CHAPTER XX. 

 THE CEREBELLUM. 



The cerebellum is situated in the inferior fossae of the occipital 

 bone, beneath the posterior lobes of the cerebrum, from which it 

 is separated by the tentorium cerebelli, a semilunar fold of the 

 dura mater. It is partially divided into hemispheres by a longi- 

 tudinal fissure, more apparent on the inferior surface, though united 

 by a central lobe, the vermiform process. Each hemisphere is con- 

 nected with the cerebrum, the pons, medulla and spinal cord by three 

 bundles of nerve-fibers known respectively as the superior, middle, 

 and inferior peduncles. The surface of the cerebellum presents a 

 series of lobes and fissures of which the former have received more 

 or less fanciful names. A section of the cerebellum shows that it is 

 composed of gray matter externally and white matter internally. 

 The general appearance presented on section is shown in Fig. 230. 



Structure of the Gray Matter. The gray matter consists 

 mainly of nerve-cells of varying size and shape, which are arranged in 

 two layers: viz., an outer or molecular and an inner or granular. 



The molecular layer consists of stellate and multipolar cells of 

 small size, from which dendrites and axons pass horizontally and 

 vertically. The granular layer consists, as its name implies, of 

 granular shaped cells and large stellate cells. These cells are 

 characterized by the possession of dendrites and axons, the course 

 and relation of which have not been clearly determined. 



The inner border of the molecular layer presents a series of large 

 cells originally described by Purkinje and known by his name. From 

 the outer end of the cell-body one or more dendrites emerge which 

 soon divide and subdivide into a number of branches which pass 

 toward the cerebellar surface. The general arrangement of these 

 dendrites gives to the entire cell a tree-like appearance (Fig. 231). 

 From the inner end of the cell an axon emerges which passes centrally 

 into the white matter. 



Structure of the White Matter. The white matter consists of 

 nerve-fibers which are arranged in association and projection systems. 



The Association System. The fibers which compose this system 

 are of variable lengths and unite adjacent as well as distant regions 

 of the cerebellar cortex. They doubtless associate them both anatom- 

 ically and physiologically. 



