THE ANATOMY OF THE WEDDELL SEAL. 837 
In the human brain this gyrus is named the isthmus of the limbic lobe, and I have used 
the same term for its description in this account of the brain of the Weddell seal. 
Posterior to the isthmus, a distinct deep fissure entered this region, 7.e. the 
basal aspect of the occipital end, as the continuation of a fissure well defined upon 
the mesial aspect of the hemisphere. Upon the basal aspect it was cut off from 
junction with the hinder end of the collateral suleus by an annectant gyrus, whereupon 
it turned abruptly backwards towards the occipital end of the hemisphere (fig. 3). It 
appears to me that that part of the fissure immediately behind the isthmus should 
be regarded as the continuation of the internal parieto-occipital sulcus (fig. 2), and 
that its extension towards the occipital end of the hemisphere is the calcarine sulcus 
(by some observers called the post-horizontal fissure). My reasons for this view will 
be further elaborated in connection with the description of the posterior cornu of the 
lateral ventricle, but meantime I may state that the calcar avis or hippocampus 
minor was closely related to the position of the deep anterior end of what I have 
named the calcarine sulcus. Resulting from appearances verified by dissection as 
well as by transverse section of the posterior cornu of the lateral ventricle (fig. 4), I 
feel warranted in concluding that the narrow gyrus which is situated on the lateral 
aspect of the calcarine fissure and connected with the hippocampal gyrus must be 
regarded as the lingual gyrus, while the larger gyrus situated on the mesial aspect 
of the calcarine fissure and posterior to the internal parieto-occipital fissure (fig. 3) 
must be regarded as the foundation for the cuneate lobule, which is found in a 
corresponding position on the mesial surface of the human cerebrum. 
Although the foregoing account shows that there was great deviation from the 
convolution pattern characteristic of a typical carnivore brain on the one hand, and by 
the human cerebrum on the other, yet the internal appearances exposed by dissection 
underwent an entire change and became simplified to a remarkable degree. So much 
was this the case that, in consideration of its size and with certain points of exception as 
to the details, the various objects were as readily recognised as they are in a human brain. 
Il. DissecTIon oF THE CEREBRUM. 
The method of procedure followed was that adopted in the dissection of the human 
brain. 
In the first place, the hemisphere was divided by a horizontal transverse section at 
about 4 mm. distance above the mesial free surface of the corpus callosum, in order to 
expose the white core or centrwm ovale minus, which, considering the total size of the 
hemisphere, was smaller than one expected. The reduction in the size of the central 
white core could be explained by the depth of the sulci. Many of the sulci at the 
frontal end were 2 ems. in depth, and at the occipital end some were 2°3 ems. deep. As, 
of course, all the sulci were bounded by a zone of grey matter, the general uous was a 
reduction in the apparent size of the central white core. 
