926 CENTRAL NERVOUS SYSTEM 



the stump of the superior or middle cerebellar peduncle inhibits the 

 rigidity of the limbs. 



The maintenance of the excessive extensor tonus known as decere- 

 brate rigidity consequently depends upon the reflex connection of a cen- 

 ter in the midbrain with the muscles. Normally the activity of this 

 reflex mechanism is held in abeyance by the inhibitory influence of the 

 fore brain. The fact that both the afferent impulses from the muscles 

 and the inhibitory impulses from the cerebrum pass through the cere- 

 bellum suggests strongly that this organ must have a very important 

 relation to the regulation of postural tone, and that this is the case is 

 indicated conclusively by what is known of the physiology of this 

 structure. 



The Function of the Cerebellum, The function of the cerebellum has 

 been the subject of exhaustive study, particularly on the part of Luci- 

 ani. 3G Stimulation of the cortex of the cerebellum with an electrical 

 current does not give rise to any detectable reactions unless the current 

 is so strong as to spread to the deeper ganglia of the brain. Conse- 

 quently it has been necessary to study the effects of removing all or 

 part of the cerebellum and to attempt to deduct from the resulting dis- 

 turbances the function which the ablated parts perform. When this is 

 done it is found that three stages can be distinguished in the condition 

 of the animal, following the operation. In the initial stage a definite 

 group of symptoms are presented which are presumably due to certain 

 immediate effects of the operation. These effects change quite com- 

 pletely after several days and a set of conditions present themselves 

 which appear to be the permanent result of the loss of the cerebellar 

 tissue. Finally, however, a certain improvement in the behavior of the 

 animal occurs which may be attributed to the compensatory action of 

 other parts of the nervous system which modify their activities so as 

 to correct for the loss of cerebellar influences. The second stage is ob- 

 viously of chief interest in the interpretation of the normal function of 

 the cerebellum. The conditions existing in this stage in man, following 

 gunshot injuries, have been exhaustively described by Holmes, 35 and as 

 they agree closely with the results obtained on animals and are of greater 

 interest to human physiology, we may draw our conclusions from them. 



The destruction of tissue in one half of the cerebellum manifests it- 

 self in the condition and behavior of the muscles on the same side of the 

 body. When compared to the muscles of the uninjured side, these ex- 

 hibit several forms of abnormality which may be designated as atonia, 

 asthenia, astasia, and ataxia. To atonia are attributed those symptoms 

 which manifest themselves as the result of a diminution in the tone of 

 the muscles. As the result of this condition the muscles feel soft and 

 flabby and the limbs tend to assume unnatural positions. When the arm 



