386 THE SENSORIAL FUNCTIONS. 



directed down the meatus, and striking against the 

 ear-drum which closes the passage, throw that 

 membrane into vibrations of equal frequency. 

 The extent of these vibrations is regulated by the 

 degree of tension given to it by the action of the 

 muscles of the malleus, the handle of which is fixed 

 to the centre of the membrane.* The vibrations 

 of the ear-drum must excite corresponding motions 

 in the air contained in the cavity of the tympanum ; 

 which, again, communicates them to the mem- 

 brane of the fenestra rotunda ; while, on the other 

 hand, the membrane closing the fenestra ovalis re- 

 ceives similar but stronger impressions from the 

 stapes, conveyed through the chain of tympanic 

 ossicula, which serve as solid conductors of the 

 same vibrations-t Thus the perilymph, or fluid 



* Savart ascertained by a series of experiments that the dry 

 membrane tympani was thrown into stronger vibrations by a given 

 sound, when it was in a state of laxity than when tense : and drew 

 the inference, that hearing becomes less acute when the tension of 

 the ear-drum is increased. See Muller's Elements of Physiology, 

 by Baly, p. 1256. 



f Savart has proved by decisive experiments, that vibrations thus 

 freely transmitted by a chain of solid bodies, such as the ossicula, 

 connected at both ends by membranes, and traversing a cavity filled 

 with air, acquire an intensity considerably greater than the vibra- 

 tions communicated by the air alone. Muller has also shown that 

 the transmission of sonorous vibrations from air to water is much 

 facilitated by the intervention of a tense membrane extended be- 

 tween these two media : and still more so if a solid conductor is 

 applied to that membrane, and affixed at its other end to the middle 

 of a tense membrane which has atmospheric air on both of its sides. 

 It is ei\sy to perceive that these are exactly the conditions ob- 

 served in the structure of the tympanic apparatus. The resonance 

 of the air in the several cavities of the external meatus, tympanum 

 and mastoid cells, and of the perilymph in those of the labyrinth 

 will also contribute to modify the vibrations, and generally increase 

 their intensity. 



