THE EYEBALL AND EYELIDS. 319 



is attached to an elastic body. This body is lodged 

 in a division or depression of the orbit on the side 

 towards the nose. When the eye is excited, the 

 eyeball is made to press on the elastic body against 

 the side of the orbit, and torce it out of its recess 

 or socket ; the consequence of which is the pro- 

 trusion of the cartilaginous third eyelid, or haw, as 

 it is termed in the horse. By this mechanism the 

 third eyelid is made to sweep rapidly over the 

 surface of the cornea, and, by means of the gluti- 

 nous fluid with which its surface is bedewed, it 

 attaches to itself and clears away offensive par- 

 ticles. 



In birds, the eye is an exquisitely fine organ, 

 and still more curiously, we might be tempted to 

 say artificially, protected. The third eyelid is more 

 perfect than that of quadrupeds : it is membranous 

 and broad, and is drawn over the surface of the 

 eye by means of two muscles attached to the back 

 part of the eyeball, one of which acts by a long 

 round tendon, that makes a course of nearly three 

 parts of the circumference of the ball.* The lach- 

 rymal gland is small, and seated low, but the mu- 

 cous gland is of great size, and placed in a cavity 

 deep and large, and on the inside of the orbit. As 

 the third eyelid is moved by an apparatus which 

 cannot squeeze the mucous gland at the same time 

 that the eyelid is moved, as in quadrupeds, the ob- 

 lique muscles are particularly provided to draw 

 the eyeball against the gland, and to force out the 



* See p. 40, vol. i. 



