ENCEPHALON OF FISHES. 



283 



Brain of Shark, CarcharUis 



example, is elongated in the axis of tlie skull, and is of a sub- 

 ccimpressed oval form, and has a large ' lateral ventricle ' in its 

 interior in the Lciiidosiren, fig. 186, I v. In the Skate, tlie pros- 

 encephala coalesce into a subdepressed transversely elongated 

 mass, their essential distinction Ijeing indicated by a mere super- 

 ficial median fissure ; in Carcharias, fig. 187, the jjrosencephalon 

 forms an almost ghjbular mass, with scarcely a trace of a median 



fissure. Amouo- Ijony fishes the 



^ -' 187 



lirosencephalic lobes are more 



or less confluent in Luciuperca 

 Sdiidrti, Tracliimis draco, S//r- 

 f/us, Mullus, Scomher trachiims, 

 Belone, Cliipea harengus, and 

 Chipea sprruttus ; they appear 

 distinct symmetrical spheroids 

 in most other fishes, their union 

 being reduced to a small trans- 

 verse medullary band (iirosencephalic commissure).' The sym- 

 metrical character of the prosencephala, as of the optic lobes, is 

 Avanting in most Pleuronectida. 



The grey vascular neurine forms the greatest part of tlie pros- 

 encephalon inmost Osseous Fishes; the white fibres radiate through 

 this, and rarely appear on any part of the exterior surface ; the 

 white substance, however, predominates in the Plagiostomcs and 

 Lepidosiren. As a rule, the proseneephalic lobes are solid ; but 

 the l^rain of Carcharias ^ shows a deep ventricular fissure at the 

 anterior and under part of the prosencephalon, with a vascular 

 fold of membrane or ' choroid plexus ' penetrating the fissure, 

 wdiich is continued forward into the orus of the olfactory lobe. 

 The lateral ventricle is more extensive in the Lepidosiren, and is 

 continued directly into the olfactory lobe. 



The ' rhinencephalon ' figs. 173 — 176, R, consists of two always 

 distinct lobes of grey matter, which receive the prolongations 

 of chiefly white fibres from the prosencephalon and its crura, and 

 give off the nerves to the olfactory caj^sule, whence they are 

 termed ' olfactory lobes,' ' tuliera,' or ' ganglia.' The rhinence- 

 phala are solid l)odies, always distinct, wide apart from each other 

 when remote from, and in mutual contact when near to, the rest 

 of the brain, but never united by a commissure. The rhinence- 



' Carus wcU recognises the homology of this commissure with that of tlic corims 

 striatum, called ' anterior commissure ' in the human brain, i. p. 24. 

 - (No. 1310, a), XX. Yol. ill. 



