294 



NORMAL HISTOLOGY. 



THE CEREBELLUM. 



The cerebellum occupies the posterior fossa of the skull and lies behind 

 the pons and medulla and the fourth ventricle (Fig. 328). By means of its 

 three peduncles inferior, middle and superior the cerebellum is connected 

 with the medulla, the pons and the mid- brain respectively. Its surface is 

 divided by the deeper fissures into lobules, each of which is subdivided by 

 shallower clefts into narrow tracts, the/0//a, from 2-4 mm. wide, that, within 

 a given lobule, in a general way parallel one another. 



With the exception of where the robust peduncles enter the hemispheres 

 and immediately above the dorsal recess of the fourth ventricle, the cere- 

 bellum is everywhere covered by a continuous sheet of cortical gray matter 

 which follows and encloses the subdivisions of the medullary layer. The 

 latter, as exposed in sagittal sections (Fig. 341), appears as a compact 



Sylvian aqueduct Corpora quadrigemina 



Cerebral peduncle 



Free surface 



Cerebellar cortex 



Roof of ventricle 



Primary limb 



Core of white matter 



Secondary limb 



Interfoliar fissure 



Cortex 



Lobule 



FIG. 341. Mesial sagittal section of brain-stem and cerebellum, showing fourth ventricle and relations 

 of the central white matter and its limbs to the continuous sheet of gray cortex. 



central mass of white matter, from which stout stems radiate into the various 

 lobules. From these primary stems, secondary branches penetrate the sub- 

 divisions of the lobules, and from the sides of these, in turn, smaller tracts 

 of white matter, the tertiary branches, enter the individual folia. Over 

 these ramifications of the white core, the cortical gray matter stretches as a 

 fairly uniform layer, about 1.5 mm. thick, that closely follows the complexity 

 ot the folia and fissures. The resulting arborization and the contrast between 

 the white and gray matte r, best seen in sections passing at right angles to the 

 folia, produce a picture known as the arbor vitts cerebelli. 



The Cerebellar Cortex. The cortical gray matter ( Fig. 342) includes 

 two very evident strata the outer and lighter molecular layer and the inner 

 and darker granule layer. 



The molecular layer is of uniform thickness, about .4 mm., and 

 contains three varieties of nerve-cells: the Purkinje cells, the basket-cells and 

 the small cortical cells. The Purkinje cells, the most distinctive elements 

 of the cerebellum, occupy the deepest part of the molecular layer, where 

 they are disposed in a single row along the junction of the outer and inner 

 layers. The cells are more numerous and closely placed upon the summit 



