1932] Poliak: Afferent Fiber Systems, Primate Cerebral Cortex 73 



nucleus and the thalamus on the one hand, and the putamen and 

 Sylvian fossa on the other. At this spot the entire sensory radiation 

 can easily be interrupted by a relatively small lesion ; whereas below 

 that spot in the ventral portion of the internal capsule and in the 

 thalamus itself, as also above it in the centrum semiovale, the fascicles 

 of the somato-sensory radiation diverge from one another. The thal- 

 amo-cortical radiation can be imagined as composed of individual fiber 

 segments or sheets ("fans") arranged in a certain definite way. 

 Bundles, or fans originating from the caudal portion of the thalamus 

 occupy a caudal position in the radiation; those from the rostral 

 portion remain anterior in the radiation; this indicates a transverse 

 segmentation of the sensory radiation perpendicular to the long axis 

 of the hemisphere and thalamus. Other factors indicate a longitudinal 

 segmentation of the radiation wherein the fascicles from the dorsal 

 portion of the lateral nucleus reach the dorsal zones of the somato- 

 sensory cortex, and those from the ventral portions of the lateral 

 nucleus reach the ventral zones of that cortex. On the one hand, this 

 would explain the functional segmentation of the thalamus, and, on the 

 other hand, could be related to the cytoarchitectural and functional 

 segmentation of the somato-sensory cortex into several parallel areas 

 along the sulcus centralis. Also such a regular or ' ' spatial ' ' arrange- 

 ment of the thalamo-cortical radiation, together with what is known of 

 the composition of the lower links of the somatic sensory paths, and 

 what is known from physiological, clinical, and pathological observa- 

 tions of the projection of the peripheral receptive surface of the body 

 upon the somato-sensory cortex, the so-called somato-topic cortical rep- 

 resentation of the body (compare Gushing, 1909; Valkenburg, 1914; 

 Vogt, 1919; Foerster, 1927; and Manlcowski, 1929), suggests the 

 mechanisms underlying that property of the somato-sensory function 

 called "localization" or spatial discrimination (see also Henschen, 

 1918, p. 456). But here further detailed investigation is necessary. 

 (Compare Chapter XIX.) 



Another feature of the thalamo-cortical radiation is the strictly 

 unilateral course of that entire fiber system. No fibers which would 

 cross to the opposite hemisphere by way of the corpus callosum, con- 

 trary to the supposition of some investigators, were seen in any of the 

 examined series where the thalamus alone or the internal capsule 

 alone was injured. They show not the slightest tendency to turn into 

 the corpus callosum although those fibers which spring from the most 

 dorsal portion of the dorso-lateral nucleus close to the caudate nucleus 



