THE ANATOMY OF THE WEDDELL SEAL. 835 
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 Henwsphere. 
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 Varoli. ‘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 sulcws 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 posterzor 
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 cwrved 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 
TRANS. ROY. SOC. EDIN., VOL. XLVIII., PART IV. (NO. 30). 122 
