PARIETAL REGION IN THE PRIMATE BRAIN 301 



short and often indicated only by a slight indentation in the an- 

 terior wall of the interparietal sulcus. It is rather accentuated by 

 the slight angle in the sulcus at this point and the intermediate 

 portion of the interparetial between these two branches, may be 

 quite distinctly marked off from the anterior more ascending 

 portion (i.e., the inferior postcentral) . The portion of the inter- 

 parietal which we have termed intermediate, would seem to repre- 

 sent the major (posterior) portion of the entire sulcus in Pero- 

 dicticus and to be the equivalent of the ramus horizontalis 

 (Cunningham) of man, sulcus interparietalis proprius. In more 

 advanced types this segment of the interparietal instead of de- 

 scribing a short curve around the superior temporal, gradually be- 

 comes somewhat lengthened, tends to lose its sharp curve and 

 become more nearly parallel with the border of the hemisphere. 

 Its length and course are largely influenced by the same factors 

 which determine the condition of the superior temporal immedi- 

 ately below, i.e., the changes in area 7, which agencies, however, 

 are at work above as well as below the sulcus. 



The posterior limb of the interparietal, behind the fossa parieto- 

 occipitalis, lies, as a rule, quite concealed beneath the operculum 

 occipitale and can be examined only by raising or removing the 

 operculum. In its simplest form the sulcus, from its highest point, 

 passes outward and backward under cover of the operculum but 

 does not usually reach the bottom of the sulcus lunatus, being 

 separated from it by the second amiectant gyrus. Although this 

 arrangement is found in Cebus it is in this form that the greatest 

 individual variation occurs. In Macacus (Zuckerkandl '03) and 

 in other forms also, the sulcus may turn suddenly upward, limit- 

 ing the first annectant gyrus behind. The first annectant gyrus 

 is subject to considerable variation in its relations; it may be well 

 defined and quite superficial, or very much depressed so that a 

 wide, open, superficial connection results between the interpa- 

 reital and parieto-occipital. This may be but the expression of 

 the degree of development of the cortex forming the first annec- 

 tant gyrus, for the most part area 19; or, if the operculum is less 

 extensive, as a result of a lessened development of the area striata. 



