Chap. VIII.] 



AND OE AMPHIBIA. 



113 



Cerebellum tend to increase in size and become more 

 and more complex as we pass from Fishes to Reptiles, 

 from Reptiles to Birds, and from Birds to Mammals. 



llie relative size of these parts, however, as well as of 

 other divisions of the Brain, will be found to vary greatly 

 in different kinds of Fishes. 



Fig. 46.— Brain and Cranial Nerves of the Perch, side view. (Gegenbaner, after 

 Cuvier.) A, Cerebral lobe with olfactory ganglion in front ; B, optic lobe : C, cere- 

 bellum ; B, medulla oblongata ; /, olfactory nerve coming from a, the nasal sac ; 

 //, optic nerve cut across ; ///, oculo-motor ; IV, trochlear nerve ; V, trigeminal ; 

 VII, auditory ; VIII, vagus, with its ganglion ; k, lateral branch of the vagus ; 

 /, upper twig of the same ; ;,i, dorsal branch of the trigeminus, which is joined by 

 n, the dorsal branch of the vagus ; a, p, y, three branches of the trigeminus ; Be, 

 facial nerve •- X, bronchial branches of the vagus. 



The Spinal Cord of Fishes is more or less cylindrical in 

 shape (fig. 47, h) and almost uniform in thickness through- 

 out, except that it tapers to a point posteriorly. It occurs 

 only rarely that there is, as in the Ray, a slight swelling 

 in the region where the nerves from the great pectoral fins 



