. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. '2H 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 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 restiforme
. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. '2H 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 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 restiforme and the basis of the vagal lol^es, again quit those columns, cowvergc, 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. 174â179, 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. Tlie cerebellum retains its earliest emliryouic form of a simple commissural bridgeorfold in the parasitic suctorial Cyclostomes, in the hea-sily- laden Sturgeon, fig. 173, C, and Polypterus,' and in tlie almost finless Lepidosiren,- fig. 186, C: it attains its highest developement, in the jiresent class, in the Sharks, where it not only covers the fourtli 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 predaccous fishes by niunerous transverse folds, fig. 187, C. Inmost Osseous Fishes the cerebellum is a smooth convex body, liemisphcroid, fig. 17o,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 l)
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Keywords: ., bookauthorowenrichard18041892, bookcentury1800, bookdecade1860
