1932] Poliak: Afferent Fiber Systems, Prim-ate Cerebral Cortex 25 



medullary stratum, less the intermediate and zonal strata. The deep 

 medullarj^ layer of the superior eolliculus and central gray substance 

 around the Sylvian aqueduct is not reached by this fiber system. Nor 

 is there evidence that these fibers cross either to the opposite superior 

 eolliculus or reach the inferior colliculi. The tract in question is the 

 occipito-parietal cortico-tectal fiber system for ocylogyric and other 

 eye and head movements. The degeneration in Experiment I of this 

 entire tract toward the superior eolliculus speaks against the latter 

 nucleus being a subcortical visual center for cortico-petal impulses. 

 (Compare also: Visual System.)^ 



Another small fascicle of fine fibers degenerated in Experiment II 

 (figs. 52 and 53). From the external geniculate body it passes along- 

 side the internal geniculate body and through the pulvinar it reaches 

 the upper strata including the stratum zonale of the superior eollic- 

 ulus, losing fibers which appear to terminate in the posterior nucleus 

 of Monakow situated between the two geniculate bodies. Here, how- 

 ever, it cannot be decided whether these fibers are special direct fibers 

 from the optic tract, or collaterals of these, or perhaps axis cylinders 

 of neurons situated in the external geniculate body.^ It is more than 

 probable that these superficial fibers which degenerated toward the 

 midbrain, whatever they may be, represent the tract for pupillary 

 light reflex (Ingvar, Brouwer-Zeeman ; see also Visual System in 

 this work). 



A number of degenerated fibers found in the thalamus may be of a 

 cortical origin. They descend from the hemisphere to the entire lateral 

 nucleus of the thalamus in its posterior segment, and to the ventral 

 portion of the pulvinar (fig. 66), while none were seen to enter the 

 dorsomedial region of the latter. These fibers are fairly numerous and 

 of various caliber, some being fairly coarse. (See also Chapter V, 

 Experiment III.) 



In a few instances fine blackened particles and "dust" found 

 within the thalamus can be interpreted as local short association fibers. 

 The dorsomedial region of the pulvinar, especially where it was 

 possible to exclude the presence of descending cortico-thalamic fibers 

 lends itself to this interpretation (fig. 36). 



1 Another experiment not reported here in full, where only the fields 18-19 of 

 Brodmann were damaged, substantiates this. No such, fibers descend from the 

 field 17, or the area striata. 



~ This latter possibility has to be discarded in view of the fact that the lateral 

 geniculate body degenerates completely after the complete destruction of the 

 ipsilateral striate area, as the Expei-iment V-E shows (see also Experiment V-d). 



