1932] Poliak: Afferent Fiber Systems, Primate Cerebral Cortex 31 



toward the operculum can they be identified. In the same way that 

 the ventral thalamo-cortical bundles turn lateralward to reach the 

 opercular region (figs. 29-31), the dorsal bundles from the dorsal 

 portion of the middle segment of the dorso-lateral nucleus of the 

 thalamus curve dorso-medially in more rostral planes to reach the 

 dorsalmost segment of the preeentral convolution (figs. 26, 27). 

 Ventral bundles, as evident from figures 30 and 31, after crossing the 

 most ventral region of the internal capsule and the triangular field 

 of Wernicke (beginning of the visual path) penetrate in part the 

 caudal portion of the putamen dividing the latter nuclear mass into 

 several islets {Put in fig. 32) ; they finally enter the external capsule 

 where they turn dorsally, ascending here pressed closely to the clau- 

 strum and appearing as a thin layer of short oblique fiber segments 

 (figs. 33, 34). The more ventral of these bundles, some of them within 

 the extreme capsule (figs. 31, 33), belong evidently to the central 

 auditory path (others to the anterior commissure). 



The course of various bundles of the thalamo-cortical radiation can 

 easily be seen in the accompanying illustrations, in which, also, the 

 most caudal fibers are shown as participating in the formation of the 

 external sagittal layer (stratum) of the parieto-occipital lobe. This 

 will be more evident in the following experiments. (On the cortical 

 region receiving thalamic fibers and on the mode of intraeortical 

 termination of these, see corresponding chapters.) 



In the accompanying illustrations it can also be seen that all 

 thalamo-cortical fibers, whether arising from the dorsal or from the 

 ventral region of the thalamus, converge, in their ascent through the 

 internal capsule, at a spot close to the dorsal comer of the thalamus 

 (fig. 30). They must all pass through this narrow passage to reach 

 the centrum semiovale situated above, and finally, the cortex. At the 

 point of convergence they can all be easily interrupted by a com- 

 paratively small lesion (compare Experiment III and V-a, figs. 5, 66), 

 whereas a small lesion situated below or above that spot or in the 

 thalamus itself will produce, on the contrary, degeneration of only 

 part of the thalamo-cortical radiation. This is due to the spread of 

 the various individual fiber bundles of the radiation when still within 

 the thalamus or in the centrum semiovale above the narrow passage 

 mentioned, and accounts for the smaller absolute number of degen- 

 erated thalamo-cortical fibers in Experiment I than in Experiment II, 

 III, and V-A. 



