THE CEREBELLUM 325 



return to the normal position. There result clonic move- 

 ments, in which both eyes take part synchronously. This 

 condition is known as nystagmus. There is no conscious 

 loss of muscle sense — that is, sensation of position or of 

 passive movement. Whatever impressions pass into the 

 cerebellum are therefore unconscious. 



Stimulation of the Cerebellum. — When the cortex of the 

 cerebellum is stimulated, movements are only induced 

 when strong currents are used. It is therefore beheved 

 that the cortex is inexcitable, the effects produced on 

 strong stimulation being due to spread of the current. 

 When the deep nuclei are stimulated, movements con- 

 cerned in preserving equihbrium follow, particularly those 

 of the head and eyes. 



It will be seen that the afferent nerves from the muscles 

 and joints and the vestibular nerve have this in common — 

 that the impressions arising in them contribute reflexly to 

 the maintenance of tonus, upon which posture depends. 

 There is, again, a close similarity between the effect of 

 destruction of the labyrinth and of removal of one lobe 

 of the cerebellum. In both cases there is a loss of tonus 

 on the same side of the body; in both cases a certain 

 degree of recovery foUows, owng to education of the 

 cerebrum. 



From the receptor organs of the muscles arise conscious 

 sensations which give us information regarding the position 

 of the hmbs. From the labyrinth arise conscious sensa- 

 tions as to the relation of the body to its environment. 

 These conscious sensations are located in the cerebrum. 

 Unconscious impressions from the afferent spinal nerves 

 play upon the centres of the cord and those from the laby- 

 rinth upon the cells of the medulla, in particular the nuclei 

 of Deiters and Bechterew. It is the function of the cere- 

 bellum to analyse these impressions and to originate from 

 them impulses which have for their object the main- 

 tenance of a condition of equihbrium and stabihty. By 



