908 SIMULTANEOUS ACTION OF TWO TONES. 



of the basilar membrane (on which the organ of Corti rests), which 

 are shortest in the first turn of the cochlea, and become longer near the 

 apex, must be considered as the organ that takes up the vibrations. 

 Hence, there would be a fiber of the basal membrane vibrating in sym- 

 pathy with each possible simple tone. According to Hensen the hairs 

 of varying length in the labyrinth may also subserve the same purpose. 

 The foregoing assumption is sufficient also to explain the perception 

 of noises. Many of these may be resolved into a confused mass of 

 simple pure tones. True noise in the physical sense must, like separate 

 explosions, be perceived by the saccules and ampullae. 



R. Ewald has proposed a new theory, the so-called sound-picture theory. 

 He believes that the impulses produced by the sound on the basilar membrane 

 give rise to a wave-picture (sound-picture), the special form of which enables 

 the basal membrane to form a link in the chain of transmitting mechanisms that 

 are interposed between the sound and its perception. 



If the parts played by the cochlea and the saccules together with the 

 ampullae be compared, it may be said that only the fundamental sen- 

 sation, the general perception of hearing from concussion of the auditory 

 nerves, as through blows and noises, is excited by the saccules and 

 ampullae; whereas, on the other hand, the pitch and the depth of the 

 vibrations and their musical character are appreciated by the cochlea. 



According to another view each nerve-cell of the cochlea hears every tone; 

 therefore separate cells are not attuned for different tones. The sharpness of 

 hearing is supposed to result from the sum of the sensitive auditory cells, all of 

 which hear the same thing. According to Held, several hair-cells are connected 

 with one nerve-fiber; hence tones of different pitch can excite one and the same 

 fiber. 



The relations between the semicircular canals and the bodily equilibrium are 

 treated in the consideration of the auditory nerve (p. 699). 



Pathological. In the presence of varying degrees of deafness, loss either 

 of all or of only certain tones in greater or lesser amount has been found. Laby- 

 rinthine affections and those of the auditory nerve both cause disturbances of 

 hearing, but with the following differences: In the presence of affections of the 

 labyrinth tones having from 12 to 64 vibrations are heard poorly with air-con- 

 duction; while, in the presence of so-called torpor of the auditory nerve, such 

 tones are well heard. Bone-conduction is good in both cases for the lowest tones. 

 In the presence of torpor of the nerve high tones are well perceived, but in that 

 of affections of the labyrinth, poorly. The hearing of spoken sounds and bone- 

 conduction are much reduced in both cases. Double hearing is rarely produced 

 by affections both of the middle and of the internal ear. 



SIMULTANEOUS ACTION OF TWO TONES. 



HARMONY. BEAT. DISCORD. DIFFERENTIAL TONES AND SUMMA- 

 TION-TONES. 



If two tones of different pitch are heard at the same time, they pro- 

 duce different sensations in accordance with the difference in pitch. 



If the number of vibrations of the two tones is in the ratio of simple 

 multiples, or as i : 2 : 3 : 4, so that, while the lower tone makes one vibra- 

 tion, the higher one completes 2, or 3, or 4, the ear obtains an impression 

 of complete harmony or concord. 



If the number of vibrations of the tones is not in the ratio of simple 

 multiples interference must result if the two are sounded together. The 

 summits and valleys of one wave can no longer always coincide with the 

 corresponding summits and valleys of the other, but in accordance with the 

 difference between the number of vibrations there must be places where 



