VISION. 469 



channel, they overflow upon the cheeks; but 

 when the quantity is not excessive, the ten- 

 dency to flow over the eye-lid is checked by an 

 oily secretion proceeding from a row of minute 

 glands, situated at the edge of the eye-lids, and 

 termed the Meibomian glands. 



The eye-brows are a further protection to the 

 eyes ; the direction of the hairs being such as 

 to turn away from them any drops of rain or 

 of perspiration, which may chance to fall from 

 above. 



Excepting in front, where the eyes are covered 

 and protected by the eye-lids, these important 

 organs are on all sides effectually guarded from 

 injury by being contained in a hollow bony 

 socket, termed the orbit, and composed of seven 

 portions of bone. These seven elements may be 

 recognised in the skulls of all the mammalia, 

 and perhaps also in those of all other verte- 

 brated animals ; affording a remarkable illustra- 

 tion of the unity of the plans of nature in the 

 construction of the animal fabric. 



§ 4. Physiology of perfect Vision. 



The rays of light, proceeding from a distant 

 object, strike upon the convex surface of the 

 cornea, which being of greater density than the 



