THE ANATOMY OF THE WEDDELL SEAL. 831 
Reil, whose boundaries are indicated by the limiting sulcus which almost completely 
separates the insula from the surrounding cortex. 
In my opinion, this interpretation of the appearances is in conformity with the facts 
elucidated by a dissection of the corpus striatum, as well as with the facts of develop- 
ment, although I am not aware that it has hitherto been advanced by any of the 
observers who have described the brain of the seal. Indeed, in his description of the 
brain of the elephant seal, TurRNER says: “I can make no definite statement as to the 
presence of the island of Reil, unless the concealed part of the anterior limb of the 
Sylvian fissure be regarded as representing it.” Again, in reference to the brain of the 
walrus, the same observer says: “1 could not speak with any precision of the island of 
Reil, unless the concealed part of the anterior limb of the sylvian convolution passed 
deeply into the fissure and was concealed by the anterior limb of the supra-Sylvian 
convolution, which for some distance therefore formed the anterior lip of the fissure of 
Sylvius.” (In the first of these quotations the reference is to the concealed part of the 
Sylvian fissure, and in the second to the concealed part of the Sylvian convolution, but 
probably this is by inadvertence. ) 
The Plates which illustrate the papers of Murig and Turner, if compared with 
fig. 1 of the present communication, will show how much minor variation the brains of 
this group of marine mammals may present, while to my own mind they emphasise 
the interpretation which [ have ventured to put forward. It is difficult to conceive a 
brain of the dimensions of those under consideration without an island of Reil; and as 
this part of the convoluted surface of the hemisphere corresponds more or less exactly 
to the surface aspect of the corpus striatum, the presence of the latter practically com- 
pels us to account for the former. 
My next endeavour was to determine which of the sulci could be accepted as the 
fissure homologous to the fissure of Rolando (sulcus centralis), because of its importance 
as a guide to the position of the sensory-motor area and its value as a dividing line 
between the frontal and parietal lobes of the cerebrum. Reference may again be made 
to the brain of the dog, in which the sulcus cruciatus is an outstanding feature, and to 
WIEDERSHEIM and ParxkeEr’s* description of the fissure, where the following occurs: 
“‘ Along the lateral surface of the hemisphere, the cruciate sulcus (the homologue of the 
central sulcus or fissure of Rolando of primates) extends upwards to the pallial fissure.” 
Now, in the Weddell seal the cruciate sulcus is well marked ; but, as may be seen by refer- 
ence to figs. 1 and 2, it is situated so far forwards that, if it be accepted as the homologue 
of the fissure of Rolando, practically not only is there no frontal lobe remaining, but 
the parietal lobe is carried forwards to a position 7 front of the basal limb of the 
fissure of Sylvius, both of which contingencies are so serious as to compel us to doubt 
whether the homology be correct in the case of this seal, in view of the importance of 
the Rolandic area as a sensory-motor centre. For these reasons, therefore, so far as the 
Weddell seal is concerned, I am driven to accept as the fissure of Rolando that fissure 
* WIEDERSHEIM and PARKER, loc. cit., p. 228, 
