612 PHYSIOLOGY 



think, to be obtained from the fact that the cochlea is filled with liquid. 

 This liquid makes the basil ar membrane ' dead beat ' because its move- 

 ments, when the liquid is still, set up eddies which quickly check the motion 

 owing to the viscosity of the liquid. On the other hand when a sound 

 is entering the ear and the fluid is therefore in"motion, this movement is the 

 more rapidly imparted to the basilar membrane because of its continuity, 

 bufe even if it consisted of separate fibres it would still be set rapidly into 

 vibration owing to the viscosity of the liquid. In this way one can explain 

 both the rapid response and the rapid damping of the cochlea. 

 In favour of Helmholtz' theory we have the^following evidence : 



(1) In boiler-makers' disease we have inability to hear high notes, and 

 we find that it is the short fibres of the basilar membrane which are degener- 

 ated. 



(2) In experiments in which the ears of animals have been stimulated for 

 long periods to the same note, subsequent examination has shown the localisa- 

 tion of degeneration to one part of the organ of Corti. With a high note the 

 short fibres are affected, with a low note the long. 



(3) If one of the ears be fatigued by prolonged stimulation to a constant 

 note, its response to the same note is found to be inhibited, but notes of slightly 

 longer or shorter pitch are found to be unaffected. This shows clearly that 

 the response to a given rate of vibration must affect a certain limited number 

 of hair cells and nerve fibres only, and is therefore strongly in favour of 

 Helmholtz' theory. 



(4) Animals whose calls have a small range of pitch (e.g. birds) have short 

 basilar membranes which vary but little in length. 



(5) Animals, in which different parts of the cochlea have been destroyed, 

 appear to give definite evidence for deafness to high notes when the fine 

 basilar fibres are damaged, and deafness to low notes when the long fibres 

 have been removed. 



(6) Patients are found in whom there are islands of deafness, that is, they 

 are deaf to a limited part of the musical scale. The Helmholtz theory readily 

 explains these cases as being due to local disease of certain basilar fibres or 

 their corresponding hair-cells. Further there are cases in which the two ears 

 give different notes, a condition called double disharmonic hearing. This is 

 easily explained by a change in the natural period of the fibres of the basilar 

 membrane in the diseased ear, cither as the result of stretching or the increased 

 mass due to inflammation. 



(7) McKendrick was able to produce a model of the cochlea with ba si la r 

 membrane and organs of Corti. He found that parts of the membrane can 

 be made to vibrate to a certain pitch and not to others as the Helmholtz 

 hypothesis requires. 



(8) McKendrick calculated that the number of fibres in the auditory nerve, 

 the number of fibres in the basilar membrane and the number of hair- cells and 

 arches of Corti were sufficient to give the total number of different pitches 

 (about 11,000) in the auditory scale. 



It would appear therefore that the evidence in favour of Hflmholt/.' 



