NERVOUS SYSTEM. VEBTEBRATA. .J2'. 



precentral and superior frontal sulcus explains the sig- 

 nificance of the obliquely-placed sulcus in the preceding 

 specimen. The orbital sulcus is simply linear, instead of 

 being irregularly triradiate, as in the last specimen. 



The other sulci do not essentially differ. In both 

 brains the collateral is represented by two sulci, the 

 anterior of which overlaps the posterior laterally, and 

 thus relieves the tension of the growing cortex, which 

 in other hemispheres is relieved by a special sulcus the 

 occipito-temporal. 0. C. 1338 o. 



D. 646. The brain of a White-cheeked Gibbon (Hylolates 

 leucogenys), (<$). The left cerebral hemisphere has been 

 separated. 



The cerebral hemispheres of this brain are much larger 

 and richer in sulci than those of the other two Gibbons' 

 brains. 



A peculiar appearance is produced in the occipital 

 region by the parieto-occipital sulcus (after crossing on to 

 the dorsal surface) being prolonged (superficially) back- 

 ward for some distance so as to form a V-shaped pattern 

 with an analogous backward extension of the Simian sulcus. 

 The two sulci, however, do not join. This peculiar appear- 

 ance seems to be the result o a very pronounced broadening 

 of the anterior limb of the gyrus called arcus occipitalis, 

 which has pushed back, so to speak, the occipital operculum 

 and the mesial end of the Simian sulcus, and driven the 

 diminutive posterior limb of the arcus under cover of 

 the mesial lip of the peculiar sagittally- directed part of the 

 parieto-occipital sulcus. As a further result, the bifid 

 extremity of the intraparietal sulcus, which forms the 

 transverse occipital sulcus, not only lies far in front of the 

 Simian sulcus and operculum, but also in front of the bent 

 parieto-occipital sulcus. 



The complexity of the intraparietal series of sulci is still 

 further increased not only by the rainus postcentralis but 

 by several other additional rami. 



The inferior occipital sulcus is present as a short deep 

 furrow, and is joined on the right hemisphere to the 

 posterior element of the middle temporal sulcus. The 



