CHAPTER C 



THE CEREBELLUM AND THE SEMICIRCULAR CANALS ; 

 FUNCTIONAL TESTS 



The cerebellum serves as the great nerve center to which are transmit- 

 ted, through the various proprioceptors, the impulses which, as it were, 

 inform it as to the exact degree of muscular effort required to maintain 

 the animal in its various postures. It is, as Sherrington puts it, the head 

 ganglion of the proprioceptive system. Such impulses from the muscles, 

 tendons, etc., could not, however, supply information regarding the exact 

 position of the body in space. For this purpose special receptors con- 

 nected with the eighth nerve are provided in the semicircular canals. 

 These, it will be remembered, are three in number on either side, each canal 

 consisting of a semicircular bone tube attached to the vestibule of the 

 internal ear ; and they are arranged so that they lie at right angles to one 

 another in the three planes of space. * The three canals on either side are 

 thus disposed so as to form an arrangement like a V-shaped armchair with 

 the back inwards. This arrangement causes the posterior vertical canal 

 of one side to be in the same plane as the superior vertical canal of the 

 opposite side, the external canals being in the horizontal plane on both 

 sides. The arangement will be plain from the diagram (Fig. 227). 



Within the osseous canals are suspended membranous tubes, which do not 

 fill the canals. The canals, etc., contain fluid, but are not completely 

 filled. The osseous as well as the membranous canals are dilated at one 

 end to form the ampulla, and it is here that the vestibular division of the 

 eighth nerve ends in a structure called the "crista acustica," consisting of 

 hair cells supported by sustentacular cells. The nerve terminates in fine 

 arborizations between the hair cells. In the saccule and utricle are struc- 

 tures similar to the crista, called the maculae acusticae. These structures are 

 receptors specialized for the purpose of responding to changes in the 

 position of the head, and therefore of the body in general. When the 

 head moves in a certain plane of space, the fluid in the membranous canals 

 and in the utricle and saccule on account of inertia undergoes a certain 

 movement, which acts on the hairs of the hair cells and thus sets up a 

 stimulus. According to the degree of the stimulation in the various 

 ampullae, which again will be dependent upon the plane or planes in 

 which the movement of the head has occurred, impulses are transmitted 



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