middle ear

Middle ear(auris media) includes the tympanic cavity (about 1 cm ³ ) lined with mucous membrane and filled with air, and the auditory (Eustachian) tube. The middle ear cavity communicates with the mammillary cave and through it with the mammillary cells located in the thickness of the mammillary process.

The tympanic cavity (cavitas tympanica, s. cavum thympani) is located in the thickness of the pyramid of the temporal bone, between the external auditory canal laterally and the bony labyrinth of the inner ear medially. The cavity has 6 walls, and its shape is compared to a tambourine placed on its edge and tilted outward.

1. The superior tegmental wall (paries tegmentalis) is formed by a thin plate of bone substance (the roof of the tympanic cavity, tegmen tympani), separating the tympanic cavity from the cranial cavity.
2. The inferior jugular wall (paries jugularis) corresponds to the inferior wall of the pyramid at the site where the jugular fossa is located.
3. The medial labyrinthine wall (paries labyrinthicus) has a complex structure and separates the tympanic cavity from the bony labyrinth of the inner ear. On this wall there is a promontory (promontorium) protruding towards the tympanic cavity. Above the promontory and somewhat posteriorly is the oval window of the vestibule (fenestra vesitibuli), leading to the vestibule of the bony labyrinth; it is closed by the base of the stapes. Somewhat above the oval window and behind it is a transverse projection of the wall of the facial nerve canal - the protrusion of the facial canal (prominentia canalis facialis). Behind and below the promontory is the cochlear window (fenestra cochleae), closed by the secondary tympanic membrane (membrana tympani secundaria). This membrane separates the tympanic cavity from the scala tympani.
4. The posterior mammillary wall (paries mastoideus) has a pyramidal eminence (eminentia pyramidalis) in its lower part, inside which the stapedius muscle (m. stapedius) begins. In the upper part of the posterior wall, the tympanic cavity continues into the mammillary cave (antrum mastoideum), into which the mammillary cells of the process of the same name also open.
5. The anterior carotid wall (paries caroticus), its lower part, separates the tympanic cavity from the carotid canal, in which the internal carotid artery passes. In the upper part of the wall is the opening of the auditory tube, connecting the tympanic cavity with the nasopharynx.
6. The lateral membranous wall (paries membranaceus) is formed by the tympanic membrane and the surrounding parts of the temporal bone.

The tympanic cavity contains three auditory ossicles covered with a mucous membrane, as well as ligaments and muscles.

The auditory ossicles (ossicula auditus, s. auditoria), miniature in size, are connected to each other, forming a chain that continues from the eardrum to the vestibular window, which opens into the inner ear. In accordance with their shape, the ossicles received the following names: malleus, incus, stapes. The malleus (malleus) has a rounded head (caput mallei), which passes into a long handle of the malleus (manubrium mallei) with two processes: lateral and anterior (processus lateralis et anterior). The incus (incus) consists of a body (corpus incudis) with an articular fossa for articulation with the head of the malleus and two legs: short (crus breve) and long (crus longum) with a thickening at the end. The thickening on the long leg - the lenticular process (processus lenticularis) serves to connect with the head of the stapes. The stirrup (stapes) has a head (caput stapedis), two legs - front and back (crus anterius et posterius), connected by means of the base of the stirrup (basis stapedis).

The malleus is fused with the eardrum along its entire length with its handle so that the end of the handle corresponds to the navel on the outer side of the membrane. The head of the malleus is connected with the body of the incus by means of a joint and forms the incudomallear joint (articulatio incudomallearis). The incus, in turn, is connected with the head of the stapes by a lenticular process, forming the incudomallear joint (articulatio incudostapedia). The joints are strengthened by miniature ligaments.

With the help of a chain of three auditory ossicles, which is mobile in the joints, the vibrations of the eardrum, caused by the impact of a sound wave on it, are transmitted to the vestibular window, in which the base of the stapes is movably fixed with the help of the annular ligament of the stapes (lig. anulare stapedis). Two muscles attached to the auditory ossicles regulate the movements of the ossicles and protect them from excessive vibrations during a loud sound. The muscle that tenses the eardrum (m. tensor tympani) lies in the semi-canal of the muscular-tubular canal of the same name, and its thin and long tendon is attached to the initial part of the handle of the malleus. This muscle, pulling up the handle of the malleus, tenses the eardrum. The stapedius muscle (m. stapedius) begins on the pyramidal eminence and is attached by a thin tendon to the posterior leg of the stapes, near its head. When the stapedius muscle contracts, the pressure of the base of the stapes, inserted into the vestibular window, is weakened.

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