CHAP, iv.] HEARING. 1013 



that the fibres are directly stimulated by the vibrations passing 

 through the bone, in canals of which the fibres lie. Such a con- 

 clusion presents great difficulties ; we shall have to refer to it 

 again later on. 



629. Leaving this view for the present on one side, and 

 assuming that the waves of sound are converted into auditory 

 impulses by means of the hair-cells, we may now turn to another 

 question, also one of great difficulty. How do the different 

 vibrations which determine the nature of different sounds so 

 differently affect the hair-cells as to give rise to sensations of 

 corresponding difference? A complex sound, consisting of vi- 

 brations of more than one period, travels as we have said, not as 

 a group of discrete waves, each corresponding to a vibration of 

 a particular period, but as a complex wave in which the simple 

 waves are compounded into one ; and the vibrations of the tym- 

 panic membrane, followed by the vibrations of the perilymph, 

 have the same composite character. When for instance a note 

 is sung, or sounded on a musical instrument, the air in the ex- 

 ternal auditory passage is not the subject of one set of waves 

 corresponding to the fundamental tone, and of other sets corre- 

 sponding to the several partial tones, but vibrates in the pattern 

 of one composite wave ; the tympanic membrane executes one 

 complex vibration, and a corresponding single complex vibra- 

 tion excites the auditory epithelium. And this holds good not 

 for a single sound only but for a mixture of sounds. We can 

 in a clumsy way take a graphic record of the vibrations of a 

 dead tympanic membrane, by attaching a marker to the stapes ; 

 could we take an adequate record of the movements of the 

 living tympanum of one of the audience at a concert, we 

 should obtain a curve, a phonogram, which though a single 

 curve only would be on the one hand a record of the multi- 

 tudinous vibrations of the concert, and on the other hand a 

 picture of the actual blows with which the perilymph had 

 struck the auditory epithelium. 



Now, whatever be the exact nature of the process by which 

 the vibrations of the perilymph give rise to auditory impulses, 

 we may consider it as probable that, in giving rise to those 

 impulses, the complex vibration is analyzed again into its con- 

 stituent simple vibrations, that the vibrations start afresh so to 

 speak in the auditory epithelium, marshalled in the same array 

 as that in which they started from the sounding instruments, 

 as if the auditory epithelium itself constituted the band playing 

 the music. And indeed that something of this kind does take 

 place is indicated by the fact that an adequately sensitive ear 

 can in a musical sound detect one or more of the partial tones 

 as distinct from the fundamental tone, or still more easily can 

 in a mixed concert detect the several notes of the several instru- 

 ments, though as we have just said in the movements of the 



