THE FUNCTIONS OF THE CEREBELLUM 457 



walk, but this is carried out by an alteration of the method of pro- 

 gression. The disorders of locomotion are quite distinct from the 

 spinal ataxia observed after interference with the afferent tracts from 

 the muscles. The difficulty now is that each diagonal movement 

 of the limbs in progression tends to throw the centre of gravity to 

 one side or other of the basis of support, and it is the mechanism for 

 maintaining the right position of the centre of gravity, i.e. the posture 

 of the body as a whole in relation to its environment, which is at fault. 

 The animal, in the case of the dog, therefore attempts to correct the 

 tendency to fall to one side or other at each step by making its basis 

 of support as wide as possible, and gradually acquires a peculiar gait, 

 consisting of a series of springs, in which the two fore limbs and two 

 hind limbs act together, the diagonal movements of the fore limbs 

 being practically abandoned. 



In lesions of the cerebellum in man the most marked symptoms 

 are the cerebellar ataxy and the occurrence of tremors, ' astasia,' on 

 the performance of willed movements. The ataxy has the same origin 

 as that in the dog ; each spinal act of locomotion tends to throw 

 the centre of gravity outside the line of support, and the tendency 

 to fall thus brought about is voluntarily compensated by abduction of 

 the corresponding limb. A staggering gait is thus produced, which 

 is practically identical with that of a drunken man, and presents 

 no trace of the over-action of muscles so characteristic of spinal 

 ataxy. That the compensation, which is slowly acquired after 

 extirpation of the cerebellum, is of cerebral origin is shown by the 

 fact that extirpation of the cerebral hemispheres, or even of the 

 motor areas of the hemispheres, after extirpation of the cerebellum, 

 at once abolishes the power of movement which has been reac- 

 quired, and after the motor areas are destroyed on both sides the 

 loss of power of progression is permanent. 



These experiments show that the cerebellum, in Sherrington's 

 words, must be regarded as the head ganglion of the proprioceptive 

 system, acting as a centre where arrive the afferent impulses from 

 the cord, the fifth nerve, and especially from the labyrinth. It 

 influences, through the superior peduncle, the cerebral cortex and 

 furnishes the subconscious basis for the guidance of the motor functions 

 of the latter organ. Through its connections with the nuclei of the 

 bulb and the efferent tracts arising therefrom, it augments the tonic 

 activity of all the muscles of the body, an effect which is especially 

 marked in the absence of the cerebral hemispheres and is responsible 

 for the condition known as decerebrate rigidity. As a centre of 

 conj unction for the afferent impressions from the muscles and^those 

 from the labyrinth it co-ordinates the segmental reflexes, which 

 determine the relative posture of each limb, with those originating 



