THE ANATOMY OF THE WEDDELL SEAL. 123 



end, where several definite sinuosities appeared, and at the widely expanded end of 

 the hippocampal gyrus were there any marked deviations from the much simpler 

 appearances presented by the limbic lobe of the human brain. 



3. The Inferior or Basal Aspect of the Hemisphere. 



As may be seen by reference to fig. 3, the general appearance and the interpretation 

 of this surface were relatively simple in comparison with the other surfaces, except in 

 the occipital region, where again there was considerable complexity due to the fact that 

 so much more of the convoluted surface of the occipital lobe was directed towards the 

 tentorium cerebelli than towards the falx cerebri. with the result that objects which 

 appear on the mesial aspect in the primate brain were found upon the tentorial aspect 

 in that of the seal. 



In the mesial plane the two hemispheres were divided from each other in the 

 frontal region by the pallial fissure as far back as the lamina terminalis, below and 

 behind which the optic chiasma was situated. The inter-peduncular space presented 

 the usual boundaries, viz. : anteriorly, the optic chiasma ; antero-laterally, the optic 

 tracts ; postero-laterally, the crura cerebri ; posteriorly, the pons Varolii. The structures 

 forming the floor of the space were the tuber cinereum, provided with a short 

 infundibulum to which the hypophysis cerebri was attached, this latter being a large 

 object in proportion to the size of other structures ; the corpora mammillaria ; and the 

 locus perforatus posticus. The occulo-motor nerves emerged from the mesial aspect 

 of the crura cerebri. 



The basal surface of the frontal lobe was clearly defined posteriorly by the fissure 

 of Sylvius and the locus perforatus anticus. This surface presented the following 

 fissures : the olfactory sulcus, which was occupied by the olfactory tract (fig. 3), 

 pursued a straight course from the locus perforatus anticus forwards towards, but not 

 quite up to, the orbital margin ; the rhinal sulcus commenced a short distance in 

 front of the Sylvian fissure and ran forwards in a curved manner, following the lateral 

 contour of the orbital surface, but separated from the margin by a convolution, then, 

 winding round the anterior end of the olfactory sulcus, it turned backwards between 

 the olfactory and pallial sulci, and terminated as a shallow groove upon the gyrus 

 rectus. 



The convolutions on the orbital surface were the following : the gyrus rectus, 

 situated between the olfactory sulcus and the mesial orbital border ; the posterior 

 orbital gyrus, forming the anterior boundary of the Sylvian fissure and the orbital 

 operculum of the island of Reil ; a triangular gyrus, occupying the space between 

 the olfactory and rhinal sulci ; and a long curved gyrus, situated between the rhinal 

 sulcus and the lateral margin of the orbital surface. The triangular and curved gyri 

 were both connected with the posterior orbital gyrus behind and with the gyrus rectus 

 in front, but otherwise they were separated throughout their length by the rhinal 



(ROY. 80C. EDIN. TRANS., VOL. XLVIII., 835.) 



