118 PBOFESSOR DAVID HEPBURN ON 



Sylvius. On the other hand, in MURIE'S * paper already referred to, there is a fairly 

 close resemblance shown in pi. Ixxviii. fig. 40, between the brain of the sea-lion and 

 that of the Weddell seal under consideration (fig. l), so far as the general position of 

 gyri and sulci is concerned ; although, having verified my interpretation of the surface 

 appearances by dissection of the interior of the hemisphere, my conclusions differ 

 considerably from those arrived at by MURIE, and so far as carnivora in general are 

 concerned I am of opinion that at least the Weddell seal presents a very novel arrange- 

 ment of the fissure of Sylvius, but still one which is quite compatible with, and readily 

 explainable by, reference to the mode of its development from the embryonic to the adult 

 condition. As is well known, the Sylvian fissure, in the course of its embryonic 

 development, results from the more or less close apposition of those portions of the 

 cerebral cortex which, as derivatives from the orbital, frontal, fron to -parietal, and 

 temporal portions of the cortex of the hemisphere, and under the term " opercula," 

 extend beyond so as to overshadow and gradually conceal from lateral observation that 

 portion of the cortex called the central lobe or island of Reil, and thus ultimately the 

 surface of the island of Reil may become completely hidden by convolutions which are 

 no longer upon the same superficial plane as those of the insula. Further, until these 

 " opercula " practically come into contact with each other, not only does the insula 

 remain more or less visible, but the lateral segment of the fissure of Sylvius is 

 represented by a gap or interval of varying width. Again, if the growth of the insula 

 kept pace with the growth of the surrounding " opercula," then the insular convolutions 

 would continue to present themselves upon the same superficial plane as that of the 

 " opercula," and thus instead of a fissure of Sylvius we should find in its place the sulcus 

 which limits and marks off the island of Reil from the surrounding cortex, viz. the 

 limiting sulcus (sulcus insulae). In other words, we should find the island of Reil pre- 

 senting or protruding between the " opercula " by whose apposition the fissure of Sylvius 

 derives its lateral characteristics. 



In my opinion, that is the interpretation of the condition which is presented by the 

 brain of the Weddell seal. As a result there appear to be two sulci extending from the 

 basal stem of the fissure of Sylvius, and between them the greater part of the island of 

 Reil not only presents itself, but is to a large extent upon the same superficial plane as 

 that of the surrounding gyri. 



The convolutions upon the surface of the insula were irregular, and neither upon 

 different brains nor upon the two sides of the same brain were they closely repeated ; 

 but I have given in fig. 1 a drawing of the brain in which they showed a tendency to 

 radiate from the basal end of the fissure of Sylvius, and I have done so because in the 

 human brain a radiating arrangement is their normal characteristic. From all this it will 

 be evident that the fissure of Sylvius as such is not represented on the lateral surface 

 of the brain of the Weddell seal ; but that in its place there is a vallecula, wide anteriorly 

 and narrower posteriorly, which is occupied by the convoluted surface of the island of 



* MURIE, loc. cit., pi. Ixxviii., fig. 40. 

 (ROY. soc. EDIN. TRANS., VOL. XLVIII., 830.) 



