MECHANICS OF THE INNER EAR 57 



It is not impossible, however, it is even probable, also 

 that a few of the sections just preceding this last convey the 

 sensation of this difference tone, instead of that of the mean 

 tone. The last section which may convey the sensation of 

 the mean tone moves only twice up and down during 

 the period, in quick succession. This double move- 

 ment is followed by a long pause during which no movement 

 occurs. Now, experimental research of recent years has prov- 

 ed that two shocks received by the auditory nerve ends may 

 be sufficient to give the sensation of the tone corresponding 

 to the frequency with which the two shocks are received — 

 but only within the middle region of the tonal series. To- 

 wards either end of this series four, six, and even more shocks 

 are found to be necessary for the sensation of the tone cor- 

 responding to the frequency of the shocks. What, then, will 

 be the consequence of choosing the tones 24 and 25 somewhat 

 higher? The section of the partition which makes the two 

 up and down movements in quick succession can no longer 

 convey the sensation of a short mean tone. If there is only 

 one period of movement, no sensation at all will then result. 

 But if many periods succeed, it is much more likely that the 

 double movement of the partition section will have the effect 

 of a single shock than no effect at all ; and the repetition of 

 this shock in each succeeding period must result in the sen- 

 sation of the tone 1, the difference tone. 



If the tones 24 and 25 are chosen still higher, it becomes 

 improbable that even three shocks received by the nerve ends 

 in quick succession between two long pauses can give the 

 sensation of a short mean tone. In this case it is highly prob- 

 able also that the second section before the last conveys the 

 sensation of the difference tone. And so a few more of those 

 more distant sections may convey the sensation of the dif- 

 ference tone instead of the mean tone. 



