NERVOUS SYSTEM Ol" FISMKS. 



the noclulus slightly swells above, and obliterates the ' calaiuus 

 scriptorius.' In tlie Lucioperca the vagal lobes are not very dis- 

 tinct, but they mark the commencement, and form the broadest part, 

 of the very long medulla oblongata, the restiform tracts diminishing 

 in size as they advance. In no other Vertebrata save fishes are the 

 vagal lobes and the nodulus present. 



The posterior pyramids, which are the encephalic continuation of 

 the posterior myelonal columns, diverging as they are pushed aside 

 by the deeper-seated tracts that form the floor of the fourth ventricle, 

 and combining with the lateral columns to form the corpus resti- 

 forme and the basis of the vagal lobes, quit those columns to 

 converge, ascend, and unite together above the anterior opening of 

 the fourth ventricle : they there form either a simple bridge or com- 

 missure (yfig- 54. c), or are developed upwards and backwards into a 

 ganglionic mass, over-arching the ventricle ; this mass is the ' cere- 

 bellum ' {,figs. 47 — 53. c). It is formed chiefly by the post-pyramidal 

 columns, but doubtless derives some share of the proper lateial or 

 restiform fibres, as the result of the previous confluence of these with 

 the post-pyramids. 



The cerebellum retains its earliest embryonic form of a simple 

 commissural bridge or fold in the parasitic suctorial Cyclostomes, 

 in the heavily laden ganoid Polypterus *, and in the almost fin- 

 less Lepidosiren {Jig. 54. c)^ ; it attains its highest development, 

 in the present class, in the Sharks, whei*e it not only covers the 

 fourth ventricle, but advances over the optic lobes, and in the Saw- 

 fish extends beyond them to rest upon the cerebrum : its surface is 

 further extended in these active predaceous fishes by numerous trans- 

 verse folds (Jig. 55. c). In most osseous fishes the cerebellum is 



a smooth convex body ; hemispheroid 

 (p/. 50. c), or transversely subelliptic 

 (Eel), or longitudinally subelliptic (Lepi- 

 dosteus. Jig. 49. o), or an oblong body 

 (Diodon,^V/. 47. c), or it is depressed and 

 tongue-shaped (Cod), or oval, or pyra- 

 midal (Perch): it is vei*y rarely found 

 extending forwards, as in Echineis and 

 Atnblyopsis spe1(pris\ {Jig. 50. c), over any 

 part of the optic lobes ; but often back- 

 wards over the whole fourth ventricle, as 

 in the Cod, the Diodon ; or over the major 

 part of the ventricle, as in the Herring, the P^el ; but sometimes 

 covering only a small portion, as in the Lump-fish, the Lepidostcus, 



* XXV. p. 24. pi. ii. figs 5. 7. + xxxiii. p. :5.Sf). pi. 27. 



\ See Dr. Wyman's excellent desci-iptioii of tliis fish, in i.xxxvi. 



Lepidosteus 

 nat. size. 



