THE ANATOMY OF THE WEDDELL SEAL. 117 



of Sir Wji. TURNER * in his account of the brain of the elephant seal, I propose to deal 

 at some length with the arrangement of the convolutions and fissures and the possibility 

 of dividing the cerebral surface into subordinate lobes, after the manner adopted in 

 describing the human brain. 



The complexity of the convolution pattern of the brain of seals has led observers to 

 devise such an elaborate terminology for the description of the separate convolutions 

 and fissures that it is a matter of considerable difficulty to correlate the different terms. 

 Consequently, bearing in mind the variations which specimens of these brains present 

 among themselves, as well as their divergence from the ordinary type of carnivore brain, 

 I have preferred to restrict the use of terms as much as possible, and to limit the attempt 

 to establish homologies to such characters as were fairly comparable to those presented 

 by the human brain. 



1. TJie Lateral Surface of the 7/f////'.s^>//r/v. (Fig. I.) 



On this aspect the convolutions and fissures were well developed both as regards 

 their size and their numbers, and yet any underlying " pattern " resulting from the 

 disposition of the primary fissures was most elusive and difficult to decide. Fortunately, 

 there was no uncertainty with regard to the fissure of Sylvius (sulcus Sylvii). Its 

 commencement in relation to the locus perforatus auticus on the basal surface of the 

 brain, and its position between the orbital and temporo-sphenoidal parts of the hemi- 

 sphere on the same surface, fixed the position of its main stem without any doubt, and 

 so by its outer end it provided one fixed point from which to unravel the complexity of 

 the lateral surface. TURNER found this fissure traceable on the lateral aspect of the 

 hemisphere " upwards and backwards for 32 mm. on the side of the right hemisphere, 

 but not so far on the left." Nevertheless, for some time I found great difficulty in 

 deciding which, and how many, of the fissures upon the lateral surface were entitled to 

 be accepted as its direct continuations, although, as the dissection proceeded, the decision 

 arrived at in the first instance was verified as correct. My initial difficulty was increased 

 by the fact that in the lateral view of the hemisphere of the brain of the dog, as may be 

 seen in the figure given by WlEDERSHEIM and PAKKKK,+ the fissure of Sylvius is 

 represented as a " closed " fissure, i.e. one provided with " opercula," forming an " arcuate 

 gyrus " which surrounds the fissure on all aspects except the basal segment of the 

 fissure. Further, in the brain of the dog, this "arcuate gyrus" is repeated twice, so 

 that altogether on the lateral aspect of its hemisphere, to quote WlEDERSHEIM and 

 PARKER, " In carnivores, cetaceans, and ungulates, three gyri arch over the Sylvian 

 fissure, one above the other, and are separated by the so-called arcuate fissures." \ 

 Certainly this was not the manner in which the convolutions and fissures were disposed 

 on the lateral aspect of the hemisphere of the Weddell seal in relation to the fissure of 



* TURNER, Challenger /.'/<//.<, vol. xxvi., Zonlmjii : lf-/n.n-i mi S> 



) U'lKinassiiKiM an.l I'AKKKU, Comparative . I iin,,,ij i,j I" . :'.! 1 '<!., 1!>"7, p. 224. 



: //>/</., p. 228. 



(ROY. SOC. EDIN. TRANS., VOL. XLVIII., 82'J.) 



