The next division is the intermediate ear; it consists of the tympanum,
mastoid cells, and Eustachian tube. The tympanum contains four small
delicate bones, viz. the malleus, the incus, the stapes, and the os
orbiculare, joined to the incus. The intermediate ear displays an
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.
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