928 CENTRA!, NERVOUS SYSTEM 



ists in the synchronous adjustment of the activities of various muscle 

 groups which should cooperate for a common end. When the fingers are 

 flexed the extensors of the wrist normally contract synergically in order 

 to prevent simultaneous flexion of the wrist. If a patient with a cere- 

 bellar lesion grasps a small object quickly, the wrist may be extended 

 excessively so that the hand is bent backwards before the fingers are 

 half flexed. Movements which involve the activities of several joints 

 are frequently decomposed into their component parts which are exe- 

 cuted one at a time instead of all at once. In bringing the finger to the 

 nose the patient will first depress the arm by a movement of the shoulder, 

 and then flex the elbow after the first act is complete. Difficulty is also 

 experienced in making rapid alternate movements such as flexion and 

 extension of the fingers. The actual movement may be made nearly as 

 rapidly as with the normal hand, but a considerable delay intervenes be- 

 tween the successive acts, indicating a difficulty in adjusting the neuro- 

 muscular mechanism for the new act. In making such rapid alternating 

 movements groups of muscles not concerned in the desired action may 

 come into play. When, for example, the ankle is voluntarily flexed and 

 extended in rapid succession the knee and hip may flex and extend also. 

 The ataxia produced by the destruction of cerebellar tissue consists then 

 in a disturbance of the normal harmony and correct cooperation in time 

 and degree of the various muscular contractions concerned in movements 

 and in the maintenance of posture. 



We may conclude that the cerebellum is actively concerned with 

 the maintenance of tone and in the adjustment of voluntary contraction 

 and plastic tone to the posture of the body, not only as it is maintained 

 in rest, but as it changes when in action. Sherrington 37 has accordingly 

 designated the cerebellum as the main ganglion of the proprioceptive 

 system. The passage of impulses from the proprioceptors and from the 

 cerebrum through the cerebellum in their course to the centers of the 

 midbrain which are involved in the maintenance of decerebrate rigidity 

 consequently assumes an important functional significance. 



No Sensory Disturbances Follow Injury to the Cerebellar Cortex. 

 In considering the function of the cerebellum it should be pointed out 

 that although it receives afferent impulses from the proprioceptors of 

 the body there is no evidence that it is concerned in any way with sen- 

 sation. The only sensory tests to which those afflicted with cerebellar 

 injuries fail to respond with normal accurac}^ are those involving the 

 comparison of weights placed in the normal and the affected hand. In 

 this test the weight in the latter is judged heavier than it should be be- 

 cause of the greater effort required to support it with the asthenic mus- 

 cles. Although the ability to maintain the equilibrium of the body may 

 be disturbed as the result of cerebellar injury, this is because the mus- 



