REPORT ON THE SEALS. 113 



depression marked the position of the future fossa and fissure of Sylvius. With a pocket 

 lens one could also see the commencement of the differentiation of a Sylvian convolu- 

 tion about this fissure. On the tentorial surface of the hemisphere a shallower fissure 

 was also seen, which was probably the beginning of the splenial fissure. 



I do not intend to enter into a discussion of the question whether the fissures on the 

 surface of the hemispheres are primarily due, either to unequal growth of the cortex 

 in different parts, or to unequal resistance offered to the growth of the cortex, or to 

 both causes acting in different parts of the same brain. I would, however, state that 

 in stripping off the pia mater from certain parts of the hemispheres of the brains 

 which I have dissected I have been struck with the tension and consequent pressure 

 exercised by the arteries on the surface of the cortex in the direction of their course. 

 This was well seen in the fissure of Sylvius occupied by the large middle cerebral 

 artery. Also in a less degree by the arteries which ran in the pia mater occupying the 

 great transverse fissure of the cerebrum, and which as they turned round the hippocampal 

 convolution undoubtedly indented its surface by their pressure. 1 In these localities there- 

 fore there seems to be sufficient evidence to show that fissures may be produced and deepened 

 by the tension of the arteries, and doubtless the same cause operates also elsewhere. 



Comparison of the Convolutions of the Seals and Walrus with 



THOSE OF THE CaRNIVORA AND OF APES AND Man. 



M. Leuret, in his well-known Anatomie comparee du Systeme Nerveux, both figures 

 and describes the cranial surface of the brain of a Seal, probably Phoca vitxdina. He 

 considers that the convolutions in this animal are analogous to those of the Ungulata, 

 especially the Pig, though without resembling them throughout, and in his arrangement 

 of the Mammalia, according to the grouping of their convolutions, he places the 

 Ungulata, Edentata, and Marsupialia between the Carnivora and the Seals. He recog- 

 nises only three convolutions in the hemisphere of the Seal — one internal, on the inner 

 surface, which is obviously the gyrus fornicatus or great limbic lobe of Broca ; one 

 external bounding the fissure of Sylvius and very irregular ; one stqierior extending 

 from before backwards on the top of the hemisphere and forming two tiers, with two 

 subdivisions in front and three behind, whilst he regards the supraorbital convolution as 

 only an offshoot of the two anterior subdivisions. Sir Richard Owen again has recog- 

 nised in the brain of Phoca a prefrontal lobe in front of the frontal crucial fissure ; an 

 orbital fold above the orbit ; Sylvian, supersylvian, medilateral, and medial folds or 

 convolutions arranged in tiers above the fissure of Sylvius ; it is obvious, however, from 



1 Johannes Seitz has recently published an elaborate memoir (Ueber die Bedeutung der Hirnfurchung, Jahrbiicher 

 fiir Psychiatre, 1887) on the signification of the fissures in the hemispheres, in which he associates them with the places 

 of entrance and emergence of the blood and lymph vessels of the brain — that they are in fact nutrient fissures. 

 (ZOOL. CHALL. EXP. — PAKT LXVIII. — 1888.) Yyy 15 



