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On The Generation Of Animals   

passages, as are the other sense-organs. Whereas those of touch and
taste are simply the body itself or some part of the body of
animals, those of smell and hearing are passages connecting with the
external air and full themselves of innate spiritus; these passages
end at the small blood-vessels about the brain which run thither
from the heart. But the eye is the only sense-organ that has a
bodily constitution peculiar to itself. It is fluid and cold, and does
not exist from the first in the place which it occupies later in the
same way as the other parts do, for they exist potentially to begin
with and actually come into being later, but the eye is the purest
part of the liquidity about the brain drained off through the passages
which are visible running from them to the membrane round the brain. A
proof of this is that, apart from the brain, there is no other part in
the head that is cold and fluid except the eye. Of necessity therefore
this region is large at first but falls in later. For the same thing
happens with the brain; at first it is liquid and large, but in course
of evaporation and concoction it becomes more solid and falls in; this
applies both to the brain and the eyes. The head is very large at
first, on account of the brain, and the eyes appear large because of
the liquid in them. They are the last organs to reach completion
because the brain is formed with difficulty; for it is at a late
period that it gets rid of its coldness and fluidity; this applies
to all animals possessing a brain, but especially to man. For this
reason the 'bregma' is the last of the bones to be formed; even
after birth this bone is still soft in children. The cause of this
being so with men more than with other animals is the fact that
their brain is the most fluid and largest. This again is because the
heat in man's heart is purest. His intellect shows how well he is
tempered, for man is the wisest of animals. And children for a long
time have no control over their heads on account of the heaviness of
the brain; and the same applies to the parts which it is necessary
to move, for it is late that the principle of motion gets control over
the upper parts, and last of all over those whose motion is not
connected directly with it, as that of the legs is not. Now the eyelid
is such a part. But since Nature makes nothing superfluous nor in
vain, it is clear also that she makes nothing too late or too soon,
for if she did the result would be either in vain or superfluous.
Hence it is necessary that the eyelids should be separated at the same
time as the heart is able to move them. So then the eyes of animals
are perfected late because of the amount of concoction required by the
brain, and last of all the parts because the motion must be very
strong before it can affect parts so far from the first principle of
motion and so cold. And it is plain that such is the nature of the
eyelids, for if the head is affected by never so little heaviness
through sleepiness or drunkenness or anything else of the kind, we
cannot raise the eyelids though their own weight is so small. So
much for the question how the eyes come into being, and why and for
what cause they are the last to be fully developed.

Each of the other parts is formed out of the nutriment, those most
honourable and participating in the sovereign principle from the
nutriment which is first and purest and fully concocted, those which
are only necessary for the sake of the former parts from the
inferior nutriment and the residues left over from the other. For
Nature, like a good householder, is not in the habit of throwing
away anything from which it is possible to make anything useful. Now
in a household the best part of the food that comes in is set apart
for the free men, the inferior and the residue of the best for the
slaves, and the worst is given to the animals that live with them.
Just as the intellect acts thus in the outside world with a view to
the growth of the persons concerned, so in the case of the embryo
itself does Nature form from the purest material the flesh and the
body of the other sense-organs, and from the residues thereof bones,
sinews, hair, and also nails and hoofs and the like; hence these are

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