NERVOUS SYSTEM. VERTEBBATA. 351 



well-known plan of suprasylvian, corono-lateral, and cal- 

 carine-intercalary arcuate sulci, which divide the hemisphere 

 into broad strips which become broken up by an extra- 

 ordinarily rich supply of sulci. 



The plan of these sulci is very constant in Odontoceti, 

 so that the description of the brain of Delphinus tursio (vide 

 infra) may be considered sufficient for the Suborder. It 

 must, however, be remembered that, rich as the supply of 

 sulci is in Phoccena, the surface of the hemisphere becomes 

 even more broken up in the larger members of the Order. 



Pari passu with the growth of the cerebral hemispheres 

 the cerebellum also attains to very great proportions, in 

 order, no doubt, that it may co-ordinate the activities of 

 the enormous body, the large sensory surface of which has 

 called into being the extensive neopallium. 



The structure of the cerebellum is essentially identical 

 with that of other large mammals. 



The other parts of the brain call for little mention. It 

 is, however, worthy of note that the olivary bodies are 

 placed some distance to the caudal side of the large pons 

 Varolii. 



Flatau and Jacobsohn, Vergl. Anat. d. Centralnerven- 

 systems, 1899, p. 449. 



D. 514. The left half of the brain of a Dolphin (Delphinus tursio} 

 (c?), which has been split in the mesial sagittal plane. 



0. C. 1333 H. 

 Presented by the Westminster Aquarium, 



D. 515. The right half of the same brain. 



This brain is larger and correspondingly richer in sulci 

 than that of the Porpoise ; but the structure of the two 

 organs is essentially the same. 



The so-called " Sylvian. fissure " begins upon the base 

 of the brain in a deep cleft- like vallecula Sylvii. It ascends 

 for only a short distance upward and slightly backward 

 (fig. 201) upon the lateral aspect. In this respect it differs 

 most markedly from the larger Carnivora, in the brains of 

 which (compare the Bears and Seals) the " Sylvian fissure " 

 becomes greatly elongated. There is a much more striking 



