THEORIES OF HEARING 613 



is very convincing. Other theories have been proposed however which will now receive 

 brief consideration. 



RUTHERFORD'S HYPOTHESIS compares the cochlea to a telephone. In 

 the same way as the diaphragm of the receiver is set into vibration by the sound waves, 

 and starts corresponding variations in the strength of the current conducted to the 

 transmitter, so the vibration of the basilar membrane as a whole causes impulses to 

 be sent up the auditory nerve which correspond with the air vibrations received by 

 the ear. Analysis does not take place in the cochlea at all but in the brain. Wrightson, 

 who has restated this theory and added much detail to it, states that the cerebral 

 analysis is effected by differences between the time intervals of the points of zero 

 pressure and of the maximum plus and minus pressures. 



The following objections may be stated against the Rutherford- Wrightson hypo- 

 thesis. 



(1) It assumes that the auditory nerve can conduct complicated wave forms, intact 

 as to pitch and amplitude, at rates up to 40,000 vibrations per second. Rutherford 

 in this connection pointed to the motor nerves of the bee's wing which are capable 

 of responding to transmitted impulses at 460 per second. Between 40,000 and 460 

 is however a big gap which will certainly have to be bridged before this view as to the 

 transmission of the vibrations intact to the brain can be accepted. 



(2) We cannot picture a cerebral apparatus which can analyse these complicated 

 nerve impulses even if they could reach it, and neither Rutherford nor Wrightson assist 

 us to do so. The relegation of the powers of analysis to the cerebral cortex is, at the 

 present at any rate, equivalent to giving up any attempt to explain the power of analysis 

 possessed by the organ of hearing. 



(3) It would seem that a very much simpler organ than that of the cochlea would 

 be sufficient to convert sound waves into nerve impulses if no analysis of the stimulus 

 took place there. 



(4) It would be very difficult to explain on this hypothesis the localisation of degene- 

 ration to certain notes, or the deafness to certain notes which accompanies disease of 

 part of the organ of Corti. 



(5) This hypothesis does not explain why fatigue to one note leaves the response 

 to all other notes apparently unaffected in intensity. 



The objections to this hypothesis are therefore of a formidable character. Much 

 additional evidence in its favour would be necessary to place it even on a par with 

 the theory of Helmholtz. 



WALLER'S HYPOTHESIS stated that the basilar membrane vibrated in the form 

 of pressure patterns w r hich are similar to those which may be seen on a vibrating plate. 

 Ewald who has elaborated this view found by experiment that the patterns take the 

 form of equidistant stationary nodes or ridges, the distance between which varies with 

 the pitch of the note entering the mechanism. The distance between the nodes is 

 measured by the hair-cells and corresponding impulses are sent to the auditory centre. 

 The advantage of this hypothesis is that like Helmholtz' it places the analysis of 

 the sound waves in the cochlea and therefore does not, like Rutherford's hypothesis, 

 require the transmission by the auditory nerve of rapid impulses or the analysis of 

 such impulses by the brain. It is clear however that so far as our present knowledge 

 goes the evidence is all in favour of Helmholtz' view. 



BEATS AND DISSONANCE. The overtones of any sound, at any rate 

 the lower ones, are at considerable distance from one another on the musical 

 scale, and therefore differ considerably in the number of vibrations of which 

 they are composed. If two tuning-forks be sounded, the vibrations of which 

 differ only by one or two per second, the phenomenon known as ' beats ' is 

 produced. This is due to the summation or interference of the waves from the 

 two tuning-forks. Let us suppose we have tuning-forks vibrating one at 100 



