THE ANATOMY OF THE WEDDELL SEAL. 125 



In the human brain this gjrus 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, i.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 sulcus 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. 



II. 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 centrum 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 cms. in depth, and at the occipital end some were 2 '3 cms. deep. As, 

 of course, all the sulci were bounded by a zone of grey matter, the general effect was a 

 reduction in the apparent size of the central white core. 



(ROT. soc. EDIN. TRANS., VOL. XLVIII., 837.) 



