104 



CONSCIOUS NERVOUS OPERATIONS 



and gives rise to the consciousness of sight (Fig. 61). 



Nerve fibers are also distributed to the numerous muscles 



of the various parts of the 

 eye and its accessories. 



139. Nerves of the Eye. 

 The second pair of cranial 

 nerves are the optic nerves 

 (Fig. 19, p. 29). The third 

 pair go to four of the mus- 

 cles which move the eyeball, 

 and are called the oculomotor 

 nerves. The fourth and sixth 

 pairs also supply muscles of 

 the eyeball. From the fifth 

 pair of nerves (the trigemi- 

 nal) are sent branches to the 

 lachrymal glands and the 

 eyelids. 



140. Course of the Visual 

 Impulse ( Fig . 01). Rays of 



light falling upon the retina produce certain changes 

 (chemical or other) in or about the peculiarly shaped 

 nerve cells called rods and cones. These changes excite a 

 nervous impulse which is conducted by the minute nerve 

 fibers from the rods and cones (the end organs of vision) 

 along the optic nerves which, after passing through the 

 opening in the back of each eye socket, unite at what is 

 called the opfic commissure. Here many of the nerve 

 fibers cross, but some from each eye pass on to the nervous 

 center of the same side ; so that if the centers of vision on 

 one side were destroyed, there would still be sight in both 

 eyes. This crossing of the optic nerve fibers is called the 

 optic chiasma (Fig. 62). 



Fig. 61 . Diagram of path 

 of optic impulse. 



Three courses are possible : (1) di- 

 rectly from cells of retina to the 

 visual center (OG") ; (2) through a 

 relay in the optic thalamus (T); 

 (3) through a relay in the anterior 

 corpora quadrigemina (Q). 



