MUSCLES OF THE EYELIDS. 



323 



perceive that, during this action, the eyeball is 

 pushed forwards. Now the lower eyelid is so 

 adapted as to slip off the convex surface of the 

 ball in this action, and thus to be depressed, while 

 the upper eyelid is elevated. 



The origin of the levator being at a, and the 

 insertion into the cartilage of the upper eyelid at 

 B, the effect of the action of the muscle must be 

 the protrusion of the eyeball c to the dotted line. 

 By the elevation of the upper eyelid, the eye 

 starts forward a little, and the lower eyelid there- 

 fore slips off the lower segment of the eyeball. 



It is curious to observe how the eyeball retreats 

 in its condition of repose, and is protruded when 

 about to be exercised in vision. High excitement, 

 as in terror, when the eyeballs are largely un- 

 closed, is attended with an increase of the sphere 

 of vision produced by the protrusion of the eye- 

 balls : a change remarkable both in the ferocious 

 and timid animals, especially in the latter. 



Such were the views of the motions of the eye- 

 ball and eyelids, introductory to a paper on the 



