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. 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
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