RESONANCE THEORY 211 



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

 swans' 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 communi- 

 cated by the perilymph to the membranes of the scala 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. Two main 

 theories are held as to the mechanism of the cochlea, viz. the 

 resonance theory and the displacement theory. 



Resonance Theory. 



Helmholtz considered that the only mechanism capable of 

 analysing compound vibrations was a system of resonators. 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 

 higher particles under the supposed conditions, would fall to the 

 lot of different nerve fibres, and hence be produced perfectly, 

 separately and independently. Now, as a matter of fact, later 

 microscopic discoveries respecting the internal construction of 

 the car, lead to the hypothesis that arrangements exist in the ear 

 similar to those which we have imagined. The end of every 

 fibre of the auditory nerve is connected with small elastic parts 

 which we cannot but assume to be set in sympathetic vibration 

 of the waves of sound." 



He considered that the fibres of the basilar membrane consti- 

 tuted this system of resonators. This membrane increases its 

 width (about three times) as it passes from its beginning in the 

 base of the cochlea to its termination in the apex and contains 

 somewhere about 10,000 of these fibres within its substance. 

 He considers that " damping arrangements exist in the ear 

 so as to quickly extinguish movements of the vibrators" 

 (M'Kendrick). 



