THE EYE AND THE SENSE OF SIGHT 



105 



Fig. 62. Diagram of optic 



chiasraa. 



Rays from 0, falling on the 

 similar regions of the retina 

 (D, D'), give rise to impulses 

 passing to the same half of 

 the brain. 



Beyond the junction of the optic nerves the course of a 

 visual impulse is called the optic tract. Some of the fibers 

 run directly to the visual center o ^ 

 from the retina, others pass to vX, 



the corpora quadrigemina and 

 other centers before reaching the 

 cortex. The gray matter of the 

 brain is regarded as the seat of 

 sense perception, and the sense 

 of sight is believed to be located 

 in certain groups of cells in the 

 hinder part of the cerebrum. 



141. The Eye. Something 

 more than the nervous apparatus 

 above described is needed to 

 enable one to perceive a definite 



image of a distant object. Light falling upon the general 

 surface of a retina with its conducting nerves and nerve 

 centers would result only in perception of light and of 

 color. But in the eye, lying in front of the retina, are cer- 

 tain refracting media which act as lenses to converge the 

 rays of light so that only points of the retina are affected 

 by them ; that is, the rays are brought to a focus, and 

 an image of the object from which the rays come is pro- 

 duced as in a photographer's camera. The eye lies in a 

 pyramidal cavity, called the orbit, having its apex directed 

 inward and backward. 



142. The Coats of the Eye. The eye is a nearly spherical 

 sac about one inch in diameter, made up of a firm wall of 

 tissues called the coats of the eye (Fig. 63). The outer of 

 these, composed of connective tissues, is opaque except at 

 the center of the front of the eye, where it becomes trans- 

 parent and is called the cornea; the remainder of this 



