620 TEXT-BOOK OF PHYSIOLOGY 



most pronounced effects. The head and the anterior part of the body are 

 at once drawn toward the pelvis on the side of the section. A voluntary 

 effort on the part of the animal causes it to lose all control of its muscles and 

 the body is rotated around its longitudinal axis from 40 to 60 times a 

 minute before it comes to rest. According as the lesion is made from be- 

 hind or before, the rotation is from or to the side of the section. In time 

 these symptoms subside, though the animal never completely recovers. 



The partial recovery of the power of coordination, observed after removal 

 of a portion or the whole of the cerebellum, indicates that the centers in the 



cord, medulla, pons, and cerebrum 

 endowed with corresponding 

 though less developed power, de- 

 velop compensatory activity and 

 acquire to some extent the capa- 

 bilities of the cerebellum itself 

 (Fig. 256). 



Clinico -pathologic facts partly 

 corroborate the results of physio- 

 logic investigations. In various 



FIG. 256. PROGRESSION AFTER DESTRUCTION or ,. , ,. 



THE V E RMis.-(M0ra/ and Day on, after Thomas.} forms of uncomplicated cerebellar 



disease, vertigo, tremor on making 



voluntary efforts, difficulty in maintaining the erect position, unsteadiness 

 in walking, opisthotonos, pleurothotonos, are among the symptoms generally 

 observed. 



Comparative anatomic investigations reveal a remarkable correspondence 

 between the development of the cerebellum and the complexity of the move- 

 ments exhibited by animals. In those animals whose movements are com- 

 plex and require for their performance the cooperation of many groups of 

 muscles the cerebellum attains a much greater development in reference to 

 the rest of the brain than in animals whose movements are relatively simple 

 in character. This relative increase in the development of the cerebellum 

 is found in many animals, as the kangaroo, the shark, the swallow, and the 

 predaceous birds generally. 



The Coordinating Mechanism. Though it is not known how the 

 cerebellum selects and coordinates groups of muscles for the performance 

 of any complex movement, it is known that its activity is largely reflex in 

 origin and excited by impulses which come to it from peripheral organs. In 

 this as in other forms of reflex activity the mechanism involves (i) afferent 

 nerves, e.g., cutaneous, muscle, optic, and vestibular, and their related end- 

 organs, tactile corpuscles, muscle spindles, retina, and semicircular canals, 

 all indirectly connected with (2) the cerebellar centers; (3) efferent nerves 

 indirectly connected with (4) the general musculature of the body. Both 

 station and progression are directly dependent on the development and 

 transmission of afferent impulses from the previously mentioned peripheral 

 sense-organs to the cerebellum. Tactile, muscle, visual, and labyrinthine 

 impressions and sensations not only cooperate in the development and or- 

 ganization of the motor adjustments necessary to the maintenance of the 

 equilibrium and locomotive coordination, but even after their organization 

 they are necessary to the excitation of cerebellar activity. The manner in 

 which they lead to the development of this capability on the part of the cere- 



