410 THE NERVOUS SYSTEM AND ORGANS OF SENSE 



body. Despite the fact that the retina is less than jV of an inch in thick- 

 ness, there are several layers of cells in its composition. The optic nerve 

 enters the eye from behind and spreads out over the surface of the retina. 

 Its finest fibers are ultimately connected with numerous elongated cells 

 which are stimulated by light. The retina is dark pm-ple in color, this 

 color being caused by a layer of cells next to the choroid 

 coat. This accounts for the black appearance of the 

 pupil of the eye, when we look through the pupil into 

 the darkened space within the eyeball. The retina acts 

 as the sensitized plate in the camera, for on it are received 

 the impressions which are transformed and sent to the 

 brain as sensations of sight. The eye, Hke the camera, 

 has a lens. This lens is formed of transparent, elastic 

 material. It is found directly behind the iris and is 

 attached to the choroid coat by means of delicate liga- 

 ments. In front of the lens is a small cavity filled with 

 a watery fluid, the aqueous humor, while behind it is the 

 main cavity of the eye, filled with a transparent, almost 

 jellylike, vitreous humor. The lens itself is elastic. This 

 circumstance permits of a change of form and, in con- 

 sequence, a change of focus upon the retina of the lens. By means of 

 this change in form, or accommodation, we are able to distinguish be- 

 tween near and distant objects. 



Defects in the Eye. — In some eyes, the lens is in focus for near objects, 

 but is not easily focused upon distant objects ; such an eye is said to be 



Diagram show- 

 ing how the 

 lens changes 

 its form. 



Diagram to show how an image is formed in the eye : a, object ; b, lens ; c, image 



upon retina. 



nearsighted. Other eyes which do not focus clearly on objects near at 

 hand are said to be farsighted. Still another eye defect is astigmatism, 

 which causes images of lines in a certain direction to be indistinct, while 



