THE CRANIAL NERVES. 



577 



VISUAL ffLD 



VISUAL HEW 



amination of a cross-section of the optic nerve shows the presence of a wedge- 

 shaped tract occupying the center of the nerve which is regarded as 

 composed of the macular fibers. At the chiasm this bundle of fibers un- 

 dergoes a partial decussation similar to that of the fibers coming from the 

 more peripheral portions of the retina. In the left optic tract, therefore, fibers 

 from at least four different regions are to be found: viz., the two-thirds of 

 the temporal side of the left retina, the temporal half of the left macula, 

 the nasal third of the right retina, and the nasal half of the right macula. 

 Corresponding fibers are to be found in the right optic tract. As the optic 

 tract passes around the crus cerebri 

 it divides into a lateral or outer, and 

 a mesial or inner bundle, which then 

 terminate in the optic basal ganglia. 

 The fibers of the lateral bundle are 

 traceable into the lateral or external 

 geniculate body (the pre-geniculum) , 

 the pulvinar of the optic thalamus, 

 and the anterior quadrigeminal body 

 (the pre-geminum) . With the excep- 

 tion of the fibers passing to the ante- 

 rior quadrigeminal body, these are 

 in all probability the true visual fibers. 

 The fibers of the mesial bundle are 

 traceable into the internal geniculate 

 body (the post-geniculum) and the 

 posterior quadrigeminal body (the 

 post-geminum). These fibers are 

 not a part of the optic nerve proper, 

 but commissural fibers associating 

 the internal geniculate bodies of the 

 two sides. (Gudden's Commissure.) 



Cortical Connections. After 

 entering the pulvinar and the lateral 

 or external geniculate body 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 

 curving backward to form the optic radiation of Gratiolet, and terminate 

 finally around nerve-cells in the gray matter of the cuneus and in the gray 

 matter bordering the calcarine fissure, both situated on the mesial aspect 

 of the occipital lobe. 



Centrifugal Fibers of the Optic Nerve. All the fibers previously alluded 

 to have been afferent or centripetal in direction; but the optic nerve also 

 contains efferent or centrifugal fibers which come from nerve-cells in the 

 basal ganglia and ramify around special cells, the amacrine 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 superior quadrigeminal body. 



37 



Optic 



FlG - 266. DIAGRAM ILLUSTRATING LEFT 

 HOMONYMOUS LATERAL HEMIANOPSIA FROM A 

 LESION OF THE RIGHT OPTIC TRACT OR THE 

 RIGHT CUNEUS. THE SHADED LINES IN THE 

 VISUAL FIELDS INDKATE THE DARKENED A*EA. 



