The middle ear together with the Eustachian tube, aduts, antrum and mastoid air cells is called the middle ear cleft. It is lined by mucous membrane and filled with air.

The middle ear extends much beyond the limits of tympanic membrane which forms its lateral boundary and is sometimes divided into

1. mesotympanum (lying opposite the pars tensa)
2. epitympanum or the attic (lying above the pars tensa but medial to Shrapnell’s membrane and the bony lateral attic wall)
3. hypotympanum (lying above the level of pars tensa). The portion of middle ear around the tympanic orifice of the Eustachian tube is sometimes called the protympanum.

Middle ear can be likened to a six-sided box with a roof, a floor, medial, lateral, anterior and posterior walls.

The roof is formed by a thin plate of bone called tegmen tympani. It also extends posteriorly to form the roof of the aditus and antrum. It separates tympanic cavity from the middle cranial fossa.

The floor is also a thin plate of bone which separates tympanic cavity from the jugular bulb. Sometimes it is congenitally deficient and the jugular bulb may then project into the middle ear, separated from the cavity by the mucosa only.

The anterior wall has a thin plate of bone which separates the cavity from internal carotid artery. Ti also has two openings; the lower one for the Eustachian tube and the upper one for the canal of tensor tympani muscle.

The posterior wall lies close to the mastoid air cells. It presents a bony projection called pyramid through the summit of which appears the tendon of the stapedius muscle to get attachment to the neck of stapes. Aditus, an opening through which attic communicates with the antrum, lies above the pyramid. Facial nerve runs in the posterior wall just behind the pyramid.

Facial recess or the posterior sinus is a depression. In the posterior wall lateral to the pyramid. It is bounded medially by the vertical part of VIIth nerve, laterally by the chorda tympani and above by the fossa incudis. Surgically facial recess is important as direct access can be made through this into the middle ear without disturbing posterior meatal wall (intact canal wall technique).

The medial wall is formed by the labyrinth. It presents a bulge called promontory which is due to the basal coil of cochlea; oval window into which is fixed the footplate of stapes; round window or the fenestra cochleae which is covered by the secondary tympanic membrane.

Above the oval window is the canal for facial nerve. Its bony covering may sometimes be congenitally dehiscent and the nerve may lie exposed making it very vulnerable to injuries or infection. Above the canal for facial nerve is the prominence of lateral semicircular canal. Just anterior to the oval window, the medial wall presents a hook-like projection called the processus cochlear informis.

The tendon of tensor tympani takes a turn here to get attachment to the neck of malleus. The cochleariform process also marks the level of the genu of the facial nerve. Medial to the pyramid is a deep recess called sinus tympani which is bounded by the subiculum below and the ponticulus above.

The lateral wall is formed largely by the tympanic membrane and to a lesser extent by the bony outer attic wall called the scutum. The tympanic membrane is semitransparent and forms a “window” into the middle ear.

Mastoid Antrum

It is a large, air-containing space in the upper part of mastoid and communicates with the attic through the aditus. Its roof is formed by the tegmen antri which separates it from the middle cranial fossa. The lateral wall of antrum is formed by a plate of bone which is on an average 1.5 cm thick in the adult. It is marked externally, on the surface of mastoid by suprameatal (MacEwen’s) triangle.

Aditus and Antrum

Aditus is an opening through which the attic communicates with the antrum. The bony prominence of ht ehorizontal canal lies on its medial side while the fossa incudis, to which is attached the short process of incus, lies laterally. Facial nerve courses just below the aditus.