THE CEREBELLUM. 519 



A voluntary effort on the part of the animal causes it to lose all 

 control of its muscles and the body is rotated in the direction of its 

 longitudinal axis from 40 to 60 times a minute before it comes to rest. 

 According as the lesion is made from behind 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, develop compensatory 

 activity and acquire to some extent the capabilities of the cerebellum 

 itself (Fig. 234). 



Clinico- pathologic facts partly corroborate the results of phys- 

 iologic investigations. In various forms of uncomplicated cere- 

 bellar 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 correspond- 

 ence between the de- 

 velopment of the cerebel- FIG. 234. PROGRESSION AFTER DESTRUCTION 

 lum and the complexity OF THE VERMIS. (Moral and Doyen, 



of the movements ex- after Thomas.} 



hibited by animals. In 



those animals whose movements are complex and require for their 

 performance the cooperation of many groups of muscles the cere- 

 bellum 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, such 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 per- 

 formance of any complex movement, it is known that its activity is 

 largely reflex in origin and excited by impulses reflected 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 con- 

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



