THE CEREBELLUM. 567 



from the same side, which also pass to the superior vermis; (3) the acoustico- 

 cerebellar tract, composed of fibers which are the axons of the sensory 

 end-nuclei (Deiters) of the vestibular portion of the auditory nerve. It is 

 probable that all these fibers decussate prior to their final termination. 



The cerebellum through this system of efferent and afferent fibers is 

 brought into relation with many different regions of the cerebrum, pons, 

 medulla, and spinal cord. Each half of the cerebellum is connected with the 

 foregoing structures of the same side, and of the opposite side. 



THE FUNCTIONS OF THE CEREBELLUM. 



From the observations of the results of experimental lesions, from the 

 analysis of clinico-pathologic facts, and from the comparative anatomic 

 development in different animals, the deduction has been drawn that the 

 cerebellum coordinates and harmonizes the action of those muscles the 

 activities of which are necessary to the maintenance of body equilibrium 

 both during station and progression. 



By equilibrium of the body is understood a condition which may be main- 

 tained for a variable length of time without displacement, and is possible only 

 so long as a vertical line passing through the center of gravity falls within the 

 base of support. The support offered by the earth to the feet neutralizes and 

 counteracts the force of gravity. In standing, when the body is in the erect or 

 military position, the arms by the side, the center of gravity lies between the 

 sacrum and the last lumbar vertebra, and the line of gravity falls between the 

 feet and within the base of support. The entire skeleton for the time being 

 is rendered fixed and rigid at all its joints by the combined action of the 

 muscles connected with it. That this position may be maintained all the 

 different groups of antagonistic but cooperative muscles must be accurately 

 coordinated in their actions. Any failure in this respect is at once attended 

 by a disturbance of the equilibrium and displacement. 



In progression, walking, running, dancing, etc., the body is translated 

 from point to point by the alternate action of the legs. Whether the direction 

 of the translation be rectilinear or curvilinear, as the legs change their position 

 from moment to moment, the center of gravity also changes, and at once the 

 equilibrium is menaced. If it is to be maintained and displacement prevented 

 there must be a prompt readjustment in the relation of all parts of the 

 body so that the line of gravity falls again within the base of support. The 

 more complicated the moments of progression, or the narrower the base of 

 support, the greater is the danger to the equilibrium, and hence the necessity 

 for rapid and compensatory changes in coordinated muscle activity. All 

 movements of this character, in man at least, are primarily volitional and 

 require for their performance the constant exercise of the attention. With 

 frequent repetition they gradually come to be performed independently of 

 consciousness and fall into the category of secondary or acquired reflexes. 



Though coordinating power is exhibited by the spinal cord, medulla, 

 and basal ganglia, it is only in the cerebellum that this power attains its 

 highest development and differentiation. To it is assigned the power of 

 selecting and grouping muscles, not in any restricted part, but in all parts of 

 the body, and coordinating their actions in such a manner as to preserve the 

 equilibrium. 



