And the two parts aforesaid display this motion more conspicuously
than the rest, because each is in a sense a separate vital organism,
the reason being that each contains vital moisture. In the case of the
heart the cause is plain, for the heart is the seat of the senses,
while an indication that the generative organ too is vital is that
there flows from it the seminal potency, itself a kind of organism.
Again, it is a reasonable arrangement that the movements arise in
the centre upon movements in the parts, and in the parts upon
movements in the centre, and so reach one another. Conceive A to be
the centre or starting point. The movements then arrive at the
centre from each letter in the diagram we have drawn, and flow back
again from the centre which is moved and changes, (for the centre is
potentially multiple) the movement of B goes to B, that of C to C, the
movement of both to both; but from B to C the movements flow by dint
of going from B to A as to a centre, and then from A to C as from a
centre.
Moreover a movement contrary to reason sometimes does and
sometimes does not arise in the organs on the occasion of the same
thoughts; the reason is that sometimes the matter which is passive
to the impressions is there in sufficient quantity and of the right
quality and sometimes not.
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