646 



MEDULLA OBLONGATA AND THE CRANIAL NERVES 



retinse to the opposite side and leaves the fibers from their outer 

 halves on the same side. Thus, the right occipital center innervates 

 the right halves of both retinae, and the left center their left halves. 

 The yellow spot of each eye, on the other hand, is innervated by both 

 centers. 1 



Posteriorly to the chiasma, these crossed and uncrossed fibers con- 

 tinue upward and backward in the form of the optic tracts. Having 

 passed the surface of the thalamus, they divide into two groups, one 

 of which terminates in the lateral geniculate body and the other in 

 the roof of the colliculus of the midbrain. In this way, certain reflex 

 centers are established which are concerned with the movements of the 



LEFT RETINA 



RIGHT RETIN/I 



FIG. 319. DIAGRAM SHOWING THE PROBABLE RELATIONS BETWEEN THE PARTS OF THE 

 RETINA AND THE VISUAL AREA OF THE CORTEX. THE BILATERAL REPRESENTATION OF THE 

 FOVEA is INDICATED BY THE COURSE OF THE DOTTED LINES. (Schafer.) 



eyeballs, the process of accommodation, and other reactions. This is 

 true especially of the colliculus, while the thalamus seems to be set 

 aside rather as a relay station in the path leading to the visual center 

 situated in the occipital cortex of the cerebrum. The latter, therefore, 

 forms a direct dependency of the cortical center and hence, its impor- 

 tance must increase with the development of the center for vision. We 

 find here, therefore, an arrangement very similar to that previously 

 noted in the case of the olfactory mechanism, i.e., the light impressions 

 received by the retinae, may actually reach the center for vision to be 

 associated or may be transferred unto a motor path in the lower reflex 

 center situated in the superior colliculus. In the former case, they must 

 first give rise to a psychic impression, and, in the latter, to a simple 

 1 Wilbrand and Sanger, Die Neurologic des Auges, Wiesbaden, 1904. 



