686 PHYSIOLOGY. 



nearly one-third of the whole cerebellar mass. Its consistency is 

 greater than that of the gray matter. 



The central nucleus of the white matter sends out an affinity 

 of arborescent prolongations which terminate in the cells of the gray 

 substance of the lamellae. It is this formation which the student knows 

 under the name of arbor vitce. 



Each one of the leaflike divisions of the white arbor vitae forma- 

 tion is enveloped by a very thin plate of yellowish substance, while 

 above this is the cortical gray substance. The latter sinks into the 

 white substance at the level of the grooves which separate the plates 

 from one another. 



A horizontal section of the cerebellum shows in the center of 

 each half of the organ an ovoid body. It is very similar to the olive of 

 the bulb in size and structure. This is the corpus dentatum. 



CORPUS DENTATUM. The corpus dentatum is formed by a yellow 

 layer folded upon itself in the form of a purse which opens in front. 

 Within the interior of this purse is found the tissue proper of the 

 corpus dentatum. It is formed of a matter which seems to be a 

 mixture of the white and gray substances. 



Under the name of accessory nucleus dentatus Meynert has de- 

 scribed two small leaves of gray substance located in front and inward 

 from the corpus dentatum. They are the nucleus globosus and nucleus 

 fastigii. Stilling has discovered two clear gray nuclei at the lower 

 border of the vermis near the median line and the roof of the fourth 

 ventricle. He calls them the nuclei emboliformes. Part of the fibers 

 of the inferior cerebellar peduncles end within these nuclei. 



Hence, there are here four gray nuclei: dentate, globosus, fas- 

 tigii, and emboHformis. The last three are in pairs, but the dentate 

 is single. 



The central white substance passes toward the lateral angles of the 

 sinus rhomboideus in three prolongations on each side. They are the 

 cerebellar peduncles. 



The superior cerebellar peduncles go forward, and pass under 

 the corpora quadrigemina, where they decussate with one another in 

 the upper level of the cerebral peduncles. They end in the optic 

 thalamus and cortex of the brain. 



The middle cerebellar 'peduncles pass forward and inward to form 

 the superficial annular fibers of the pons. These fibers form a true 

 commissure between the two hemispheres of the cerebellum; other 

 fibers decussate in the pons to terminate in the islands of gray sub- 



