538 THE SENSE OF HEARING. 



isolated by an atmosphere of air, and not continuous with the 

 bones of the cranium; for every solid body thus isolated by a 

 different medium propagates vibrations with more intensity 

 through its own substance than it communicates them to the 

 surrounding medium, which thus prevents a dispersion of the 

 sound ; just as the vibrations of the air in the tubes used for 

 conducting the voice from one apartment to another are pre- 

 vented from being dispersed by the solid walls of the tube. 

 The vibrations of the membraua tympani are transmitted, 

 therefore, by the chain of ossicula to the fenestra ovalis and 

 fluid of the labyrinth, their dispersion in the tympanum being 

 prevented by the difficulty of the transition of vibrations from 

 solid to gaseous bodies. The mernbrana tympani being a 

 tense, solid body, bounded by free surfaces, the sonorous undu- 

 lations will be partially reflected at its surfaces, so as to cause 

 a meeting of undulations from opposite directions within it; it 

 will, therefore, by resonance, increase the intensity of the vi- 

 brations communicated to it, and the undulations, thus ren- 

 dered more intense, will act, in their turn, upon the chain of 

 auditory bones. 



The necessity of the presence of air on the inner side of the 

 membrana tympani, in order to enable it and the ossicula au- 

 ditus to fulfil the objects just described, is obvious. Without 

 this provision, neither would the vibrations of the membrane 

 be free, nor the chain of bones isolated, so as to propagate the 

 sonorous undulations with concentration of their intensity. 

 But while the oscillations of the membrana tympani are readily 

 communicated to the air in the cavity of the tympanum, those 

 of the solid ossicula will not be conducted away by the air, but 

 will be propagated to the labyrinth without being dispersed in 

 the tympanum. Equally necessary is the communication of 

 the air in the tympanum with the external air, through the 

 medium of the Eustachian tube, for the maintenance of the 

 equilibrium of pressure and temperature between them. 



The propagation of sound through the ossicula of the tym- 

 panum to the labyrinth must be effected either by oscillations 

 of the bones, or by a kind of molecular vibration of their par- 

 ticles, or, most probably, by both these kinds of motion. 1 



1 Edouard Weber has shown that the existence of the membrane 

 over the fenestra rotunda will permit approximation and removal of 

 the stapes to and from the labyrinth. When by the stapes the mem- 

 brane of the fenestra ovalis is pressed towards the labyrinth, the mem- 

 brane of the fenestra rotunda may, by the pressure communicated 

 through the fluid of the labyrinth, be pressed towards the cavity of 

 the tympanum. 



