THE SPECIAL SENSES 



26l 



When the light is very strong and brilliant the iris spreads 

 its curtain farther over the pupil in order to shut out some 

 of the rays. When the light is faint the curtain is drawn 

 back, making the pupil larger in order to admit as many 

 rays of light as possible. 



The black appearance of the pupil is due to the thick 

 black coating which lines the inside of the choroid and pre- 

 vents all light from passing through the wall of the eyeball. 

 It is like looking through a small window into a dark room. 



FIG. 162. 



Diagram showing the Change in the Lens during 

 Accommodation. 



On the right the lens is arranged for distant vision, the ciliary muscle is relaxed, 

 and the ligament D is tense, so flattening by its compression the front of the 

 lens C; on the left the muscle A is acting, and this relaxes the ligament and 

 allows the lens B to become more convex, and so fitted for the vision of 

 near objects. 



360. The Crystalline Lens. Just behind the iris is a clear, 

 transparent, jellylike body, called the crystalline lens. It is 

 convex, or rounded, both back and front, and is about one 

 third of an inch in diameter. It is shut up in a kind of 

 transparent bag and is held in its place by a number of 

 little bands. The crystalline lens separates the front 

 chamber of the eye from the back chamber. 



The front chamber is filled with a clear, watery fluid 

 called the aqueous, or watery, humor, This fluid keeps the 



