126 Audition
than that of air (a 32-dB change). The middle ear compensates
for this impedance difference via the lever action of the ossic-
ular chain (a chain of three bones—malleus, incus, stapes—
connecting the tympanic membrane to the inner ear) in com-
bination with the pressure increase between the large area
of the malleus’s connection to the tympanic membrane and
the small area of the footplate of the stapes’ connection to the
oval window of the inner ear. Over a significant portion of
the audible frequency range, the middle ear in combination
with the resonances of the outer ear canal delivers the sound
to the inner ear with no pressure loss due to the high imped-
ance of the inner ear structures. The eustachian tube connects
the middle ear to the nasal cavities so that pressure on each
side of the tympanic membrane remains the same, a necessary
condition for efficient middle- and inner-ear functioning.
Inner Ear
The inner ear contains the hearing organs and those of the
vestibular (balance-equilibrium) system (Fay & Popper, 1992;
Webster, Fay, & Popper, 1992). The anatomy of the inner
ear differs significantly across the animal kingdom. In mam-
mals, the cochlea, a snail-like tube that spirals on itself three
to four times, is the hearing organ of the inner ear (see Fig-
ure 5.3). The cochlea contains an inner tube, the cochlear par-
tition, which contains supporting structures and the hair cells,
the biological transducers for hearing. The cochlea is thus
divided into three canals or scala: scala vestibuli (above the
cochlear partition), scala media (the cochlear partition), and
scala tympani (below the cochlear partition). Scala vestibuli
and scala tympani contain a viscous fluid, perilymph, whereas
scala media contains a different fluid, endolymph. In a cross-
section (see Figure 5.4), the cochlear partition is bounded
above by Resiner’s membrane and below by the basilar mem-
brane. The metabolic engine for the cochlea resides within
stria vascularis on the outer wall of the cochlea. Fibers from
the auditory part of the VIIIth cranial nerve innervate the hair
cells along the basilar membrane and course through the mid-
dle (modiolus) of the cochlea before picking up myelination
on the way to the auditory brain stem. There are two types of
hair cells (see Figure 5.5): outer hair cells, which in mammals
are arranged in three rows toward the outside of the cochlearFigure 5.4 Main components of the inner ear in relation to the other structures of the ear. (From Yost, 2000, adapted from drawings by Dorland, 1965,
with permission.) Schematic diagram of middle ear and partially uncoiled cochlea, showing the relationship of the various scalae. Source:From Yost
(2000), adapted from similar drawings from Zemlin (1981), with permission.