The middle ear is an air-containing cavity in the petrous temporal bone. It contains the auditory ossicles, which function to transmit vibrations of the tympanic membrane (TM) to the perilymph of the internal ear. It communicates with the nasopharynx anteriorly by the eustachian tube and with the mastoid air spaces posteriorly by the aditus ad antrum (Fig.^231-1).

FIG. 231-1. Sagittal section of the middle ear and related structures.

The TM is a thin, pearly gray, fibrous membrane, which, when illuminated, produces a cone-shaped light reflex anteroinferiorly. Superiorly, the pars flaccida is a relatively slack portion of the membrane lying between the malleolar folds; the remainder of the membrane is tense and is called the pars tensa. The auditory ossicles are the malleus, incus, and stapes. Both the incus and the handle and lateral processes of the malleus are typically visible through the TM ( Fig 2.3.1-2.). The relationships of the facial nerve, sigmoid sinus, and internal carotid artery to the tympanic cavity are shown in Fig 2.31.-1...

FIG. 231-2. Right tympanic membrane as seen through the otoscope.

Inner Ear

The inner ear consists of the cochlea, which contains the auditory sensory receptors, and the vestibular labyrinth, which contains balance receptors. Cristae in the semicircular canals detect angular acceleration and macules detect linear acceleration. Afferent nerves from the vestibular labyrinth connect to brain stem nuclei to maintain smooth movement of the eyes during head movement and to the cerebellum to control oculomotor and postural functions. Blood supply is from the vertebrobasilar system. A branch of the anteroinferior cerebellar artery, the internal auditory artery, supplies the vestibular labyrinth. (See Fig..231-3).

FIG. 231-3. Schematic drawing of the bony labyrinth containing the vestibular and auditory sensory organs. The otolithic organs (utricle and saccule) lie in the vestibule. The internal auditory artery divides into the common cochlear artery and the anterior vestibular artery. The anterior vestibular artery provides the blood supply to the anterior and horizontal semicircular canals but not to the cochlea. Isolated occlusion of the anterior vestibular artery may therefore cause acute vestibular syndrome without hearing loss.

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