THE EYE 355 



The Iris. The colored part of the eye is called the iris. Its 

 function is to regulate the amount of light that enters the eye. The 

 circular opening in the center of the iris is called the pupil. Circular 

 muscular fibers run around the pupil; and when they contract the 

 pupil is made smaller. Other muscje fibers run in from the outer 

 edge of the iris to the pupil; and when these contract the pupil is 

 enlarged. The size of the pupil is under involuntary control. When 

 a strong light enters the eye, the pupil is diminished, while in a weak 

 light the pupil is enlarged to admit more light. It takes a few 



Pupil of eye expanded to let in plenty Same pupil contracted to shut out 



of light when illumination is dim. excessive light. 



FIG. 236. 



minutes for the pupil to adjust itself to sudden changes in the quan- 

 tity of light. 



A cat's eyes show very distinctly the changes in the size of the 

 pupil. Examine the eyes of a cat in the light, and after shutting 

 the cat in a dark room examine the eyes again. 



Owls have very large pupils which enable them to see at night 

 better than most animals and birds; but a bright light dazzles their 

 eyes, as the pupils can not be made sufficiently small! Many animals, 

 as the cat and horse, can see better at night than man can see, 

 because their pupils can be opened wider than the pupils of the 

 human eye. 



Darken the room. Look at the iris of someone's eye. Quickly bring a candle 

 near the eye. Notice how the iris slowly closes. 



