The mastoid (posterior) wall of the middle ear is only partially complete. The lower part of this wall consists of a bony partition between the tympanic cavity and mastoid air cells. Superiorly, the epitympanic recess is continuous with the aditus to the mastoid antrum (Figs. 8.120 and 8.121).
Associated with the mastoid wall are: the pyramidal eminence, a small elevation through which the tendon of the stapedius muscle enters the middle ear; and the opening through which the chorda tympani nerve, a branch of the facial nerve [VII], enters the middle ear.
The anterior wall of the middle ear is only partially complete. The lower part consists of a thin layer of bone that separates the tympanic cavity from the internal carotid artery. Superiorly, the wall is deficient because of the presence of: a large opening for the entrance of the pharyngotympanic tube into the middle ear, and a smaller opening for the canal containing the tensor tympani muscle.
The foramen for the exit of the chorda tympani nerve from the middle ear is also associated with this wall (Fig. 8.120).
The labyrinthine (medial) wall of the middle ear is also the lateral wall of the internal ear. A prominent structure on this wall is a rounded bulge (the promontory) produced by the basal coil of the cochlea, which is an internal ear structure involved with hearing (Fig. 8.120).
Associated with the mucous membrane covering the promontory is a plexus of nerves (the tympanic plexus), which consists primarily of contributions from the tympanic branch of the glossopharyngeal nerve [IX] and branches from the internal carotid plexus. It supplies the mucous membrane of the middle ear, the mastoid area, and the pharyngotympanic tube.
Additionally, a branch of the tympanic plexus (the lesser petrosal nerve) leaves the promontory and the middle ear, travels across the anterior surface of the petrous part of the temporal bone, and leaves the middle cranial fossa through the foramen ovale to enter the otic ganglion. Other structures associated with the labyrinthine wall are two openings, the oval and round windows, and two prominent elevations (Fig. 8.120):
The oval window is posterosuperior to the promontory, is the point of attachment for the base of the stapes (footplate), and ends the chain of bones that transfer vibrations initiated by the tympanic membrane to the cochlea of the internal ear.
The round window is posteroinferior to the promontory.
Posterior and superior to the oval window on the medial wall is the prominence of the facial canal, which is a ridge of bone produced by the facial nerve [VII] in its canal as it passes through the temporal bone.
Just above and posterior to the prominence of the facial canal is a broader ridge of bone (prominence of the lateral semicircular canal) produced by the lateral semicircular canal, which is a structure involved in detecting motion.
Posterior to the epitympanic recess of the middle ear is the aditus to the mastoid antrum, which is the opening to the mastoid antrum (Fig. 8.121).
The mastoid antrum is a cavity continuous with collections of air-filled spaces (the mastoid cells), throughout the mastoid part of the temporal bone, including the mastoid process. The mastoid antrum is separated from the middle cranial fossa above by only the thin tegmen tympani.
The mucous membrane lining the mastoid air cells is continuous with the mucous membrane throughout the middle ear. Therefore infections in the middle ear can easily spread into the mastoid area.