(su PHULOSOPHY OF ZOOLOGY. 
ed the Upper eye-lid; the other, which covers the inferior 
portion, is termed the Under eye-lid. These meet in a line, 
which usually observes a horizontal direction. The margins 
of both eye-lids are thickened by the cells of the meibomian 
glands, and rounded, so that when they meet on the eye- 
ball, there is a conduit formed for the tears within. ‘The 
Tarsus, as the margin is termed, is still farther strengthen- 
ed for supporting a row of hairs, called Cilia, or eye-lashes. 
The eye-lids exhibit considerable difference in their mo- 
tions. In some animals, both eye-lids are capable of ap- 
proaching each other, while in others, only one eye-lid is 
able to move, either the under rising upwards to join its an- 
tagonist, or the upper descending for the same purpose. In 
one fish, (Tetraodon mola or Sun-fish,) the eye-lid is single, 
circular, with a perforation in the centre, the aperture con- 
tracting or enlarging according to circumstances. 
The Nictitating Membrane, or third eye-lid, as it is 
sometimes called, is of a more delicate texture, and more 
liberally supplied with bloodvessels than the eye-lids, and 
is even transparent in some animals. In many animals it 
is single, situated at the nasal angle of the eye, within the 
eye-lids, or on the interior of the under eye-lid. In others 
it is double, situated at each corner of the eye. In some 
cases it is destitute of motion, while in others it is capable 
of covering the eye-ball, by extending in a horizontal or 
vertical direction, according to its position. 
The Conjunctiva forms a permanent and continuous co- 
vering on the eye-ball itself. In man, it adheres so closely 
to the cornea, that it cannot be separated unless by macer- 
ation. In other cases, where there are no eye-lids, and 
where the skin passes directly over the eye, the adhesion is 
very slight. In almost all animals the conjunctiva is trans- 
parent where it passes over the cornea, but is usually thick- 
ened and coloured when it covers the sclerotic coat, form- 
