Middle Ear Anatomy

The middle ear together with the eustachian tube, aditus,
antrum and mastoid air cells is called the middle ear
cleft  . It is lined by mucous membrane and filled
with air.
The middle ear extends much beyond the limits of
tympanic membrane which forms its lateral boundary
and is sometimes divided into (i) mesotympanum (lying
opposite the pars tensa) , (ii) epitympanum or the attic
(lying above the pars tensa but medial to Shrapnell's
membrane and the bony lateral attic wall), (iii) hypotympanum
(lying below the level of pars tensa) (Fig, 1.5).
The portion of middle ear around the tympanic orifice of
the eustachian tube is sometimes called the protympanum.
Middle ear can be likened to a six-sided box with a
roof, a floor, medial, lateral, anterior and posterior walls.
The roof is formed by a thin plate of bone called
tegmen tympani. It also extends posteriorly to form the
roof of the aditus and antrum. It separates tympanic cavity
from the middle cranial fossa.
The floor is also a thin plate of bone which separa tes
tympanic cavity from the jugular bulb. Sometimes, it is
congenitally deficient and the jugu lar bulb may then
project into the middle ear; separated from the cavity
only by the mucosa.

The anterior wall has a thin plate of bone which separates
the cavity from internal carotid artery. It also has
two openings; the lower one for the eustachian tube and
the upper one for the canal of tensor tympani muscle,
The posterior wall lies close to the mas toid air cells.
It presents a bony projection called the pyramid through
the summit of which appears the tendon of the stapedius muscle to get attachment to the neck of stapes. Aditus, an opening through which attic communicates with the
antrum, lies above the pyramid . Facial nerve runs in the
posterior wall just behind the pyramid. Facial recess or
the posterior sinus is a depression in the posterior wall lateral
to the pyramid. It is bounded medially by the vertical
part of VlIth nerve, laterally by the chorda tympani and
above, by the fossa incudis (Fig. l.7) . Surgically, facial
recess is important, as direct access can be made through
this into the middle ear without disturbing posterior
canal wall (intact canal wall technique, see page 72).
The medial wall (Fig. 1.8) is formed by the labyrinth.
It presents a bulge called promontory which is due to the
basal coil of cochlea; oval window into which is fixed the
footplate of stapes; round window or the fenestra cochleae
which is covered by the secondary tympanic membrane.
Above the oval window is the canal for facial nerve. Its
bony cove ring may sometimes be congenitally dehiscent

and the nerve may lie exposed making it very vulnerable
to injuries or infection. Above the canal for facial nerve
is the prominence of lateral semicircular canal. Just anterior
to the oval window, the medial wall presents a hooklike
projection called the processus cochleariformis. The
tendon of tensor tympani takes a turn here to get attachment
to the neck of malleus. The cochleariform process
also marks the level of the genu of the facial nerve which
is an important landmark for surge ry of the facial nerve.
Med ial to the pyramid is a deep recess called sinus tympani
which is bounded by the subiculum be low and the
ponticulus above .
The lateral wall is formed largely by the tympanic
membrane and to a lesser extent by the bony outer attic
wall called the scutum. The tympanic membrane is semi transparent
and forms a 'window' into the middle ear. It is

possible to see some struc tures of the middle ear through
the normal tympanic membrane, e.g. the long process of
incus, incudostapedial joint and the round window.
Mastoid Antrum
It is a large, air-containing space in the upper part of mastoid
and communicates with the attic through the aditus.
I ts roof is formed by the tegmen antri which is a continuation
of the tegmen tympani and separa tes it from the middle
cranial fossa. The lateral wall of antrum is formed by
a plate of bone which is on an average 1.5 cm thick in the
adult. It is marked externally on the surface of masto id by
suprameatal (MacEwen's) C1-iangle (Fig. 1.9).
Aditus ad Antrum
Aditus is an opening through wh ich the attic communicates
with the antrum. The bony prominence of the horizonta
l canal lies on its medial side while the fossa
incudis, to which is attached the short process of incus,
lies laterally. Facial nerve courses just below the aditus.

The Mastoid and its Air Cell System:
The mastoid consists of bone cortex with a "honeycomb"
of ai r cells underneath. Depending on development of air
cell, three types of mastoid have been described.
(i) Well-pneumatised or cellular. Mas toid cells are well
developed and intervening septa are thin.
(ii) Diploetic. Mastoid consists of marrow spaces and
a few air cells.
(iii) Sclerotic or acellular. There are no cells or marrow
spaces.
With any type of mastoid pneumatisation, antrum is always
present. In sclerotic mastoids, antrum is usually small and
the sigmoid sin us is anteposed.
Depending on the location, mastoid air cells are
d ivided into:
(i) zygomatic cells (in the root of zygoma).
(ii) tegmen cells (extending into the tegmen tympani).
(iii) perisinus cells (overlying the sinus plate).
(iv ) retrofacial cells (round the facial nerve).
(v) perilabyrinthine cells (located above, below and
behind the labyrinth, some of them pass through
the arch of superior semicircular canal. These cells
may communicate with the petrous apex).
(vi) peritubal (around the eustachian tube. Along with
hyporympanic cells they also communicate with
the petrous apex).
(vii) tip cells which are quite large and lie medial and
lateral to the digastric ridge in the tip of mastoid.
(viii) marginal cells (lying behind the sinus plate and
may extend into the occipi tal bone).
Ox) squamosal cells (lying in the squamous part of temporal
bones).

