ADDITIONAL TOPICS IN HUMAN BIOLOGY 168 



is white in color, except in front, where it becomes the transparent 

 cornea. 



Inside of the outer coat is a second layer which is seen beneath 

 the cornea as a colored ring known as the iris. In the center of the 

 iris is a circular opening, the pupil, which is black in appearance. 

 Through the pupil enter the rays of light into the interior of the 

 eyeball. If one comes suddenly from a dark room into the light, it 

 is possible to see this opening 

 quickly decrease in size. The inner 

 lining of the eyeball is extremely 

 thin and black in color ; it is known 

 as the retina, and connected with 

 it are the many nerve fibers that 

 carry messages to the brain. 



Behind the iris is a beautiful 

 transparent object, the crystalline 

 lens, both surfaces of which are 

 convex. The space within the eye- 

 ball in front of this lens is occupied 

 by a liquid, and behind the lens is 

 a jellylike substance. 



FIG. 52. Section of the eye. 



246. The eye as a camera. 



Any one who is at all familiar with 



a camera knows that by means of 



a lens, or a combination of lenses, 



the scene to be photographed is 



made to appear upside down on the ground glass plate at the back 



of the camera. If the image is not clear, it is brought into focus 



by moving the lens nearer to, or farther from, the object. 



In the eye, too, we have an arrangement similar to that of a 

 camera, since the convex surfaces of the cornea and crystalline lens 

 (Fig. 53) focus the rays of light so that an image is formed on the 

 sensitive retina at the back of the eye. Since, however, the lenses 

 within the eye cannot be moved backwards and forwards, as in a 

 camera, focusing or accommodation of the eye must be accomplished 



C = Cornea. 

 I = Iris. 



L = Crystalline lens. 

 ON = Optic nerve. 



R = Retina. 

 V. H. = Jellylike substance. 



