.JOG PKOF. G. ELLIOT SMITH OX THE 



The only -n-riter t\ ho has attempted to seek for the common mammalian homologue of 

 this fronto-orbital sulcus of the Lemurs is Ziehen *. He came to the conclusion that it 

 represented the presylvian (orbital) sulcus of other mammals ; hut the fallacy of his 

 argument is quite patent and capable of easy refutation. Ziehen re2:ards the sulcus 

 which I have called " orbital " in the Lemurs as the representative of the " intraorbital " 

 sulcus of the Carnivora, and the diagonal (fronto-orbital) sulcus as the presylvian (which 

 I call orbital). Thus he attempts to homologize a sulcus which is one of the most stable 

 features of the Prosimian brain with an unimportant furrow like the intraorbital which 

 is found in only a few of the larger Carnivora and is morphologically of no importance ; 

 and, on the other hand, he suggests the identity of the exceedingly stable presylvian 

 (orbital) sulcus of the Carnivora with the fronto-orbital sulcus which is found only in the 

 o-enera Lemur, Propilheciis, and Indris of all the Prosimia?, and not constantly even in 

 these. He attempts to justify this by the statement that the presylvian sulcus may bo 

 absent in a Carnivore, and quotes the example of Eerpcstcs. I have, however, examined 

 ten cerebral hemispheres of Herpestes, and have never found the orbital (presylvian) 

 sulcus absent, though it is always very small f. But even if it were always absent not 

 only in one but in several genera of Carnivora, this would not justify the homology of 

 one of the most constant sulci of tiiis Order with perhaps the most unstable sulcus in 



Lemurs. 



In the literature of cerebral morphology the orl)ital (presylvian) sulcus of the 

 Carnivora has been represented as the homologue of many sulci liy different writers. 

 According to Owen, Meynert, and many others (even in Edinger's recent text-book, 

 1899), it represents part of the Sylvian fissure; according to Pansch it is the precentral 

 sulcus ; and according to Broca and Eberstaller (and also Soury in his great work ' Le 

 Systeme Nerveux Central,' 1899) it is the homologue of the central (PiOlando's) sulcus. 



The evidence of Comparative Anatomy (and by this I do not mean the futile attempt 



to compare the brain of one specialized Carnivore like the Cat with that of the highest 



Primate) shows decisively that the presylvian sulcus of the Mammalia cannot possibly be 



x-epresented in the Primates by any of these four sulci — Sylvian (Owen), precentral 



(Pansch), central (Broca), or fronto-orbital (Ziehen). The orbital (presylvian) sulcus is 



perhaps the most constant sulcus (in the neopallium) in the Mammalia. It is found in 



the brain of almost every Carnivore f, Ungulate, Cetacean, Edentate (excepting Chlami/- 



dophoi'us), ami in all Piodents, Insectivores, and Marsupials which have any cerebral sulci. 



It is therefore highly improbable that such a stable sulcus should either be entirely absent 



or be represented by an inconstant element in the Primates. Moreover, the brain of the 



J'rimates possesses a sulcus in the same position in the hemisphere as the presylvian 



occupies in other mammals ; and this sulcus is the most constant furrow in front of the 



Sylvian fissure. This can be no other than the orbital sulcus (the " triradiate " sulcus of 



'J'urner ; the " presylvian " sulcus of Owen). The correctness of this identity of presylvian 



and orbital sulci becomes more conclusively demonstrated as the range of comj)arison 



increases. If we compare, for example, the pi-esylvian sidcus of the Rodent JDoUcholis, 



* Aroh. f. Psych. M. xxviii. 



t It is actuallj- absent in the Ilctpcbtiue Crossai-c/ivf, 



