CH. XLIX.] SEMICIRCULAR CANALS 709 



Artificial stimulation of the canals produces movements of the head and orbits, 

 and giddiness. Similar movements occur during bodily rotation, and giddiness is 

 the result of a rivalry of sensations which afford conflicting ideas of the position of 

 the body relatively to external objects. A certain proportion of deaf mutes lose their 

 sense of direction under water, cannot maintain their equilibrium when their eyes are 

 shut, exhibit no orbital movements when rotated, and never suffer from sea-sickness 

 or giddiness. This proportion (36 per cent.) is approximately the frequency in which 

 abnormal conditions of the canals have been found post-mortem in deaf mutes. 



It will be noticed that the eanals 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 left horizontal canal, and 

 diminished pressure on the corresponding nerve-endings 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. 



" 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. 515). 



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 canals 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 stimu- 

 lated. Stimulation of the cerebellum itself and this has been done 

 through the skull in man causes giddiness, and consequent muscular 

 efforts to correct it. The results of stimulation, indeed, are precisely 

 analogous to those of extirpation, only in the reverse direction. Loss 

 of muscular tone which follows extirpation of the canals is probably 

 the result of secondary changes in the brain. 



