V 



THE SENSE OF HEARING 



243 



Tin.- interior of the chamber is thus divided into two portions by the 

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

 -p'-nding 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 </, also covered with a rubber membrane, is made in the 

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

 lYesMire upon the membrane of the I'ene.-tra ovalis causes that of the fenestra 

 rotunda to bulge out, and the little elastic membrane of the aluminium di.-c 

 is necessarily displaced in the same direction. 



'The camera acoustica is lixed hori/ontally by the metal support c and 

 screw h to the pillar a (Fig. !>!)). It is then connected with a simple con- 

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

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

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



Fn;. 9H. EwaM's runiciu aroust ir.i. Explanation in text. 



the iron rod <7, 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 iirst 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 clnmber, 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 

 i- 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, hut 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: 



