The tympanic cavity (cavum tympani) is the oblique space between the petrosal and tympanic parts of the temporal bone. It contains the three auditory ossicles that transmit vibrations of the tympanic membrane to the perilymphatic space of the vestibule of the inner ear and numerous nerves cross through the tympanic cavity. The epitympanic recess is dorsal to a dorsal plane through the osseous external acoustic meatus. It is a small portion of the tympanic cavity occupied almost entirely by the head of the malleus and the incus at their articulation.
The tympanic cavity adjacent to the tympanic membrane is irregularly quadrangular in shape, being flattened laterally by the tympanic membrane that forms its lateral wall. In the caudal portion, but facing rostrally, is the secondary tympanic membrane closing the cochlear window (fenestra cochleae), formerly round window. Just within the cochlear window is the cochlear canaliculus.
The ventral portion of the cavity within the tympanic bulla, may be compared in shape to the interior of an eggshell, having an elliptical opening on the side that faces dorsally. It communicates with the tympanic cavity proper through this opening. The long axis of the tympanic cavity is approximately 15 mm in length and at an angle of approximately 45 degrees with the sagittal plane in a caudolateral direction. The width and depth are approximately equal, measuring 8 to 10 mm. The tympanic membrane is slanted ventromedially.
On the medial wall of the tympanic cavity is a bony eminence, the promontory, that houses the cochlea; it lies opposite the tympanic membrane medial to the epitympanic recess. The vestibular window (fenestra vestibuli), formerly oval window, is occupied by the base of the stapes. It is located on the dorsolateral surface of the promontory just medial to the pars flaccida. The ostium of the auditory tube (ostium tympanicum tubae auditivae) is the rostral extremity of the tympanic cavity proper. The tendon of the m. tensor tympani descends ventrolaterally through an arch in a thin lamina of bone that overlies the muscle. It inserts on the muscular process of the malleus. The ossicles form a short chain across the dorsal part of the tympanic cavity.
Nerves that are found coursing through the tympanic cavity include the chorda tympani, from the facial nerve that passes through the tympanic cavity medial to the malleus to join the lingual nerve. The tympanic plexus arising from the tympanic nerve (n. tympanicus) of the glossopharyngeal nerve lies on the promontory and supplies the tympanic mucosa and the minor petrosal nerve to the otic ganglion. The caroticotympanic nerves from the internal carotid plexus also contribute to this tympanic plexus.
The auditory tube (tuba auditiva), formerly Eustachian tube, is a short canal that extends from the nasopharynx to the rostral portion of the tympanic cavity. Its short bony wall is formed rostrally by the squamous part of the temporal bone, and ventrally its floor is formed by the tympanic part of the temporal bone. The lateral wall, which is approximately 8 mm long, is nearly twice the length of the medial wall. The tube is oval in cross-section, with its greater diameter 1.5 mm. The medial wall of the membranous part of the tube is supported by a plate of hyaline cartilage, the rostral end of which curves medially to form a short hook.
The m. tensor veli palatini arises in the groove of the petrous part of the temporal bone ventrolateral to the m. tensor tympani. It supports the lateral wall of the auditory tube. The branch of the fifth cranial nerve that supplies the m. tensor tympani enters the tympanic cavity in association with the tendon of origin of the m. tensor veli palatini.