i?i 
Mr. Bell on the motions of the eye , &c. 
third eye-lid, or haw, as it is termed in the horse. By this 
mechanism the third eye-lid is made to sweep rapidly over the 
surface of the cornea, and by means of the glutinous fluid 
with which its surface is bedewed, it attaches and clears away 
offensive particles. 
In birds, the eye is an exquisitely fine organ, and still more 
curiously, and as we might be tempted to say, artificially 
protected. The third eye-lid is more perfect ; it is mem- 
branous and broad, and is drawn over the surface of the eye 
by means of two muscles which are attached to the back part 
of the eye-ball, and by a long round tendon, that makes a 
course of nearly three parts of the circumference of the ball. 
The lacrymal gland is small, and seated low, but the mucous 
gland is of great size, and seated in a cavity deep and large, 
and on the inside of the orbit. As the third eye-lid is moved 
by an apparatus which cannot squeeze the mucous gland at 
the same time that the eye-lid is moved, as in quadrupeds, 
the oblique muscles are particularly provided to draw the 
eye-ball against the gland, and to force out the mucus on the 
surface of the third eye-lid. It flows very copiously ; and 
this is probably the reason of the smallness of the proper 
lacrymal gland which lies on the opposite side of the orbit. 
We already see two objects attained through the motion 
of these parts : the moistening the eye with the clear fluid 
of the lacrymal gland, and the extraction or protrusion of 
offensive particles. 
There is another division of this subject no less curious ; 
the different conditions of the eye during the waking and 
sleeping state, remain to be considered. If we approach a 
person in disturbed sleep when the eye-lids are a little apart, 
