ENCEPHALON OF FISHES. 



283 



187 



g a 



Brain of Shark, Carcharias 



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

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

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

 encephala coalesce into a subdepressed transversely elongated 

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

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

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

 fissure. Among bony fishes the 

 prosencephalic lobes are more 

 or less confluent in Lucioperca 

 sandra, Tracliinus draco, Sar- 

 f/us, Mullus, Scomber trachinus, 

 Belone, Clupea harengus, and 

 Clupea sprattus ; they appear 

 distinct symmetrical spheroids 

 in most other fishes, their union 

 being reduced to a small trans- 

 verse medullary band (prosencephalic commissure). 1 The sym- 

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

 wanting in most Pleuronectidce. 



The grey vascular neurine forms the greatest part of the 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 Plagiostomes and 

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

 the brain of Carcharias 2 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, 

 which is continued forward into the cms 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 capsule, whence they are 

 termed 4 olfactory lobes,' ' ttibera,' or ( ganglia.' The rhinence- 

 phala are solid bodies, 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- 



1 Cams well recognises the homology of this commissure with that of the corptis 

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

 3 (No. 1310, A), xx. vol. iii. 



