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. 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 hairy animals. In these animals, then, the skin on the head is
hard, and so does not allow of the formation of an upper eyelid,
whereas lower down the integument is of a flesh-like character, so
that the lower lid can be thin and extensible.
The act of blinking is performed by the heavy-bodied birds by
means of the membrane already mentioned, and not by this lower lid.
For in blinking rapid motion is required, and such is the motion of
this membrane, whereas that of the lower lid is slow. It is from the
canthus that is nearest to the nostrils that the membrane comes. For
it is better to have one starting-point for nictitation than two;
and in these birds this starting-point is the junction of eye and
nostrils, an anterior starting-point being preferable to a lateral
one. Oviparous quadrupeds do not blink in like manner as the birds;
for, living as they do on the ground, they are free from the necessity
of having eyes of fluid consistency and of keen sight, whereas these
are essential requisites for birds, inasmuch as they have to use their
eyes at long distances. This too explains why birds with talons,
that have to search for prey by eye from aloft, and therefore soar
to greater heights than other birds, are sharpsighted; while common
fowls and the like, that live on the ground and are not made for
flight, have no such keenness of vision
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