PARIETAL REGION IN THE PRIMATE BRAIN 297 



The fissura Sylvii presents some interesting variations. In 

 Cebus it is superficially confluent with the superior temporal, 

 whereas in the lower Old World Apes this or more advanced con- 

 ditions, in which these two fissures are separated, may be found. 

 In Cercopithecus and in Cercocebus the slightly submerged an- 

 nectant gyrus between the fissura Sylvii and the superior temporal 

 may become superficial on one or both sides, thus separating more 

 or less widely the two sulci, with the possibility of this separation 

 being more marked on the left side. In ]\Iacacus the separation 

 of the two sulci is more common ; it is still wider and more frequent 

 in Cynocephalus and quite constant in Semnopithecus. In both 

 these latter forms the Sylvian fissure has become more horizontal, 

 and there may be a slight branching of its posterior end, its course 

 is in addition less regular, on account of the appearance of small 

 transverse temporal gyri, and it may in addition receive a poste- 

 rior subcentral. . 



The sulcus temporalis superior (parallel sulcus) is quite char- 

 acteristic in all cases, as a long deep furrow extending from near 

 the tip of the temporal pole almost to the interparietal sulcus. 

 Its relations to the Sylvian fissure have been noted. With the 

 separation of these sulci, the superior temporal sulcus also tends 

 to becoine more horizontal, describing more of a curve around 

 the Sylvian and its posterior end may be branched or directed 

 forward. WTien unbranching and continuing its original course, 

 , there may be found, immediately in front of it, above the end of 

 the Sylvian, a small shallow furrow which is compensatory in 

 character. Both the branching of the superior temporal and the 

 compensatory furrow may be found together or more rarely a 

 similar furrow appears behind, just in front of the sulcus lunatus. 



The sulcus interparietahs is of special interest and importance 

 since it is situated in a region whose great development is a char- 

 acteristic feature of the human brain. Its simplest form is seen 

 in Perodicticus and its extension backward and outward to reach 

 the lunate, in Pithecia is shown in figure 2. In its curved form in 

 Pithecia it is very similar to the type now under discussion. In 

 Cercopithecus, where it is still quite simple, it appears as a very 



