152 HUMAN BIOLOGY 



^ Read this line slowly. Can you see the star all the 

 time ? (If so, hold the book farther or closer and repeat.) 

 Within the coats of the ball, like the pulp within the 

 rind of an orange, are the soft contents, divided into three 

 parts. The first is a watery liquid in front, which serves 

 to keep the cornea bulged out (Fig. 122). It is called the 

 a'qne-oiis humor. The main cavity of the ball is occupied 

 by a clear, jelly like substance called the vit^re-ous hninor, 

 which serves to keep the ball distended. Back of the iris, 

 and separating the two humors just named, is the crystal- 

 line leus, a beautiful clear lens, convex or rounded out on 

 both sides (Exp. 14). It serves to bring the light to a 

 focus on the retina, thereby forming images of outside 

 objects. 



The eye, like a camera, has a dark lining, the choroid; 

 the retina corresponds to the sensitive plate, and the lens 

 brings the rays to a focus on it and forms the image. 



The Path of Light in the Eye. The light enters through 

 the transparent cornea and passes through the aqueous 



humor. As it goes through 

 the pupil, the iris shuts off all 

 the light that is not needed. 

 The crystalline lens receives 

 the light that has been al- 

 lowed to pass, and so bends 

 the rays that by the time they 

 FIG. 124. CROSSING OF OPTIC have passed through the vit- 



NERVES showing that one nerve ^^ ^^ th ^ R 



reaches same half of both eyes. 



the retina in just the right 



way to form a tiny image of anything outside (Exp. 11). 

 The choroid absorbs any light that passes the retina. 

 The iris and choroid of albinos have no pigment; hence 

 albinos squint their eyes to shut out some of the light. 



