iv] PoNtinifra/ or Sensor;/ Area 77 



appearance. (-5) The interradiary Held undergoes important changes, and in fact assumes an 

 entirely different aspect, tor although the fibres of delicate calibre remain as before, those large 

 mednllated fibres which cross the radiations at all angles and form such a prominent and character- 

 istic feature of the typical postcentral cortex disappear, to be replaced by sparsely distributed 

 fibres of medium size only. ((>) Large fibres are absent from the white substance immediately 

 underlying the cortex. 



In the lower parts of this "intermediate postcentral" field, the fibres generally suffer 

 a reduction in calibre, a change which accentuates the difficulty of defining its lower 

 boundaries. 



In the anthropoid brain, the " postcentral " area is bounded behind by an " intermediate " 

 zone of cortex, of which the characters are uncommonly like those described above. 



B. TYPE OF CELL LAMINATION. (Plate VI, fig. 2.) 



Judgment on the differences between the primary and intermediate types of postcentral 

 cortex is not made so readily from sections stained for nerve cells as from sections showing 

 the fibres, still there are a few points worthy of mention. 



On following round the layers of large pyramidal cells, both external and internal, it is of 

 importance to notice that the moment the intermediate field is reached those larger chromo- 

 philous elements, which were described at length in discussing the postcentral area proper, 

 cease to be represented, while at the same time the common remaining constituents undergo 

 a slight reduction in size and number. It may also be observed that the inequality of repre- 

 sentation between these two large cell laminae becomes more decided, and on account of the 

 relative paucity of large substellate elements an appearance of advanced development is imparted 

 to the underlying fusiform layer. In this respect the " intermediate " type assumes a feature 

 which will be seen to be possessed by the ordinary "parietal cortex." 



The lamina of stellate cells in the " intermediate " field seems to be of greater depth," 

 but not so closely packed as it is in the primary area. 



Not so obvious on simple microscopic inspection, all the above-mentioned points come 

 out plainly when camera lucida drawings of cells from the two areas are made and compared. 



Distribution of the Intermediate Postcentral Cortex. (Plate I.) 



The definition of the boundaries of this area does not present much difficulty. Anteriorly 

 it merges with the " typical postcentral " field. On the mesial surface it occupies the hinder 

 part of the paracentral lobule, being limited by the calloso-niarginal fissure, it thus forms 

 a bed for the diverticulum of postcentral cortex situated here. Below this it usually conies in 

 contact with the " intermediate precentral " area, but the dividing line here i.s by no means 

 definite. At the upper margin of the hemisphere the upper extremity of the calloso-niarginal 

 fissure forms a constant posterior limit. The next link in the boundary is formed by a line 

 dropped from the calloso-niarginal incisure to the sulcus postcentralis superior, and this sulcus 

 and the sulcus postcentralis inferior both form limits. In some cases the intermediate 

 arrangement of fibres extends down the anterior wall of the postcentral sulci for some distance, 

 but it never seems to cross them, hence the calloso-margiual and the postcentral sulci may be 

 regarded as fixed boundaries 



