114 University of California Publications in Anatomy [Vol.2 



and the thalamus must be left open (either special neurons^ or col- 

 laterals of incoming peripheral optic fibers or such collaterals given 

 off by fibers of the visual radiation) . 



The degenerated cortico-petal visual fibers after leaving the tri- 

 angular field of Wernicke and after crossing the internal capsule where 

 they lie somewhat scattered (fig. 52, degenerated area vr above the 

 external geniculate body) gradually gather and become condensed into 

 one, single degenerated zone corresponding with the dorsal horizontal 

 branch of the external sagittal stratum {vr in figs. 53-57). It is 

 natural that the \asual path while still in the internal capsule does 

 not form a compact fiber system ; the majority of capsular fibers 

 belong to other fiber systems. Only when visual fibers reach anterior 

 levels of the occipital lobe do they approach close enough to form a 

 dense fiber sheet composed almost exclusively of visual fibers (figs. 

 55-57). Here these fibers occupy, consequently, a smaller territory. 

 When the sagittal strata of the parieto-occipital lobe take their 

 proper shape, the afferent visual fibers remain strictly within the 

 limits of these fiber formations and remain so till they approach their 

 cortical termination. 



The bundle of the visual radiation springing from the internal seg- 

 ment of the external geniculate body forms, accordingly, the dorsal 

 horizontal branch of the external sagittal layer {vr in fig. 55). This 

 bundle represents the dorso-medial rib or radius of the fiber fan of 

 the visual radiation. Its degenerated fibers, similarly as in Experi- 

 ment III and also in Experiments I, II, and V-a, do not mix to any 

 considerable extent with the postero-dorsal fascicles of the somatic sen- 

 sory (thalamo-cortical) radiation {sr in figs. 52, 53 ; a prolonged con- 

 fusion was produced when these thalamic fibers were proclaimed a por- 

 tion of the visual system). The visual fibers, although close to and 

 immediately below the somatic sensory fibers, occupy in reality a sepa- 

 rate zone. Soon, however, both visual and somato-sensory fibers separate 

 from each other. The degenerated zone of the visual portion of the 

 parieto-occipital sagittal strata occupies from one-half to two-thirds 

 or even three-quarters of the thickness of these layers which is the 

 external sagittal stratum. Although the latter gradually decreases 

 in size occipitalward, it becomes relatively larger as the internal 

 stratum is reduced. Fibers of the degenerated visual bundle are 



1 This does not appear probable because in the experiments of Brouwer and 

 Heuven after destruction of the striate area, all the cells of the external geniculate 

 body degenerated. The same is proved by our Experiment V-d and V-E. See these.) 



