Still
further, the liver contains no spacious receptacle in its substance,
as does the heart; but its blood is in a vessel as in all the other
viscera. The vessel, moreover, extends through it, and no vessel
whatsoever originates in it; for it is from the heart that all the
vessels take their rise. Since then one or other of these two parts
must be the 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.
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