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. 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
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