1932] PoUak: Afferent Fiber Systems, Primate Cerehral Cortex 65 



tion, the somato-sensory region reveals in general a close kinship to 

 the auditory projection cortex. (See: Auditory System, in the 

 present work.) 



The caliber of the intracortical somato-sensory fibers exceeds some- 

 what that of the fibers constituting the visual radiation (see: Visual 

 System, in the present work) , and is about equal or slightly superior 

 to that of the auditory radiation (see : Auditory System, in the present 

 work). Yet even the coarsest among the thalamo-cortical fibers, those 

 entering the postcentral convolution, do not quite attain the size of the 

 strongest efferent fibers of the precentral region, the so-called pyra- 

 midal fibers. There are also some regional differences in respect to 

 the size of fibers, since, in general, the thalamic fibers which tend 

 toward the operculum of the precentral and parietal regions are less 

 coarse than the rest, especially less than those entering the postcentral 

 convolution. It also appears that quite coarse afferent fibers found in 

 the postcentral convolutions are absent in the precentral gyrus. 

 Somato-sensory fibers forming a close bundle or system destined for a 

 certain cortical region, for a segment of a convolution, have approxi- 

 mately the same caliber; this is well exemplified, for instance, by fibers 

 in Experiment III which reach the dorso-caudal portion of the 

 postcentral-parietal convolution. Taking the thalamo-cortical radia- 

 tion as a whole, there is an appreciable variation in the size of fibers 

 according to different localities. 



