CEREBRAL CONVOLUTIONS " SALLY." 69 



chimpanzee brains this portion of the frontal lobe below the 

 '' anterior limb " is much more developed, and forms practically 

 an operculum — overlapping, though only to a slight extent, the 

 insula. 



Cunningham states that the submerged portion of the insula 

 presents very little trace of an anterior boundary, and that it 

 ascends gradually along an inclined plane until it finally reaches 

 the free surface of the frontal lobe. I do not find this to be 

 the case at all generally. In two brains in the Oxford Museum 

 this overhanging anterior lobe exhibits more or less distinct 

 traces of a subdivision into two lobules. 



The most distinct case was photographed, and is represented 

 in fig. 8. Here the " anterior limb " of the Sylvian fissure is 

 forked ; one branch is nearly vertical (%'.), and forms as usual 

 the boundary of the fronto-parietal operculum ; it is the 

 ''ramus ascendens " of the anterior limb. Just below it there 

 is another slight but distinct fissure {Sy" .) separating a small 

 triangular lobe from the rest of the anterior boundary of the 

 insula (/r. op.). This little lobe, I suggest, is the "frontal 

 operculum " or pars triangularis, and the second fissure is the 

 homologue of the '' anterior horizontal limb " of the Sylvian 

 fissure. Below it the remainder of the lobe will then corre- 

 spond with the orbital operculum, which here distinctly over- 

 laps the insula. 



A second brain (Oxford Museum, 947/.) shows somewhat 

 similar conditions, but in a less degree. These may be com- 

 pared with Cunningham's fig. 1, pi. iv, of the brain of a human 

 foetus of the eighth month. 



The brain represented in fig. 8 resembles still more closely 

 the figures given by Schafer in ' Quain's Anatomy ' (tenth 

 edition, vol. iii, part 1). He shows that on the right side of 

 this human brain the two branches of the anterior limb of the 

 Sylvian fissure practically join at their commencement, i. e. 

 the "pars triangularis" or frontal operculum is here less 

 developed than on the left side, where it entirely separates the 

 two rami. The greater development on the left side of the pars 

 triangularis may be in relation with the " centre of speech." 



