ISO 



HUMAN BIOLOGY 



eye (Fig. 121). The opening of one of the canals may 

 be seen by looking into a mirror. Sometimes these canals 

 are stopped up, and what is called a " weeping eye ' 

 results. A temporary stoppage may occur during a cold 

 in the head. 



Tears prevent friction between eye and lid. Winking 

 applies the tears to the ball. Small glands along the 

 edges of the lids form a kind of oil which usually prevents 

 the tears from flowing over the lids. Sometimes this oily 

 secretion is so abundant, especially during sleep, as to 

 cause the lids to stick together. The mucous membrane 



of the eyelids 

 continues as a 

 transparent 

 membrane (the 

 c o n j un c t iva) 

 which passes 

 over the front of 

 the ball. 



The globe of 

 the eye consists 

 of its outer wall 

 and the soft con- 

 tents (Fig. 122). 

 The wall has three layers or coats. The outer coat is the 

 tough sclerotic (Greek, skleros, hard), composed of dense 

 connective tissue (Exp. 14). It gives strength and firm- 

 ness to the eyeball. It shows between the lids as the 

 ''white of the eye." It is white and opaque except in 

 front; there it bulges out to form the transparent cornea. 

 This clear portion of the wall may be seen by looking at 

 the eye of another from the side. 



The second coat, called the cJwroid, consists of blood 



VITREOUS HUMOR 



Iri 



''Retina 

 Chore id 

 Vclerotic coat 



FIG. 122. THE ANATOMY OF THE EYE. 



