Anatomy of Middle Ear - Walls & Parts

Anatomy of the Middle Ear - Parts & Walls

In this post, we'll focus on the Parts and Walls of the Middle Ear cavity.

🌬 What is the Middle Ear Cleft?

The Middle Ear Cleft is a continuous air-filled space within the temporal bone, comprised of:

1. The Tympanic Cavity (Middle Ear Cavity)
2. The Eustachian Tube
3. The Mastoid Air Cell System

The Tympanic Cavity is the irregular space located between the Tympanic Membrane laterally and the Osseous Labyrinth (Inner Ear) medially.

👂 The Middle Ear Cavity

The middle ear cavity is an irregular shape. Due to the convexity of the medial wall (promontory) and the concavity of the lateral wall (tympanic membrane), it is constricted in the center.

• Width:

  • ~2 mm at the center (at the umbo)
  • ~6 mm superiorly (in the attic/epitympanum)
  • ~4 mm inferiorly (in the hypotympanum)
  • Widest in the attic, thinnest at the center.

• Vertical & Anterior-Posterior Length: ~15 mm

• Volume: 1-2 cm³ of air. A minimum volume of 0.5 cm³ is necessary for normal function.

🧭 Relations of the Middle Ear Cavity:

The middle ear cavity has six "walls" with important relations:

• Superior: Middle Cranial Fossa and Temporal Lobe.
• Inferior: Jugular Bulb (posteriorly) and Internal Carotid Artery (anteriorly).
• Lateral: Tympanic Membrane (separating it from the External Auditory Meatus).
• Medial: Inner Ear (Osseous Labyrinth).
• Anterior: Openings related to the Tensor Tympani muscle and Eustachian Tube.
• Posterior: Mastoid Air Cell System (communicates with the Mastoid Antrum via the Aditus ad Antrum).

✨ Parts of the Middle Ear Cavity

The middle ear cavity can be divided into three main portions:

1- Mesotympanum: The part lying opposite the Pars Tensa of the Tympanic Membrane.

• Contents: Stapes, long process of Incus, Oval Window, Round Window.

• Eustachian tube exits from its anterior aspect (Protympanum).

• Posterior wall contains Facial Recess and Sinus Tympani.

What are the boundaries of Facial Recess?

What are the boundaries of Sinus Tympani?

2- Epitympanum (Attic): The portion above the level of the short/lateral process of the Malleus. Lies within the notch of Rivinus (dehiscence in the tympanic bone).

• Lateral Boundary: Pars Flaccida and the Bony Outer Attic Wall.

• Contents: Head of the Malleus, Body of the Incus, associated ligaments and mucosal folds.

What is Cog?

3- Hypotympanum: The lowest portion, lying below the level of the inferior part of the Tympanic Sulcus (or bony ear canal).

• Floor is related to the Internal Jugular Vein and Internal Carotid Artery.

4- Protympanum: The portion of the middle ear around the tympanic orifice of the Eustachian Tube, located in the anterior wall.

🧱 Anatomy of the Middle Ear Walls

The middle ear cavity has six walls:

1. Lateral Wall

• Partly bony, partly membranous.

• Formed by:

  • Bony lateral wall of the Epitympanum superiorly (part of the squamous bone).
  • Tympanic Membrane centrally.
  • Bony lateral wall of the Hypotympanum inferiorly.

What is Scutum? What is it's clinical importance?

What are the boundaries of Prussak's space?

• Openings (Tympanic Canaliculi):

  • Petrotympanic Fissure

  • Canal of Huguier

  • Posterior Canaliculus (for Chorda Tympani)

What is the other name of Petrotympanic Fissure? What structures pass through it?

Which nerve passes through Canal of Huguier?

2. Floor (Inferior Wall)

• Very narrow.

• Thin plate of bone separating the middle ear from the Jugular Bulb (posteriorly) and Internal Carotid Artery (anteriorly).

• Inferior Tympanic Canaliculus: Small canal near the fossula petrosa (between jugular bulb fossa and carotid canal). Houses the inferior ganglion of the glossopharyngeal nerve.

What structures pass through the Inferior Tympanic Canaliculus?

• Clinical Importance:
  • Dehiscence
  • High-riding Jugular Bulb

3. Posterior Wall

• The highest wall (~14 mm), formed by petrous bone. Wider superiorly.

