Now that these vessels exist on
account of the blood has already been stated. For every fluid requires
a receptacle, and in the case of the blood the vessels are that
receptacle. Let us now explain why these vessels are two, and why they
spring from one single source, and extend throughout the whole body.
The reason, then, why these two vessels coalesce into one centre,
and spring from one source, is that the sensory soul is in all animals
actually one; and this one-ness of the sensory soul determines a
corresponding one-ness of the part in which it primarily abides. In
sanguineous animals this one-ness is not only actual but potential,
whereas in some bloodless animals it is only actual. Where, however,
the sensory soul is lodged, there also and in the selfsame place
must necessarily be the source of heat; and, again, where this is
there also must be the source of the blood, seeing that it thence
derives its warmth and fluidity. Thus, then, in the oneness of the
part in which is lodged the prime source of sensation and of heat is
involved the one-ness of the source in which the blood originates; and
this, again, explains why the blood-vessels have one common
starting-point.
The vessels, again, are two, because the body of every sanguineous
animal that is capable of locomotion is bilateral; for in all such
animals there is a distinguishable before and behind, a right and
left, an above and below.
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