846 A MANUAL OF PHYSIOLOGY 



gave up the pillars of Corti, and, adopting a suggestion of 

 Hensen, substituted the radial fibres of the basilar mem- 

 brane as his hypothetical resonators. But while it is true 

 that these are much more adequate to the task imposed on 

 them, since their range of length is far greater (41 /JL at the 

 base to 495 /u, at the apex of the cochlea Hensen) ; and 

 while the structure of Corti's organ certainly suggests that 

 some one or other of its elements may be endowed with 

 such a function, the theory of peripheral analysis of pitch 

 tends upon the whole rather to break down than to be 

 strengthened as evidence gathers. 



When two notes of different frequency are sounded together, they 

 * interfere' with each other. If two tuning-forks A and B, making 

 100 and 101 vibrations a second respectively, be started together, 

 at the end of the first vibration of A, B will be T J(jth of a vibration 

 ahead, at the end of the second T ^ths of a vibration; at the end of 

 the fiftieth half a vibration. Here the crest of B's wave will coincide 

 with the trough of A's, and if the forks are vibrating with the same 

 amplitude the resultant for this vibration will be zero, the wave will 

 be blotted out. If the amplitudes are not the same, the wave will 

 still be weakened. At the end of the hundredth vibration of A, B 

 will have gained a whole vibration, the tops of the two waves will 

 coincide, and the sound will be strengthened. We recognise the 

 alternate changes in the amplitude of the interfering sound-waves by 

 a change in the auditory sensation, which is called a beat; and in the 

 case supposed there will be one beat a second. If the difference in 

 the frequency of the forks is five there will be five beats a second. If 

 the difference is twenty there will be twenty beats a second. As the 

 difference is increased the beats will ultimately follow each other 

 so rapidly that they will themselves be fused into a note a beat-tone 

 as it is called, whose pitch will correspond to the frequency of the 

 beats. Now, Hermann has found that the ear may perceive a beat- 

 tone which elicits no response from a resonator attuned to its note and 

 readily set into vibration by the same note when sounded by a tuning- 

 fork. This shows that the process by which pitch is appreciated, 

 whatever it may be, is not entirely explicable on the theory of 

 resonance. 



(2) The second theory, in accordance with the simile used 

 by Rutherford, to whom we owe it in its present form, may 

 be conveniently labelled the ' telephone theory.' He sup- 

 poses that the organ of Corti (or, at any rate, the hair-cells) 

 is set into vibration as a whole by all audible sounds, and 

 that its vibrations are translated into impulses in the auditory 



