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.
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