smaller than the tympanic membrane, the force per unit area
(i.e., the pressure) is increased 15 to 20 times. Additional
advantage is gained through the lever action of the middle-
ear bones. The amount of energy transmitted to the inner
ear can be lessened by the contraction of two small skeletal
muscles in the middle ear. The
the malleus and tympanic membrane, and its contraction
dampens their movement. The
attaches to the sta-
pes and similarly controls its mobility. These muscles con-
tract reﬂ exively to protect the delicate receptor apparatus of
the inner ear from continuous, loud sounds. They cannot,
however, protect against sudden, intermittent loud sounds,
which is why it is crucial for people to wear ear protection
in environments like a gun ﬁ ring range. These muscles also
contract reﬂ exively when you vocalize to reduce the loudness
of your own voice, and optimize hearing over certain fre-
The entire system described thus far involves the trans-
mission of sound energy into the cochlea, where the recep-
tor cells are located. The cochlea is almost completely divided
lengthwise by a ﬂ uid-ﬁ lled membranous tube, the
which follows the cochlear spiral (see Figure 7–35) and
contains the sensory receptors of the auditory system. On
either side of the cochlear duct are endolymph-ﬁ lled compart-
which is on the side of the cochlear
The human ear. In this and the following two ﬁ gures, violet indicates the outer ear, green the middle ear, and blue the inner ear. The malleus,
incus, and stapes are bones and components of the middle ear compartment. Actually, the auditory tube is closed except during pharynx
movements such as swallowing or yawning.
Stapes at oval window
Middle ear cavity
Relationship between the middle ear bones and the cochlea.
Movement of the stapes against the membrane covering the oval
window sets up pressure waves in the ﬂ uid-ﬁ lled scala vestibuli.
These waves cause vibration of the cochlear duct and the basilar
membrane. Some of the pressure is transmitted around the
helicotrema directly into the scala tympani. The cochlea is shown
uncoiled for clarity.
Redrawn from Kandel and Schwartz.