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. There is a heart, then, in all sanguineous
animals, and the reason for this has already been given. For that
sanguineous animals must necessarily have blood is self-evident.
And, as the blood is fluid, it is also a matter of necessity that
there shall be a receptacle for it; and it is apparently to meet
this requirement that nature has devised the blood-vessels. These,
again, must necessarily have one primary source. For it is
preferable that there shall be one such, when possible, rather than
several. This primary source of the vessels is the heart. For the
vessels manifestly issue from it and do not go through it. Moreover,
being as it is homogeneous, it has the character of a blood-vessel.
Again its position is that of a primary or dominating part. For
nature, when no other more important purpose stands in her way, places
the more honourable part in the more honourable position; and the
heart lies about the centre of the body, but rather in its upper
than its lower half, and also more in front than behind. This is
most evident in the case of man, but even in other animals there is
a tendency in the heart to assume a similar position, in the centre of
the necessary part of the body, that is to say of the part which
terminates in the vent for excrement. For the limbs vary in position
in different animals, and are not to be counted with the parts which
are necessary for life. For life can be maintained even when they
are removed; while it is self-evident that the addition of them to
an animal is not destructive of it
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