44 ELEMENTARY PHYSIOLOGY. 



sation is very painful. No light passes through the 

 sclerotic coat; but the cornea is very transparent. 



Next to the tough outside white coat 

 The Black Hes ^ chor aid. This is a soft black 



t^oat. 



membrane. It prevents the reflection 



of strong light from the inner surf ace of the eyeball, 

 and this serves an important part in making the sight 

 sharp and clear. The front part of the choroid coat 

 is arranged like a circular curtain. This is called 

 the iris. This is what gives the eye its so-called 

 color. The difference between a black eye and a 

 blue eye is, that the cells of the iris of the one have 

 a black coloring matter in them, while the cells of 

 the iris of the other contain blue coloring matter. 

 In the center of the iris is a circular opening called 

 the pupil which you can see by looking directly into 

 the eye of another person who stands close before you. 

 Through this little circular window, surrounded by 

 the curtains of the iris, the light must pass on its 

 way to the back inner part of the eyeball. The 

 amount of light which passes through the pupil is 

 regulated by an interesting action of the iris. When 

 the light is strong, the little muscles which are 

 threaded through the curtain produce the effect of 

 making the pupil smaller so as to pass less light. 

 When we go from a light place into a dark, these 

 same muscles bring about an opposite effect, that is, 

 the pupil is made larger so as to admit more rays. 

 This adjustment of the curtain of the eye is not in- 

 stantly done. It requires some time. This you can 



