THE ANATOMY OF THE CHIMPANZEE. 49 



have seen equalled in any figure of a chimpanzee brain. The sulcus praecentralis 

 inferior almost reaches the fissure of Sylvius. Owing to the absence of the ante- 

 rior limb ot" the fissure of Sylvaus, the third frontal convolution at this region 

 has the shape of a solid wedge placed base downward with an indentation on 

 its surface which represents the anterior limb. It then winds over the sulcus 

 orhito-frontalis. The second frontal fissure is interrupted by an annectant gyrus 

 between the second and third convolutions. On the inferior surface, the olfactory 

 fissure does not run to the median line. There is a deep fissure in the posterior 

 part of the orbital surface curving forward and outward, which has no represen- 

 tation on the left. Tlie median surface of the frontal lobe is left to be taken 

 later. 



The parietal lobe. The external parieto-occipital fissure, by which is meant that part 

 of the internal one of the same name which extends into the convexity of the brain, is 

 about 2 cm. long and similarly disposed in each hemisphere. It is surrounded by the 

 arcus parieto-occiintalls, the classical jili de passage, which is quite uncovered by the 

 operculum, but a little shut in anteriorly by the parietal lobe. The sulcus occipitalis 

 transversus (Affenspalte), also symmetrical in the two halves, runs from the median line, 

 just behind the arcus to the outer, lower, border of the hemisphere. In the first three 

 quarters of its course it runs outward and slightly forward, then it bends strongly down- 

 ward. The intra-]>arietal fissure is simple and nearly symmetrical. It consists of an 

 inferior vertical part which is continuous with the main (horizontal) portion which runs 

 into the Affenspalte just below the arcus. The superior vertical part is distinct. It can 

 be called " vertical " only by homology, for it is decidedly more horizontal. The main 

 limb just before entering the Affenspalte sends off a small branch which runs just in 

 front of the arcus toward the median line. 



The posterior central convolution is small. On the right it is evidently continuous 

 along the whole length of the fissure of Rolando. It is continuous also on the left, 

 though less evidently, for it is nearly cut through at the two knees of the fissure of 

 Rolando above by the upper detached portion of the intra-parietal fissure, and below by 

 a sulcus from the main stem. At the lower end this convolution expands on the left side 

 into a triangular mass with a centr.il imprint, probably- representing the sulcus suhcen- 

 tralis posterior. The superior parietal convolution consists of two gyri separated in front 

 by the upper part of the intra-parietal fissure which is hei'e essentially horizontal. They 

 unite behind it and are continued into the parieto-occipital arcus, bounding the fissure of 

 that name, which is quite uncovered by the operculum. 



The inferior parietal convolution divides into the supra-marginal gyrus surrounding 

 the fissure of Sylvius and the angular gyrus which descends to the edge of the hemisphere 