Abscesses may form in relation to these air cells and
may sometimes be located far from the mastoid region.
Development of Mastoid
Mastoid develops from the squamous and petrous bones.
The petrosquamosal suture may persist as a bony platethe
Komer's septum, separating superficial squamosa l
cells from the deep petrosal cells. Korner's septum is surgically
important as it may cause difficulty in locating the
antrum and the deeper cells, and thus lead to incomplete
removal of disease at mastOidectomy (Fig. 1.11). Masto id
antrum cannot be reached unless the Korner's septum
has been removed.
Ossicles of the Middle Ear (Fig. 1.12)
There are three ossicles in the middle ear- the malleus,
incus and stapes.
The maLLeus has head, neck, handle (manubrium), a
lateral and an anterior process. Head and neck of malleus

lie in the attic. Manubrium is embedded in the fibrous
layer of the tympanic membrane. The lateral process
forms a knob-like projection on the outer surface of the
tympanic membrane and gives attachment to the anterior
and posterior malleal (malleolar) folds.
The incus has a body and a short process, both of
which lie in the attic, and a long process which hangs
vertically and attaches to the head of stapes.
The stapes has a head, neck, anterior and posterior
crura and a footpl ate. The footplate is held in the oval
window by annular ligament.
The ossicles conduct sound energy from the tympanic
membrane to the oval window and then to the inner ear
fluid .
Intratympanic Muscles
There are two muscles-tensor t)'mpani and the stapedius;
the former attaches to the neck of malleus and tenses the
tympanic membrane while the latter attaches to the neck

f tapes and helps to dampen very loud sounds thus pre\'
enting noise trauma to the inner ear. Stap dius is a 2nd
arch muscle and is supplied by a branch of CN VII while
tensor tympani develops from the 1st arch and is supplied
by a branch of mandibular nerve (V3)·
Tympanic Plexus
It lies on the promontory and is formed by (i) tympanic
branch of glossopharyngeal and (ii) sympathetic fibres from
the plexus round the internal carotid artery. Tympanic
plexus supplies innervation co the medial surface of the
tympanic membrane, tympanic cavity, mastoid air cells
and the bony eustachian tube. It also carries secretomotor
fibres for the parotid gland. Section of tympanic
branch of glossopharyngeal nerve can be carried out in
the middle ear in cases of Frey's syndrome.
Course of secretomotor fibres to the parotid:
Inferior salivary nucleus --- CN IX --- Tympanic
branch --- Tympan ic plexus --- Lesser petrosal nerve ---
Otic ganglion --- Auriculotemporal nerve --- Parotid
gland.
Chorda Tympani Nerve
It is a branch of the facial nerve which enters the middle
ear through posterior canaliculus, and runs on the medial
surface of the tympanic membrane between the handle
of malleus and long process of incus, above the attachment
of tendon of tensor tympani. It carries taste from
anterior two-thirds of tongue and supplies secretomotor
fibres to the submaxillary and sublingual salivary glands.
Lining of the Middle Ear Cleft
Mucous membrane of the nasopharynx is continuous
with that of the middle ear, aditus, antrum and the mastoid
air cells. It wraps the middle ear structures-the ossicles,
muscles, ligaments, and nerves-like peritoneum wraps
various viscera in the abdomen-raising several folds and
dividing the middle ear into various compartmenrs. Middle
ear contains nothing but the air; all the structures lie
outside the mucous membrane.
Histologically, the eustachian tube is lined by ciliated
epithelium which is pseudostratified columnar in the
cart ilaginous part, columnar in the bony part with several
mucous glands in the submucosa. Tympanic cavity is
lined by ciliated columnar epithelium in its anterior and
inferior part which changes to cuboidal type in the posterior
part. Epitympanum and mastoid air cells are lined
by flat, nonciliated epithelium.
Blood Supply of Middle Ear
Middle ear is supplied by six arteries, out of which two
are the main, i.e.
(i) Anterior tympanic branch of maxillary artery which
supplies tympanic membrane.
(ii) Stylomastoid branch of posterior auricular artery
which supp lies middle ear and mastoid air cells.

Four minor vessels are:
(i) Petrosal branch of middle meningeal artery (runs
along greater petrosal nerve).
(ii) Superior tympanic branch of middle meningeal
artery traversing along the canal for tensor tympani
muscle.
(iii) Branch of artery of pterygoid canal (runs along
eustachian tube).
(iv) Tympanic branch of internal carotid.
Veins drain into pterygoid venous plexus and superior
petrosal sinus.
Lymphatic Drainage
Lymphatics from the middle ear drain into retropharyngeal
and parotid nodes while those of the eustachian tube
drain into retropharyngeal group.