irregular cavity, having a membrane, called the membrana tympani,
stretched across its extremity; and this cavity has a communication with
the external air, through the Eustachian tube, which leads into the fauces,
or throat. The membrane of the tympanum is intended to carry the
vibrations of the atmosphere, collected by the outward ear, to the chain
of bones which form the peculiar mechanism of the tympanum. Besides the
effect of the hard and bony parts of the ear in increasing the power of
sound, the tension of the different membranes is also a requisite: thus
various muscles are so situated as to put the membrane on the stretch,
that the sound, striking upon it, may, from its tension, similar to that
of the parchment of a drum-head, have full influence upon the sense. In
respect to its tension, the membrane of the tympanum may be also compared,
not unaptly, to the string of a violin, or musical instrument, even more
properly than to a drum; as the state of tension and relaxation in such
chords produces a variety of sound in the instrument, so, in the same
manner, circumstances, which affect the tension and relaxation of the
tympanum, vary most perceptibly its powers of action, and the customary
agency of the organ. Its four bones act mechanically, in consequence of
the power of the local muscles: they strike like the key of an instrument,
and produce a percussion on the nerves of the tympanum. Not only may the
membrane of the tympanum be partially destroyed, and hearing be preserved,
but the small bones of the tympanum have been in certain cases lost, or
have come away, from ulceration, and through a constitutional or other
cause; but in such cases it appears that the stapes was, in most instances,
left, and thus the openings of the fenestra ovata and fenestra rotunda
were preserved, which prevented the escape of sound from the labyrinth and
internal parts. With respect to the Eustachian tube, its aperture into
the throat seems indispensable to hearing; and whenever closed, from
malconfirmation or disease, deafness is the certain consequence.