Anatomy of middle ear

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Anatomy of the Middle Ear

The middle ear is an air-filled, mucous membrane-lined space within the temporal bone, situated between the tympanic membrane laterally and the lateral wall of the internal ear medially. Its primary function is to transmit and amplify sound vibrations from the tympanic membrane to the oval window of the inner ear.

(Gray's Anatomy for Students, p. 1098)

Parts

The middle ear consists of two main parts:

Tympanic cavity proper - the space immediately adjacent to the tympanic membrane
Epitympanic recess (attic) - the superior extension above the level of the tympanic membrane, which houses the heads of the malleus and incus

Parts of the Middle Ear - Gray's Anatomy

Communications

Posteriorly - via the aditus to the mastoid antrum, which communicates with the mastoid air cells
Anteriorly - via the pharyngotympanic (Eustachian) tube to the nasopharynx

Walls / Boundaries

The middle ear has six walls:

Detailed view of the middle ear walls and contents

Wall	Also Called	Key Contents / Relations
Roof	Tegmental wall	Tegmen tympani (thin bone) separates the middle ear from the middle cranial fossa
Floor	Jugular wall	Thin bone over the internal jugular vein; tympanic branch of CN IX enters here
Lateral	Membranous wall	Largely formed by the tympanic membrane; upper part is bony (lateral wall of epitympanic recess)
Medial	Labyrinthine wall	Lateral wall of the inner ear; features the promontory, oval window, round window, prominence of facial canal, prominence of lateral semicircular canal
Anterior	Carotid wall	Separates tympanic cavity from internal carotid artery; has openings for the pharyngotympanic tube and tensor tympani canal
Posterior	Mastoid wall	Lower part separates tympanic cavity from mastoid air cells; superiorly opens into aditus ad antrum; contains pyramidal eminence (for stapedius tendon) and opening for chorda tympani

(Gray's Anatomy for Students, p. 1098-1099)

Tympanic Membrane

The tympanic membrane separates the external from the middle ear and has four layers (from outside in):

Squamous epithelium (outer)
Radiating fibrous layer
Circular fibrous layer
Mucosal layer (inner)

Average total area: 70-80 mm²
Average vibrating surface: 55 mm²
Divided into the pars tensa (the larger, taut portion) and the pars flaccida (Shrapnell's membrane) (the looser superior portion above the lateral process of the malleus)

(K.J. Lee's Essential Otolaryngology, p. 270)

Auditory Ossicles

Three tiny bones form a chain from the tympanic membrane to the oval window, transmitting sound vibrations:

Malleus (largest ossicle)

Head - in the epitympanic recess; articulates posteriorly with the incus
Neck - constriction below the head
Anterior process - attached by a ligament to the anterior wall
Lateral process - attached to the malleolar folds of the tympanic membrane
Handle (manubrium) - embedded in the tympanic membrane
Receives tensor tympani tendon on the medial surface of its upper manubrium

Incus

Body - in the epitympanic recess; articulates with the head of the malleus
Long limb (process) - runs downward parallel to the manubrium of the malleus; ends in the lenticular process, which articulates with the stapes head
Short limb (process) - runs posteriorly and is anchored by a ligament to the posterior incudal fossa
The long process of the incus receives the least blood supply and is most frequently necrosed in chronic ear disease

Stapes (smallest bone in the body)

Head - articulates with the long process of the incus
Anterior and posterior crura - two bony limbs diverging from the head
Footplate (base) - oval in shape (avg 1.41 mm × 2.99 mm); fits into the oval window, attached by the annular ligament

(Gray's Anatomy for Students, p. 1101-1102; K.J. Lee's Essential Otolaryngology, p. 271)

Ossicular Joints

Malleoincudal joint - diarthrodial (synovial)
Incudostapedial joint - diarthrodial (synovial)
Stapediovestibular (stapediolabyrinthine) joint - syndesmotic (via annular ligament)

Muscles of the Middle Ear

Muscle	Origin	Insertion	Innervation	Action
Tensor tympani	Cartilaginous pharyngotympanic tube, greater wing of sphenoid, its bony canal	Medial surface of upper manubrium of malleus (via cochleariform process)	Medial pterygoid nerve (branch of V3)	Pulls handle of malleus medially, tensing the tympanic membrane to dampen vibrations
Stapedius	Inside the pyramidal eminence	Posterior surface of neck of stapes	Branch of facial nerve (CN VII)	Pulls stapes posteriorly, dampening oscillation in response to loud sounds

The contraction of both muscles in response to loud sounds is called the acoustic (stapedial) reflex, which protects the inner ear.

