THE CEREBELLUM. 571 



head over heels. Similar phenomena follow division of the superior vertical 

 canals. 



Bilateral destruction of both sets of canals is attended by extraordinary 

 disturbances in the equilibrium. From the moment of the operation the 

 animal, the pigeon, loses all control of its motor mechanisms. 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 somersaults, dashing itself against surrounding objects 

 until life is endangered. If the animal be protected from injury, these dis- 

 turbances gradually subside, and in the course of a few months the equilibra- 

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

 possible. In this condition, however, the coordinating power is directly de- 

 pendent 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 coordina- 

 tive 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 concerned in the main- 

 tenance 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 transmitted from a variety of 

 peripheral sense-organs. 



