1932] Poliak: Afferent Fiber Systems, Primate Cerebral Cortex 201 



identical with the fibers of the human striate area found by Ramon y 

 Cajal (1909-11, vol. 2, p. 614) and reg'arded by him as exogenous 

 afferent fibers orig-inating in the external geniculate body. 



2. INTEENAL OEGANIZATION OF THE VISUAL RADIATION 



The visual radiation is composed of individual bundles, each hav- 

 ing' its definite subcortical origin, course, and cortical termination. 

 Neither mixing- nor diffuse spreading of the fibers belonging- to dif- 

 ferent though neighboring bundles, nor appreciable overlapping of 

 the territories of the individual bundles, is discernible either during 

 their course or at their cortical termination. The small segments of 

 the striate cortex supplied by individual bundles or segments of the 

 visual radiation are clearly delimitable and even have sharp, linear 

 boundaries. The shape of such cortical segments is triangular or 

 approximately so. The shape, however, varies according to the region. 

 In the fissura calcarina the triangles are narrow and lie parallel to the 

 fissure, with their lateral side exactly parallel to the floor of the 

 fissure. The sharp wedge of such a triangle is turned oralward, toward 

 the splenium of the corpus callosum, that is, it lies at the rostral 

 beginning of the striate area where that area occupies only a small 

 portion of the floor of the fissure. As the striate cortex becomes larger 

 toward the occipital pole, extending in the direction of the brim of the 

 lips of the fissure, the extent of these triangles also becomes larger. 

 Over the occipital pole, and, in the monkey, over the occipital oper- 

 culum the triangles thus supplied are of more compact form, com- 

 parable to the calotte of a ball. Here the triangles remind one of the 

 triangular scotomata of the "central" portion of the visual fields; 

 this would indicate a subcortical rather than a cortical origin of these 

 scotomata. 



The mutual arrangement of bundles or sectors of the visual 

 radiation drawn from the present investigations is as follows (figs. 

 22 and 23) : 



The dorsal horizontal branch has its exclusive origin in the internal 

 segment of the external geniculate body, closest to the thalamus, and 

 terminates exclusively in the upper lip of the fissura calcarina, where 

 it descends by bending around the dorsal comer of the posterior horn 

 of the lateral ventricle. That branch remains dorsal during its entire 

 course, forming the dorsal "rib" of the fiber "fan" of the \'isual 

 radiation. Its course is, therefore, comparable to that of a spiral slowly 



