162 University of California Puhlications in Anatomy [Vol.2 



Since each of the constituent structures of the afferent visual system 

 and of the visual cortex has its own function, its destruction will be 

 followed by an irreparable and lasting loss of that particular func- 

 tion. The replacement or compensation of the lost function must be 

 achieved by what can be called a more economic utilization of the 

 parts remaining undamaged. 



1. VISUAL RADIATION; ITS SUBCORTICAL ORIGIN, ITS COURSE, AND 

 ITS CORTICAL TERMINATION 



In passing to the discussion of arguments for the above statements, 

 to remain on firm ground, the organization of the peripheral portion 

 of the visual system must first be considered. Here, naturally, we 

 must rely upon the fundamental studies of Brouwer and his collabo- 

 rators. As is evident from Brouwer-Zeeman's, Minkowski's, and 

 Overbosch's experiments, all peripheral optic fibers originating in the 

 retina terminate either in the external geniculate body and in the 

 corpus praegeniculatum, or in the superior colliculus of the midbrain. 

 This alone excludes the thalamus proper from the rank of a sub- 

 cortical visual center. Neither does there exist, as explained pre- 

 viously, other evidence which will withstand a critical examination, 

 to support the opinion of a threefold subcortical origin of the visual 

 radiation (Monakow), in man and other primates. The present experi- 

 ments are also clearly detrimental to that view since it was found that 

 the brachium of the superior colliculus degenerates in the cortico- 

 fugal sense, from the parieto-occipital cortex to the colliculus (figs. 

 36, 96), and no cells of the colliculi degenerated after the destruction 

 of the striate area (Experiment V-d and V-e). On the other hand, 

 no ascending fibers were found which might connect the pulvinar with 

 the occipital lobe. Equally the thalamus proper, its dorso-lateral and 

 its ventro-lateral nucleus, must be excluded from the direct eortieo- 

 petal visual path. Since all evidence points to the external geniculate 

 body as the only and exclusive internuncial station of the cortico- 

 petal visual fiber system, that nucleus wall therefore be considered 

 exclusively in all further discussions.^ 



The visual radiation with its exclusive origin in the external 

 geniculate body forms, on the whole, a single, compact, and well 

 defined fiber system. It is composed of fine and medium sized fibers, 

 preponderantly, and of coarser fibers. The coarser fibers are scattered 



The corpus or griseuin praegeniculatum must also be excluded. 



