THE CENTRAL NERVOUS SYSTEM. 75 



is found in the fact that the cerebral hemispheres consciously direct 

 the movements which, when the cerebellum is intact, are unconsciously 

 co-ordinated. In this way, the cerebral mechanism compensates to 

 some extent for the loss of cerebellar function. It will be remembered 

 that two of the long tracts concerned with conveying afferent impulses 

 from the muscles on each side terminate in the cerebellum, while the 

 remaining two end in the thalamus, and are brought into relation with 

 the cortex of the cerebral hemisphere by other neurons. 



If one lateral half only of the cerebellum is removed, the weakness 

 and tremors are limited to that side of the body. For two or three 

 weeks the animal is unable to stand, and lies on the affected side with 

 its head and trunk turned in the same direction (fig. 21). Later, it 

 succeeds in standing and walking by abducting the limbs of the weak 

 side or by availing itself of the support of a wall. 



FIG. 21. Dog with right half of its cerebellum removed. 

 (From Schafer's Text-book of Physiology.} 



Stimulation of the cortex of the cerebellum gives rise to movements 

 of the body, but stronger stimuli are required than in the case of 

 stimulation of the cortex of the cerebral hemispheres. Weak stimuli 

 applied to the central nuclei of the cerebellum are effective in producing 

 movements, especially of the eyes and head. Stimulation of Deiters' 

 nucleus in the pons, on the other hand, is followed by movements of 

 the trunk and limbs. 



Observation of the effects produced by localised lesions, experimental 

 or pathological, shows that there is localisation of function in the 

 cerebellar cortex. The immediate effect of such a partial injury is the 

 occurrence of muscular contractions, giving rise to so-called "forced 

 movements." Injury to the anterior part of the vermis is followed by 

 movements of the head, injury of the anterior part of a lateral lobe by 

 movements of the fore-limb of the same side, and injury of the posterior 

 part of the lateral lobe by movements of the hind limb of the same side. 



An additional fact which has some bearing on the function of the 

 cerebellum is that in an animal from which the cerebral hemispheres 

 have been removed, the cerebellum being intact, the muscles frequently 

 pass into a condition of _tonic contraction leading to what isJuiown as 

 decerebrate^ rigidity ; this condition disappears on s division of the 

 posterior nerve roots. 



