AN ATYPICAL CEREBRAL CORTEX 363 



ment of the cortex in the latter region is illustrated in the pre- 

 ceding description by Areas 2 and 3, while that of the former 

 which shows greatest variation from the general cortical pattern, 

 is well shown in Areas 4 and 5. 



Projection fibers entering mainly via zonal layer 



Let us first consider the effect of the entering thalamic fibers 

 when distributed mainly by way of the outer or zonal layer of 

 the cortex. This is the case especially in Area 5, at the region of 

 junction of the thalamus and cerebrum. The second layer of the 

 cortex, which is, elsewhere, for the most part, made up of poorly 

 differentiated pyramidal, polymorphic and embryonal cells, 

 shows here a fewer number of elements on the whole and of these 

 the greater number are quite large well differentiated pyramidal 

 or polymorphic cells. It is of significance to note that the average 

 size of the elements decreases while their number increases in 

 passing more deeply into the cortex in this region. The various 

 cortical laminae are much disturbed and difficult to make out. 

 Certain very large giant cells occur in the deepest stratum and it 

 is to be noted that this stratum bears the same relation to the 

 projection fibers in the medullary layer as the second layer of the 

 cortex bears to these fibers in the zonal layer. 



In Area 4, large polymorphic cells are present in the second 

 layer of the cortex in considerable numbers, though this condition 

 is not so marked as in Area 5. A considerably smaller number of 

 thalamic fibers are coursing in the zonal layer in Area 4. The 

 cortical lamination here also shows great variation from that ob- 

 taining over the greater part of this cortex. As in Area 5, the 

 polymorphic layer, which is also reached by thalamic fibers via 

 the medullary center, here shows many well developed large 

 cells, though none of these attain the size of the giant cells in the 

 former area. 



The thalamic fibers entering by way of the zonal layer are able 

 to come into immediate contact with the dendritic processes of 

 the elements of the second layer of the cortex. The effect is such 

 as to cause elements which normally show least signs of spe- 



