CHAPTER XXII. 

 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 longitudinal fissure, more apparent on the 

 inferior surface, though united by a central lobe, the vermiform process. 

 Each hemisphere is connected 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. 257. 



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 den- 

 drites 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 dendrities emerge which soon divide and 

 subdivide into a number of branches which pass toward the cerebellar sur- 

 face. The general arrangement of these dendrities gives to the entire cell a 

 tree-like appearance (Fig. 258). 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 cer- 

 bellar cortex. They doubtless associate them both anatomically and physio- 

 logically. 



The Projection System. The fibers composing this system connect the 

 cerebellar cortex with certain structures in the cerebrum, pons, medulla, 

 and spinal cord. They may be divided into efferent and afferent systems. 



The efferent fibers have their origin in the cells of Purkinje and the 

 dentate nucleus. Some of these fibers emerge from the cerebellum in the 

 superior peduncles through which they pass toward and beneath the corpora 



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