THE ANATOMY OF THE WEDDELL SEAL. 119 



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, TURNER says : " I can make no definite statement as to the 

 presence of the island of Eeil, 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 : " I 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 MURIE 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 I 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 crucintns is an outstanding feature, and to 

 WIEDERSHEIM and PARKER'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 fissu re 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 in 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 ih<- 

 Weddell seal is concerned, I am driven to accept as the fissure of Rolando that fissure 



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(ROY. SOC. KDIN. TKANS., VOL. XLVIII., 831.) 



