184 



NATURE 



N-i . EMBER ~. 1918 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return . or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of NATURE. No notice is 

 taken of anonymous communications.] 



The Perception of Sound. 



Prof. Keith has replied to the physiological 

 criticisms in Prof. Bayliss's most thoughtful letter in 

 Nature of October 17. 1 will therefore confine myself 

 principally to the physical objections he has raised. 



With regard to the difference between the molecular 

 movement of a liquid subjected to the pressure of a 

 sound-wave, and the molar motion of a minute mass 

 of liquid iii the cochlea, it cannot be forgotten that 

 liquids could not conduct sound "unless they were 

 both elastic and compressible." 



The fact that wain- is so difficult to compress, and 

 this only at very high unit pressures, is proof of 

 its high degree of elasticity or tendenq for its .mole- 

 cules to return to their undisturbed positions. Hut 

 an extremely low unit pressure will cause molar 

 motion in a small mass of liuuid moving in a vessel 

 of suitable shape. It is to such molar motion thai I 

 refer. A reference to my footnote (p. 56 of my book) 

 shows that Helmholtz in his later studies recognised 

 that "an incompressible fluid contained between solid 

 walls is distinguishable from a compressible one in 

 this : that every impulse which reaches any part of 

 its surface communicates itself immediately" through- 

 out the whole fluid and sets every part instantly in 

 motion, while in a compressible fluid a wave pro- 

 ceeds from the spot of action and travels with a 

 certain velocity, setting in motion the several particles 

 of the fluid consecutively. Thus if the dimensions of 

 the whole mass are infinitely small in comparison 

 with the wave-length (as it is 'in the labyrinth water), 

 and if the walls of the petrous bone which enclose it 

 are so solid that we can consider them as absolutelv 

 so when compared with the minute pressure under 

 consideration, we deduce the following : The com- 

 munication of action throughout the whole mass is 

 practically instantaneous, and the labyrinth water 

 under the influence of the sound waves moves virtually 

 as a fluid absolutelv incompressible (and therefore in- 

 capable of sound waves) would move under the same 

 relations." 



I am glad that Prof. Bayliss directs attention to 

 the difficulty he finds in seeing how there is a differ- 

 -I pressure on the upper and lower surfaces of 

 the basilar membrane when the column is a con- 

 tinuous one. 



The answer is that the high pressure on tb- upper 

 side of the membrane is carried through the bent 

 lexer, of the Corti arches to do mechanical work in 

 bending i| lf . hairlels (see p. (38). In science it is true, 

 more homety affairs, that we cannot both eat 

 our cake and have it. As the pressure is passed out 

 on the uppi r side of the membrane to do work in the 

 scala media in bending thousands of hairlets, there is 

 an equivalent loss of pressure in the passage below 

 the membrane. In a double-acting hvdraulic engine 

 we have a somewhal similar condition of things. 

 The high-pressure water is admitted into a cvlinder 

 and the piston is pushed Forward and backward 

 alternately. The piston-rod carries the pressure to the 

 crank-shaft, thus doing external work.' The equiva- 

 lent unit pressure disappear? in thi exhaust water 

 which is pushed out at atmospheric pressure. The 

 same displacement of liuuid takes place in the exhaust, 

 NO. 2558, VOL. I02] 



but thi displacement is reduced in pressure bj thi 

 equivalent amount of work carried out of the system. 

 Prof. Bayliss asks whj thi man} Corti 



aides when a few might suffice. This I 



n order to extend the sui fa< i ol excitation, 



which I havi endeavoured to describi on p, 72; also 

 to provide for the probability that timing a lifetime 

 ': 1 as work many o! thi I arches would 



fail to function. 

 With regard to Yoshii's experiments, 1 



seen them, hut Prof. Keith lias informed me that 

 such observations have been made, though he did not 

 di cribe Hie conditions under which the experiments 

 lie! been carried out If the observations depended 

 upon prolonged exposun to the vibrations ol a musical 

 note, i scarcely think the} should be regarded as a 

 proof ol the localisation of certain strings in th< 

 basilar membrane to produce certain notes. It 

 mighl only mean that some weak part in the 

 system gave waj and ceased to function under a 

 stre-- lit Fatigue. Ii seems quite possible, as sug- 

 gest, d b\ Prof. Keith, that notes of high frequency, 

 rising ver\ rapidly to their maximum pressure, might 

 tend to short-circuit towards the narrow end of the 

 membrane, and notes of low frequency, rising slowly 

 to their maximum, might impress the wider end of 

 the membrane, but it must be remembered that the 

 anas at the fenestra! end are extremely small, and that 

 Helmholtz himself recognised the practically instant- 

 aneous action of the pressure throughout the passage. 



The strongest argument against the siting instru- 

 ment theory is that in the basilar membrane all the 

 so-called strings are cemented together sideways, and 

 cannot, therefore, vibrate freely or respond each I 

 ow 11 vibration rate. 



In the displacement theory the striate or inelastic 

 portion of the membrane moves downwards 

 whole, following the same spiral plane on a hinge 

 coinciding with the tapered edge of the aperture, each 

 strip of its breadth descending a distance increasing 

 from the fenestral end to a maximum at the helico- 

 tremal end, and in this way obliging the triangular 

 displacements to increase towards that end uniformly. 



In a compound-wave form entering the ear we have 

 indications not onlv of the original impulses of the 

 pure tones from which the compound is built up, but 

 also of the differential tones, the summational tones, 

 and harmonics. 



When we find these all reappearing as sensations in 

 the brain, the conclusion appears to be forced upon us 

 that the ear must be a machine adapted to sustain 

 on their passage to the brain these impulses of which 

 we find indications in the wave-form. 



My endeavour has been to examine the structure 

 and working of this marvellous machine which causes 

 the accurate transference of such impulses. 



Thomas YYriciitson. 



Neasham Hall, Darlington. 



The objections against Sir Thomas Wrightson's 

 theory raised by Prof. Bayliss in Nature of 

 October 17, though grave, no doubt, to those who 

 can .appreciate their cogency, yet appear to one who 

 has attempted to approach the problem from the 

 side of phonetics comparatively slight; while there are 

 objections to the prevalent theory which, if I may be 

 allowed brieflv to state and develop them, ma\ perhaps 

 throw some light on the question at large. 



I would put the matte,- this way. The human ear 

 as imagined by Helmholtz is no great improvement 

 on Nature, for it could neither (1) hear a note of 

 music, nor (2) hear a large percentage of the words 

 in the Fnglish language. 



