SENSE OF HEARING. 619 



there it passes to the nerves. Here it is converted into impulses 

 which are transmitted to the brain, where sound is produced. 



It has been supposed that the rods of Corti are so arranged as 

 to vibrate with particular tones, one rod for each tone, but it is 

 doubtful whether such a differentiation can be made out in the 

 auditory apparatus. The rods are not present in the ears of birds, 

 and there is no reason to believe that birds cannot appreciate 

 musical tones. In the basilar membrane there are' fibers enough 

 to respond to all the notes that can be appreciated that is, from 

 33 waves to 38,000 waves in a second. It is more probable that 

 the rods simply act as levers to communicate the vibrations of 

 the fibers of the basilar membrane to the terminal nerve-filaments 

 in the auditory cells. 



Just how one is able to distinguish the differences in the in- 

 tensity (loudness), pitch, and quality of sounds is not understood. 

 The explanation most generally accepted at the present time, as 

 to pitch at least, is that as when a tone is sung over the strings of 

 a piano, certain strings are set in vibration sympathetically, so in 

 the basilar membrane, where, as in the piano, there are fibers of 

 different lengths, these respond to different tones, and that in con- 

 nection with each tone there is a separate filament of the auditory 

 nerve, so that if the note is a high one a certain fiber is set in 

 vibration, and the nerve-filament in communication with it 

 transmits an impulse to certain cells in the brain, which when 

 excited give the impression of a high note, and so with other 

 notes and other nerve-cells. 



The Telephone Theory. The introduction of the telephone and 

 a study of its mechanism have led some writers to question the 

 explanation which is generally accepted of the mechanism of 

 hearing, and to suggest that as the single telephone wire transmits 

 the complex sounds produced by an orchestra to a distance where 

 they are reproduced in all their variety of intensity, pitch, and 

 quality, so " the cochlea does not act on the principle of sympa- 

 thetic vibration, but that the hairs of all its auditory cells vibrate 

 to every tone, just as the drum of the ear does ; that there is no 

 analysis of complex vibration in the cochlea or elsewhere in the 

 peripheral mechanism of the ear ; that the hair-cells transform 

 sound-vibrations into nerve-vibrations similar in frequency and 

 amplitude to the sound-vibrations ; that simple and complex vibra- 

 tions of nerve-molecules arrive in the sensory cells of the brain, 

 and there produce, not sound again, of course, but the sensations 

 of sound, the nature of which depends not upon the stimulation 

 of different sensory cells, but on the frequency, the amplitude, 

 and the form of the vibrations coming into the cells, probably 

 through all the fibers of the auditory nerve." This explanation 

 has been put forth by Prof. William Rutherford under the title of 

 the " Telephone Theory of the Sense of Hearing." 



