1,12 PEOF. G. ELLIOT SMITH OX THE 



upi^er [or caudal] part of the " Sylvian fissure " is purely suprasylviau, but the lower 

 [or anterior] part of the sulcus is formed by the meeting of the dorsal lip of the supra- 

 sylviau sulcus with the ventral lip of the pseudosylvian sulcus. 



In nrany Prosimian brains, especially of the genera Lemur, Fropithecus, and Iiiclris 

 (l)ut also, to a less marked degree, in Galngo, Perodicticus, and Nycticehm), the lower 

 end of tlie suprasylviau sulcus may be seen emerging from the " Sylvian fissure " some 

 distance above the rhinal fissure. Several writers have noticed this. Ziehen, for instance, 

 Ciills it the " Vorderster Abschnitt der Fissura circularis externa (s. Reilii) " (Arch. f. 

 Psych. Ed. xxviii. p. 929). 



These relations of the pseudosylvian and suprasylviau sulci are identical with those 

 foTuid in the smaller Cebidse and CercopithecidiB ; but in these Monkeys the lower end 

 of the suprasylviau sulcus is not exposed. It begins to emerge in the Cynocepludi and 

 Se^nnopUheci, and in ILjlohatcs and the Antliropoid Apes it becomes prolonged forward 

 (as the " opercular sulcus " of Marchand) so as to almost meet the fronto-orbital 

 [diagonal] sulcus. Even in the Simiidje the dorsal lip of this forward extension of the 

 suprasylvian sulcus and the anterior lip of the diagonal sulcus tend to become opercular, 

 and extend downward and backward until in m(«t hiunan brains they meet the 

 opercular caudal lip of the pseudosylvian sulcvis, and thus completely cover the anterior 

 insvilar area, wliich is always exposed in non-hiunan brains. In Man alone is tlie 

 Sylvian fissure complete. In the Apes the "stem" of tlie fissure is sometimes formed, 

 but the so-called Sylvian fissure of Monkeys really represents only a jmrt of the posterior 

 limb of the human Sylvian fissure. The fully ibrmed anterior limbs are never found, 

 except in the brain of Man. 



The extent of submergctl area varies considerably in the different Prosimian genera. 

 In Lemur there is a vei-y considerable area, chiefly overlapped by the opercular lip of 

 the pseudosylvian sulcus: in Tar^vH*-, Ji/t•;"0(?t'Z//^s■, and Ayf^/Zcei^s there is practically no 

 submerged area, so that if the term " insida " be used as synonymous with " overlapped 

 neopallium," the opinion of Flower and Ziehen that there is an insula in Lemur but not 

 in Isiicticclus is justified. 



Put it has Ijcen demonstrated that in the Apes a considerable part of the insula is not 

 submerged and extends forward as far as the frocto-orbital sidcus. If my identification 

 of the latter sulcus in the Lemurs is correct (and in this matter I am supported by the 

 opinion of Ziehen and Flatau and Jacobsohn), the insula must be regarded as being 

 exposed in a very considerable part of its extent. We have no certain criterion to inform 

 us which neopallial areas should be regarded as " insula." la the human brain the 

 insida is that area of neopallium (1) which is suliinerged in the Sylvian fissure, (2j is 

 circumscribed (almost completely) by limiting sulci, (o) is applied to the lateral aspect 

 of the nucleus lenticularis, and (4) is split to form a claustrum. If we adopt any of 

 these criteria in the Lemiu's, the region so mapped out will fidfil )ione of the other 

 three conditions. 



The next group of sulci to be considei'ed includes the lateral, coronal, and crucial. 

 The sulcus ^^hich I have called "latcr;d" in the Lemurs is regarded by Ziehen as 

 the lepreseutative of the " Bogenfurche " of the Dng's brain, which is composed of the 



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