77 



extremity of the superior occipital sulcus by a dotted line to indicate 

 the fact that it is less constant than the ventral limb, which is really 

 the posterior extremity of the main sulcus. (Zuckerkandl has 

 emphasized a similar fact in the case of Hylobates, op. cit. supra.) 

 The two limbs of the sulcus occipitalis superior embrace the caudal 

 extremity of the sulcus retrocalcarinus (Cunningham's "posterior 

 calcarine fissure"): when the latter is bifid its upper branch usually 

 (though not always) extends above the dorsal limb of the sulcus occ. 

 superior. The latter sulcus often extends on to the mesial surface 

 and the sulcus retrocalcarinus often crosses on to the lateral surface. 

 It then frequently happens that the two furrows appear superficially 

 to be concurrent. But in only one instance have I found the absence 

 of a submerged gyrus in such cases. These facts will explain why 

 Cunningham calls the sulcus occ. superior "external calcarine". 



This sulcus is exceedingly variable in both the human and the 

 simian brain. It is often converted into a horizontally-placed U-shaped 

 furrow surrounding the lateral prolongation of the retrocalcarine sulcus 

 and situated concentrically within the sulcus lunatus. A good exan)i)le 

 of this is seen in Gustaf Retzius's "Das Menschenhirn", Taf. LXIV, 

 Fig. 2. 



In many cases it is broken up into a group of irregular pit-like 

 depressions. When it presents the characteristically pithecoid form 

 (Fig. 1) a sulcus accessorius (sulci occ. superioris) is also, as a rule, 

 present as a compensatory furrow below it. A similar sulcus occurs 

 in the Gorilla and often in other large Apes. 



The sulcus occipitalis inferior is often present in the human brain 

 (Fig. 1) in such a characteristic form that its identity is beyond all 

 question. It is, however, subject to disintegrative changes such as 

 atiect its homologue in Semnopithecus and all the Siraiidae. So that 

 in Man we are not surprised to find that this sulcus lacks the con- 

 stancy of form, which it presents in most of the Cercopithecidae. In 

 the larger Cebidae and the smallest Cercopithecidae the caudal part 

 of this furrow becomes swept on to the ventral (tentorial) surface 

 and its course becomes deflected toward the sulcus collateralis. It 

 sometimes happens (though probably for vastly difi'erent reasons) that 

 a similar phenomenon occurs in the human brain. As a rule its 

 anterior extremity joins the fragmentary sulcus temporalis secundus. 



There can be no reasonable doubt in the mind of anyone acqua- 

 inted with the structure of the Gorilla's brain that the furrow which 

 I have called "sulcus occipitalis lunatus" represents the "Affenspalte" 

 and that its overhanging posterior (ventral) lip is the occipital operculum. 



