8!8 hensen's experiments on mysis. 



threads (Hensen). Thus, a string-like fibre of the membrana basilaris, which is 

 capable of vibrating, corresponds to every possible simple tone. According to 

 Hensen, the hairs of the labyrinth, which are of unequal length, may serve this 

 purpose. Destruction of the apex of the cochlea causes deafness to deeper tones 



(Baginsky). . , . . . ., 



[Hensen's Experiments. That the hairs in connection with the hair-cells vibrate 

 to a particular note is also rendered probable by the experiments of Hensen on the 

 crustacean My*is. He found that certain of the minute hairs (auditory hairs) in 

 the auditory organ of this animal, situate at the base of the antennae, vibrated when 

 certain tones were sounded on a keyed horn. The movements of the hairs were 

 observed by a low-power microscope. In mammals, however, there is a difficulty, 

 as the hairs attached to the cells appear to be all about the same length. We must 

 not forget that the perception of sound is a mental act] 



This assumption also explains the perception of noises. 



Of noises in the strictly physical sense, it is assumed that they, like single 

 impulses, are perceived by the aid of the saccules and the ampullae. 



It is assumed that the saccules and the ampullae are concerned in the general 

 perception of hearing, i.e., of shocks communicated to the auditory nerve (by 

 impulses and noises); while by the cochlea we estimate the pitch and depth of the 

 vibrations, and musical character of the vibrations produced by tones. 



The relation of the semicircular canals to the equilibrium of the body is referred 

 to in 350. 



417 SIMULTANEOUS ACTION OF TWO TONES HARMONY BEATS 

 DISCORDS DIFFERENTIAL TONES. When two tones of different pitch 

 fall upon the ear simultaneously, they cause different sensations according to the 

 difference in pitch. 



1. Consonance. If the number of vibrations of the two tones is in the ratio of 

 simple multiples, as 1 : 2 : 3 : 4, so that when the low notes make one vibration the 

 higher one makes 2 : 3 or 4 ... . then we experience a sensation of complete 

 harmony or concord. 



2. Interference. If, however, the two tones do not stand to each other in the 

 relation of simple multiples, then when both tones are sounded simultaneously 

 interference takes place. The hollows of the one sound-wave can no longer coincide 

 with the hollows of the other, and the crests with the crests, but, corresponding to 

 the difference of number of vibrations of both curves, sometimes a wave-crest must 

 coincide with a wave-hollow. Hence, when wave-crest meets wave-crest, there 

 must be an increase in the strength of the tone, and when a hollow coincides with 

 a crest, the sound must be weakened. Thus we obtain the impression of those 

 variations in tone intensity which have been called " beats." 



The number of vibrations is of course always equal to the difference of the number of vibra- 

 tions of both tones. The beats are perceived most distinctly when two organ tones of low 

 pitch are sounded together in unison, but slightly out of tune. Suppose we take two organ 

 pipes with 33 vibrations per second, and so alter one pipe that it gives 34 vibrations per second, 

 then otw distinct beat will be heard every second. The beats are heard more frequently the 

 greater the difference between the number of vibrations of the two tones. 



Successive Beats. The beats, however, produce very different impressions upon 

 the ear according to the rapidity with which they succeed each other. 



1. Isolated Beats. When they occur at long intervals, we may perceive them 

 as completely isolated, but single intensifications of the sound with subsequent 

 enfeeblement, so that they give rise to the impression of isolated beats. 



2. Dissonance. When the beats occur more rapidly they cause a continuous 

 disagreeable whirring impression, which is spoken of as dissonance, or an unhar- 

 monious sensation. The greatest degree of unpleasant painful dissonance occurs 

 when there are 33 beats per second. 



