Middle ear

The middle ear (or tympanic cavity) is an air-filled chamber in the petrous part of the temporal bone that. It is split into two parts:separated from the tympanic cavity proper (the space directly internal toexternal ear by the tympanic membrane), and the epitympanic recess or attic (the space superior to the membrane). It sits between the external ear (divided from each other by the tympanic membrane) and the inner ear (divided by by the medial wall of the tympanic cavity). It contains the three auditory ossicles whose whose purpose is to transmit sound vibrations from the tympanic membrane to the lateral wall of the inner ear.

Gross anatomy

The middle ear is shaped like a narrow box with concave sides. It has six walls:

1. The tegmental wall (roof): Formed by the tegmen tympani, a thin plate of petrous bone that also roofs in the canal for the tensor tympani and the tympanic antrum. It separates the dura mater on the floor of the middle cranial fossa from the tympanic cavity.

2. The jugular wall (floor): Formed by a thin layer of bone that separates the tympanic cavity from the superior bulb of the internal jugular vein.

3. The membranous (lateral) wall: This is formed entirely by the peaked convexity of the tympanic membrane and superiorly by the lateral bony wall of the epitympanic recess.

4. The labyrinthine (medial) wall: This separates the tympanic cavity from the inner ear. The initial part (basal turn) of the cochlea forms the promontory of the labyrinthine wall. It also features the oval and round windows, which communicate with the inner ear.

5. The mastoid (posterior) wall: This features the aditus (opening) to the mastoid antrum in its superior part, which connects the tympanic cavity to the mastoid cells. The canal for the facial nerve descends between the posterior wall and the antrum, medial to the aditus. Above this canal is the convex bulge of the lateral semicircular canal. Below the aditus there is a projection of a hollow cone, called the pyramid, which is perforated by the tendon of the stapedius.

6. The carotid (anterior) wall: This separates the tympanic membrane from the carotid canal. In its superior aspect there is an opening of the Eustachian tube (also known as the pharyngotympanic tube) and the canal for the tensor tympani.

Contents

Bones

Middle ear ossicles consist of three small bones (the malleus, incus and stapes), which form a mobile chain across the tympanic cavity from the tympanic membrane to the oval window.

Muscles

There are two muscles, one for the malleus and one for the stapes, which act to damp down over-vibration from low-pitched sound waves. These are, respectively:

• tensor tympani
• stapedius

Nerves

The chorda tympani leaves the facial nerve in the facial canal and enters the tympanic cavity through the posterior wall, lateral to the pyramid, lying just underneath the mucous membrane. It runs over the pars flaccida of the tympanic membrane, and the neck of the malleus. It leaves at the anterior margin of the tympanic notch.

Blood supply

• arterial supply
  • anterior tympanic artery from the maxillary artery
  • stylomastoid artery from the posterior auricular or occipital arteries
  • numerous small vessels from the external carotid artery
• venous drainage to the ptyerygoid venous plexus or the superior petrosal sinus

Lymphatic drainage

Lymphatic drainage is to the parotid, retropharyngeal and upper deep cervical nodes.

Innervation

This is by the tympanic branch of the glossopharyngeal nerve (Jacobson nerve), which forms the tympanic plexus by combining with sympathetic fibres from the internal carotid nerve. Branches from the plexus supply sensory and vasomotor fibres to the mucous membrane of the tympanic cavity, as well as to the tympanic membrane and external auditory meatus.

The middle and external ear are also supplied by branches of the trigeminal, facial, glossopharyngeal and vagus nerves which results in referred pain in the ear from other areas supplied by these nerves, e.g. the teeth, posterior part of the tongue, pharynx and larynx.

The tympanic plexus gives off the lesser petrosal nerve, which supplies the parotid gland via the otic ganglion.

Related pathology

• middle ear granulation tissue
• otomastoiditis
• middle ear effusion
• tympanic membrane retraction
• eustachian tube dysfunction
• middle ear tumours
• cholesterol granuloma
• acquired cholesteatoma