336 DE- (i- ELLIOT SMITH ON 



the downward beading of the posterior part of the pallium, which results in the 

 production of the large post-rhinal tongue-like process, necessarilj' implies a restriction 

 to, or rather a tension on, the growing pallium at the seat of the bending. It is not 

 unlikely that this flexure produces a kink — the fossa Sylvil — at the place of bending, 

 and that the tension of the growing cortex in this region is relieved by the outgrowtli of 

 lips of iiallium at the margins of the kink. In the human brain both of these factors 

 become greatly exaggerated, and a much larger area becomes involved, but the process 

 is probably analogous to that which takes place in the Dog. 



In the Dog's brain the Sylvian fissure is formed by the meeting of two lips of the pallium, 

 \\hich extend toward their place of meeting by overlapping a depressed area of cortex. 

 If these lips be cut away, we expose a triangular depressed ai'ea of exactly the same 

 shape as the fossa Sylvih in Ilyrmecophaga. The anatomy of this region of the Dog's 

 brain has been most lucidly demonstrated by Dr. Langley *. In the dissection 

 represented in his ligvire 16 we find a triangular depressed area ov fossa Sijlvii (which 

 he calls the " Island of Ueil "), limited on the ventral side by tlie convex junction of the 

 two parts of the rhinal fissure ; on the caudal side bounded by an almost vertical 

 limiting furrow, which is formed by the outgrowth of overhanging lip (which has been 

 cut away in the dissection) ; and bounded in front and dorsally by an oblique furrow 

 which is formed by the overhanging anterior lip. The superior limiting furrow joins 

 the posterior limiting furrow above, and the two are analogous to the sulcus Umitans 

 insidce of the human brain. It is also noteworthy that a narrow horizontal depression 

 extends forward from the fossa Si/Uni above the anterior rhinal fissure, just as we find in 

 Myrmecophaga, but in the Dog this depression is overhung by the anterior extremity of 

 the upper Up or operculum. 



Thus the representative of the sulcus /3 in the Dog's brain appears in a superficial view 

 to join the anterior rhinal fissure. 



In Myrmecophaga there is no Sylvian fissure, even in the sense in wliicli this term is 

 applied to the Carnivore's brain. But we have a fossa Sylvil, which is limited posteriorly 

 by an operculum that has not yet overlapped the depression, and anteriorly by the very 

 fjiintest trace of an anterior operculum. Before we can say that a Sylvian fissure exists, 

 these two lips must increase considerably in size, overlap the depressed area, and 

 ultimately meet to form a fissure. 



The exposure of the fossa Syloli in the Edentata is indicative of the small extent of 

 pallium as compared with the Carnivore. 



The sulcus [5 is represented in the Dog by a sulcus which is called siqjraorbilal 

 by FloAver f and others, or ^j/r - Sylvian by Owen and others. The sulcus y is 

 represented in the brain of the Carnivora by a longitudinal sulcus knoflm as the 

 coronal or sagittal sulcus. In the Carnivora, however, the cephalic extremity of 

 the coronal sulcus is usually bent in the lateral direction to accommodate the crucial 

 sulcus, which is lacking in the Edentata. The caudal extremity of this sulcus is 



* J. N. Laugley, " The Structure of the Dog's Brain," .loumal of Physiology, vol. iv. .See especially, in this 

 connection, pi. vii. tig. 16. 



t W. H. Flower, " Anatomy of Protdes,'' Proc. Zool. Soc. Loudon, 1809, p. 479. 



