CH. XLIX.] SEMICIRCULAR CANALS. . 689 



It is these canals which enable all of us to know in which 

 direction we are being moved, even though our eyes are bandaged, 

 and the feet are not allowed to touch the ground. On being 

 whirled round, such a person knows in which direction he is 

 being moved, and when the whirling stops he seems, especially if 

 he opens his eyes, to be whirling in the opposite direction, owing 

 to the rebound of the fluid in the canals. The forced movements 

 just described in animals are due either to the absence of the 

 normal sensations from the canals, or to delusive sensations 

 arising from their irritation, and the animal makes efforts to 

 correct the movement which it imagines it is being subjected to. 



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 auditory 

 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 ampullary nerve-endings of the right horizontal 

 canal, and diminished pressure on the corresponding nerve-endings 

 of the left side. 



" One canal can be affected by, and transmit the sensation of 

 rotation about one axis in one direction only ; and for complete 

 perception of rotation in any direction about any axis, six canals 

 are required in three pairs, each pair being in the same or parallel 

 planes, and their ampullae turned opposite ways. Each pair 

 would thus be sensitive to any rotation about a line at right 

 angles to its plane or planes, the one canal being influenced by 

 rotation in one direction, the other by rotation in the opposite 

 direction." (Crum-Brown.) 



The two horizontal canals are in the same plane ; the posterior 

 vertical of one side is in a plane parallel to that of the superior 

 vertical of the other side (see fig. 512). 



These four sets of impressions (tactile, muscular, visual, and 

 labyrinthine) reach the cerebellum by its peduncles ; from the 

 eyes through the superior peduncle, from the semicircular c:ui:il 

 through the middle and inferior peduncles, and from the body 

 generally through the restiform body or inferior peduncle. Section 

 and stimulation of the peduncles cause inco-ordination, chiefly 

 evidenced by rotatory and circus movements similar to those that 

 occur when the nerve-endings in the semicircular canals are 

 destroyed or stimulated. Stimulation of the cerebellum itself 

 and this has been done through the skull in man causes 



K.P. y Y 



