ti 9 6 THE EAR. 



mesial plane, but the right superior and the left posterior canals are not 

 quite parallel — the posterior making with the mesial plane a somewhat 

 greater angle than the superior — as much as 8° more in one instance. 



Thus the ,°„ superior canal is nearly parallel to the . . , posterior, 

 left l right r 



and in each pair of approximately parallel canals the ampulla of one 



canal is at the one end, and that of the other at the other end. To sum 



up — (1) The six canals are sensibly parallel, two and two; and (2) the 



two horizontal canals are on the same plane, while the superior canal on 



one side is nearly parallel with the posterior canal of the other. These 



facts point to the two sets of canals and ampullae acting as one organ, 



in a manner analogous to the action of two retinae for single vision. 



For many years the view has been developing that the semicircular 

 canals communicate nervous impulses, which give a consciousness of the 

 position of the head in space, and thus bring into play the muscular 

 mechanisms necessary for equilibrium. Suppose that the canals and 

 ampullae were free to move in the perilymph surrounding them, a very 

 slight movement of the head in space might cause a small pull or 

 pressure on the nerve-endings in the ampullae, and thus a nervous 

 impulse would be aroused in the nerve fibres distributed to these 

 structures. Such a mechanism would be equivalent to a variation of 

 pressure ; but, as we have seen that sound-waves also correspond to 

 variations of pressure, it is evident that such variations might be pro- 

 duced by movements of the stapes or by movements of the head as a 

 whole. Thus the ultimate mechanism of hearing, and that assumed to be 

 the function of the semicircular canals, is essentially the same in kind, 

 only, according to this view, the pressures communicated by the base of 

 the stapes give rise to sensations of hearing, while those brought about by 

 the varying tensions of the ampullae originate those vague sensations 

 which preside over and regulate the muscular movements required to 

 maintain equilibrium. 



These views may be traced to two important researches, the one by 

 Purkinje 1 in 1820, and the other by Flourens 2 in 1828. Purkinje 

 directed attention to the well-known vertigo that follows rapid rotation 

 of the body in the erect position in a vertical axis. The vertigo is felt, 

 often with nausea, on suddenly stopping the rotatory motion. During 

 rotation, if the eyes are open, objects appear to be moving in an opposite 

 direction to that of the real movement ; and when the movement of the 

 body is arrested, a sensation of movement in the same direction con- 

 tinues for a few instants, and external objects still appear to be moving 

 in the opposite direction. This may be termed visual vertigo. It has its 

 counterpart in tactile sensations, when there is a tactile vertigo, on 

 touching bodies during the rotation. In these experiments we may sup- 

 pose external objects to be rotating round an imaginary axis, and, as we 

 have seen, the rotation may be in the same direction as, or in the opposite 

 direction to, that round the vertical axis of the body. Purkinje made the 

 important observation, that the position of the imaginary axis of rotation 

 for external objects depends on the axis of rotation executed at the head ; 

 and that if we change the position of the head, after having arrested the 

 movement of the body, we find that the axis of imaginary rotation 



1 Med.Jahrb., Wien, 1820, Bd. vi. ; also Bull, der schlesischen Gesellsch., 1825-1826. 

 - Mem. Acad. d. sc. de VInst. de France, Paris, 1828, tome ix. p. 5 ; also " Recherches 

 experimentales sur lcs propri£t<5s du systeme nerveux," Paris, 1842, p. 438. 



