THE HIND-BRAIN. 689 



The Pyramid and Biventral Lobes. The pyramid is a conical projection, forming 

 the largest prominence of the lower worm. It is separated from the hemispheres 

 by the sulcus valleculae, across which it is connected to the biventral lobe by an 

 indistinct band of gray matter, analogous, to the furrowed band already described. 

 The biventral lobe is triangular in shape, with the apex pointing inward and 

 backward to become joined by the connecting band to the pyramid. The external 

 border is separated from the slender lobe by the post-pyramidal fissure. The base is 

 directed forward, and is on a line with the anterior border of the amygdala, and is 

 separated from the flocculus by the post-nodular fissure. 



The Tuber Valvulae or Tuber Posticum, and Posterior Inferior Lobes. The tuber 

 valvulae is the posterior division of the inferior worm. It is of small size, and 

 laterally spreads out into the large posterior inferior lobes of the hemispheres. 

 These lobes, which, as stated above, comprise at least two-thirds of the inferior 

 surface of the hemisphere, are divided into two by the post-gracile fissure. The 

 anterior lobe is named the slender lobe, and the posterior, the inferior semilunar 

 lobe. Both these lobes show a tendency to subdivision into two ; that of the 

 slender lobe is well marked, and its subdivisions are sometimes described as 

 distinct lobes and named the anterior and posterior slender lobes, the fissure 

 between them being termed the intra-gracile fissure. 



INTERNAL STRUCTURE OF THE CEREBELLUM. 



The cerebellum consists of white and gray matter. 



The White Matter. If a sagittal section (Fig. 366) is made through either 

 hemisphere of the cerebellum, the interior will be found to consist of a central 

 stem of Avhite matter, which contains in its interior a gray mass, the corpus den- 

 tatum. From the surface of this central stem a series of plates of medullary 

 matter are detached, which, covered with gray matter, form the laminae. In 

 consequence of the main branches from the central stem dividing and subdividing, 

 the section presents a characteristic appearance, which is named 'the arbor viice. 

 If a vertical section is made in the median plane of the cerebellum it will be found 

 that the central stem divides into two main branches, which, from their direction, 

 may be named respectively the vertical and the horizontal branch. The vertical 

 branch passes upward to the culmen, where it subdivides freely, some of its ramifi- 

 cations passing forward and upward to the central lobe. The horizontal branch 

 passes backward to the folium cacuminis, considerably diminished in size in 

 consequence of having given off large secondary branches : one. from its upper 

 surface, ascends to the clivus ; the others descend, and enter the lobes in the 

 inferior vermiform process, the tuber valvulse, the pyramid, the uvula, and the 

 nodule. It is not necessary to describe in detail the various divisions of the white 

 matter, as they correspond to the lobes on the surface. 



The white matter of the cerebellum includes two varieties of nerve matter : 

 (1) the peduncular fibres, which are directly continuous with those of the pedun- 

 cles of the cerebellum ; (2) the fibres proper (fibrce proprice) of the cerebellum 

 itself. 



The Peduncles of the Cerebellum. From the anterior part of each hemisphere 

 arise three large processes or peduncles superior, middle, and inferior by which 

 the cerebellum is. connected with the rest of the encephalon. 



The superior peduncles form the upper lateral boundaries of the floor of the 

 fourth ventricle. As they extend forward and upward they converge on the 

 dorsal aspect of the ventricle, and thus assist to roof it in. They may be 

 traced as far as the corpora quadrigemina, under which they pass. They enter 

 the upper and mesial part of the medullary substance of the hemispheres, beneath 

 the ala lobuli centralis and the frgenulum, and pass to a great extent into the 

 interior of the corpus dentatum, though some of their fibres wind round it and 

 reach the gray cortical matter, especially of the inferior surface. 



The fibres of the superior peduncles mainly emerge from the hilum of the corpus 



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