ENCEPHALON OF FISHES. 



183 



small portion of a circle, fig. 182, h ; and in some bony fish, as tho 

 Garpike, Loach, and Lumpfish, they are scarcely raised above 

 the level of the floor of the ventricle. They are not deve- 

 loped in the Polypterus, the Lepidosiren, 

 or the higher Plagiostomes ; and both tori 

 and tuberculi are peculiar ichthyic develope- 

 ments in the ventricles of the optic lobes. 

 The bottom of the optic ventricle, fig. 184, v, 

 anterior and external to the 'tori,' is grey, and 

 usually prominent, with white fibres radiating 

 through it to rise and expand upon the walls 

 of the lobes. The optic lobes have almost 

 coalesced in the Sturgeon, fig. 274, o, Poly- 

 pterus, Lepidosiren, Amblyopsis, and Loach 

 (Cobitis). Where they are quite distinct 

 externally, as in most Osseous Fishes, they 

 are brought into mutual communication by one 

 or two commissures ; the anterior ' commissura 

 transversa ' is the most constant ; it is shown 

 in the Perch, fig. 182, and in the Herring, 

 fig. 184, s; it passes in front of the entry to 

 the third ventricle. 



In the Myxine and Lepidosiren the prepyramidal fibres curve 

 suddenly forward and upward before expanding into the 

 floor and sides of the third ventricle, and they thus form a 

 small protuberance beneath the basis of the optic lobes, fig. 

 186, n. In the Shark the same columns swell out laterally, 

 and form two small protuberances, fig. 187, ?z, separated below 

 by the vascular (hypophysial) floor of the third ventricle. In 

 most Osseous Fishes the corres- 

 ponding fibres of the prepyrami- 

 dal tracts swell out suddenly, be- 

 neath the optic lobes, into two 

 protuberant well - defined oval 

 ganglions ( c hypoaria,' fig. 185, n, 

 fig. 181, e) : their bulk is increased 

 by added grey matter, which 

 variegates their outer surface ; 

 they are well developed in the 

 common Cod, in which, as in some 

 other fishes, they contain a cavity (hypoarian ventricle). In some 

 SalmonidcB their surface is striated ; in some Cyprinida (Tench) 

 they are confluent : but commonly they are distinct, and have in 



Bruin and portion of niyelon 

 Cod. CCXVT. 



184 



185 



Optic ventricles ; 

 Herring 



Base of train ; Cod 



