834 A MANUAL OF PHYSIOLOGY 



vestibular division of the nerve which is especially related to the 

 vestibule (p. 823). 



There is very strong evidence that the semicircular canals are con- 

 cerned, not in hearing, but in equilibration. A pigeon from which 

 the membranous canals have been removed still hears perfectly 

 well so long as the cochlea is intact, but exhibits the most profound 

 disturbance of equilibrium. If the horizontal canal is destroyed 

 or divided the pigeon moves its head continually from side to 

 side around a vertical axis ; if the superior canal is divided, the 

 head moves up and down around a horizontal axis. The power 

 of co-ordination of movements is diminished, but not to the same 

 extent in all kinds of animals. Thrown into the air, the pigeon is 

 helpless ; it cannot fly ; but a goose with divided semicircular canals 

 can still swim. The condition is only temporary, even when the 

 injury involves the three canals on one side ; but if the canals on 

 both sides are destroyed, recovery is tardy, and often incomplete. 

 In mammals the loss of co-ordination is much less than in birds ; 

 and movements of the eyes, the direction of which depends on the 

 canal destroyed, take to a large extent the place of movements of 

 the head. The effects of destructive lesions have their counterpart 

 in the phenomena caused by stimulation ; excitation of a posterior 

 canal, for example, in the pigeon causes movements of the head from 

 side to side. 



Lee's results in fishes are, on the whole, of similar tenor. 

 Mechanical stimulation of the ampullae in the dogfish, by pressing 

 on them with a blunt needle, calls forth characteristic movements 

 of the eyes and fins, and electrical stimulation of the auditory nerve 

 causes movements compounded of the separate movements obtained 

 by stimulation of the ampullae one by one. Lee concludes that the 

 semicircular canals are the sense-organs for dynamical equilibrium 

 (i.e., equilibrium of an animal in motion), and the utricle and saccule 

 for statical equilibrium (i.e., equilibrium of an animal at rest). 



The evidence from all sources points strongly to the conclusion 

 that afferent impulses are actually set up in the fibres of the auditory 

 nerve, through the hair-cells, by alterations of pressure or by stream- 

 ing movements of the endolymph when the position of the head is 

 changed. Rotation of the head to the right may be supposed to 

 cause the endolymph in the right external canal, in virtue of its 

 inertia, to lag behind the movement, and to press upon the anterior 

 surface of the ampulla. The disorders of movement after lesions of 

 the canals may be explained as the result of the withdrawal of 

 certain of these afferent impulses, and the consequent overthrow of 

 that equipoise of excitation necessary for the maintenance of equi- 

 librium. Even in man there is evidence of the existence of some 

 mechanism not depending on the muscular sense or on impressions 

 passing up the channels of ordinary or special sensation, by which 

 orientation (the determination of the position of the body in space) 

 is rendered possible. For a man lying perfectly still, with eyes shut, 

 on a horizontal table which is made to rotate uniformly, can not 

 only judge whether, but also in what direction, and approximately 

 through what angle, he is moved (Crum Brown). The phenomena 

 of pathology afford weighty additional testimony in favour of the 

 equilibratory function of the semicircular canals. For many cases 

 of vertigo are associated with changes in the internal ear (Meniere's 

 disease). And while nearly every normal individual becomes dizzy 

 when rapidly rotated, 35 per cent, of deaf-mutes are entirely un- 



