ISO I'iili.OSOl'HY 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 meiboinian 

 glands, and rounded, so that when they meet on the eye- 

 ball, there is a conduit formed for the tears within. The 

 Tarsus, as tire 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. 



Tlie 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- 



