HEARING. THE INTERNAL EAR. 195 



pose that apparatus so different in form, and so distinct 

 in the different parts of the organ, should have a special 

 object, and that they combine their functions to produce the 

 complex sensation of hearing. Miiller has demonstrated that 

 the same aerial vibrations act with much more intensity on 

 the fluid of the labyrinth, after having traversed the chain of 

 small bones and the fenestra ovalis, than they do after 

 traversing the air in the cavity of the tympanum, and the 

 membrane of the fenestra rotunda ; he thinks that the waves 

 of the same sound transmitted through the two fenestrse 

 differ not only in intensity, but also in their timbre up to a 

 certain point, since those reaching the fenestra rotunda are 

 aerial vibrations, and those reaching the fenestra ovalis, by 

 the chain of small bones, are in the state of vibrations of 

 solid bodies. But the cochlea also receives sonorous waves 

 of both kinds by the scala tympani and the scala vestibuli; 

 and farther, the cavities which form the labyrinth communi- 

 cate with them, all being filled with a common fluid, and 

 all united by their walls ; they would seem therefore to be 

 bound together up to a certain point as regards auditory im- 

 pressions, and nothing demonstrates that vibrations are elec- 

 tively directed in their movement on leaving the vestibule, 

 either toward the cochlea or the semicircular canals. 



It must be admitted notwithstanding, that authors gener- 

 ally agree in placing the principal, and indeed only seat of 

 auditory impressions in the cochlea, and this is the doctrine 

 now professed by M. Helmholtz, to whom we owe our know- 

 ledge of the origin and mechanism of the timbre of sounds. 

 We will briefly state his theory of hearing. 



We have already seen that the terminal filaments of the 

 acoustic nerve spread themselves regularly side by side over 

 the lamina spiralis of the cochlea, like the cords of a key- 

 board; the eminent professor of Heidelberg compares these 

 nervous filaments to the strings of a piano, and explains their 

 functions in the following manner. If the piano be opened, 

 and a person sings loudly above the strings any note what- 

 ever, the sonorous waves cause the strings which respond 

 to the harmonics of the voice, to vibrate also; each one of 

 these strings vibrates exclusively in unison with one har- 



