FUNCTIONS OF THE MIDDLE EAR. 685 



with the water. This fact elucidates the action of the 

 fenestra ovalis, and of the plate of - the stapes which 

 occupies it, and, with the preceding fact, shows that both 

 fenestrse that closed by membrane only, and that with 

 which the moveable stapes is connected transmit very 

 freely the sonorous vibrations from the air to the fluid of 

 the labyrinth. 



A small, solid body, fixed in an opening by means of a 

 border of membrane, so as to be moveable, communicates 

 sonorous vibrations from air on the one side, to water, or 

 the fluid of the labyrinth, on the other side, much better 

 than solid media not so constructed. But the propagation 

 of sound to the fluid is rendered much more perfect if the 

 solid conductor thus occupying the opening, or fenestra 

 ovalis, is by its other end fixed to the middle of a tense 

 membrane, which has atmospheric air on both sides. 



A tense membrane is a much better conductor of the 

 vibrations of air than any other solid body bounded by 

 definite surfaces ; and the vibrations are also communi- 

 cated very readily by tense membranes to solid bodies in 

 contact with them. Thus, then, the membrana tympani 

 serves for the transmission of sound from the air to the 

 chain of auditory bones. Stretched tightly in its osseous 

 ring, it vibrates with the air in the auditory passage, as 

 any thin tense membrane will when the air near it is 

 thrown into vibrations by the sounding of a tuning-fork 

 or a musical string. And, from such a tense vibrating 

 membrane, the vibrations are communicated with great 

 intensity to solid bodies which touch it at any point. If, 

 for example, one end of a flat piece of wood be applied to 

 the membrane of a drum while the other end is held in 

 the hand, vibrations are felt distinctly when the vibrating 

 tuning-fork is held over the membrane without touching 

 it ; but the wood alone, isolated from the membrane, will 

 only very feebly propagate the vibrations of the air to the 

 hand. 



