^\;^^^v;a 



Fig. 75, Experiment IV. In this experiment, to whieli figure 76 also belongs, 

 a portion of the dorsal horizontal branch and in addition to it, the nearby por- 

 tion of the vertical branch of the visual radiation were interrupted. In contrast 

 to Experiments II and III (figs. .55-57, 69-73, and 74), where the dorsal rib 

 of the radiation supplying the upper lip of the calcarine fissure degenerated 

 alone and was interrupted completely, here the interruption of the upper 

 horizontal branch was incomplete, its most internal bundle being left unaltered; 

 yet the destruction extended to the nearby portion of the vertical branch. 

 The degenerated bundle of the visual radiation is therefore somewhat different 

 and supplies in oral planes corresponding with figure 75, only the lateral 

 half of the striate cortex of the upper lip closer to the bottom of the 

 fissure (zone a-b), leavang the actual "boundary zone" between the letter (a) 

 and the arrow with normal fibers. In this experiment the interrupted bundle 

 becomes a "boundary bundle" only in more caudal sections corresponding with 

 figure 76, where its fibers spread as far medially as the striate cortex extends 

 (boundary of the striate cortex containing a double intracortical stripe marked 

 by an arrow). No afferent visual fibers whatever enter into the parastriate 

 cortex marked with number 18. Furthermore this figure illustrates the manner 

 in which the dorsal horizontal branch reaches the upper lip. Its fibers do not 

 turn in one and the same plane medially, but only gradually by slowly ascend- 

 ing in their course oceipitalward and slowly bending around the narrow ridge 

 of the fiber fold formed by the tapetum, and again descending toward the 

 upper lip where they turn again in part in the rostral direction. Thus the 

 bundles before reaching the striate cortex describe a spiral, those destined 

 for the oral segments of the upper lip forming at the same time an arc in the 

 sagittal plane with its concavity turned rostrally; this arc, therefore, remains 

 in figure 75 incomplete, being completed in caudal levels represented by 

 figure 76. Note the sharp line of demarkation between the degenerated and 

 normal segments of the external sagittal layer. Lower lip (Li), upper lip (Ls) 

 of the fissura calcarina (FC). 



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