DAMPING 261 



cochlea acts as a progress! \cly graduated load on the basilar 

 libres. " Supposing a small traiisxerse sector of the basilar 

 membrane to mo\ c from its central position towards the scala 

 tympani, its movement will displace a certain amount of fiuid, and 

 the displacement will travel along the scala till the membrane 

 closing the round window is bulged to an extent sullicient to 

 accommodate the amount of fluid displaced. It follows that an 

 equivalent quantity of fluid will be displaced across each cross- 

 section of the scala from the level of the vibrating sector to that 

 of the round window. The mass of the fluid moved will equal 

 that of a column the base of which is equal to the area of the 

 vibrating sector, and the height to the distance from the sector to 

 the round window. A similar mass of fluid will be displaced in the 

 scala vestibuli, but in the opposite direction " (Wilkinson). The 

 mass (weight of double column) progressively increases from the 

 base to the apex of the cochlea. This Avill cause a differentiation 

 in the periodicity of the vibrations in the same sense as the varia- 

 tions of length and tension, i.e. length and mass are greatest on the 

 longer fibres, while tension is greatest on the shorter fibres. 



Now if we consider the mean length of the basilar membrane as 

 35 mm., we may state as an approximation that the shortest 

 distance (following round the inner curve of the scalae) between 

 the longest fibre of the membrane and the windows woidd be 

 2 X 20 = 40 mm. The distance between the windows and the 

 shortest fibre is about 2 mm., i.e. a twenty-fold variation in fluid 

 load is possible. 



Damping. Some mechanism must exist to prevent a resonating 

 fibre from continuing its vibrations after the stimulating sound 

 has ceased. Hartridge has devised many demonstrations of the 

 presence of dampers. In one experiment he used a siren arranged 

 to produce instantaneously a change of phase of half a wave-length 

 in the successive pulses emitted by it. When the change of phase 

 occurred, there was apparently a period of silence. That is, at that 

 moment, the resonators were damped before a fresh set of resonators 

 took up the new vibrations. 



This theory receives confirmation from the following facts : — 



(1) Birds and other animals whose calls have a short range of 

 pitch have short basilar membranes. 



(2) Prolonged subjection to a definite note produces degenera- 

 tion in a definite part of the membrane — e.g. in boiler-makers' 

 disease there is inability to hear high notes with degeneration of 

 the short fibres ; animals give evidence of deafness to low notes 

 when the long fibres of the membrane have been destroyed. 



(3) If one ear be fatigued by prolonged stimulation from a 



