Figure 243

Horizontal section of a human temporal bone. The relationships of the three divisions of the ear within the temporal bone are shown. The tympanic membrane (TM) separates the external acoustic meatus from the tympanic cavity (TC). Within the tympanic cavity, sections of the malleus (M) and incus (I) can be seen. The posterior wall of the tympanic cavity is associated with the mastoid air cells (AC). The lateral wall of the cavity is formed principally by the tympanic membrane. The opening to the internal ear or oval window (arrowhead) is seen in the medial wall of the cavity (the stapes has been removed). The facial nerve (F) can be observed near the oval window. The cochlea (C), the vestibule (V), and a portion of the lateral semicircular canal (LSC) of the bony labyrinth are identified. The cochlear and vestibular divisions of cranial nerve VIII (N) can also be observed within the internal acoustic meatus. x65.

the middle ear (Fig. 24.4). The layers of the tympanic membrane from outside to inside are

• The skin of the external acoustic meatus

• A core of radially and circularly arranged collagen fibers

• The mucous membrane of the middle ear

One of the auditory ossicles, the malleus, is attached to the tympanic membrane (see Fig. 24.1). Sound in the form of airwaves causes the tympanic membrane to vibrate, and these vibrations are transmitted to the attached auditory ossicles that link the external ear to the internal ear. Perforation of the tympanic membrane may cause transient or permanent hearing impairment.

The auditory ossicles connect the tympanic membrane to the oval window

The three small ossicles, or bones, the malleus, the incus, and the stapes, cross the space of the middle ear in series (Fig. 24.5) and connect the tympanic membrane to the oval window. These bones help to convert sound waves (i.e., vibrations in air) to mechanical (hydraulic) vibrations in tissues and fluid-filled chambers. Movable joints connect the bones, which are named according to their approximate shape:

• Malleus (hammer), attached to the tympanic membrane

• Stapes (stirrup), whose footplate fits into the oval window

• Incus (anvil), linking the malleus to the stapes

Two muscles attach to the ossicles and affect their movement

The tensor tympani muscle lies in a bony canal above the auditory tube; its tendon inserts on the malleus. Contraction of this muscle increases tension on the tympanic membrane. The stapedius muscle lies in a bony eminence on the posterior wall of the middle ear; its tendon inserts on the stapes. Contraction of the stapedius tends to dampen the movement of the stapes at the oval window. The stapedius is only a few millimeters long and is the smallest skeletal muscle.

The two muscles of the middle ear are responsible for a protective reflex called the attenuation reflex. Contraction of the muscles makes the chain of ossicles more rigid, thus reducing the transmission of vibrations to the internal ear. This reflex protects the internal ear from the damaging effects of very loud sound.

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