On The Parts Of Animals   

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. Now as the front is more honourable and of
higher supremacy than the hinder aspect, so also and in like degree is
the great vessel superior to the aorta. For the great vessel is placed
in front, while the aorta is behind; the former again is plainly
visible in all sanguineous animals, while the latter is in some
indistinct and in some not discernible at all.
Lastly, the reason for the vessels being distributed throughout the
entire body is that in them, or in parts analogous to them, is
contained the blood, or the fluid which in bloodless animals takes the
place of blood, and that the blood or analogous fluid is the material
from which the whole body is made. Now as to the manner in which
animals are nourished, and as to the source from which they obtain
nutriment and as to the way in which they absorb this from the
stomach, these are matters which may be more suitably considered and
explained in the treatise on Generation. But inasmuch as the parts
are, as already said, formed out of the blood, it is but rational that
the flow of the blood should extend, as it does, throughout the whole
of the body. For since each part is formed of blood, each must have
blood about and in its substance.
To give an illustration of this. The water-courses in gardens are so
constructed as to distribute water from one single source or fount
into numerous channels, which divide and subdivide so as to convey it
to all parts; and, again, in house-building stones are thrown down
along the whole ground-plan of the foundation walls; because the
garden-plants in the one case grow at the expense of the water, and
the foundation walls in the other are built out of the stones. 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.
As the blood-vessels advance, they become gradually smaller and
smaller, until at last their tubes are too fine to admit the blood.
This fluid can therefore no longer find its way through them, though
they still give passage to the humour which we call sweat; and
especially so when the body is heated, and the mouths of the small
vessels are dilated. Instances, indeed, are not unknown of persons who