THE ANATOMY OF THE WEDDELL SEAL. 121 



fissure, which was deepest at its ends and shallowest about half way between them, 

 divided the parietal region of the brain into an upper and a lower lobule, and it might 

 quite fairly be termed the intra-parietal sulcus. Each of the lobules above and below 

 the intra-parietal sulcus presented in its turn a short and less defined fissure whose 

 course was roughly parallel to that of the intra-parietal sulcus, but neither of these 

 short fissures opened into the fissure of Rolando. Thus the frontal ends of the convolu- 

 tions both above and below the intra-parietal sulcus were united together, with the 

 result that the arrangement suggested an interrupted post-central (ascending parietal) 

 gyrus. 



It has already been stated that there was no definite guide which could be selected 

 as a demarcation between the parietal and occipital lobes, and therefore I can only say 

 that, as a whole, the convolutions in the occipital region ran from behind forwards, and 

 more or less parallel to each other, to make connections with the parietal and temporo- 

 sphenoidal convolutions. One of these connections seems worthy of special notice. It 

 joined the hinder end of the island of Reil and the hinder end of the temporo-sphenoidal 

 operculum to one of the occipital convolutions. In this relation it should be 

 remembered that the Sylvian fossa (which ultimately becomes the posterior limb of 

 the Sylvian fissure in the primate brain) is shallowest in this region during the process 

 of its development. 



The lateral aspect of the temporo-sphenoidal lobe, which provided one of the opercula 

 of the island of Reil, was situated below and behind the Sylvian fossa. It presented 

 two fairly well defined convolutions, an upper and a lower, separated by a definite 

 sulcus, with irregular sulci of smaller dimensions, suggesting the possibility of further 

 subdivision. 



2. The Mesial Surface of the Hemisphere. (Fig. 2.) 



This aspect of the hemisphere presented considerable elaboration and complexity as 

 regards the structures belonging to the pallium, but in the basal region it was simpler 

 and more easy of interpretation. As on the lateral surface, the convolutions and fissures 

 were large and well defined, although the determination of their homologies was a 

 matter of considerable difficulty. 



The corpus callosum measured 5 cms. in length and 4 mm. in vertical depth over 

 the greater part of its length. The genu was 10 mm. long and 9 mm. in vertical depth, 

 while the vertical depth of the splenium was 5 mm. From the anterior end of the 

 genu to the frontal end of the hemisphere the distance was 2 cms., and from the posterior 

 margin of the splenium to the occipital end of the hemisphere the distance was 4 cms. 

 Therefore, as a whole, the corpus callosum was situated nearer to the frontal end of the 

 brain. The rostrum of the corpus callosum was very short, but the lamina terminalis 

 (lamina cinerea), extending from the rostrum to the optic chiasma, was a well-defined 

 object. 



The callosal sulcus, which separated the dorsal surface of the corpus callosum and 



(ROY. soc. EDIN. TRANS., VOL. ZLVIII., 833.) 



