serves to equalize the air pressures on the two sides of the tympanic membrane. If
these two pressures become moderately unequal, there is greater tension upon the
tympanic membrane; this reduces (dampens) mechanical oscillations of the membrane.
Extreme pressure differences cause severe pain. The passage of the auditory tube into
the nasopharynx opens when one swallows; therefore, the pressure differences are
controlled somewhat by the swallowing reflex.
(2) Associated spaces. The middle ear cavity extends into the mastoid
bone as the mastoid air cells. The relatively thin roof of the middle ear cavity separates
the middle ear cavity from the middle cranial fossa.
c. Auditory Ossicles. There is a series of three small bones, the auditory
ossicles, which traverse the space of the middle ear cavity from the external ear to the
internal ear. The auditory ossicles function as a unit.
(1) The first ossicle, the malleus, has a long arm embedded in the
tympanic membrane. Therefore, when the tympanic membrane is set into mechanical
oscillation, the malleus is also set into mechanical oscillation.
(2) The second ossicle is the incus. Its relationship to the malleus
produces a leverage system which amplifies the mechanical oscillations received
through the malleus.
(3) The third ossicle, the stapes, articulates with the end of the arm of the
incus. The foot plate of the stapes fills the oval (vestibular) window.
d. Auditory Muscles. The auditory muscles are a pair of muscles associated
with the auditory ossicles. They are named the tensor tympani muscle and the
stapedius muscle. The auditory muscles help to control the intensity of the mechanical
oscillations within the ossicles.
13-13. THE INTERNAL EAR
a. Transmission of the Sound Stimulus. The foot plate of the stapes fills
the oval (vestibular) window, which opens to the vestibule of the internal ear (Figure
13-7A). As the ossicles oscillate mechanically, the stapes acts like a plunger against
the oval window. The vestibule is filled with a fluid, the perilymph. These mechanical,
plunger-like actions of the stapes impart pressure pulses to the perilymph.