THE FUNCTIONS OF THE CEREBELLUM 455 



and originate either in the muscles and tendons or in the labyrinth. 

 Activity of this apparatus is roused as a rule by the movement of the 

 organism itself, and is only a secondary result of the environmental 

 stimulation which provoked the original movement. By its efferent 

 tracts starting in the roof- and paracerebellar nuclei, the cerebellum 

 is able to affect the musculature of the same side of the body by 

 a direct influence on the anterior horns. It also enters to a much 

 greater extent into relation with the opposite cerebral hemispheres, 

 so that it is in a position to control or modify the activity of these, 

 whether excited on their sensory or on their motor sides. 



The view here laid down as to the essential functions of the cere- 

 bellum is borne out by experiments involving stimulation and ablation 

 of this organ. 



STIMULATION OF THE CEREBELLUM. It was first shown by 

 Ferrier that movements of the same side of the body can be excited 

 by stimulation either of the cerebellar hemispheres or of the superior 

 vermis. These results have been confirmed by subsequent observers, 

 and point to each half of the cerebellum being connected functionally 

 with the skeletal muscular apparatus of the corresponding side of the 

 body. The cortex cerebelli is not excited with ease. To evoke move- 

 ments much stronger stimuli are necessary than, e.g. for the excitation 

 of the motor area of the cerebral cortex. This again is in accordance 

 with what we should expect from the anatomy of the organ, knowing 

 as we do that the cortex is an end-station for a number of afferent 

 paths, but has no direct efferent paths from it to the lower motor 

 mechanisms of the cord. On the other hand, movements 

 are excited by minimal stimuli from the intrinsic nuclei of the 

 cerebellum. 



As a result of his experiments Horsley has concluded that the 

 cortex cerebelli must be regarded as an afferent receptive centre from 

 which axons pass to the ventrally placed efferent nuclei, viz. the nuclei 

 dentati, fastigii, emboliformes, as well as Deiters' nuclei. Whereas 

 excitation of the roof nuclei produces more especially movements of 

 the eyes, head, the paracerebellar (e.g. Deiters' nucleus) are responsible 

 more especially for the movements of the trunk and limbs. The 

 movements of the body which are thus evoked are those concerned in 

 maintaining equilibrium and are involved in every alteration in the 

 position of the body. 



EFFECTS OF ABLATION OF THE CEREBELLUM. Complete 

 unilateral extirpation of the cerebellum, after the irritation effects 

 of the lesion itself have passed away, brings about a condition of 

 the animal characterised by : 



(1) Slight loss of power on the same side of the body. 



(2) Considerable loss of tone on the same side. 



