OF THE OBANG UTANG. 15 



filled up or not filled up, bridged or not bridged over, tbat the 

 absence or presence of an external perpendicular fissure, the exist- 

 ence or non-existence of an ' operculum,' depends. 



In the figures referred to, and to some extent in those appended 

 to this paper, the anterior edge of the occipital lobe is seen to rise 

 wave-like as it were against the table-land of the fronto-parietal 

 lobes. The wave-like edge is the ' operculum.' Along the middle 

 line on each in Fig. i, Fig. iii, and Fig. iv, the wave-like edge, 

 speaking of disruption of continuity between the occipital lobes and 

 the mass of brain anterior to them, is absent ; a convolution, «, a, 

 passes across what would else be a chasm. This convolution is the 

 ■ premier pli de passage ' of Gratiolet ; it comes, according to that 

 authority, thus to the surface, and thus bridges the chasm in man, 

 in the orang, and in the ateles, but in no other ape. Our first canon 

 can be immediately applied in the estimation of the value of this 

 structure upon the data thus put before the reader upon the autho- 

 rity of M. Gratiolet. Leaving the task of so applying it to the 

 reader, I shall proceed to show that the superficial position of this 

 bridging convolution is by no means an universally present charac- 

 teristic either of the human brain or of the orang's ; and, thirdly, 

 that it is sometimes both present and superficially visible in the 

 brain of the chimpanzee. 



Of seven human brains at present in the University Museum, 

 three possess this bridging convolution on both sides entirely super- 

 ficial in position : in the fourth we find it wanting on one side, two 

 spurs thrown out from the declivity of the occipital representing 

 what is a perfectly continuous chain on the other side : in the fifth 

 it is concealed on one side by the overhanging edge of the occipital 

 lobes : in the sixth it does not quite reach, on the left side, the level 

 held by the occipital and parietal lobes which it connects : in the 

 seventh, a deep chasm is visible on both sides ; but on the left the 

 convolution, which seems to fail to bridge the fissure, does really 

 cross and fill it up, though at a distance of as much as an inch 

 from the longitudinal fissure ; whilst on the right side the connect- 

 ing convolution dips vertically downward, and leaves a deep valley 

 between the occipital and parietal lobes. This seventh brain be- 

 longed to a man who, by trade a gardener, was possessed of more than 

 an average share of intelligence, and whose brain was carefully pre- 

 served for this reason, as well as on account of its great size, and 



