SENSE OF SIGHT 



221 



nels are insufficient for their escape, and the tears roll over 

 the cheek. 



Fig. 22. 



a, the eye-ball, and b, b, are the upper and lower sides. Now in order 

 to prevent dust and other bodies from working their way between the 

 call and the lid, through passages at c, c, into the socket of the eye, 

 where they would excite great inconvenience and pain, we find the 

 common skin of the eye-lids d, d, after covering their edges, turn in a 

 little way between the lid and the ball, and then turn back and are 

 reflected over the surface of the cornea ; where, to prevent the obstruc- 

 tion of vision, it becomes perfectly transparent. 



Fig. 23. 



The eye-lids separated, and viewed from behind ; a, the lachrymal 

 gland ; 6, the ducts from the lachrymal gland ; c, the mouths of these 

 ducts ; d, the puncta lachrymalia ; e, the meibomian glands, which 

 secrete the oily fluid. 



19* 



