CEREBRAL CONVOLUTIONS *' SALLY." 55 



ment of the parieto-occipital in man, but in none of them do we 

 find quite these relations. In three instances in which the fissure 

 is duplicated (see his figures on p. 40), the portion which cuts 

 into the upper surface of the hemisphere lies in front of the 

 vertical portion, and not behind it as in " Sally." In one case 

 (his fig. 18) this " doubling " is brought about by the gyrus 

 cunei rising to the surface and the " stem " of his -<-shaped 

 fissure being prolonged into the cuneus. This was only met 

 with in 4 out of 127 hemispheres examined. In the other case 

 (his fig. 19), which he found only once in the course of his re- 

 search, the gyrus intercuueatus is oblique and becomes super- 

 ficial, the upper part of the parieto-occipital being prolonged 

 downwards in front of the main part of the fissure. 



Now, in " Sally " it appears to me that the cause of the 

 " doubling " is the same, viz. the rising up of the gyrus 

 intercuueatus (a. in my figures), but the upper part of the 

 original fissure cuts downwards into the cuneus, behind the 

 main part of the fissure, which extends into the precuneus and 

 only just reaches the surface. 



This part of the parieto-occipital fissure lying on the upper 

 surface and separated from that on the mesial surface may be 

 termed the ''superior or lateral parieto-occipital." 



I wish to avoid the term " external,^^ which would perhaps 

 be more appropriate, since the term " external parieto-occipital" 

 has been used in a variety of senses. The only true sense in 

 which the expression should be used, as Cunningham and 

 others have pointed out, is to refer to that part of the vertical 

 incision which appears as the upper surface of the hemisphere, 

 and varies in extent, naturally, with the depth of the incision. 

 But the term has had two other significances attached to it, 

 especially by English writers, namely, to imply (1) true 

 Affenspalte (Bischoff''s " fissura perpendicularis externa," in the 

 foetal human brain) — in this sense it is used by Beddard ; and 

 (2) the groove between the anterior free edge of the operculum 

 and the parietal lobe. 



We will now examine the conditions of the parieto-occipital 

 fissure in the normal chimpanzee brain. 



