NERVOUS SYSTEM. VERTEBRATA. 177 



The interpretation of these raises a question of great 

 difficulty. There can be little doubt that the most anterior 

 represents the orbital or presylvian sulcus of other mam- 

 mals*. It does not actually open into the rhinal fissure, 

 but begins near it, and proceeds obliquely upward and 

 forward. In this course it is joined by the anterior ex- 

 tremity of a great inverted V-shaped sulcus, which may 

 possibly represent the suprasylvian sulcus. But upon this 

 question it is impossible to express a decided opinion at 

 present f. 



The other two sulci which diverge from the angle of the 

 rhinal fissure are probably to be regarded as essentially 

 " Sylvian " (in the sense in which this term is applied, say, 

 in speaking of the Cat's brain) . The more posterior, which 

 we may distinguish as sulcus B, freely communicates with 

 the rhinal fissure and with the sulcus tentatively called 

 suprasylvian in such a manner that the presylvian and 

 suprasylvian sulci, the sulcus B, and a short basal piece of 

 rhinal fissure form a pentagonal pattern. The deep ver- 

 tical sulcus which springs from the rhinal fissure between 

 the lower ends of the presylvian sulcus and sulcus B may 

 be distinguished as A. It ascends almost as far as the 

 angle of the so-called " suprasylvian " sulcus. 



The sulcus which it is customary to call " Sylvian fissure " 

 in mammals other than the Primates is a feature of little 

 morphological stability, and is to be regarded as essentially 

 of the nature of a kink produced by the flexure of the 

 hemisphere. Or perhaps it would be more accurate to 

 speak of it as being developed in response to the stress 

 produced in this region of the neopallium by the ventral 

 extension of its ventro-caudal region. In the Kangaroo 

 this stress seems to be relieved by the two sulci A and B 



* Not the orbital pure and simple, but the orbital joined to a second more 

 dorsal (mesial) sulcus, the direction and situation ofv which is probably 

 determined by the bias given to the expanding cortex by the presence of the 

 orbital sulcus. Compare the left hemisphere of specimen D. 216. 



t Close examination shows that the anterior limb of the V which is the 

 smaller and more unstable element and may represent the coronal sulcus is 

 not joined to the caudal limb, which is the supposed homologue of the 

 suprasylvian. 



VOL. II. N 



