On The Parts Of Animals   

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.
There are some who say that the vessels commence in the head. In this
they are clearly mistaken. For in the first place, according to their
representation, there would be many sources for the vessels, and these
scattered; and secondly, these sources would be in a region that is
manifestly cold, as is shown by its intolerance of chill, whereas the
region of the heart is as manifestly hot. Again, as already said, the
vessels continue their course through the other viscera, but no vessel
spreads through the heart. From this it is quite evident that the
heart is a part of the vessels and their origin; and for this it is
well suited by its structure. For its central part consists of a dense
and hollow substance, and is moreover full of blood, as though the
vessels took thence their origin. It is hollow to serve for the
reception of the blood, while its wall is dense, that it may serve to
protect the source of heat. For here, and here alone in all the
viscera and indeed in all the body, there is blood without
blood-vessels, the blood elsewhere being always contained within
vessels. Nor is this but consistent with reason. For the blood is
conveyed into the vessels from the heart, but none passes into the
heart from without. For in itself it constitutes the origin and
fountain, or primary receptacle, of the blood. It is however, from
dissections and from observations on the process of development that
the truth of these statements receives its clearest demonstration. For
the heart is the first of all the parts to be formed; and no sooner is
it formed than it contains blood. Moreover, the motions of pain and
pleasure, and generally of all sensation, plainly have their source in
the heart, and find in it their ultimate termination. This, indeed,
reason would lead us to expect. For the source must, when. ever
possible, be one; and, of all places, the best suited for a source is
the centre. For the centre is one, and is equally or almost equally
within reach of every part. Again, as neither the blood itself, nor
yet any part which is bloodless, is endowed with sensation, it is
plain that that part which first has blood, and which holds it as it
were in a receptacle, must be the primary source of sensation. And
that this part is the heart is not only a rational inference, but also
evident to the senses. For no sooner is the embryo formed, than its
heart is seen in motion as though it were a living creature, and this
before any of the other parts, it being, as thus shown, the
starting-point of their nature in all animals that have blood. A
further evidence of the truth of what has been stated is the fact that
no sanguineous animal is without a heart. For the primary source of
blood must of necessity be present in them all. It is true that
sanguineous animals not only have a heart but also invariably have a
liver. But no one could ever deem the liver to be the primary organ
either of the whole body or of the blood. For the position in which it
is placed is far from being that of a primary or dominating part; and,
moreover, in the most perfectly finished animals there is another
part, the spleen, which as it were counterbalances it. 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