754 



FUNCTIONS OF THE CEREBELLUM 



[CH. LI. 



cells which act as supports (fig. 463). When the endolymph in the 

 interior of the canals is thrown into vibration, the hairs of the hair- 



Fio. 462. Section through the wall of the ampulla of a semicircular canal, passing through the crisia 

 acoustica. 1, Epithelium ; 2, tunica propria ; 3, fibrous layer of canal ; N, bundles of nerve-fibres ; 

 C, cupula, into which the hairs of the hair-cells project. (After Schiifer.) 



cells are affected, and a nervous impulse is set up in the contiguous 

 nerve-fibres, which carry it to the central nervous system. 



The walls of the saccule and 

 utricle are similar in composition, 

 and each has a similar hillock, called 

 a macula, to the hair-cells on which 

 nerve-fibres of the vestibular nerve 

 are distributed. 



It will be noticed that the canals 

 of each side are in three planes at 

 right angles to each other, and we 

 learn the movements of our body 

 with regard to the three dimensions 

 of space by means of impressions 

 from the ampullary endings of the 

 vestibular nerve; these impressions 

 are set up by the varying pressure of 

 the endolymph in the ampullae. 



Thus a sudden turning of the 

 head from right to left will cause 

 movement of the endolymph towards, 

 and therefore increased pressure on, 

 the hair-cells connected to the am- 

 pullary nerve-endings of the left horizontal canal, and diminished 

 pressure on the corresponding apparatus of the right side. It 

 is probable that resulting from such a movement two impulses 

 reach the brain, one the effect of increased pressure in one ampulla, 

 the second the effect of decreased pressure in its fellow. It may 



FIG. 463. 1, Hair-cell ; 3, hair-cell, showing 

 the hair broken, and the base of the hair 

 split into its constituent fibrils; 2, fibre- 

 cell; N, bundle of nerve-tibres which 

 have lost their medullary sheath, and 

 terminate by arborising round the base 

 of the hair-cells ; A B, surface of tunica 

 propria. (After Retxius.) 



