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Show 68 MR. F. E. BEDDARD ON THE [Feb. 7, of this brain. It was visible also in the Oxford brain to about the same extent; and equally clearly in one of the three other brains at m y disposal. The appearance of the island of Reil upon the surface of the brain completely shut off by sulci from surrounding regions would thus appear to be a fairly common feature of the Gorilla brain'. As to the Chimpanzee, the same exposure of the island at a lower level than the rest of the surface occurred in one of the two brains which I examined. I simply record, as to the Chimpanzee, m y own observations without attempting any statistics. In the brain which I have selected for figuring the exposed island of Reil was exceedingly conspicuous on account of the fact that it is depressed below the surface of the brain and completely surrounded by furrows. It is thus cut off from other gyri in all of the three brains to which reference has just been made. In two of the remaining brains which I have examined the island of Reil appeared at first sight to be not exposed upon the surface of the brain. This appearance I believe to be delusive and to be clue to the fact that there is no anterior sulcus dividing off the island from the gyri of the frontal lobe; the level of the island gradually rises and it becomes continuous with a gyrus of the frontal lobe. Parieto-occipital fissure.-The Gorilla's brain shows precisely the same variability in the continuity of the fissure separating the parietal and the occipital lobes that is exhibited by the Chimpanzee and the Orang. The operculum, in fact, is not always equally developed. In only one of the five brains at m y disposal-that belonging to the University Museum at Oxford (tig. 3)-was the occipital lobe cut off from the parietal by a complete fissure reaching the mesial surface of the brain. The result is, of course, an appearance which is very like that which is so characteristic of the common Chimpanzee. The brain of " Sally," therefore, is so far more like that of the Gorilla. In the four remaining Gorillas' brains there is thus no apparent continuity between the parieto-occipital fissure and the " Affenspalte " or Simian fissure. Between the two is a " pli de passage." W e will commence with some account of the parieto-occipital fissure itself in the four brains where the operculum is absent. The simplest arrangement of this fissure agrees precisely with what Benham has described and figured (15. fig. 21) as the simplest arrangement observable in the Chimpanzee. It is a long fissure showing for about half an inch on the dorsal aspect of the brain ; on the mesial surface it runs forwards and is ultimately parallel to the calcarine. I only discovered this simplest state of affairs in two separate half-brains. In the corresponding half to one of these the fissure was the same, excepting for the addition of a forward branch. In the half corresponding to the other of the two brains just mentioned there was an apparent difference of 1 Dr. v. Bischoff found it in all the brains that he examined. |
