THE VESTIBULAR APPARATUS AND CEREBELLUM 191 



nuclei within the cerebellar hemispheres. Within the vermis 

 are other smaller centers, called the roof nuclei, because they lie 

 immediately above the fourth ventricle (nuclei emboliformis, 

 globosus, and fastigii, see Fig. 96). Some of the afferent fibers 

 which enter the cerebellum end in these nuclei, but most of them 

 end in the cortex. The efferent fibers, on the other hand, arise 

 from the deep nuclei, especially the dentate nuclei (Fig. 88). 



The cerebellar cortex has three distinct layers. External to 

 the central white matter (Fig. 89, C) is a wide layer composed of 

 very minute granule cells (Fig. 89, B) densely crowded together, 

 with scanty cytoplasm, short, claw-like dendrites, and slender 

 unmyelinated axons which ascend to the superficial molecular 

 layer (Fig. 89, A), where they bifurcate (their branches running 

 lengthwise of the folium) and end among the dendrites of the 

 Purkinje cells, to be described immediately. The middle layer 

 of the cortex is composed of a single row of Purkinje cells (Fig. 

 89, a); these have large globose bodies with massive bushy 

 dendrites directed outward and slender axons directed inward. 

 These axons are myelinated and constitute the chief efferent 

 pathway from the cortex; they do not, however, leave the cere- 

 bellum, but end in the deep gray nuclei (chiefly the dentate 

 nuclei), from which other neurons carry the impulses out of the 

 cerebellum. The dendrites of the Purkinje cells are widely ex- 

 panded transversely to the length of the folium, but are very 

 narrow in the opposite direction; thus each cell comes into 

 contact with the largest possible number of axons of the 

 granule cells which run lengthwise of the folium. The outer- 

 most or molecular layer contains the dendrites of the Purkinje 

 cells, termini of the axons of the granule cells and of other 

 fibers, and a small number of neurons with short axons, among 

 which are the basket cells illustrated in Figs. 16 and 89, b. 



Afferent fibers terminate in the cerebellar cortex in two ways. 

 They may pass directly out to the molecular layer as ascending 

 or climbing fibers, where they end in very intimate relation with 

 the dendrites of the Purkinje cells (Figs. 15 and 89, n), or they 

 may end as moss fibers (Fig. 89, h) among the cells of the gran- 

 ule layer. Here the granules take up the nervous impulses and 

 deliver them to the dendrites of the Purkinje cells. Ramon 

 y Cajal is of the opinion that the moss fibers are the terminals 



