THE CEREBELLUM. 201 



THE CEREBELLUM. 



The cerebellum is situated in the inferior fossae of the occipital 

 bone, beneath the posterior lobes of the cerebrum. It attains its 

 maximum weight, which is about five ounces, between the twenty- 

 fifth and fortieth years, the proportion between the cerebellum and 

 cerebrum being as i to 8^. 



It is composed of two lateral hemispheres and a central elongated 

 lobe, the vermiform process ; the two hemispheres are connected with 

 each other by the fibers of the middle peduncle, forming the super- 

 ficial portion of the pons Varolii. The cerebellum is brought into 

 connection with the medulla oblongata and spinal cord through the 

 prolongation of the restiform bodies ; with the cerebrum, by fibers 

 passing upward beneath the corpora quadrigemina and the optic 

 thalami, and then forming part of the diverging cerebral fibers. 



Structure. It is composed of both white and gray matter, the 

 former being internal, the latter external, and is convoluted, for 

 economy of space. 



The white matter consists of a central stem, the interior of which 

 is a dentated capsule of gray matter, the corpus dentatum. From 

 the external surface of the stem of white matter processes are given 

 off, forming the lamina, which are covered with gray matter. 



The gray matter is convoluted and covers externally the laminated 

 processes ; a vertical section through the gray matter reveals the 

 following structures : 



1. A delicate connective-tissue layer, just beneath the pia mater, 

 containing rounded corpuscles, and with branching fibers passing 

 toward the external surface. 



2. The cells of Purkinje, forming a layer of large, nucleated, branched 

 nerve-cells sending off processes to the external layer. 



3. A granular layer of small but numerous corpuscles. 



4. A nerve-fiber layer, formed by a portion of the white matter. 



Properties and Functions. Irritation of the cerebellum is not 

 followed by any evidences either of pain or convulsive movements ; 

 it is, therefore, insensible and inexcitable. 



Coordination of Movements. Removal of the superficial portions 

 of the cerebellum in pigeons produces feebleness and want of har- 

 mony in the muscular movements ; as successive slices are removed, 



