CHAP. XVII.] THE BRAIN OF QUADRUMANA. 299 



anterior extremity of the Temporal Lobe, as it is in the 

 Gorilla (fig. 117), is more nearly opposite the middle of 

 the Sylvian Fissure. This peculiar disposition of the 

 fissure of Rolando in the Orang coincides with a greater 

 comparative development of the lower (or third) tier of 

 * frontal convolutions,' and with a notable falling off in the 

 size of the lower half of the ' ascending parietal ' con- 

 volution. On the other hand, the disposition met with 

 in the Gorilla seems to 

 be due principally to the 

 greater development in 

 it of the lower part of 

 the parietal region of the 

 Hemispheres. Thus, the 

 great size of the ' supra- 

 marginal lobule ' and of 

 the lower part of the ' as- 

 cending parietal' convo- 

 lution, seems to cause 

 the lower half of the 

 fissure of Eolando to be 

 pushed decidedly for- 

 wards. These peculiari- 

 ties do not appear to FIG. US. Brain of Orang, view of base or 

 i r -i under aspect. (Owen, after Tiedemann.) Com- 



have been previously pare with p bise ^ Human Brain Fig 144 / 

 noticed by anatomists. 



The External Perpendicular Fissure is particularly 

 well marked in the Chimpanzee (fig. 115), though it is 

 seldom distinctly visible in the human brain. In the 

 Chimpanzee it is not crossed by any superficial ' bridging 

 convolutions,' so that its posterior border (or ' Operculum ' 

 as it is called in lower forms of Quadrumana) is uninter- 

 rupted. This fissure is continued on the inner side of the 

 brain as the 'Internal Perpendicular Fissure ' (fig. 120, fp). 



