188 HAND-BOOK OF PHYSIOLOGY. 



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 vibra- 

 tions are also communicated 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 audi- 

 tory 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 mem- 

 brane without touching it; but the wood alone, isolated from the mem- 

 brane, will only very feebly propagate the vibrations of the air to the 

 hand. 



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

 is necessary to point out certain important differences. 



When a drum is struck, a certain definite tone is elicited (fundamental 

 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 fun- 

 damental tone. The tympanic membrane can take up an 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 tympanic membrane 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. Further, if 

 the membrane were quite free in its centre, it would go on vibrating as a 

 drum does some time after it is struck, and each sound would be pro- 

 longed, leading to considerable confusion. This evil is obviated by the 

 ear-bones, which check the continuance of the vibrations like the "dam- 

 pers" in a pianoforte. 



The ossicula of the ear 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 vibra- 

 tions 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 conduct- 

 ing the voice from one apartment to another are prevented from being 



