CETACEA. 



583 



spinal chord (a, J?g. 272) gently expanding 

 into the medulla ohlongata, on the anterior 

 surface of which the corpora pyramidalia (b, 

 Jig. 272) are seen well defined and prominent. 

 At the point where they begin to rise above 

 the surface of the medulla, there is a manifest 

 decussation of their internal fibres ; they pro- 

 ceed through the pans Varoli (c), and are 

 continued into the crura cerebri. 



The corpora ollvuria are situated near the 

 pyramidalia; they do not, however, project 

 from the surface as in the human brain, but 

 are distinguishable by the internal grey sub- 

 stance (corpus dentatum alivx). Their medul- 

 lary fibres proceed through the pons and enter 

 the bigeminal bodies, in which they converge 

 and decussate each other. 



The transverse medullary fibres, which are 

 seen in most Mammalia extending across the 

 under surface of the medulla oblongata imme- 

 diately behind the pons, and which Treviranus 

 has called the trapezium, are wanting in the 

 brain of the Dolphin, as in that of the Orang 

 Utan and the Human subject. 



The two posterior columns of the spinal 

 chord are continued (according to Tiedemann) 

 as the corpora restiformia to the cerebellum. 

 Between these is situated the fourth ventricle, 

 from the floor of which the acoustic nerves 

 take their origin. 



The very large size of the cerebellum in 

 proportion to the spinal chord and cerebrum, 

 which Hunter noticed in the Piked Whale, is 

 equally remarkable in the Dolphin. The cere- 

 bellum is deeply divided into lobes, of which 

 six may be distinguished on the upper surface 

 of each hemisphere. Of these, two small lobes 

 correspond to the posterior superior lobes of 

 the human cerebellum. 



On the under surface we remark 

 the posterior inferior lobes (e), the 

 anterior inferior lobes (fj, one lobe 

 corresponding to the amygdaloid 

 lobe of Reil (g), and the floccus 

 (ti). Each lobe is subdivided by 

 deep fissures into smaller lobes, and 

 these again by shallow anfractu- 

 osities into lamelke. The middle 

 or vermiform portion of the cere- 

 bellum (a, fig. 273) is not sym- 

 metrical, but inclined, like the cra- 

 nium itself, to the right side. The 

 internal medullary substance of the 

 cerebellum resulting from the di- 

 vergent fibres of the cms, corpus 

 restiforme, and procesms ad testcs, 

 and the superadded commissural 

 fibres, has a well-marked internal 

 grey substance or corpus firnbri- 



probably not calculated to smell water. It becomes 

 difficult therefore to account for the manner in 

 which such animals smell the water ; and why the 

 others should not have had such an organ, which 

 seems to be peculiar to the large and small Whale- 

 bone Whales ( Baleena mysticetus and Balcenoptera 

 rostrata); the organ, in those which have it, is ex- 

 tremely small, when compared with that of other 

 animals, as well as the nerve, which is to receive 

 the impression." Phil. Trans, pp. 428, 430. 



aturn, and is covered by the usual external 

 layer of similar material. Between the columns 

 which extend from the cerebellum to the bige- 

 minal bodies, the medullary lamella called 

 valvula Vieussenii is situated. The pons or 

 commissure of the cerebellum (c, fig. 272) is of 

 large size, corresponding to the hemispheres of 

 the part which it seems to associate in action. 



The cerebrum is extended backwards over 

 the cerebellum, but the posterior parts of the 

 hemispheres diverge from one another so as to 

 expose a part of the cerebellum. The most 

 striking feature of the cerebrum is its great 

 breadth, which exceeds its length, a disposition 

 of this organ peculiar among Mammalia to 

 the Cetaceous order. Each hemisphere is seen 

 at its inferior surface to be divided by the 

 fissura rnagna (k, Jig. 272) into an anterior 

 (/) and middle lobe (i), which latter is con- 

 tinued above the cerebellum into the posterior 

 lobe. The whole external surface of the he- 

 mispheres is divided by deep anfractuosities 

 into convolutions, which are proportionally 

 more numerous and narrower even than in the 

 human brain. This structure seems common 

 to all the Cetacea ; besides the observations of 

 Tiedemann and Cuvier in the common Dol- 

 phin, the numerous convolutions have been 

 remarked by Tyson in the brain of the Por- 

 pesse, and by Scoresby in that of the Mysticete 

 Whale. 



The crura cerebri (i,fig. 272) are of large size; 

 the eminentiae mammillares (/j) are as usual 

 situated between them, and anterior to these 

 are the infundibulum and pituitary gland (o). 



The two hemispheres in the Dolphin's brain 

 described by Tiedemann, measured each two 

 inches and eleven and a half lines in length, 

 and were united by a corpus callosum (b,jig. 



Fig. 273. 



273,) of one inch and three lines in length. The 

 chief peculiarity of this part is its position, 

 which is not horizontal, but inclined down- 

 wards and forwards. The bigeminal bodies 

 are of considerable size ; the anterior ones are 

 rounded and lie closer together than the pos- 

 terior. These have an oval form, and are 

 separated by a depression which receives the 



