178 I'HI.I.OSOI'HV OV ZOOLOGY. 



4. J///.sT/t',v of the Rye- In the vertebral animals, each 

 eye is placed in a socket, situated in the bones of the face, 

 and termed the Orbit. The eye-ball does not fill this cavi- 

 ty at its posterior part, but the remaining space is occupied 

 with fat or gelatinous matter. The ball rests upon these, 

 and is thus able to move, without being injured by friction. 

 As the fat, in lean and old animals, is in a great measure 

 absorbed, the eye appears, in such cases, sunk in the orbit. 

 In some cases, the ball of the eye is supported on a foot- 

 stalk, which enables the muscles to act on a longer lever, 

 and admits of a greater extent of motion. 



The muscles by which the eye is moved, have their ori- 

 gin in the walls of the orbit, and their insertion in the scle- 

 rotic coat. Tendinous fibres, however, pass on to the cor- 

 nea, with which they become incorporated. In man, the 

 four straight muscles of the eye can be separated from the 

 sclerotica, and the external layer of the cornea demonstrat- 

 ed to be a continuation of their tendons. In birds, the 

 same connection between the tendons and cornea prevails *. 

 The muscles vary in number in different animals. 



5. Glands of' the Eye. These bodies vary in form and 

 number in the different classes of animals. In man, they 

 consist of three kinds, the Lachrymal, and Meibomian, and 

 the Caruncula Lachrymalis. 



The Lachrymal gland is situated between the ball of the 

 eye and the upper arch of the orbit, a little towards the 

 temples. It consists of a number of small whitish granu- 

 lar bodies collected together into lobes. From these pro- 

 ceed several canals which descend through the substance of 

 the upper eye-lid, and open on its internal surface. The 

 fluid which is secreted is the Tears. This fluid, accord- 

 ing to the observations of FOURCROY and VAUQUELIN, con- 



* Phil. Trans. 1795, p. 11. and 263, and 1796, p. 1* 



