THE FUNCTIONS OP THE CEREBELLUM. 275 



that any disturbance in the center of gravity of the body may be 

 subconsciously rectified by appropriate action of the various 

 muscular groups. It evidently represents the nerve center hav- 

 ing supreme control over other nerve centers, so that these may 

 not bring about such movements as would disturb the equili- 

 brium of the animal. 



In order that the cerebellum may perform this function it 

 must, however, be informed of two things. In the first place, it 

 must know the existing state of contraction of the muscles and 

 the tightness of the various tendons that pull upon the joints, 

 and in the second, it must know the exact position of the center 

 of gravity of the body. 



Information of the condition of the muscles and tendons is 

 supplied through the nerves of muscle sense, which run in 

 every muscular nerve and are connected in the muscles with 

 peculiar sensory nerve terminations called muscle spindles. 

 When the muscles contract, or the tendons are put on the stretch, 

 these spindles are compressed and sensory or afferent stimuli 

 pass up the nerves of muscle sense, enter the cord by the pos- 

 terior roots and reach the cerebellum by way of the lateral col- 

 umns (see p. 249). 



Information regarding the center of gravity of the body is 

 supplied through the vestibular division of the eighth nerve, 

 which, it will be recalled, is connected with the semicircular can- 

 als and vestibule. In these structures are membranous tubes or 

 sacs containing a sensory organ (called the crista or macula 

 acoustica), which consists essentially of groups of columnar cells 

 furnished with very fine hair-like processes at their free ends 

 and connected at the other end with the fibers of the eighth 

 nerve. The hair-like processes float in the fluid which is con- 

 tained in the membranous canals or sacs. This fluid does not, 

 however, completely fill these structures, so that it moves when- 

 ever the head is moved. This movement affects the hair-like 

 processes and thus sets up nerve impulses which are carried 

 to the cerebellum. 



To make the hair cells of this receiving apparatus capable 

 of responding to every possible movement of the head, it is, 



