NERVOUS SYSTEM OF FISHES. 183 



equals the cerebellum in size, but is less than the optic lobes in the Perch 

 and Bream ; it equals the optic lobes, but is less than the cerebellum in 

 the Eel; in the Stickleback and Gurnard the prosencephalon exceeds the 

 cerebellum ; still more so in the Lepidosteus, but is less than the optic 

 lobes ; in the Lucioperca, the Amblyopsis, and the Skate, neither the 

 cerebellum nor the optic lobes are so large as the prosencephalon ; in 

 the large Sharks their united size scarcely equals that of the prosen- 

 cephalon ; and in the Salamandroid Polypterus and the Lepidosiren 

 the prosencephalic lobes surpass all the rest of the brain, and manifest 

 their true cerebral character and importance. In the Amblyopsis the 

 relative magnitude of the prosencephalon is due to the diminution of 

 the optic lobes in that blind fish ; in the Plagiostomes it is due to 

 absolute development ; as it is, also, in the Polypterus and Lepido- 

 siren, where the prosencephalon presents the closest similarity in 

 form and structui'e to that division of the brain in the Batrachian 

 Reptiles ; each lobe, for example, is elongated in the axis of the 

 skull, and is of a subcompressed oval form, and has a large ' lateral 

 ventricle ' in its interior in the Lepidosiren {fig. 54. Iv.) In the Skate 

 the prosencephala coalesce into a subdepressed transversely elongated 

 mass, their essential distinction being indicated by a mere superficial 



median fissure; in theCarcharias 

 {fig. 55. p.), the prosencephalon 

 forms an almost globular mass, 

 with scarcely a trace of a median 

 fissure. Amongst bony fishes 

 the prosencephalic lobes are 

 more or less confluent in Lucio- 

 perca Sandra, Trachinus draco. 

 Brain of Shark, c«rcA«n«5. Sargus, Mullus, Scombertra- 



chimis, Belone, Clupea harengus, and Clupea sprattus ; they appear as 

 distinct symmetrical spheroids in most other fishes, their union being 

 reduced to a small transverse medullary band (prosencephalic com- 

 missure, _/%. 52. ?/ *). The symmetrical character of the prosence- 

 phala, as of the optic lobes, is wanting in most rieuronectidce. 



The grey vascular neurine forms the greatest part of the prosence- 

 phalon in most osseous fishes ; the white fibres radiate into this 

 substance, and rarely appear on any part of the exterior surftice ; the 

 white neurine, however, predominates in the Plagiostomes and Lepido- 

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

 of the brain of Carcharias (No. 1310, A.) shows a deep ventricular fis- 

 sure at the anterior andunder partof the prosencei)halon,witli a\ascular 



* Cams well recognises the Iioinology of tliis coinmissure with that of the corjius 

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



N 4 



