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.
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