NEBVOUS SYSTEM. VERTEBRATA. 447 



with the left hemisphere of this individual (D. 656), but the 

 sulcus around which the partially submerged arcus occipitalis 

 is arranged seems to join the posterior limb of the Y-shaped 

 arrangement of the other two elements ; and a branch of 

 the intraparietal cuts into and thereby complicates the 

 arcus occipitalis. In D. 660 there seems to be a long, simple, 

 vertical, parieto-occipital sulcus cutting deeply and trans- 

 versely into the hemisphere : its dorsal limb is surrounded 

 by a broad extensive simple arcus occipitalis. A peculiar 

 sulcus, however, emerges from the dorsal part of the 

 parieto-occipital sulcus and pursues a sagittal course 

 backward. 



There is on the left hemisphere of this individual (D. 656) 

 u simple collateral sulcus, beginning below the bend in the 

 postcalcarine, from which it is separated by a partially 

 submerged gyrus, and ending immediately below the 

 anterior end of the calcarine. It is linked to the posterior 

 element of the occipito-temporal sulcus, which is very 

 deep. It crosses on to the external surface and fulfils the 

 function of the inferior occipital sulcus of other Apes. Its 

 anterior segment is separate and deepened at the expense 

 of the inferior occipital. There is a very complete calloso- 

 niarginal sulcus. But it is peculiar that in all these 

 hemispheres of the Gorilla the sulcus is prolonged much 

 further back than the neighbourhood of the upper end of 

 the central sulcus (where it usually ends in the human 

 brain). The sulcus, however, which represents its caudal 

 part in the human subject, has all the appearance in the left 

 hemisphere (also in the right hemisphere of specimen D. 660 

 and in both sides in D. 659) of being merely a vertical 

 branch of the sulcus and in the right hemisphere is quite 

 separate from it. 



In the left hemisphere of specimen D. 660 the manner in 

 which these variations are brought into line with the 

 human condition is shown. The form of the " compen- 

 satory " (Broca's " postlimbic ") varies with the form of 

 the calloso-marginal. It is best developed as a large 

 triradiate figure in the right hemisphere of this specimen. 



This brain affords an admirable demonstration of the 

 mesial structures corpus callosum, fornix, septum lucidum, 



