95 



Put briefly then my contention is that since the anthropoids alone 

 (and of these only the highest) have a large parietal area, the great 

 furrows which in them separate it into four specialised parts (Elliot 

 Smith, Brodmann loc. cit.) must be new furrows. 



Cebus and the new-world apes have no sulcus postcentralis 

 superior, and it is usually stated (e. g. Cunningham 1. c.) that this 

 furrow makes its appearance for the first time in the next highest 

 families, the baboons and old-world apes generally. A new sulcus 

 undoubtedly does appear in these animals but it is not a true post- 

 centralis superior. If it was it would occupy a position such that it 

 separated the sensory area from the parietal field. The typical sulcus 

 postcentralis superior lies between the area praeparietalis and the area 

 parietalis superior. Now the furrow which appears in the pithecidae 

 has no such position. It is situated actually within the sensory area, 

 as a glance at Brodmann's map will show. If this map is correct 

 — and here it is fully borne out by Schuster's investigation of Papio 

 hamadryas (8) — the newly-developed sulcus lies between two specia- 

 lised parts of sensory cortex, those two parts being area praeparieta- 

 lis and area post-centralis caudalis (the terms are Brodmann's). 



This essential difference of position has not been previously 

 pointed out. It is however of the very greatest importance that we 

 .should realise that this sulcus is not the same as the superior post- 

 central of man and other anthropoids, for, as I have already said, it 

 occupies a fundamentally different position. To emphasise this it is 

 only proper that this newly-recognised furrow should receive a special 

 designation. I have ventured to call it sulcus praeparietalis. 



Brodmann's diagrams further show how very large the sensory 

 area of the pithecidae is in proportion to the rest of the brain. Indeed 

 the majority of the retro-central half of the cerebrum is occupied by 

 the enormous visual and common sensory areas. The parietal area is 

 of small size only, intercalated between the two areas named. As 

 evolution progresses the whole parietal area increases in size, separat- 

 ing the visuo-sensory cortex from that of general sensibility. The 

 sensory area then becomes a comparatively narrow strip and the cau- 

 dally bent portion of the post-central sulcus (hitherto erroneously 

 believed to be sulcus interparietalis) becomes pushed up till it assumes 

 a direction almost parallel with the sulcus centralis Eolandi. 



This change is very materially assisted b}' the enormous increase 

 in the lower parietal area which is seen to have taken place in man. 



