40 University of California PuhUcations in Anatomy [you. 2 



degenerated fibers also reach the lower parietal convolution (fig. 67). 

 There are also a few degenerated fibers which reach the region 

 around the most dorsal extremity of the sulcus centralis (fig. 66), 

 From a close study of the course and cortical distribution of the 

 degenerated fibers of the radiation it becomes evident that from 

 the caudal segment of the thalamus fibers emanate supplying the 

 postcentral-parietal half of the somatic sensory region, while from the 

 rostral segment of the thalamus come those reaching the anterior half 

 of the somato-sensory region in front of the sulcus centralis. For, 

 that zone of the somato-sensory cortex supplied b}^ the degenerated 

 fibers in this experiment corresponds with the convexity of the post- 

 central gyrus including the narrow strip of cortex covering both the 

 anterior and posterior slopes of that convolution. The bottom of the 

 central sulcus in contrast with the results in Experiments I, II, and 

 V-A, receives only a few scattered degenerated fibers. The sharpness of 

 the anterior boundary of the supplied cortical zone is remarkable ; it 

 runs below the anterior margin or lip of the postcentral convolution, in 

 the sulcus centralis, along the entire dorso-ventral extent of that sulcus 

 (except at its most dorsal extremity), thus leaving the anterior portion 

 of area 3 of Brodmann which extends to the precentral gyrus, and the 

 whole precentral region (areas 4 and 6) with normal afferent fibers. 



The segmentation of the thalamo-cortical radiation into individual 

 ''fans," each arranged parallel to its neighboring fan and all fans with 

 their planes more or less perpendicular upon the longitudinal axis of 

 the hemisphere, which segmentation corresponds with the segmenta- 

 tion of the pre-postcentral cortical region into cytoarchitectural areas, 

 appears fairly evident from this experiment. The position of the 

 fiber fans within the internal capsule and in the white matter of the 

 hemisphere corresponds, however, only approximately with the planes 

 vertical to the long axis of the hemisphere. The posterior fans with 

 their upper extremities closer to the cortex are inclined occipitalward, 

 the oral fans oralward, thus shaping the entire thalamo-cortical radia- 

 tion somewhat similar to that of an umbrella or an irregular mush- 

 room. (Compare, especially. Experiments I, II, and V-a.) Thus the 

 somatic sensory fibers are assembled into definite sub-systems already 

 in the white substance of the hemisphere. The form or shape of the 

 fiber "fan" of the thalamic radiation which degenerated in Experi- 

 ment III, as well as the shape of the cortical area supplied by it, would 

 be different if the arrangement of fibers in the subcortical white sub- 

 stance were another, or irregular, or a "diffuse" one. It appears 



