On The Parts Of Animals   

however, more conspicuous in the other organs of sense. For there are
two ears and two eyes, and the nostrils, though joined together, are
also two. Were these latter otherwise disposed, and separated from
each other as are the ears, neither they nor the nose in which they
are placed would be able to perform their office. For in such animals
as have nostrils olfaction is effected by means of inspiration, and
the organ of inspiration is placed in front and in the middle line.
This is the reason why nature has brought the two nostrils together
and placed them as the central of the three sense-organs, setting them
side by side on a level with each other, to avail themselves of the
inspiratory motion. In other animals than man the arrangement of these
sense-organs is also such as is adapted in each case to the special
requirements.
Part 11
For instance, in quadrupeds the ears stand out freely from the head
and are set to all appearance above the eyes. Not that they are in
reality above the eyes; but they seem to be so, because the animal
does not stand erect, but has its head hung downwards. This being the
usual attitude of the animal when in motion, it is of advantage that
its ears shall be high up and movable; for by turning themselves about
they can the better take in sounds from every quarter.
Part 12
In birds, on the other hand, there are no ears, but only the auditory
passages. This is because their skin is hard and because they have
feathers instead of hairs, so that they have not got the proper
material for the formation of ears. Exactly the same is the case with
such oviparous quadrupeds as are clad with scaly plates, and the same
explanation applies to them. There is also one of the viviparous
quadrupeds, namely the seal, that has no ears but only the auditory
passages. The explanation of this is that the seal, though a
quadruped, is a quadruped of stunted formation.
Part 13
Men, and Birds, and Quadrupeds, viviparous and oviparous alike, have
their eyes protected by lids. In the Vivipara there are two of these;
and both are used by these animals not only in closing the eyes, but
also in the act of blinking; whereas the oviparous quadrupeds, and the
heavy-bodied birds as well as some others, use only the lower lid to
close the eye; while birds blink by means of a membrane that issues
from the canthus. The reason for the eyes being thus protected is that
nature has made them of fluid consistency, in order to ensure keenness
of vision. For had they been covered with hard skin, they would, it is
true, have been less liable to get injured by anything falling into
them from without, but they would not have been sharp-sighted. It is
then to ensure keenness of vision that the skin over the pupil is fine
and delicate; while the lids are superadded as a protection from
injury. It is as a still further safeguard that all these animals
blink, and man most of all; this action (which is not performed from
deliberate intention but from a natural instinct) serving to keep
objects from falling into the eyes; and being more frequent in man
than in the rest of these animals, because of the greater delicacy of
his skin. These lids are made of a roll of skin; and it is because
they are made of skin and contain no flesh that neither they, nor the
similarly constructed prepuce, unite again when once cut.
As to the oviparous quadrupeds, and such birds as resemble them in
closing the eye with the lower lid, it is the hardness of the skin of
their heads which makes them do so. For such birds as have heavy
bodies are not made for flight; and so the materials which would
otherwise have gone to increase the growth of the feathers are
diverted thence, and used to augment the thickness of the skin. Birds
therefore of this kind close the eye with the lower lid; whereas
pigeons and the like use both upper and lower lids for the purpose. As
birds are covered with feathers, so oviparous quadrupeds are covered
with scaly plates; and these in all their forms are harder than hairs,
so that the skin also to which they belong is harder than the skin of