PARIETAL REGION IN THE PRIMATE BRAIN 311 



panzee there seems to be a wider range of variation and the ramus 

 occipitahs is more completely covered by the operculmn. The 

 postcentral sulci show considerable changes, are more branched 

 and irregular, the superior is often separated and may unite with 

 a superior parietal. The ramus horizontaUs is much more ir- 

 regular on account of ventral and dorsal offshoots which show 

 a certain compensatory relationship to the terminal branches of 

 the Sylvian and particularly the parallel sulcus. Any of the con- 

 ditions found in the Gibbon may occur, but with the natural 

 tendency for the new furrows to increase in number; they are 

 placed in general more or less parallel with the superior temporal 

 or in a radiating manner perpendicular to the interparietal, they 

 may unite with any neighboring furrows or be independent. The 

 dorsal branches of the interparietal are especially variable and 

 irregular. 



In the great majority of brains the ramus occipitalis disappears 

 beneath the operculum. This part of the parietal lobe which is 

 covered by the operculum, namely the anterior sloping wall of 

 the lunate sulcus, is an important area, taken up in large part by 

 the three or even four and five external annectant gyri, out of 

 which is completed (Zuckerkandl) the parietal lobe. As pre- 

 viously stated, the extent of the operculum will determine the 

 degree to which these gyri are submerged, and its recession will 

 bring them to the surface in their natural order from within out- 

 ward. The first annectant gyrus of Gratiolet, or arcus parieto- 

 occipitalis of Eberstaller is looped around the fossa parieto-occi- 

 pitalis as the latter cuts the margin of the hemisphere. It is 

 bounded laterally by the ramus occipitalis and its size is largely 

 dependent upon the depth of the fossa. It is the first of the an- 

 nectant gjrri to make its appearance on the surface and indeed its 

 anterior limb is always uncovered. The second annectant gyrus 

 usually forms a loop around the termination of the occipital ramus, 

 separating it from the bottom of the lunate sulcus, while the re- 

 maining annectant gyri are small variable sagittal foldings, the 

 fate and importance of which are much more problematical. In 

 the Chimpanzee and Orang there is frequently given off under 

 the operculum a branch which passes upward and backward 



