PROSENCEPHALON OF MAMMALS. 



119 



ing : — The prefrontal lobe, »", w x , as defined by the fissure 14, 

 fig. 93, is shorter than in the Felines; both medial /, medi- 

 lateral m, and supersylvian g, folds, are longitudinal and parallel, 

 but with outlines wavy through secondary fissures. The sylvian 

 fissure, .*, marks the sylvian fold, which is not continuously or 

 well defined, the lateral fissures being too irregular and inter- 

 rupted. The ectorhinal fissure, through the vertical extension of 

 the natiform protuberance, is more interrupted than in other 

 Carnivora ; but it is continued backward from the sylvian fissure 

 and defines the mesial non-convoluted part of that protuberance 

 (basirhinal tract). The orbital fold, bounding the outer crus and 

 side of the rhinencephalon, is defined by the entorbital fissure, 

 more extensively and definitely in the Seal than in most other 

 Carnivora. The mass of the hemispheres behind the sylvian 

 fissures is relatively greater than in other Carnivora, and a larger 

 proportion of the cerebellum is covered thereby. 



The general parallel longitudinal 

 course of the primary folds, e, g, I, cha- 

 racteristic of the hemispheres of Car- 

 nivora, is more strictly preserved in the 

 Cetacea, fig. 94 ; but with great increase 

 of cerebral substance, and especially 

 of the exterior layer, by the very nume- 

 rous secondary fissures, which mask the 

 true character of the primary ones, n 

 and 8, until a comparison of the relative 

 depth unfolds it. 1 



Although the olfactory nerve be not 

 developed in Delphinidce, the rhinal, fig. 

 GO, b, and basirhinal, b' ', tracts are de- 

 fined by the ectorhinal fissure, 2, a circumstance significative of the 

 derivation of this marine family from some form of the Mammalian 

 class, retaining, as in Balcenidce, the usual provision of nerves of 

 special sense. In fig. 95 are shown the extent of the corpus 

 callosum, b, the depth of the cerebral fissures, and the proportions 

 of the ' corpus striatum,' d, k, the optic thalamus, t, and the 

 hippocampus, g, with its unusually broad ( tamia,' h } or free border 

 continued into the ' fornix.' A beginning of the * posterior horn' 

 of the lateral ventricle is now first shown. 



1 The primary cerebral convolutions in the hoofed Mammals have 



1 The preparation of the Porpoise's brain, by which I showed this structure in the 

 ' Hunterian Course' of 1842, is probably still preserved in the Museum of the Royal 

 College of Surgeons : the primary convolution g is removed. 



Delphinus ; upper surface of right 

 hemisphere. 



