THE EYE 



335 



in the retina. The retina corresponds to the photographic 

 plate of a camera. A bulging transparent tissue, called 

 the cornea, forms a round window through the front part 

 of the eyeball, and admits light to the retina. Behind the 

 cornea is hung a curtain called the iris, in whose center is 

 a hole called the pupil. The iris is colored to a shade 



a bone of the orbit. 



b muscle which moves 



the eyeball. 

 c sclerotic coat. 

 d choroid coat. 



The human eye. 



e retina. 

 / eyelid. 

 g iris. 

 h lens. 



* cornea. 

 j muscle which changes 



the shape of the 



lens. 

 k optic nerve. 



varying from blue to dark brown, and it is this which gives 

 the color to the eye. 



The iris is composed of muscle fibers which can contract so as to 

 make the pupil smaller. A strong light acts in a reflex way to cause 

 the iris to contract and make the opening of the pupil smaller, but in a 

 dim light the pupil is large, so as to admit all the light possible. Thus 

 the iris regulates the amount of light admitted to the retina. 



606. Sight Behind the pupil is a lens whose shape 

 can be changed at will by the action of muscles. The 



