

THE HIND-BRAIN OR RHOMBENCEPHALON 791 





only be distinctly seen after the cerebellum has been separated from the medulla 

 oblongata and pons. On either side of the nodule is a thin layer of white sub- 

 stance, named the posterior medullary velum. It is semilunar in form, its convex 

 border being continuous with the white substance of the cerebellum; it extends 

 on either side as far as the flocculus. The flocculus is a prominent, irregular 

 lobule, situated in front of the biventral lobule, between it and the middle cere- 

 bellar peduncle. It is subdivided into a few small laminae, and is connected to 

 the inferior medullary velum by its central white core. The flocculi, together 

 with the posterior medullary velum and nodule, constitute the lobus noduli. 



The Uvula and Tonsilla. The uvula (uvula vermis; uvular lobe) forms a consid- 

 erable portion of the inferior vermis; it is separated on either side from the tonsil 

 by the sulcus valleculae, at the bottom of which it is connected to the tonsil by a 

 ridge of gray matter, indented on its surface by shallow furrows, and hence called 

 the furrowed band. The tonsilla (tonsilla cerebelli; amygdaline nucleus) is a rounded 

 mass, situated in the hemispheres. Each lies in a deep fossa, termed the bird's 

 nest (nidus avis), between the uvula and the biventral lobule. The uvula and ton- 

 sillae form the lobus uvulae. 



!The Pyramid and Biventral lobules constitute the lobus pyramidis. The pyramid 

 is a conical projection, forming the largest prominence of the inferior vermis. 

 It is separated from the hemispheres by the sulcus valleculae, across which it is 

 connected to the biventral lobule by an indistinct gray band, analogous to the 

 furrowed band already described. The biventral lobule is triangular in shape; 

 its apex points backward, and is joined by the gray band to the pyramid. The 

 lateral border is separated from the inferior semilunar lobule by the postpyramidal 

 fissure. The base is directed forward, and is on a line with the anterior border of 

 the tonsil, and is separated from the flocculus by the postnodular fissure. 



The Tuber Vermis (tuber valvula) and the Inferior Semilunar Lobule (lobulus semi- 

 lunaris inferior; postero-superior lobule) collectively form the lobus tuberus (tuberw 

 lobe). The tuber vermis, the most posterior division of the inferior vermis, is of 

 small size, and laterally spreads out into the large inferior semilunar lobules, 

 which comprise at least two-thirds of the inferior surface of the hemisphere. 



Internal Structure of the Cerebellum. The cerebellum consists of white and gray 

 substance. 



White Substance. If a sagittal section (Fig. 704) be made through either hemi- 

 sphere, the interior will be found to consist of a central stem of white substance, 

 in the middle of which is a gray mass, the dentate nucleus. From the surface of this 

 central white stem a series of plates is prolonged; these are covered with gray 

 substance and form the laminae. In consequence of the main branches from the 

 central stem dividing and subdividing, a characteristic appearance, named the 

 arbor vitae, is presented. If the sagittal section be made through the middle of 

 the vermis, it will be found that the central stem divides into a vertical and a hor- 

 izontal branch. The vertical branch passes upward to the culmen monticuli, 

 where it subdivides freely, one of its ramifications passing forward and upward 

 to the central lobule. The horizontal branch passes backward to the folium vermis, 

 greatly diminished in size in consequence of having given off large secondary 

 branches; one, from its upper surface, ascends to the clivus monticuli; the others 

 descend, and enter the lobes in the inferior vermis, viz., the tuber vermis, the 

 pyramid, the uvula, and the nodule. 



The white substance of the cerebellum includes two sets of nerve fibers: (1) 

 projection fibers, (2) fibres proprise. 



Projection Fibers. The cerebellum is connected to the other parts of the bradn 

 by three large bundles of projection fibers, viz., to the cerebrum by the superior 

 peduncle, to the pons by the middle peduncle, and to the medulla oblongata by 

 the inferior peduncles (Fig. 705). 



