1932] Poliak: Afferent Fiher Sijstem.s, Primate Cerebral Cortex 163 



more in the external half of the external sagittal layer and in both 

 horizontal branches. The visual radiation is identical with the fiber 

 lamina called by descriptive anatomy the external sagittal layer of 

 the parieto-occipital lobes (H. Sachs). This fiber lamina on cross- 

 sections through the hemisphere is shaped like a horse-shoe with its 

 concavity facing inward and embracing the internal sagittal layer, 

 the tapetum, the posterior horn of the lateral ventricle, and the fissura 

 calcarina (figs. 39, 55, 95). It can roughly be subdivided into an 

 upper and a lower horizontal branch, each of them entering its cor- 

 responding lip of the fissura calcarina, and into a vertical or per- 

 pendicular branch linking together both horizontal branches, and 

 reaching the occipital pole. 



The fibers of the external sagittal stratum have, on the whole, a 

 longitudinal oral-caudal course. They are grouped into bundles which, 

 in parallel arrangement, run their entire course occipitalward. The 

 dorsal and ventral fiber bundles very gradually accomplish a spiral 

 turn of ninety degrees or more around the lateral ventricle. Indi- 

 vidual fibers and bundles, especially those extremely medial in the 

 upper and lower horizontal branches, must twist considerably more. 

 Both upper and lower branches describe a spiral in the reverse sense 

 (fig. 2'2). The bundles of the vertical branch have a straighter course 

 toward the occipital pole. The totality of the fibers of the visual 

 radiation form a fiber ' ' fan ' ' with handle near the external geniculate 

 body, its "ribs" gradually but not equally diverging from one another 

 and radiating toward the cortex. The visual or external geniculo- 

 cortical radiation can be looked upon as a separate, specialized portion 

 of the huge diencephalo-cortical afferent fiber system. Near its origin 

 (cgl) it is close to the somatic sensory and auditory paths {sr and ar 

 in figs. 30-34, 51, 52) ; toward the cortex it diverges from these two 

 paths. However, it is accompanied for a considerable distance by 

 the posterior bundles of the somatic sensory (thalamocortical) fibers; 

 the dorsal bimdles of the visual path which form the upper hori- 

 zontal branch of the external sagittal layer are closest to the somatic 

 sensorj^ path (figs. 52-55, 67-69, 71). The intermediate and ventral 

 bundles forming the perpendicular and ventral horizontal branch, 

 especially the latter, are near their origin beneath the temporal lobe 

 close to the auditory path. Although the visual radiation forms a 

 well defined fiber system, it is penetrated by other systems: by 

 ascending and descending callosal fibers, by association fibers (figs. 

 87-94), and by corticofugal fibers descending from the occipito- 



