The middle ear is an air-filled space within the temporal bone which lies between the tympanic membrane laterally and the inner ear medially. It contains 3 ossicles which connect the tympanic membrane to the inner ear at the oval window. The ossicles transmit sound vibrations from the external auditory meatus into the inner ear apparatus. It is divided into two parts:
- Tympanic cavity (adjacent to the tympanic membrane)
- Epitympanic recess/attic (space superior to the tympanic cavity)
- Roof: separates the middle ear from the middle cranial fossa. Thin layer of bone – tegmen tympani.
- Floor: thin layer of bone separating from the internal jugular vein. Contains a small aperture which transmits the tympanic branch from the glossopharyngeal nerve
- Lateral wall: formed mainly by the tympanic membrane. Superior aspect formed by bony wall of the epitympanic recess
- Posterior wall: inferiorly – bone separating middle ear cleft from mastoid air cells; superiorly – aditus to mastoid antrum (continuous with the epitympanic recess). Pyramidal eminence – stapedius tendon enters middle ear. Opening for chorda tympani, a branch of facial nerve (VII).
- Anterior wall: inferiorly – thin layer of bone separating middle ear from the carotid canal, superiorly – opening of eustachian tube (pharyngotympanic tube), opening for tensor tympani muscle (contained with a canal).
- Medial wall: Formed by the lateral aspect of the inner ear. Promontory – convex protrusion formed by the bone overlying the basal turn of the cochlea; covered by the tympanic plexus (branches from glossopharyngeal nerve (IX) and branches of the internal carotid plexus.
- 2 openings: Oval window – posterosuperior to cochlear promontory; attachment for stapes footplate (final point of attachment for ossicular chain). Round window – posteroinferior to cochlear promontory
- Facial canal prominence: bony ridge formed by facial nerve canal
- Lateral semicircular canal prominence: sticks out laterally above the facial nerve canal
There are 3 ossicles which bridge the middle ear cleft, attaching laterally to the tympanic membrane and medially to the oval window of the inner ear, the malleus, the incus and the stapes:
- Largest ossicle
- Handle of malleus (manubrium) – attaches to tympanic membrane
- Head – rounded part, lies in epitympanic recess superiorly, articulates with the incus via its posterior surface.
- Anterior process – attaches to anterior wall
- Lateral process – attaches to anterior/posterior malleolar folds of tympanic membrane
- Second bone in ossicular chain
- Body – articulates with head of malleus at the incudomalleolar joint
- Short process – extends posteriorly, attaches to the posterosuperior wall of the middle ear
- Long process – extends inferior, parallel to the handle of the malleus. Distally it curves medially to articulate with the stapes at the incudostapedial joint.
- Smallest of the ossicles (and smallest bone in the human body)
- Head – concave, articulates with the long process of the incus
- Neck – beneath the head
- Anterior and Posterior limbs/processes – connect the head/neck to the footplate
- Base/footplate – attaches to the oval window on the medial wall of the middle ear.
There are two muscles associated with the ossicular chain, the tensor tympani and the stapedius. These muscles contract in response to loud noises, reducing the amount that the ossicles vibrate and subsequently reducing transmission of sound to the inner ear structures.
- Lies within a bony canal parallel to the eustachian tube
- Origin – cartilaginous part of eustachian tube, greater wing of sphenoid, and bony canal
- Insertion – handle of malleus
- Action – pulls handle of malleus medially to tense the tympanic membrane; reduces the force of vibrations.
- Innervation – Branch of the mandibular nerve (V3)
- Origin – inside the pyramidal eminence (small protrusion on the posterior wall of the middle ear)
- Insertion – Neck of stapes; tendon emerges from the apex of the pyramidal eminence.
- Action – Pulls the stapes posteriorly, preventing excessive vibration in response to loud noise.
- Innervation – Branch of the facial nerve (VII)
The mastoid area forms the posterior component of the temporal bone.
- Mastoid antrum: an irregularity cavity contained within the anterosuperior aspect of the mastoid area
- Aditus to mastoid antrum (aditus ad antrum): opening to the mastoid antrum, communicates anteriorly with the epitympanic recess
- Mastoid air cells: collections of air-filled spaces continuous with the mastoid antrum, extending into the mastoid process
- Tegmen mastoideum: the roof of the mastoid, which forms the posterior extension of the tegmen tympani. Separates the mastoid area from the middle cranial fossa.
- Sigmoid plate: thin lamina of bone separating mastoid air cells from the sigmoid sinus.
Eustachian Tube (Pharyngotympanic Tube)
The pharyngotympanic tube connects the middle ear with the nasopharynx, equalising pressure between the middle ear and external auditory meatus.
- Superior opening: anterior wall of the middle ear
- Inferior opening: nasopharynx
- Superior third – bony; inferior two thirds – cartilaginous