Anatomy of the Middle Ear

ANATOMY OF THE MIDDLE EAR

The middle ear, an air-filled cavity, includes the tympanic mem-brane laterally and the otic capsule medially. The middle ear cleft lies between the two. The middle ear is connected by the eusta-chian tube to the nasopharynx and is continuous with air-filled cells in the adjacent mastoid portion of the temporal bone.

The eustachian tube, which is approximately 1 mm wide and 35 mm long, connects the middle ear to the nasopharynx. Nor-mally, the eustachian tube is closed, but it opens by action of the tensor veli palatini muscle when performing a Valsalva ma-neuver or when yawning or swallowing. The tube serves as a drainage channel for normal and abnormal secretions of the middle ear and equalizes pressure in the middle ear with that of the atmosphere.

Tympanic Membrane

The tympanic membrane (ie, eardrum), about 1 cm in diame-ter and very thin, is normally pearly gray and translucent. The tympanic membrane consists of three layers of tissue: an outer layer, continuous with the skin of the ear canal; a fibrous mid-dle layer; and an inner mucosal layer, continuous with the lining of the middle ear cavity. Approximately 80% of the tympanic membrane is composed of all three layers and is called the parstensa.

The other 20% of the tympanic membrane lacks the middle layer and is called the pars flaccida. The absence of this fibrous middle layer makes the pars flaccida more vulnerable to pathologic disorders than the pars tensa. Distinguishing land-marks of the tympanic membrane include the annulus, the fi-brous border that attaches the eardrum to the temporal bone; the short process of the malleus; the long process of the mal-leus; the umbo of the malleus, which attaches to the tympanic membrane in the center; the pars flaccida; and the pars tensa (Fig. 59-2).

The tympanic membrane protects the middle ear and con-ducts sound vibrations from the external canal to the ossicles. The sound pressure is magnified 22 times as a result of transmission from a larger area to a smaller one.

Ossicles

The middle ear contains the three smallest bones (ie, ossicles) of the body: malleus, incus, and stapes. The ossicles, which are held in place by joints, muscles, and ligaments, assist in the transmis-sion of sound. Two small fenestrae (ie, oval and round windows), located in the medial wall of the middle ear, separate the middle ear from the inner ear. The footplate of the stapes sits in the oval window, secured by a fibrous annulus, or ring-shaped structure. The footplate transmits sound to the inner ear. The round win-dow, covered by a thin membrane, provides an exit for sound vibrations (see Fig. 59-1).