(Gray's Anatomy for Students, p. 1102)

Medial Wall - Key Landmarks

The labyrinthine (medial) wall contains several important structures:

Promontory - rounded bulge formed by the basal coil of the cochlea; covered by the tympanic plexus of nerves
Oval window (fenestra vestibuli) - posterosuperior to the promontory; accepts the stapes footplate; transmits vibrations to the perilymph of the scala vestibuli
Round window (fenestra cochleae) - posteroinferior to the promontory; faces posteroinferiorly and is protected by an anterior lip; closed by the secondary tympanic membrane; allows pressure release
Prominence of the facial canal - indicates the path of CN VII above the oval window
Prominence of the lateral semicircular canal - above the facial canal prominence

(Gray's Anatomy for Students, p. 1099)

Nerves

Tympanic Plexus

Formed on the surface of the promontory. Contributions:

Tympanic branch of CN IX (Jacobson's nerve) - major contributor; carries parasympathetics that continue as the lesser petrosal nerve to the parotid gland via the otic ganglion
Auriculotemporal nerve (V3)
Auricular branch of CN X
Branches from the internal carotid sympathetic plexus

The plexus supplies the mucous membrane of the middle ear, mastoid cells, and pharyngotympanic tube.

Chorda Tympani (CN VII branch)

Enters the middle ear through the posterior wall (via a canal near the pyramidal eminence)
Crosses the middle ear between the handle of the malleus and the long process of the incus
Exits through the anterior wall via the petrotympanic fissure (canal of Huguier)
Carries taste from the anterior 2/3 of tongue and preganglionic parasympathetics to the submandibular and sublingual glands

(K.J. Lee's Essential Otolaryngology, p. 268; Gray's Anatomy for Students, p. 1100-1101)

Ligaments of the Ossicles

Malleus:

Superior malleal ligament (head to tegmen/roof of epitympanum)
Anterior malleal ligament (neck/anterior process to sphenoid via petrotympanic fissure)
Lateral malleal ligament (neck to tympanic notch)

Incus:

Superior incudal ligament (body to tegmen)
Posterior incudal ligament (short process to incudal fossa)

Stapes:

Annular ligament (footplate to margins of oval window)

Clinically Important Spaces

Prussak's space - the space between Shrapnell's membrane (laterally) and the neck of the malleus (medially), bounded superiorly by the lateral malleal fold and inferiorly by the lateral process of the malleus. This is the most common site for acquired cholesteatoma formation.
Aditus ad antrum - the channel connecting the epitympanic recess to the mastoid antrum
Pouches of von Troeltsch - anterior and posterior recesses between the malleal folds and the tympanic membrane

Blood Supply

Source	Supplies
Anterior tympanic artery (from maxillary a.)	Tympanic membrane
Posterior tympanic artery (from stylomastoid a.)	Middle ear posterior
Superior tympanic artery (from middle meningeal a.)	Tensor tympani, tegmen area
Inferior tympanic artery (from ascending pharyngeal a.)	Promontory
Internal carotid artery branches	Anterior middle ear

Venous drainage - to the superior petrosal sinus and the pterygoid plexus.

Note: The long process of the incus has the most precarious blood supply and is most prone to ischemic necrosis.

(K.J. Lee's Essential Otolaryngology, p. 269-270)

Sources:

Gray's Anatomy for Students, pp. 1098-1103
K.J. Lee's Essential Otolaryngology, pp. 268-272
Cummings Otolaryngology Head and Neck Surgery

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