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human brains as that represented in Fig. 1. The simple fact that 

 the sulcus occipitalis transversus is overlapped b}^ the operculum in the 

 Anthropoid Apes and not in Man has been unduly magnified as a 

 distinctive feature. Those writers who, for this reason, would compare 

 the brain of Man directly with that of the Cebidae thereby sacrifice 

 the obvious means of explaining the presence of the paramesial sulcus 

 and numerous other features in the occipital region, which are so 

 clearly illuminated by a direct comparison with the Gorilla's brain. 



The sulci on the lateral aspect of the human brain are subject 

 to a very wide range of variation. The process of progressive dwind- 

 ling of the occipital operculum, which begins in Semnopithecus and 

 goes a stage further in many individual Simiidae, is carried to an 

 extreme degree in most human brains. The operculation of the caudal 

 lip of the sulcus arcuatus occurs in almost a half of the specimens 

 of Egyptian brains; that a similar condition is not unknown in European 

 brains is shown by Gustaf Retzius' excellent illustrations ("Das 

 Menschenhirn", Taf. LV, Fig. 3 and elsewhere). Concerning this 

 specimen Retzius says : "Dicht hinter dieser Querfurche (Sulcus occipi- 

 talis transversus Ecker's) erkennt man in beiden Hemisphären noch 

 je eine starke Querfurche, einen Sulcus occipitalis transversus secundus, 

 an welcher sich von hinten her eine operkelartige Rindenpartie 

 hervorgewölbt hat." Finally he adds: "Es liegt in solchen Fällen in 

 der That wenigstens eine 'Analogie' mit den Verhältnissen bei den 

 Affen vor." This is something more than a mere analogy, it is a 

 case of true homology. Elsewhere in Retzius's great monograph a 

 number of instances of undoubted "Affenspalten" are found (Taf. XXI, 

 Fig. 8; Taf. XXIII, Fig. 6; Taf. XXX, Fig. 3; Taf. XL VIII, Fig. 4; 

 Taf. LI, Fig. 1 ; Taf. LIX, Fig. 2 among others). In the Egyptian 

 brain every possible stage in the dwindling of the operculum is re- 

 presented until extreme cases such as those represented in Figs. 5 

 and 6 are reached. 



But before I discuss the effects of the disappearance of the oper- 

 culum I must call attention to the variations in the form and position 

 of the operculated form of the sulcus lunatus. 



Fig. 1 represents the commonest and the most Gorilla-like form. 

 In Fig. 2 (examples of which will be found in Retzius's "Menschenhirn") 

 the lunate sulcus is not so oblique nor so laterally-placed as it is in 

 the type (Fig. 1). The lateral branch of the sulcus occipitalis trans- 

 versus is slightly overlapped by the occipital operculum. Fig. 4 re- 

 presents the appearance of the more typical lunate sulcus when seen 

 from above and comparison with Fig. 3 (which represents the condition 



