686 THE SENSE OF HEARING. 



The ossicula of the ear, which are represented in this 

 experiment by a piece of wood, are the better conductors 

 of the sonorous vibrations communicated to them, on 

 account of being 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 prevented from 

 being dispersed by the solid walls of the tube. The 

 vibrations of the membrana tyrapani 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 membrana 

 tympani being a tense, solid body, bounded by free sur- 

 faces, the sonorous undulations 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 vibrations commu- 

 nicated to it, and the undulations thus rendered 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 auditus to fulfil the objects, just described, is 

 obvious. Without this provision, neither would the vibra- 

 tions 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 oscilla- 

 tions 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 



