THE SPECIAL SENSES. 



135 



of the lids keep the edges smooth and prevent them sticking 

 to each other. 



The eye-brows are also shades for the eyes, and help to 

 protect them from the fierce rays of the sun. These fringes 

 of hair incline outwards so as to divert the perspiration from 

 the forehead off to the sides and thus prevent it trickling 

 down into the eyes. 



13. The Tears. While we are awake, the eye^ball is in 

 almost constant motion. With every movement the eye rubs 

 against the lids, and in 

 order that there may be 

 no friction, the surface 

 is moistened by a wa- 

 tery fluid which when 

 it overflows forms tears. 

 This fluid is secreted by 

 a gland about the size 

 of an ordinary bean, 

 situated in a hollow in 

 the upper and outer part 

 of the socket. This tear- 

 gland has several small FlG - 41. The Eye-lashes and the Tear Glands: 

 T. i B, tear duct; C, C, tear canals; D, tear sac; G, 



ducts, which open at teargland . 

 the outer corner of the 



eye beneath the upper eye-lid. The motion of the eye and 

 the winking of the lids spread the fluid over the exposed 

 surface. The same movements carry the fluid to the inner 

 angle of the eye, where it collects in a small hollow between 

 the two lids, and is drained off by small canals, one in the 

 upper and one in the lower lid. These canals run inward 

 to a sac in the corner of the nose, and from this sac the fluid 

 descends into the nose. Any local irritation of the eye or 

 mental emotion, such as weeping or crying, causes more fluid 

 to form than can be carried off by the canals. It overflows 

 and runs down the cheeks in tears. 



