THE CEREBELLUM. 521 



horizontal canal* in the pigeon is followed by oscillations of the 

 head in a horizontal plane around a vertical axis. Bilateral section 

 so increases these oscillations that the pigeon is unable to maintain 

 equilibrium and forced to fall and turn continuously around the 

 vertical axis. Bilateral section of the posterior vertical canals;gives rise 

 to oscillations around a horizontal axis which frequently become so 

 exaggerated as to eventuate in the turning of backward somersaults, 

 head over heels. Similar phenomena follow division of the superior 

 vertical canals. 



Bilateral destruction of both sets of canals is attended by extra- 

 ordinary disturbances in the equilibrium. From the moment of the 

 operation the animal, the pigeon, loses all control of its motor mechan- 

 isms. It can neither maintain a fixed attitude nor execute orderly 

 movements of progression ; its activity, continuous and uncontrollable, 

 is characterized by spinning around a vertical axis, turning somer- 

 saults, dashing itself against surrounding objects until life is endan- 

 gered. If the animal be protected from injury, these disturbances 

 gradually subside, and in the course of a few months the equilibratory 

 power is so far regained that standing and walking at least become 

 possible. In this condition, however, the coordinating power is 

 directly dependent on visual impulses, for with the closure of the 

 eyes all the previous motor disturbances at once recur. These and 

 similar facts indicate that the semicircular canals are the peripheral 

 sense-organs from which come the nerve impulses most essential to 

 the excitation of the cerebellar coordinative centers in their control of 

 equilibrium and of progression. 



The cerebellum may therefore be regarded as the essential, most 

 highly differentiated portion of the coordinating mechanism con- 

 cerned in the maintenance of equilibrium, during both station and 

 progression. The manner in which the cerebellum accomplishes 

 this result is unknown, though it is certain, from the foregoing facts, 

 that its special mode of activity is dependent on the excitatory action 

 of nerve impulses reflected from a variety of peripheral sense-organs. 



* The physiologic anatomy of the semicircular canals is described in the chapter 

 devoted to the ear, to which the reader is referred. 



