NERVOUS SYSTEM. VERTEBRATA. 473 



the complete covering of the insula. The anterior lip of tho fronto- 

 orbital (or, as we may now call it, anterior limiting sulcus of Reil) grows 

 backward to meet the temporal operculum, and thus gives rise to the 

 " stem " of the Sylvian fiasure. The dorsal lip of the forward exten- 

 sion of the superior limiting sulcus grows down to meet the temporal 

 operculum (thus forming the anterior part of the posterior limb of the 

 Sylvian fissure) and also the orbital operculum (which is the anterior 

 operculated lip of the f ronto-orbital sulcus). The latter meeting gives 

 rise to the anterior limb of the Sylvian fissure. It often happens, 

 however, that the expanding cortex in the neighbourhood of the 

 meeting place of the anterior and superior limiting sulci becomes ac- 

 commodated by the formation of an additional operculum the frontal. 

 As the result two anterior limbs of the Sylvian fissure (instead of one) 

 are produced. 



It follows from this account that a complete Sylvian fissure exists 

 only in the human brain, and that the so-called Sylvian fissure of 

 even the Anthropoid Apes lacks properly-constituted anterior limbs, a 

 small part of the posterior limb, and generally also the " stem " of the 

 complete sulcus. 



The full development of the opercula leads to the abortion of the 

 upper part of the fronto-orbital sulcus in the human brain. 



The lateral, post-lateral, and ansate sulci of the Carnivora and 

 other Mammalian Orders become in the Primates the intraparietal, 

 transverse occipital, and ramus postcentralis superior respectively. 

 It is a moot point whether the coronal sulcus, which is so constant 

 and precocious, in the Carnivora, Ungulata, and Edentata, forms 

 the ramus postcentralis inferior of the intraparietal system. The 

 evidence seems to point to the sulcus rectus and the lower part of the 

 central sulcus as being the real derivatives of this furrow. 



In most Apes the region lying behind the transverse occipital sulcus 

 undergoes a peculiar modification leading to the formation of a great 

 operculum from the posterior lip of a new sulcus, called Simian or, as 

 the Germans say, " Affenspalte." This does not usually occur in the 

 human brain, probably because the cortical areas around tho transverse 

 occipital sulcus undergo a greater expansion than is the case in the 

 Apes. 



I have seen, however, in the brain of an Egyptian fellah a small 

 indubitable Simian sulcus like that of the Gorilla. It was separated 

 by a considerable interval from the mesial plane. 



It thus happens that this region of the human brain more closely 

 resembles the condition found in many of the larger Cebida) (in which 

 the opercular formation has either not begun or is only just com- 



