THE SENSES. 577 



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 musi- 

 cal 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, vibra- 

 tions 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. 



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 (fundamen- 

 tal 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 4,000 or 5,000 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. Fur- 

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

 ing 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 ossicula 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 vi- 

 brations with more intensity through its own substance than it commu- 

 nicates them to the surrounding medium, which thus prevents a disper- 

 sion 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 tympani 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 vibra- 

 tions from solid to gaseous bodies. 



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 ob- 

 37 