• Separates middle ear from mastoid air cells, except at the Aditus ad Antrum.

• Aditus ad Antrum: Triangular communication (~4x4x4 mm) between the Epitympanum (anteriorly) and the Mastoid Antrum (posteriorly).

• Divisions: Upper 1/3 (corresponds to Aditus ad Antrum), Lower 2/3 (Retrotympanum).

What is Incudal Buttress?

What is Incudal Fossa and what structure is present in it?

• Lower 2/3: Bony wall from bony annulus to labyrinth. Houses the vertical segment of the Facial Nerve, Styloid eminence, Pyramidal eminence, Fossa Incudis.

• Note: The lower posterior wall presents structures dividing the retrotympanum into spaces:

  • 3 Eminences: Pyramidal, Caudal, Styloid.
  • 7 Bony Ridges: Caudal, Pyramidal, Styloid, Ponticulus, Subiculum, Fustis, Funiculus.
  • 4 Sinuses/Spaces: Facial Sinus, Lateral Tympanic Sinus (Lateral); Posterior Tympanic Sinus, Sinus Tympani (Medial).

To know more about the eminences, Bony ridges and sinuses in the posterior wall of Middle ear, get access to my full notes by purchasing it.

What is Subiculum?

What structure is present in the Pyramidal eminence?

What is Ponticulus? What are it's 2 variations?

4. Anterior Wall

• Separates middle ear cavity from the Petrous Carotid Canal (Internal Carotid Artery).

• Narrow as medial/lateral walls converge anteriorly.

• Divisions:

  • Lower Portion: Largest, anterior wall of Hypotympanum. Thin bone separating middle ear from vertical segment of carotid artery. Contains two tiny openings for Caroticotympanic nerves (superior/inferior) transmitting sympathetic fibers from the carotid sympathetic chain to the tympanic plexus on the promontory.

  • Middle Portion (Protympanum): Contains two tunnels (separated by the Septum Canalis Musculotuberius).

    • Upper tunnel: Transmits the Tensor Tympani muscle.
    • Lower tunnel: Bony portion of the Eustachian Tube.

  • Upper Portion: Root of the Zygoma, forms the anterior wall of the Epitympanum.

5. Medial Wall (Labyrinthine Wall)

• Separates the middle ear from the Inner Ear.

• Important Structures: Cochlear Promontory, Tympanic segment of Facial Nerve, Oval Window, Round Window, Tensor Tympani canal, Cochleariform Process, Lateral Semicircular Canal.

• Divisions (using Tensor Tympani canal and Facial Nerve as landmarks):

  • Upper 1/3: Medial wall of the Epitympanum. Demarcated posteriorly by the Lateral Semicircular Canal.
  • Lower 2/3: Medial wall of the Mesotympanum. Contains the Cochlear Promontory, Oval Window, and Round Window.

Which nerve plexus is found on the Cochlear Promontory? What nerves contribute to it?

• Facial Nerve Canal (Tympanic Segment): Runs obliquely posterior-superiorly above the oval window. Joins the vertical segment at the Second Genu.

• Oval Window (Fossa Vestibuli): Situated posterior and superior to the promontory. Closed by the Stapes Footplate surrounded by the Annular Ligament.

• Round Window: Situated posterior and inferior to the promontory. Covered by the Round Window Membrane.

• Tensor Tympani Bony Canal: Tensor Tympani semicanal runs from protympanum, ends just anterior to oval window niche.

• Lateral Semicircular Canal: Above the level of the facial nerve canal, forms the medial wall of the Attic.

What is the Cochleariform Process ? What structure passes through it?

6. Superior Wall (Roof)

• Formed by a thin plate of bone called the Tegmen Tympani.

• Separates the middle ear cavity from the overlying Middle Cranial Fossa, Dura, and Temporal Lobe.

• Parts:

  • Tegmen Tubae: Overlying the Eustachian tube.
  • Tegmen Tympani: Overlying the Tympanic Cavity.
  • Tegmen Antri: Overlying the Mastoid Antrum.

• Superior surface is covered by dura, inferior surface by middle ear mucosa. Separates CSF from air space.

• Formed by the tegmental plate of the petrous bone and the horizontal plate of the squamous bone.

• Petrosquamous Suture: Between petrous and squamous parts. Unossified in newborns, closes in adulthood. Dura can be tightly adherent here.