts