124 THE VOYAGE OF H.M.S. CHALLENGER. 



to the gyrus and lobus hippocampi on the other, these four convolutions in the Carnivora 

 and Pinnipedia might seem at first sight as if they approximated to the temporo- 

 sphenoidal and occipito-temporal convolutions in Man and Apes, though in Man they are 

 greatly elongated and approach the horizontal in their direction, in conformity with the 

 direction of the fissure of Sylvius. Moreover, they project in front of the uncus or 

 lobus hippocampi so as to form the tip of the temporo-sphenoidal lobe and the greater 

 part of the boundary of the Sylvian fossa, so that the lobus hippocampi with the short 

 postrhinal fissure is not visible at the base of the human brain, but is displaced inwards 

 on to the tentorial aspect. But further, in the brains of the Carnivora and Pinnipedia 

 the lobus hippocampi appears as a distinct protuberance on the base of the brain, and 

 itself forms the posterior boundary of the Sylvian fossa. These differences in the two 

 types of brain might seem to be accounted for simply by the great development and the 

 change in direction of the convolutions of the temporo-sphenoidal lobe in the brains of 

 Man and Apes, causing in them displacement of the lobus hippocampi to the inner 

 surface of the hemisphere, and its concealment, when the hemisphere is looked at from 

 the cranial aspect, by the greatly elongated temporo-sphenoidal convolutions. 



But I am of opinion that this does not express the whole difference between these 

 brains in this region. In the description of the brain both of the Walrus and the Seals 

 I have indicated that the Island of Reil may find its representative in these animals in 

 the anterior limb of the Sylvian convolution, which is more or less concealed within the 

 fissure of Sylvius ; and in the brain of the Polar Bear I have shown that an entire 

 arched convolution is concealed within that fissure. If I am right in this indication, 

 then I believe that the Island of Reil, which in the brain of the Ape and still more in 

 that of Man is entirely concealed within the Sylvian fissure, is either the homologue of 

 the Sylvian convolution of the carnivorous brain, or that the Sylvian convolution in 

 the Carnivora potentially represents both that convolution and a rudimentary insula. 

 In the true Carnivora the Sylvian convolution was as a rule superficial and on the 

 cranial aspect, though in the Otter and Badger indications of the depression of its 

 anterior limb within the fissure were seen. In the Seals and Walrus the concealment 

 of this convolution was still more marked, so that the brains of these animals form 

 apparently in this particular a transition to those of Man and Apes, in which the 

 concealment of the Island is complete. On the supposition therefore that the Island 

 of Reil in Man and Apes is morphologically related to the Sylvian convolution of 

 the Carnivora, the superior temporo-sphenoidal convolution in the Human and Ape's 

 brain cannot be regarded as corresponding with the posterior limb of the Sylvian con- 

 volution, but with that of the convolution of the tier immediately above and behind the 

 Sylvian convolution, i.e., the 3rd external convolution of Ferrier or the suprasylvian 

 convolution of my description. The sinking of the Sylvian convolution into the fissure 

 may perhaps to some extent be associated with a diminution in magnitude of the 



