32 University of California Puhlications in Anatomy [^oi" 2 



The region of the thalamus more extensively destroyed in Experi- 

 ment I is its posterior and middle third ; the anterior (rostral) seg- 

 ment remained uninjured. Accordingly, all thalamo-cortical fibers 

 degenerated in the present experiment must originate in the caudal 

 and middle segments of the thalamus. When considering the region 

 of the cortex receiving thalamic fibers in the present experiment 

 (shaded areas around the central sulcus in fig. 1, see also chapter on 

 Extent, etc., of the Somatic Sensory Cortex), it is evident from the 

 shape of that region, even more so in Experiments III (fig. 3) and V-a 

 (fig. 4), that the fiber bundles of the thalamo-cortical radiation must of 

 necessity be arranged in ''fans" placed perpendicularly though some- 

 what obliquely to the long axis of the hemisphere, in order that each of 

 them may supply a narrow strip of cortex approximately parallel to 

 the sulcus centralis of Rolando. (A similar view on the vertical seg- 

 mentation of the hemisphere has been expressed by Ch. Jakob and 

 Onelli.) The segmentation of the thalamus and of the whole thalamo- 

 cortical radiation found in the present investigation harmonizes well 

 with the areal differentiation of the cerebral cortex found in cytoarchi- 

 tectural and other investigations. This arrangement which seems to he 

 especially well marked on both sides of the sulcus centralis (Brod- 

 mann's areas 1, 2, 3, 4 and 6 in fig. 7), may be viewed as a cortical 

 segmentation rooted in functional differentiation. 



Another result of the present experiment, the complete absence 

 of degenerated thalamo-cortical fibers passing through the corpus 

 callosum to the opposite hemisphere speaks clearly in favor of strictly 

 unilateral connections and functional relations between the thalamus 

 and cortex (see identical finding in Experiments II, III, and V-a). 

 This finding is the more valuable since the lesion in this experiment 

 involves no callosal fibers of the middle portion of the brain. 



Experiment II 



In this experiment, the instrument was introduced at a spot marked 

 by the small dotted area in figure 2, in the ventral portion of the 

 frontal lobe close to the arcuate sulcus and ventral to the sulcus 

 frontalis principalis. The spot was chosen to avoid injury to the 

 occipito-parietal lobe. The lesion (L in corresponding figures), a nar- 

 row channel scarcely more than half a millimeter wide, penetrates 

 obliquely inward and caudalward through the most oral extremity of 

 both the extreme and external capsules, through the rostral portion 



