258 OUTPOSTS OF THE INTELLIGENCE SERVICE 



hair cells, is composed of a row of inner " rods," shaped like ulnar 

 bones, attached by their terminal end to the basilar membrane, and 

 fitting on to the heads of the outer " rods." The latter resemble 

 swan's heads and necks, the backs of the heads fitting into the 

 hollows of the inner " rods." 



The mechanism of the internal ear. 



It is obvious that every movement of the stapes is communicated 

 by the perilymph to the membranes of the sccda media, from 

 them to the endolymph, and so to the organ of Corti. There is 

 also no doubt about the hair cells as being the final instruments 

 for the transmission of the impulses to the nerve. The only 

 structures placed so that their movements can be transmitted to 

 these hair cells are («) the basilar membrane, {b) the tectorial 

 membrane, and (c) the pillars of the arches of Corti. We may 

 dismiss Corti's arches as the basis whereby fluid motion is converted 

 into the movements of the hairs, because the cochlea of birds is 

 free from them. Opinion is sharply divided as to the comparative 

 importance of the two membranes. Because of its structure, 

 position and marked differentiation at different levels, most 

 modern investigators are of the opinion that the basilar membrane 

 plays the more important part. 



Several theories have been put forward as to the mechanism of 

 the inner ear. The most important are (i.) the resonance theory 

 associated with the names of Helmholtz, McKendrick, Hartridge 

 and Wilkinson; (ii.) the displacement theory of Wrightson and 

 Keith; and (iii.) the pressure-pattern theory of Rutherford as 

 elaborated by Ewald. 



Resonance Theory. 



Although Helmholtz was not the first to suggest that possibly 

 the fibres of the basilar membrane acted as resonators like the 

 strings of a piano, he may be considered the originator of this 

 theory, as he worked out a connected theory of hearing based on 

 the physics of sympathetic resonation. 



In his own words, " Suppose we were able to connect every string 

 of a piano with a nerve fibre in such a way that this fibre would be 

 excited and experience a sensation every time the string vibrated. 

 Then every musical tone which impinged on the instrument would 

 excite in the ear, as we know to be really the case, a series of 

 sensations corresponding to the pendular vibrations into which 

 the original motion of the air had to be resolved. By this means, 

 then, the existence of each tone would be exactly so perceived, as 

 it is really perceived by the ear. The sensations excited by the 



