1932] Poliak: Afferent Fiber Systems, Primate Cerebral Cortex 75 



characteristics which are common to all. The densest bundles of the 

 thalamo-cortical radiation enter the cortex lining the floor of the sulcus 

 centralis and its immediate vicinity along both walls of that sulcus. 

 The number of afferent fibers gradually decreases toward both lips of 

 the central sulcus belonging to both precentral and postcentral con- 

 volutions. Since, according to cytoarchitectural investigations, the 

 postcentral granular type of the cortex, besides occupying the whole 

 postcentral region (that is, the convexity of the postcentral convolu- 

 tion and the posterior wall of the sulcus centralis), pushes out for a 

 short distance to the anterior wall of the sulcus centralis, it is more 

 than probable that the zone which, according to the present investiga- 

 tions, receives the most abundant afferent fiber supply in the entire 

 somato-sensory cortex, is identical with the most anterior narrow strip 

 of the postcentral cortex around the fundus of the sulcus centralis, 

 where the inner granular layer has also its best development (area 

 3 of Brodmann). This zone must, therefore, be considered as the 

 "nuclear or focal zone" of the entire somato-sensory region of the 

 hemisphere (deeply shaded, narrow areas on both sides of the sulcus 

 centralis C in fig. 6, in reality entirely sunk in the sulcus) . The num- 

 ber of the afferent fibers, as has been said, decreases gradually in both 

 the anterior and posterior walls of the central sulcus toward the con- 

 vexities of the precentral and postcentral convolutions. This slow and 

 gradual decrease does not. however, show any particular difference 

 between the two walls. A further decrease is noticeable in the cortex 

 covering the convexities of both the central convolutions. Thus the 

 areas 1, 2, and 4 of Brodmann (wider doubly shaded areas in front and 

 behind the sulcus centralis in fig. 6) being about equally supplied with 

 the afferent fibers, though less abundantly than the "focal zone" 

 around the bottom of the sulcus centralis (area 3 of Brodmann), form 

 a belt in front of and behind the "focal zone." However, in all three 

 areas mentioned which do not belong to the ' ' focal zone ' ' the number 

 of afferent fibers is still quite considerable. In front of area 4 and 

 behind the postcentral region there is the third or outer belt of the 

 somato-sensory cortex where the number of sensory fibers shows a 

 further gradual decrease until tlieir complete disappearance toward 

 the fringes of the somato-sensory region (areas 6, 5, and the oral por- 

 tion of area 7, lightly shaded areas in fig. 6). Area 6 is nevertheless 

 almost entirely included in this region. On the internal face of the 

 hemisphere the somatic sensory cortex extends as far as the sulcus 

 cinguli (So in fig. 6), and occupies about the same areas as on the 



