194 THE GENERAL ANATOMY OF GORILLA [SECT. A 



and orbital opercula as in the human brain. As these opercula extend 

 laterally they diverge, and expose a small triangular depressed area of insula. 

 The orbital operculum is limited by a distinct fronto-orbital sulcus, which is 

 really the anterior limiting sulcus of the insula. A dorsal opercular fold 

 demarcates the lateral (superior) margin of the insula, and makes so 

 complete a superior limiting fissure, that it appears to join the anterior 

 limiting (fronto-orbital) sulcus, though closer examination reveals a strip of 

 cortex dividing the two sulci. In most brains of Gorilla this strip is wide 

 and quite easily seen. The mesial end of the fronto-orbital sulcus does not 

 usually join the stem of the Sylvian fissure. The orbital surface of the 

 frontal lobe is marked by a tri-radiate orbital sulcus to which smaller sulci 

 may be accessory. The central sulcus presents a well-marked "genu." 

 On the lateral surface of the frontal lobe, well-marked representatives of 

 the sulci rectus and arcuatus are seen, and are evidently representative 

 of the inferior frontal and inferior pre-central sulci of human anatomy. 

 The superior pre-central and superior frontal sulci are arranged almost 

 diagrammatically. A small sulcus frontalis medius and a representative 

 of the sulcus fronto-marginalis (of Wernicke) may occur. The inferior 

 transverse sulcus is commonly absent, but Eberstaller's sulcus diagonalis 

 occurs. 



The parallel sulcus is hooked round the extremity of the Sylvian fissure, 

 much as it is in the Cynocephalous apes. The intra-parietal sulcus is clearly 

 recognizable : the ramus post-centralis 1 superior is commonly independent 

 of the last-named sulcus which dips posteriorly under the occipital operculum. 

 The arcus occipitalis is very large, and wholly exposed, the mesial end of the 

 occipital operculum having been pushed completely outward and backwards. 

 A Y-shaped lateral occipital sulcus cuts into the occipital pole of the hemi- 

 sphere. The occipital operculum is bounded by an almost semicircular 

 Affenspalte. 



The calcarine sulcus (cf. Fig. 128 a) consists of an anterior deep (true 

 calcarine) portion, and a shallower retro-calcarine element, the latter being 

 strongly inflected at its posterior end. A dorsal post-calcarine sulcus runs 

 parallel to this last-mentioned portion. 



The compound sulcus parieto-occipitalis of the mesial aspect is constant 

 in occurrence, but very variable in extent and connections. The collateral 

 sulcus may join the posterior element of the occipito-temporal sulcus, and 

 operculation occurs in this region as it does in the genera Cynocephalus 

 and Macacus. The calloso-marginal sulcus is very complete, and Broca's 

 " compensatory " sulcus also occurs. 



The cerebellum closely resembles that of Man. The floccular lobe 

 however is larger than in human brains, and consists of two lobules each 

 attached by its own stalk. The mesial lobule is much the larger and is 



1 The two sulci are continuous in the right cerebral hemisphere of a Gorilla 

 preserved in the Anatomical Collection at Munich. 



