THE CRANIAL NERVES. 



529 



VISUAL FIELD 

 , Ha 



VISUAL FIELD 



the mesial bundle are traceable into the internal geniculate body 

 (the post-geniculum) and the posterior quadrigeminal body (the post- 

 geminum). 



Cortical Connections. After entering the basal ganglia the 

 visual fibers terminate in end-tufts which arborize* around nerve- 

 cells. From these cells new axons arise which ascend through the 

 posterior part of the internal 

 capsule, at the same time curv- 

 ing backward to form the optic 

 radiation of Gratiolet, and 

 terminate finally around nerve- 

 cells in the gray_rnjjtter of the 

 cuneus and in the gray matter 

 Horcterlng the calcarine fissure, 

 both situated on the mesial 

 aspect of the occipital lobe. ^ 



CentrifugaTjFl6erT~ol the 

 Optic Nerve. All the fibers 

 previously alluded to have been 

 afferent or centripetal in direc- 

 tion ; but the optic nerve also con- 

 tains efferent or centrifugal fibers 

 which come frorrT nervScens" in 

 the basal ganglia and ramify 

 around special cells, the amac- 

 rine cells, in the retina. Their 

 function is unknown. It has 

 been suggested that they regulate 

 the vascular supply to the retina. 

 Centrifugally coursing fibers also 

 connect the visual areas of the 

 cortex with the basal ganglia. 



Functions. The optic nerve 

 apparatus in its entirety con- 

 nects the visual cells of the retina 

 with the cells of the cerebral cor- 

 tex. The excitation of this ap- 

 paratus evokes the sensation of light and its different qualities colors. 



The specific physiologic stimulus of the visual retinal cells is the 

 impact of the undulations of the ether. The energy set free excites 

 in the optic nerve, nerve impulses which are transmitted first to the 

 optic ganglia and then to the cerebral cortex, where they evoke the 

 sensations of light and color. 



Iris Reflex. The optic nerve also assists in the automatic reg- 

 ulation of the size of the pupil. Some of its fibers form the afferent 



34 



FIG. 241. DIAGRAM ILLUSTRATING LEFT 

 HOMONYMOUS LATERAL HEMIANOPSIA 

 PROM A LESION OF THE RIGHT OPTIC 

 TRACT OR THE RIGHT CUNEUS. THE 

 SHADED LINES IN THE VISUAL FIELDS 

 INDICATE THE DARKENED AREA. 



