Now
just after the same fashion has nature laid down channels for the
conveyance of the blood throughout the whole body, because this
blood is the material out of which the whole fabric is made. This
becomes very evident in bodies that have undergone great emaciation.
For in such there is nothing to be seen but the blood-vessels; just as
when fig-leaves or vine-leaves or the like have dried up, there is
nothing left of them but their vessels. The explanation of this is
that the blood, or fluid which takes its place, is potentially body
and flesh, or substance analogous to flesh. Now just as in
irrigation the largest dykes are permanent, while the smallest are
soon filled up with mud and disappear, again to become visible when
the deposit of mud ceases; so also do the largest blood-vessels remain
permanently open, while the smallest are converted actually into
flesh, though potentially they are no whit less vessels than before.
This too explains why, so long as the flesh of an animal is in its
integrity, blood will flow from any part of it whatsoever that is cut,
though no vessel, however small, be visible in it. Yet there can be no
blood, unless there be a blood-vessel. The vessels then are there, but
are invisible owing to their being clogged up, just as the dykes for
irrigation are invisible until they have been cleared of mud.
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