210 THE NERVOUS SYSTEM 



follow closely the dendritic ramifications of these neurons and terminate in free 

 varicose endings. 



It would seem reasonable to suppose that the two kinds of afferent fibers, 

 just described, have a separate origin and functional significance. According 

 to Cajal (1911) it is probable that those entering the cerebellum through the 

 brachium pontis are distributed as climbing fibers, and those from the restiform 

 body as mossy fibers. The accompanying diagram represents the probable 

 course of impulses through the cerebellum (Fig. 152). The mossy fibers, prob- 

 ably derived from the restiform body, transfer their impulses to the granule 

 cells; and these, in turn, relay them, either directly or through the basket neu- 

 rons, to the Purkinje cells. The climbing fibers, which probably come from the 

 brachium pontis, transfer their impulses directly to the dendrites of the Purk- 

 inje cells. We do not known to which class the fibers of the vestibulocerebellar 

 tract should be assigned. The efferent path may be said to begin with the 

 Purkinje cells, whose axons terminate in the central cerebellar nuclei. From 

 these nuclei, especially the dentate, arise the fibers of the brachium conjunc- 

 tivum, the great efferent tract from the cerebellum. By means of the axons 

 of the granule cells, basket cells, and neurons of Golgi's Type II, as well as by 

 the collaterals from the axons of the Purkinje cells, an incoming impulse may be 

 diffused through the cortex. 



The cerebellum probably receives fibers from all the somatic sensory centers, 

 but especially from those of the proprioceptive group, through which afferent 

 impulses are conveyed to it from the muscles, joints and tendons, and from 

 the semicircular canals of the ear. Its connection with the vestibular appa- 

 ratus is especially intimate. In fact, as already stated, it may be regarded from 

 the standpoint of development as a very highly specialized portion of the ves- 

 tibular nucleus. It is the great proprioceptive correlation center. Further- 

 more, it sends efferent impulses to the various somatic motor centers and plays 

 an important part in the coordination of muscular contraction and in the main- 

 tenance of muscular tone. It is the chief center for equilibration, which depends 

 upon the proper adjustment of the muscles in response, very largely, to the 

 impulses from the semicircular canals. In man and mammals it also receives 

 impulses from the cerebral cortex by way of the pons, which probably set the 

 coordinating cerebellar mechanism into activity to bring about the proper 

 adjustment of voluntary movements. For additional details concerning the 

 functions of the cerebellum the reader should consult the recent paper by 

 Holmes (1917). 



