THE SENSE OF HEAKING 



243 



The interior of the chamber is thus divided into two portions by the 

 partition wall c : the anterior chamber a, and posterior chamber &, corre- 

 sponding with the scala vestibuli and scala tympani. In the wall of the 

 anterior chamber a there is a hole / which corresponds with the fenestra 

 ovalis, and is covered with a rubber membrane. 



A similar hole d, also covered with a rubber membrane, is made in the 

 floor of the posterior chamber, which represents the fenestra rotunda. 

 Pressure upon the membrane of the fenestra ovalis causes that of the fenestra 

 rotunda to bulge out, and the little elastic membrane of the aluminium disc 

 is necessarily displaced in the same direction. 



The camera acoustica is fixed horizontally by the metal support c and 

 screw b to the pillar a (Fig. 99). It is then connected with a simple con- 

 trivance for the transmission of sound-waves, which is attached to the 

 pillar/, and consists of a receiving funnel , closed at the bottom by an elastic 

 membrane (the tympanum) ; this is connected with the fenestra ovalis by 



Fin. 99. Ewald's camera acoustica. Explanation in text. 



the iron rod c, which carries two little discs at its two ends (chain of auditory 

 ossicles). 



When a note is whistled or otherwise sounded in front of the receiver, 

 the sound, as in the ear, is first transmitted to the tympanum, then by means 

 of the rod (the ossicles) to the fenestra ovalis. The vibrations then pass 

 through the water of the chamber, and throw the elastic membrane into 

 vibration. But here, too, as in the ear, it is possible for sound to be trans- 

 mitted without the interposition of the rod. If the transmitting apparatus 

 is removed, and the sound-waves are produced at a short distance from the 

 fenestra ovalis, the stationary waves will equally be visible upon the 

 membrane. And if a vibrating tuning-fork is placed directly upon one of. 

 the walls of the chamber, an acoustic image is formed (bone conduction). 



Ewald's theory, unlike all other theories of audition (including 

 that of Helmholtz), is based, not on purely hypothetical con- 

 siderations, but on a physical fact which can be experimentally 

 established by a model which adequately reproduces the funda- 

 mental conditions under which the auditory apparatus acts. He 

 formulates the fundamental principle of his theory as follows : 



