086 HANDBOOK OF PHYSIOLOGY. 



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 movable 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 movable, 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 propa- 

 gation 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 communicated very readily by tense mem- 

 branes 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 ossicles. 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 vibra- 

 tions of the air to the hand. 



In comparing the membrana tympani to the membrane of a drum, 

 however, it is necessary to point out certain important differences. 



When a drum is struck, a certain definite tone is elicited (funda- 

 mental tone) ; similarly a drum is thrown into vibration when certain 

 tones are sounded in its neighborhood, while it is quite unaffected by* 

 others. In other words it can only take up and vibrate in response to 

 those tones whose vibrations nearly correspond in number with those of 

 its own fundamental tone. The tympanic membrane can take up aiW 

 immense range of tones produced by vibrations ranging from 30 to 4000 

 or 5000 per second. This would be clearly impossible if it were an/ 

 evenly stretched membrane. 



The fact is, that the membrana tympani is by no means evenly 

 stretched, and this is due partly to its slightly funnel-like form, and 

 partly to its being connected with the chain of auditory ossicles. Fur- 

 ther, if the membrane were quite free in its centre, it would go on 



