THE CENTRAL NERVOUS SYSTEM 733 



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 move- 

 ments compounded of the separate movements obtained by stimula- 

 tion 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 equilibrium. 

 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 hori- 

 zontal 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 unaffected (James), 

 and the proportion seems to be much higher among congenital deaf- 

 mutes. Kreidl and Bruck, too, have found that abnormalities of loco- 

 motion and equilibration are much more common in deaf and dumb 

 children than in others. Now, in these cases the defect is usually 

 in the internal ear. We must conclude, then, that the co-ordination 

 of muscular movements necessary for equilibrium is achieved in 



