On The Parts Of Animals   

are unable to discern the reason for this; they see also that the
brain is the most peculiar of all the animal organs; and out of these
facts they form an argument, by which they link sensation and brain
together. It has, however, already been clearly set forth in the
treatise on Sensation, that it is the region of the heart that
constitutes the sensory centre. There also it was stated that two of
the senses, namely touch and taste, are manifestly in immediate
connexion with the heart; and that as regards the other three, namely
hearing, sight, and the centrally placed sense of smell, it is the
character of their sense-organs which causes them to be lodged as a
rule in the head. Vision is so placed in all animals. But such is not
invariably the case with hearing or with smell. For fishes and the
like hear and smell, and yet have no visible organs for these senses
in the head; a fact which demonstrates the accuracy of the opinion
here maintained. Now that vision, whenever it exists, should be in the
neighbourhood of the brain is but what one would rationally expect.
For the brain is fluid and cold, and vision is of the nature of water,
water being of all transparent substances the one most easily
confined. Moreover it cannot but necessarily be that the more precise
senses will have their precision rendered still greater if ministered
to by parts that have the purest blood. For the motion of the heat of
blood destroys sensory activity. For these reasons the organs of the
precise senses are lodged in the head.
It is not only the fore part of the head that is destitute of flesh,
but the hind part also. For, in all animals that have a head, it is
this head which more than any other part requires to be held up. But,
were the head heavily laden with flesh, this would be impossible; for
nothing so burdened can be held upright. This is an additional proof
that the absence of flesh from the head has no reference to brain
sensation. For there is no brain in the hinder part of the head, and
yet this is as much without flesh as is the front.
In some animals hearing as well as vision is lodged in the region of
the head. Nor is this without a rational explanation. For what is
called the empty space is full of air, and the organ of hearing is, as
we say, of the nature of air. Now there are channels which lead from
the eyes to the blood-vessels that surround the brain; and similarly
there is a channel which leads back again from each ear and connects
it with the hinder part of the head. But no part that is without blood
is endowed with sensation, as neither is the blood itself, but only
some one of the parts that are formed of blood.
The brain in all animals that have one is placed in the front part of
the head; because the direction in which sensation acts is in front;
and because the heart, from which sensation proceeds, is in the front
part of the body; and lastly because the instruments of sensation are
the blood-containing parts, and the cavity in the posterior part of
the skull is destitute of blood-vessels.
As to the position of the sense-organs, they have been arranged by
nature in the following well-ordered manner. The organs of hearing are
so placed as to divide the circumference of the head into two equal
halves; for they have to hear not only sounds which are directly in
line with themselves, but sounds from all quarters. The organs of
vision are placed in front, because sight is exercised only in a
straight line, and moving as we do in a forward direction it is
necessary that we should see before us, in the direction of our
motion. Lastly, the organs of smell are placed with good reason
between the eyes. For as the body consists of two parts, a right half
and a left, so also each organ of sense is double. In the case of
touch this is not apparent, the reason being that the primary organ of
this sense is not the flesh or analogous part, but lies internally. In
the case of taste, which is merely a modification of touch and which
is placed in the tongue, the fact is more apparent than in the case of
touch, but still not so manifest as in the case of the other senses.
However, even in taste it is evident enough; for in some animals the
tongue is plainly forked. The double character of the sensations is,