THE SENSES. 687 



vibrating as a drum does some time after it is struck, and each sound 

 would be prolonged, leading to considerable confusion. This evil is 

 obviated by the ear-bones, which check the continuance of the vibrations 

 like the " dampers" in a pianoforte. 



The ossicles of the ear are the better conductors of the sonorous vi- 

 brations 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 com- 

 municates them to the surrounding medium, which thus prevents a 

 depression 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 vibra- 

 tions of the membrana tympani are transmitted, therefore, by the chain 

 of ossicula to the fenestra ovalis and fluid of the labyrinth, their disper- 

 sion in the tympanum being prevented by the difficulty of the transition 

 of vibrations from solid to gaseous bodies. 



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

 brana tympani, in order to enable it and the ossicula auditus 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 ossi- 

 cula will not be conducted away by the air, but will be 

 propagated to the labyrinth without being dispersed in 

 the tympanum. 



The propagation of sound through the ossicula tym- 

 pani to the labyrinth, must be affected either by oscil- 

 lations of the bones, or by a kind of molecular vibration 

 of their particles, or, most probably, by both these kinds 

 of motion. 



It has been shown that the existence of the mem- 

 brane over the fenestra rotunda will permit approxima- 

 tion and removal of the stapes to and from the laby- 

 rinth. When by the stapes the membrane of the ffon ?f soSnd 

 fenestra ovalis is pressed toward the labyrinth, the internal ear - 

 membrane of the. fenestra rotunda may, by the pressure communicated 

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

 tympanum. 



The long process of the malleus receives the undulations of the mem- 

 brana tympani (fig. 405, , a) and of the air in a direction indicated by 



