274 



ANATOMY OF VERTEBRATES. 



173 



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 Vertebrates save Fishes are the vagal lobes 

 and the nodulus present. 



The posterior pyramids, which are the anterior continuation of 

 the posterior myclonal 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 restiforme and the basis of the vagal lobes, again quit 

 those columns, converge, ascend, and unite together above the 

 anterior opening of the fourth ventricle : they there form either 

 a simple bridge or commissure, fig. 173, C, or are developed 

 upwards and backwards into a ganglionic mass, overarching the 

 ventricle ; this mass is the 'cerebellum,' figs. 174 — 179, c. It is 



formed chiefly by the post-pyramidal columns, 

 but doubtless derives some share of the proper 

 lateral 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 Sturgeon, fig. 173, c, and Polypterus, 1 

 and in the almost finless Lepidosiren, 2 fig. 186, 

 C : it attains its highest developement, in the 

 present class, in the Sharks, where 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 transverse 

 folds, fig. 187, c. Inmost Osseous Fishes the 

 cerebellum is a smooth convex body, hemispheroid, fig. 175, C, or 

 transversely subelliptic (Eel, fig. 176, c), or longitudinally subel- 

 liptic (Lepidosteus), fig. 174, c ; but it may be an oblong body 

 (Diodon), fig. 171, C, or be depressed and tongue-shaped (Cod, 

 fig. 183,/*), or oval, or pyramidal (Perch, fig. 182, a) ; it is very 

 rarely found extending forward, as in Echeneis and Amblz/opsis, fig. 

 175, c, over any part of the optic lobes ; but often backward 

 over the whole fourth ventricle, as in the Cod, fig. 183, f, and the 

 Diodon, fig. 171, C ; or over the major part of the ventricle, as in the 



Brain : Sturgeon, cxcix. 



xxv. p. 24, pi. ii. figs. 5, 7. 



2 xxxin. p. 339, pi. 27. 



