THE EYES 235 



rule, having dark eyes, and a blonde, eyes of a light 

 color. 



By looking into a mirror we can see a little hole in 

 the iris which lets the rays of light pass into the inner 

 eye. This little round door, called the pupil, is con- 

 trolled by some wonderful muscles which have the 

 power of contracting, or making it grow smaller, when 

 the light is strong, and expanding, or making it become 

 larger, as the light grows weaker. All may have noticed 

 that on coming suddenly into bright light, the eyes 

 squint. This results from the quick effort of the 

 muscles to protect the eye. 



In the cut on page 234 we notice that in front of the 

 iris is the aqueous (watery) humor, which seems to keep 

 the cornea bulged out. Back of the iris we also see 

 a convex-shaped humor called the crystalline lens, 

 which performs the same work as the lens of a camera, 

 for its office is to bring the rays of light to a focus on 

 the retina. The latter, we observe in the same illus- 

 tration, is the innermost coat of the eye. We also see 

 that between the lens and the retina there is the vitreous 

 humor. This is a jellylike substance, occupying about 

 four-fifths of the interior of the eyeball. It is color- 

 less and transparent, and besides maintaining the 

 form of the eyeball, helps to protect the sensitive retina. 

 The optic nerve, which the picture shows extending 

 from the back of the eye, transmits to the brain the 

 impressions received by the retina. 



Comparing the eye to a camera helps us to under- 



