116 PROFESSOR DAVID HEPBURN ON 



in its fronto-occipital diameter ; 115 mm. in its greatest transverse diameter at a point 

 well forward on the temporo-sphenoidal lobes ; and 71 mm. in vertical height, measured 

 from the pons varolii to the vertex of the cerebrum. Thus, apart from the peculiarity 

 of its general outline in total size, it was only slightly less than an average human brain. 

 Throughout the anterior two-thirds of their extent the cerebral hemispheres were, as 

 already indicated, in very close apposition, and the falx cerebri only dipped into the 

 pallial or superior longitudinal fissure to a slight extent ; but in its posterior third this 

 cleft opened to form a wide interval, measuring 65 mm. in the transverse direction at 

 its hinder end and narrowing as it ran forwards towards the posterior end of the corpus 

 callosum. In the deep level of this interval the pineal body and the upper surface of 

 the vermis were visible, as well as part of the upper surface of the cerebellar hemispheres. 

 It should be stated that the backward extension of the occipital lobes of the cerebrum 

 carried them 2 mm. beyond the cerebellar hemispheres. 



In its essential features the basal aspect of the brain conformed to current 

 descriptions of the mammalian brain ; but it presented many special points of interest, to 

 which reference will be made in the course of my survey. 



I. CEREBRAL CONVOLUTIONS AND FISSURES. 



Regarded as a whole, the cerebral convolutions (gyri) were large and well defined 

 from each other by deep, well-marked fissures (sulci), and yet many furrows not deep 

 enough to be regarded as sulci were seen crossing the surfaces of convolutions. In- 

 variably these shallow furrows were in the position of blood-vessels ramifying in the pia 

 mater, and it was clearly demonstrable that the furrows were produced by the blood- 

 vessels. In appearance they resembled the arterial grooves upon bony surfaces, and 

 their presence upon the surface of the brain suggested either arterial pulsation or resist- 

 ance to brain growth as their determining cause. Indeed, from the distinct character of 

 many of them it would not be difficult to credit these vessels with the possibility of 

 determining the position of new fissures in a rapidly expanding hemisphere. In their 

 chief and outstanding characters the two hemispheres corresponded with each other ; 

 but in the matter of intimate detail they presented a considerable amount of asymmetry, 

 although neither hemisphere could be said to be more elaborate!)' convoluted than the 

 other. 



The general plan of the convolutions and fissures was not simple or easy to determine. 

 In fact, the whole arrangement bore very little if any resemblance to that presented by 

 the brain of a typical member of the carnivora, e.g. the dog ; and this is somewhat 

 remarkable and unexpected when we remember that the seals are themselves carnivores 

 notwithstanding their numerous adaptations to an aquatic habitat. Partly for this 

 reason, and partly because my observations do not altogether harmonise with those of 

 MURIE * in his description of another seal (Otaria jubata, the sea-lion), nor with those 



* MUKIE, Trans. Zovl. .S"c. /,</., vol. viii., 1874. 



(ROY. 80C. EDIN. TRANS., VOL. XLVII1., 828.) 



