R0LLEST0N ON THE BRAIN OF THE ORANG TJTANG. 211 



of this structure upon the data thus pat before the reader upon the au- 

 thority 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 con- 

 volution is by no means an universally present characteristic either of 

 the human brain, or of the Orangs ; 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 superficial in 

 position ; in the fourth vre 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 pa- 

 rietal 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 it and fill it up, though at a distance 

 of as much as an inch from the longitudinal fissure ; whilst on the 

 right side the connecting convolution dips vertically downward, and 

 leaves a deep valley between the occipital and parietal lobes. This 

 seventh brain belonged to a man who, by trade a gardener, was pos- 

 sessed of more than an average share of intelligence, and whose brain 

 was carefully preserved for this reason, as well as on account of its great 

 size, and the development of its convolutions. This last of the seven 

 brains will allow us to apply our second canon to test the value of the 

 absence of this structure in the particular relation of superficial position 

 as a mark of serial degradation. 



But a structure which exhibits so much variability, as to conform to 

 the rule in but three, and to diverge more or less from it in four, out 

 of seven brains chosen at haphazard for examination, as being all at that 

 moment which a particular museum contained, will scarcely seem to 

 merit a high place as a zoological differentia. With reference to the 

 " premier Pli de Passage" in the orang, a careful comparison of the re- 

 lations of the parts lettered aa, in fig. 3, with the same relations in 

 fig. 4, will show that this convolution is by no means superficial in its 

 entire extent on the left side of that brain. And, secondly, in our second 

 specimen of an orang' s brain, this convolution is concealed on both sides 

 within the fissure ; and the cerebral hemispheres in this specimen pre- 

 sent, in consequence, as perfectly wave-like an opercular edge as in any 

 other monkey. In confirmation of this, I would appeal to Tiedemann's* 

 and Wagner' sf figures, already referred to, as giving typical representa- 

 tions of an external perpendicular fissure in the brain of orang utangs, in 

 which, according to M. Gratiolet, it should be invariably half- filled up 

 by his " premier pli de passage." 



Lastly, with reference to the chimpanzee : one specimen possesses 



* Wagner, Icones Zoot, viii., 3. t Ap. Wagner, Icon., taf. viil., fig. 2. 



