132 THE VOYAGE OF H.M.S. CHALLENGE?.. 



at a corresponding spot did for about 4 mm. indent the marginal convolution in the same 

 reo-ion as in Ursus arctos, and a somewhat longer one was present in the left hemisphere. 

 These fissures were so short and shallow that they gave me the impression of being quite 

 subordinate furrows. On the other hand, the parieto-occipital fissure in the brain of 

 Man and Apes is one of primary importance ; it appears in the human foetus at about 

 the fifth month, and is especially marked on the inner surface of the hemisphere ; whilst 

 neither in Ursus arctos nor Ursus maritimus was there any evidence of a fissure which 

 corresponded with the internal parieto-occipital fissure of the human brain. 



Dr. Murie, in the course of his description of the brain of Otaria jubata, employs to a 

 large extent the terminology of human anatomy, and believes that he can recognise in 

 the brain of this Eared Seal the majority of the convolutions and fissures present in the 

 human brain. Amongst other fissures he describes, by the name of internal perpendicular, 

 the fissure which is more usually named parieto-occipital. He figures it as indenting 

 the marginal convolution towards the hinder end of the hemisphere, and as present both 

 on the mesial and cranial surfaces. In conformity with the method of nomenclature 

 which he has adopted he has named the convolution in front of the inner part of this 

 fissure the quadrilateral lobule of the parietal lobe, whilst that which lies behind it he 

 names the internal occipital lobule. In the brains both of the Elephant Seal and Walrus 

 the marginal convolution was indented in a position almost corresponding to that in 

 Otaria jubata, by a continuous fissure both on the cranial and mesial surfaces, the 

 length of which was, however, variable in the different brains, especially on the mesial 

 -surface. Partly owing to this variability, and partly because we have no information on 

 the development of this fissure in the Pinnipedia, I am not prepared to say that it is 

 homologous with the parieto-occipital fissure of the human brain. 



The evidence obtained from experiments on the cerebral cortex has established the 

 important fact that stimulation of the occipital lobe in the brain of the Monkey produces no 

 definite reaction ; whilst stimulation of the angular gyrus, both in its anterior and posterior 

 limbs (13), affects the pupils and occasions movements of the eyes to the opposite side, so 

 that this convolution is a visual centre. 1 In the Dog also the most posterior parts of the 

 1st and 2nd external convolutions do not respond to stimulus, whilst a portion of the 

 2nd external convolution in front of the most posterior part (13), when stimulated, gives 

 reactions similar to those obtained from the angular gyrus in the Monkey. There is 

 reason to think, therefore, that the most posterior parts of the 1st and 2nd external 

 convolutions of the Dog are potentially equivalent to the occipital lobe in the brain of 

 the Monkey, although they are not differentiated by a parieto-occipital fissure, whilst the 

 2nd external convolution immediately in front of the part which does not respond to 

 stimulus and the angular gyrus are homologous with each other physiologically. In all 

 probability these convolutions are also anatomically identical, for Gratiolet, who was the 



1 See the researches of Ferrier and other experimentalists. 



