266 



OUR BODIES AND HOW WE LIVE 



The eyebrows form a protecting and shading ridge over 

 the eyes, while. the thick fringe of hairs, arranged some- 

 what like the straw on a thatched roof, prevents the per- 

 spiration from rolling into the eyes as it trickles down the 



forehead. 



366. The Tears. 

 Nature provides a 

 special fluid to pro- 

 tect the eye. This 

 fluid is call ed the 

 tears. 



The tear apparatus 

 consists of the glancl 

 for secreting a thin, 

 watery fluid, the 

 tears, and the pas- 

 sages for draining 

 them off. The tears 

 moisten the surface 

 of the eye whenever 

 the lids wink, and 

 wash away the par- 

 ticles of dust. 



The tears are 

 carried off by two 

 fine tubes, one in the 

 upper and one in the 

 lower lid, which unite 

 and form a sac from which the tear duct leads into the nose. 

 The ordinary flow of tears is thus drained off. 



Emotion may excite an excessive flow which the canals 

 cannot carry off. The tears then overflow and run down 



FTG. 165. The Relative Position of the 

 Lachrymal Apparatus and the Eyelids. 



A and C, lachrymal canals; B, lachrymal sac; 

 D, small muscle which serves to compress the 

 lachrymal sac; E, lining membrane of the upper 

 and iower eyelids; F, glands upon the inner 

 surface of the eyelids, with ducts opening upon 

 the free margins of the eyelids ; H, great open- 

 ing, or antrum, of the upper jawbone. The oil 

 glands of the nose are plainly shown. 



