CHAPTER XCIX 

 FUNCTIONS OF THE CEREBELLUM 



In our discussion of reflex action we have so far considered only those 

 receptors coming from the exterior of the body, although we have recog- 

 nized that a considerable number of the afferent nerve roots contain fibers 

 coming from receptors situated in the muscles, the tendons and the joints, 

 and called proprioceptors because they respond not to changes in the 

 environment but to alterations in the body itself. We have seen that the 

 proprioceptors consist structurally of muscle spindles and of the nerve 

 endings in the tendons and ligaments and synovial membranes. They are 

 receptors that are attuned to respond to differences in tension caused either 

 by bulging of the muscles or by stretching of the fibers of tendons and 

 ligaments. 



The impulses are transmitted in the spinal cord, either by the posterior 

 columns or by the lateral cerebellar tracts. Those traveling by the pos- 

 terior columns are sent mainly to the cerebral cortex of the opposite side, 

 whereas those in the cerebellar tracts enter the cerebellum by the inferior 

 peduncles of the same side. The cerebral impulses connect with neurons, 

 which transmit the impulse back again to the cerebellum of the opposite 

 side, so that ultimately the cerebellar cortex is connected with the spinal 

 cord of the same side either directly or indirectly through the cerebral 

 cortex. These anatomical facts indicate in a general way that we may 

 expect the function of the cerebellum to be that of the chief nerve center 

 concerned in the integration of the proprioceptive impulses originated by 

 the condition of contraction or relaxation of the different groups of 

 muscles in the body, and by the amount of tension existing in the various 

 tendons, ligaments and other membranes surrounding the joints. 



Experimental investigation has justified these expectations. The re- 

 moval of the entire cerebellum an operation which has usually been 

 performed on birds, particularly pigeons, because of the ease with which 

 it can be done in these animals leads immediately to a condition in which 

 muscular activity is entirely uncontrolled. A pigeon after this operation 

 flies about in an incoordinate way, turning summersaults, dashing itself 

 against the walls of its chamber, and ultimately after constant futile move- 

 ments, exhausting itself. If one cerebellar lobe is removed, the body when 

 at rest is curved toward the side of the lesion, and the movements of the 



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