68 ME. r. E. BEDDAED OHf THE [Feb. 7, 



of this brtiin. It was visible also in the Oxford brain to about the 

 same extent ; and equally clearly in one of the three other brains 

 at my 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, 

 my own observations w-ithout attempting any statistics. 



In the brain M'hich I have selected for figuring the exposed 

 island of Eeil was exceedingly conspicuous on account of the fact 

 that it is depressed below the surface of the brain and coinpletely 

 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 Eeil appeared at first sight to be not exposed upon the 

 surface of the brain. This appearance I believe to be delusive 

 and to be due 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-occijntal 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 deve- 

 loped. In only one of the five brains at my disposal — that belonging 

 to the University Museum at Oxford (fig. 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 hke 

 that of the Gorilla. In the four remaining Gorillas' brains there 

 is thus no apparent continuity between the parieto-occipital fissure 

 and the " Aff'enspalte " or Simian fissure. Between the two is a 

 " pli de passage." 



We 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 diff'erence of 



^ Dr. V. Eischoff found it in all the brains that he examined. 



