FUNCTIONS OF THE CEREBELLUM. 907 



cerebelli produced in the head and eyes a turning to the homonymous 

 side, the reverse direction to that produced by ablation even in the 

 so-called irritative stage. Through the variety of forced attitudes 

 assumed under cerebellar lesions, there is repeated a group of features, 

 suggesting that the cerebellum reacts upon some focus of innervation in 

 which the motor elements of that group are knit together to an entity. 

 The turning of the head, the lateral deviation of the eyeballs, the 

 pleurosthotonus, the extension of elbow, can all be evoked from the 

 extreme posterior end of the optic thalamus, as if from a focus 

 for a co-ordinate movement in which these components existed. It 

 is perhaps on this that a cerebellar influence is exerted. As to the 

 direction of its influence, it seems probable that the clean removal by a 

 skilled experimenter of a portion of cerebellum leads, though its effect 

 at first be compounded of irritation and deficiency, almost at once to a 

 result in which deficiency is dominant. The interpretation of the 

 irritative symptoms would then be, that in the organ from which 

 Luciani's tonic and trophic cerebellar influence was removed, there 

 ensued for a time an overaction, comparable to the profuse paralytic 

 secretion of a submaxillary gland after section of its nerves — comparable, 

 in fact, to an escape from control ; such overaction would of itself entail 

 after a time exhaustion. This phenomenon would come under the class 

 of " isolation dystrophies " already spoken of. Luciani, in describing the 

 influence which the cerebellum exerts upon the cerebral cortex, compares 

 it luminously to that exerted by the afferent spinal root upon the motor 

 nerve cells. It would represent his view hardly fully, perhaps, to say 

 that the cerebellum maintains the tone of certain " muscular " regions 

 of the cerebral cortex. From the construction of the cerebellum it is 

 clear that it may act as an adjuvant to the Eolandic cortex in one or 

 all of several ways as follows : — 



1. The afferent impulses gathered up from the cord, the trigeminus, 

 and especially the vestibular nerve, may, acting through the cerebellar 

 cortex, and then the corpus dentatum and crossed rubro-thalamic 

 nuclei, finally react on the cerebral cortex, mainly of the side opposite 

 to that whence they started. Such a path of action is implied in the 

 view of cerebellar action sketched by Gowers. Impulses coming from 

 muscles and ligaments, originated by contractions and tensions, impulses 

 from the semicircular canals and from the muscles of the eyes, 

 impressions not affecting consciousness, reach the cerebellar cortex, and 

 thence react mainly on the Eolandic region of the cerebral cortex. 

 Such a view does not deal with the middle peduncle nor with the 

 afferent portion of the posterior. It brings forward clearly, however, 

 a cerebellar factor in the " tonus " of the cerebral cortex. 



2. The afferent impulses gathered up from the cord and cranial 

 nerves, especially from the vestibular, may, acting through the vermis, 

 react upon the cerebellospinal and vestibulo-spinal efferent systems, and 

 through these last contribute a necessary factor to the normal tonus 

 and excitability of the spinal and cranial motor root cells. If this 

 factor in the normal condition of the motor root cells were wanting, 

 they might be expected to reply in various abnormal ways under 

 excitation from the cortex. 



3. Each half of the cerebellum receives a great incoming tract from 

 the pontine nuclei across the median line. There is little doubt that 

 these nuclei are themselves end stations for fibres efferent from centres 





