Anatomy of Middle Ear

Anatomyof

Middle Ear© 2014 ARJrockzzz

Middle Ear (Tympanum)

The entire middle ear cavity is lined by mucous membrane. It is extending from medial surface of tympanic membrane to cochlear promontory of internal ear (It lies in between external ear and inner ear).It is shaped like a Biconcave disc. The vertical and antero posterior diameters are 15mm, transverse diameter is 6mm at upper part, 2mm at the centre and 4mm at the lower part.

Tympanic Cavity :-

6 Walls - Roof, Floor, Anterior wall, Posterior wall, Medial wall, Lateral wall

4 Chambers - Meso tympanum, Epitympanum, Hypo tympanum, Posterior tympanum

Contents of Tympanum -1. Air2. Ossicles3. Tympanic plexus4. Intra tympanic muscles5. Chorda tympani nerve6. Ligaments7. Arteries and Veins

Communication -

i. Anteriorly to naso pharynx through Eustachian tube.ii. Posteriorly to mastoid antrum through the aditus.

Walls of the Tympanum

1. Roof or Tegmental wall : It separates the tympanum from the middle cranial fossa by a thin bony plate known as Tegmen tympani.

2. Floor or Jugular wall : The middle ear cavity is separated from the jugular bulb by a thin bony plate.

3. Lateral wall : It is formed by the tympanic membrane and partly by bone above, behind and below.

4. Anterior wall or Carotid wall : From above downwards the following openings are present on the anterior wall -

a) Canal for chorda tympani nerveb) Canal for tensor tympani musclec) Eustachian tube openingd) Carotid canal wall separates middle ear cavity from internal carotid

artery

5. Posterior wall or Mastoid wall : Aditus antrum connects the epitympanum with the mastoid antrum; Stapedius tendon passes through pyramid and inserts into the neck of stapes; Lateral to the pyramid is the opening for the chorda tympani nerve of facial nerve.

Lateral wall of tympanic cavity. Parts of the roof, floor, anterior and posterior walls (adjoining the lateral wall) are also seen. The position of the upper part of the malleus, and of the incus is shown in dotted line.

6. Medial wall or Labyrinthine wall : It separates middle ear cavity from internal ear. Fenestra ovalis (Oval window) lies between middle ear and scala vestibule of the cochlea. Below & behind the promontory Round window is present which separates middle ear from scala tympani of the cochlea (Sinus tympani is a depression between the two openings Oval & Round).

Medial wall of middle ear. Parts of the anterior and posterior walls, and of the mastoid antrum and mastoid air cells, are also seen.

Tympanic Cavity : Middle ear cavity can be divided into 4, they are –

1) Meso tympanum : It is the middle ear proper and the biggest of the four, it corresponds with the pars tensa of tympanic membrane.

2) Epitympanum : It is also known as epitmpanic recess or Attic, the area above to meso tympanum (the upper part of tympanum).

3) Hypo tympanum : The lower part lying below the tympanic membrane (or below the meso tympanum).

4) Posterior tympanum : The posterior part lying behind the level of tympanum.

Communications of Middle Ear Cavity :

1. Anteriorly middle ear cavity communicates with Eustachian Tube. Eustachian tube is about 3.5cm in length which connects the middle ear with Naso pharynx. The outer third of this is bony, that adjoins with middle ear; The inner ⅔ ͬ ͩ is cartilaginous and leads into Nasopharynx. In adults the tube is Obliquely placed where as in infants it is short, wide & horizontally placed so Naso pharyngeal infections easily spread to middle ear in infants.

2. Posteriorly middle ear cavity communicates with Mastoid antrum through aditus.

a) Aditus ad Antrum - a short canal connecting epitympanum with mastoid antrum.

b) Mastoid antrum - It is the largest air cell in mastoid bone.

Contents of Middle Ear : 3 Ossicles (3 tiny bones) they are –

1. Malleus (Hammer) : Largest & most lateral ossicle, measuring 8mm in length. It has a head, neck, handle, anterior and lateral processes. Handle is firmly attached to the pars tensa of ear drum, head is situated in the epitympanum & articulate with body of incus.

2. Incus (Anvil) : It has a body, a short process & long process. Body articulates with head of malleus, short process projects backwards in the attic, long process projects downwards behind the handle of malleus & articulates with head of stapes.

3. Stapes (Stirrup) : Smallest ossicle measuring about 3.5mm and consists of a head, neck, foot plate & also anterior and posterior crura. Head articulates with long process of incus and foot plate of stapes is held to the Oval window by the annular ligament.

Ossicles of the ear as seen from the medial side

Muscles :

1. Tensor tympani is inserted into the neck of Malleus.2. The Stapedius is inserted into the neck of Stapes.

Blood Supply :

1. Middle meningeal artery2. Maxillary artery3. Ascending pharyngeal artery4. Stylo mastoid branch of posterior auricular artery

Nerve Supply :Sensory - Tympanic branch of Glosso-pharyngeal nerve.

Motor - Tensor tympani by mandibular nerve; stapedius by facial nerve.

Lymphatic Drainage :5. Pre auricular lymph nodes.6. Retro pharyngeal lymph nodes.

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