. Cunningham's Text-book of anatomy. Anatomy. HISTOGENESIS AND MINUTE STBUCTUKE OF CEEEBELLUM. 579 Fiss. suprapyramidalis. ' Fiss. prima. In its inferior part the roof of the ventricle is exceedingly thin and is not all formed of nervous matter. The posterior medullary velum is a mere ridge which can hardly be said to enter into its formation: the epithelial lining of the cavity, supported by pia mater, is carried downwards towards the inferior boundaries of the floor of the ventricle. At the lowest part of the calamus scriptorius, and also along each lateral boundary of the floor, the epithel
. Cunningham's Text-book of anatomy. Anatomy. HISTOGENESIS AND MINUTE STBUCTUKE OF CEEEBELLUM. 579 Fiss. suprapyramidalis. ' Fiss. prima. In its inferior part the roof of the ventricle is exceedingly thin and is not all formed of nervous matter. The posterior medullary velum is a mere ridge which can hardly be said to enter into its formation: the epithelial lining of the cavity, supported by pia mater, is carried downwards towards the inferior boundaries of the floor of the ventricle. At the lowest part of the calamus scriptorius, and also along each lateral boundary of the floor, the epithelial roof becomes thickened at its attachment to the parts of the medulla oblongata. The small semilunar lamina which stretches across between the inferior parts of the two clavae at the calamus scriptorius and overhangs the opening of the central canal is termed the obex (Fig. 482, p. 550). A downwardly directed protrusion of the epithelial roof is often found behind the Parafloc. Velum medullare posherius. ' -Floe. Recessus la^ralis vent: quarri. Tuberc'ulum acusHcum. Medulla oblongata. Nodulus. Obex. Taenia venlriculi quarl-i. Fig. 513.—The Posterior Aspect of a Fcetal Cerebellum and Medulla Oblongata. The Histogenesis and Minute Structure of the Cerebellum. The developmental history of the cerebellum presents certain peculiar features which seem quite enigmatic unless considered from the point of view of the evolu- tion of the connexions and functions of the organ. The cerebellum is derived from part of the alar lamina of the rhombencephalon, and at an early stage of its develop- ment the rudiment shows the regular lamination into ependyma, mantle layer, and marginal layer, which has already been de- scribed as distinctive of the corresponding place of development in the whole nervous system. The cells of this mantle layer are to be looked upon as an outlying (superior) part of the receptive nucleus of the vestibular nerve, the cells to which information con- cernin
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