158 University of California Puhlications in Anatomy [yoh. 2 



(5) The fibers of the visual radiation are, on the whole, finer than 

 the somatic sensory (thalamo-cortical) fibers. Most of the visual fibers 

 are of medium size, only a small number having^ a larg^e caliber. The 

 coarse visual fibers almost exclusively enter the fissura calcarina and 

 the pole of the occipital lobe, the thin or medium sized fibers enter 

 the pole of the occipital lobe and the occipital operculum. The visual 

 fibers, however, exceed in caliber the association and callosal fibers of 

 the striate area and of the occipital lobe generally. 



(6) The visual radiation is composed of fibers grouped into fiber 

 bundles arranged, on the whole, in parallel fashion. Each fiber bundle 

 originates in a definite small segment of the external geniculate body 

 and terminates in a definite small segment of the striate area. 



(7) The visual radiation or the external sagittal layer of the 

 parieto-occipital lobe can be subdivided into three anatomical-functional 

 portions discernible on cross-sections perpendicular to the long axis 

 of the hemisphere: (a) the dorsal horizontal branch, (b) the ventral 

 horizontal branch, and (c) the vertical or the perpendicular branch 

 connecting both horizontal branches. The dorsal horizontal branch is 

 composed of fibers originating in primates from the medial (internal) 

 segment of the external geniculate body close to the thalamus which 

 corresponds with the upper extramacular segments of both homony- 

 mous hemiretinae, including the upper monocular portion of the 

 crossed retina. This branch supplies the striate cortex of the upper 

 (dorsal) lip of the fissura calcarina, with the most dorso-medio- 

 anterior bundle, the dorsal ' ' boundary bundle ' ' of the radiation, sup- 

 plying the most internal longitudinal "boundary segment" of the 

 upper lip, and with the successive lateral bundles supplying the more 

 lateral and somewhat caudal segments closer to the floor of the cal- 

 carine fissure and to the axis of the striate area. The ventral hori- 

 zontal branch is composed of fibers originating from the lateral 

 (external) segment of the external geniculate body close to the lenti- 

 forme nucleus, corresponding with the lower extramacular quadrants 

 of both homonymous hemiretinae, including the lower monocular por- 

 tion of the crossed retina. It supplies the striate cortex of the lower 

 (ventral) lip of the fissura calcarina. The narrow zones of the striata 

 area supplied here are probably similarly arranged longitudinally 

 as are those in the upper lip, but in reverse order, with the most 

 medial zones being supplied by the most ventro-medio-anterior bundle, 

 and so forth. The perpendicular (intermediate) branch of the visual 

 radiation is composed of fibers originating from the large inter- 



