ANATOMY OF THE MIDDLE EAR CLEFT

THE MIDDLE EAR CAVITY (TYMPANIC CAVITY):
It is located in the temporal bone between the external ear and the inner ear. It is a six-walled cavity about 15 mm in height, 15 mm antro-posteriorly and 2-6 mm from side to side. All the walls are bony, except the lateral wall (tympanic membrane) and are lined with mucous membrane, which is ciliated columnar epithelium except its postero-superior part that has flattened squamous epithelium.

• It is divided into three regions:

1- Meso-tympanum (middle ear cavity proper): lies opposite to the tympanic membrane.

2- Epi-tympanum (attic): lies above the tympanic membrane level.

3- Hypo-tympanum: lies below the tympanic membrane level.

• Contents: It is filled with air and contains:

1. Three bones (auditory ossicles): malleus, incus and stapes.

2. Two nerves: The chorda tympani nerve and the tympanic plexus.
3. Two muscles: The tensor tympani muscle and the stapedius muscle.

Walls of the middle ear:

1. Lateral wall:

• It separates the middle ear from the external ear.

• Formed by:

1) The tympanic membrane with bones above and below.

2) The outer attic mass or scutum (part of the squamous temporal bone) forms the upper part of the lateral wall of the attic ( epi-tympanum).

2. Medial wall: It separates the middle ear from the inner ear.

• Formed mainly by:

1. The first (basal) turn of the bony cochlea (of the inner ear) It produces a central rounded bulge called the promontory.
2. Two openings connect the middle ear (functionally) with the inner ear:

a- The oval window: lies above and behind the promontory. It is closed by the footplate of stapes.

b- The round window: lies below and behind the promontory. It is closed by the secondary tympanic membrane.

3. The horizontal part of the facial nerve: runs in a bony canal above the oval window.

3. Superior wall: It separates the middle ear from the middle cranial fossa and temporal lobe of the brain. It is a thin bony plate called the tegmen tympani.
   

4. Inferior wall: A thin bony plate separates the middle ear from the bulb of internal jugular vein.
   

5. Anterior wall: It separates the middle ear from the internal carotid artery. It has an opening for the Eustachian tube. The tensor tympani muscle enters the middle ear through this wall.

6. Posterior wall: It separates the middle ear from the mastoid process.

• It has an opening (the aditus ad antrum) which connects the epitympanum (attic) with the mastoid antrum.

• The stapedius muscle enters the middle ear through this wall through a bony ridge (the pyramid).

• The vertical part of the facial nerve runs in a bony canal in this wall.

THE EUSTACHIAN TUBE (THE AUDITORY TUBE):

The Eustachian tube communicates the middle ear cavity with the nasopharynx where it opens 1.5 cm behind the posterior end of the inferior turbinate. Its length is about 36 mm and is directed medially, forwards and downwards. The tube is shorter, wider, and more horizontal in children than in adults. Its lateral third is bony while its medial two thirds are cartilaginous. It is lined with mucous membrane with ciliated columnar epithelium. The tube is closed at rest. It opens by contraction of the tensor veli palatine muscle during swallowing and yawning to ventilate the middle ear.

THE MASTOID AIR CELLS

They are located within the mastoid process of the temporal bone which is a pyramidal bony projection directed inferiorly behind the auricle. They are small air-filled bony cavities that communicate with each other and lined with flattened squamous epithelium. They communicate anteriorly with the middle ear cavity through the aditus ad antrum. The largest air cell is the mastoid antrum, which lies behind the attic. The number and size of the other cells varies and usually arranged in groups named after the anatomical structure they are near to e.g. retrofacial, tip, perisinus, periantral, subdural, sinodural and zygomatic air cells, according to the degree of cellularity. The mastoid process is not present in the newborn. It forms from traction on the squamous and petrosal parts of the temporal bone by cervical muscles, since the child starts to support his head.


The functions of the mastoid air cells are still unknown but they are thought to share in pressure-regulating mechanism of the middle ear cavity and in decreasing the weight of the skull bones.

Category: Anatomy Notes