250 NERVOUS SYSTEM OF VERTEBRATES. 



taining the equilibrium, or is it necessary for the coordination of 

 muscular contractions with reference to definite movements ? In 

 the results of experimental investigations on mammals the function 

 of the cerebellum v^hich stands out most prominently is different 

 from either of these. Dogs which have lost one cerebellar 

 hemisphere, although they are unable to stand or walk, can swim 

 well in water (which supports their body weight), both coordinating 

 their movements and maintaining their equilibrium. Such animals 

 learn after a time to compensate for the loss of the cerebellum by 

 certain voluntary modifications of their movements; e.g., curving 

 the spine so as to bring the center of gravity over the sound legs, 

 spreading the feet wide apart, etc. They can then stand and 

 walk. These and other facts show that the loss of the cerebellum 

 does not involve loss of the power of equilibration nor of cutaneous 

 or muscle sense on which the power of coordinated movements 

 depends, but does result in weakness of muscular action on the 

 injured side. It seems, therefore, that the cerebro-spinal mechan- ' 

 isms are sufficient to carry out all voluntary movements without 

 the aid of the cerebellum, but the movements are lacking in strength, 

 precision and regularity. The cerebellum is not shown to be a 

 necessary link in the nervous mechanisms which control muscular 

 action but it seems to add something to the voluntary movement. 

 According to Luciani the function of the cerebellum is to maintain 

 the tone of muscles during rest, to increase the energy of contrac- 

 tion when called forth by voluntary impulses and to determine 

 the rhythm of motor impulses. In this way the imperfect actions 

 of the dog deprived of its cerebellum would be perfected into 

 normal movements. Whether the cerebellar cortex actually 

 serves other functions, such as the coordination of specific move- 

 ments, remains for further investigation to decide. 



The fact that the cerebellum receives fibers from so many and 

 various sources is important in this connection. It would seem 

 that the maintenance of tone is not an abstract thing which is 

 unrelated to present activity or sensory stimuU. If the function 

 of the cerebellum is to maintain the tone of muscles, it is evident 

 that it sends out the necessary impulses in response to impulses 

 brought to it from related sensory areas or special sense organs. 



