THE EYEBALL AND EYELIDS. 317 



between them and the cornea. If the eyeball 

 were to remain without motion, the margins of 

 the eyelids would meet in such a manner on the 

 surface of the cornea, that a certain portion would 

 be left untouched, and the eye would have no 

 power of clearing off what obscured the vision, at 

 that principal part of the lucid cornea which is in 

 the very axis of the eye ; and if the tears flowed, 

 they would be left accumulated on the centre of 

 the cornea, and winking, instead of clearing the 

 eye, would suffuse it. To avoid these effects, and 

 to sweep and clear the surface of the cornea, at 

 the same time that the eyelids are closed, the eye- 

 ball revolves, and the cornea is rapidly elevated 

 under the eyelid. 



Another effect of this motion of the eyeball is 

 to procure the discharge from the lachrymal ducts ; 

 for by the simultaneous ascent of the cornea, and 

 descent of the upper eyelid, the membrane on 

 which the ducts open is stretched, and then the 

 tears flow unimpeded. 



By this simultaneous motion, also, the descent 

 of the eyelid and the ascent of the cornea, the ra- 

 pidity with which the eye escapes from injury, is 

 increased. Even creatures which have imperfect 

 eyelids, as fishes, by possessing this rapid revolv- 

 ing motion of the eye, avoid injury and clear off 

 impurities. 



I may observe, in passing, that there is a pro- 

 vision for the protection of the eye, in the manner 

 in which the eyelids close, which has not been no- 

 28* 



