1. ANATOMY OF THE MIDDLE EAR
By
Dr. Syed Salman Hussaini

2. THE MIDDLE EAR CLEFT

The middle ear cleft consists of the
− tympanic cavity,
− Eustachian tube and
− mastoid air cell system.

The tympanic cavity is an irregular, air-filled space within
the temporal bone between the tympanic membrane
laterally and the osseous labyrinth medially.

It contains the ossicles, muscles and structures, like the
tympanic segment of the facial nerve,

3. THE TYMPANIC CAVITY

5. THE TYMPANIC CAVITY

Divided into three compartments

the epitympanum (upper),

the mesotympanum (middle) and

hypotympanum (lower).

The epitympanum or attic, lies above the level of the
malleolar folds and is separated from the mesotympanum
and hypotympanum by a series of mucosal membranes
and folds.

The mesotympanum lies opposite the tympanic
membrane.

The hypotympanum lies below the level of the inferior
part of the tympanic sulcus and is continuous with the
mesotympanum above.

6. THE LATERAL WALL

The lateral wall of the tympanic cavity is formed by the

bony lateral wall of the epitympanum superiorly,

tympanic membrane centrally and

bony lateral wall of the hypotympanum inferiorly.

The lateral epitympanic wall is wedge-shaped in section
and its sharp inferior portion is also called the outer attic
wall or scutum (Latin: 'shield').

It is thin and easily eroded by cholesteatoma, leaving a
telltale sign on a high resolution coronal CT scan.

8. 
Three holes are present in the bone of the medial surface
of the lateral wall of the tympanic cavity.

The petrotympanic fissure is 2 mm long which opens
anteriorly just above the attachment of the tympanic
membrane.

It receives the anterior malleolar ligament and transmits
the anterior tympanic branch of the maxillary artery to the
tympanic cavity.

The chorda tympani nerve enters the medial surface of the
fissure through a separate anterior canaliculus (canal of
Huguier) which is sometimes confluent with the fissure.

It then runs posteriorly between the fibrous and mucosal
layers of the tympanic membrane, across the upper part of
the handle of the malleus and then continues within the
membrane, but below the level of the posterior malleolar
fold.

9. 
The nerve reaches the posterior bony canal wall just
medial to the tympanic sulcus, enters the posterior
canaliculus.

It then runs obliquely downwards and medially through the
posterior wall of the tympanic cavity until it reaches the
facial nerve.

The point of entry of the chorda tympani into the facial
nerve bundle is usually at the level of the inferior third of
the facial canal on its anterior wall.

During cortical mastoidectomy, the fibrous strands of the
tympanomastoid suture line can often be confused with
the chorda tympani although the angle of the white strands
of the suture line is different from the angle of the chorda.

The nerve carries taste sensation from the anterior two-
thirds of the same side of the tongue and secretomotor
fibres to the submandibular gland.

10. THE ROOF

The roof of the epitympanum is the tegmen tympani

It is a thin bony plate that separates the middle ear space
from the middle cranial fossa.

It is formed by both the petrous and squamous portions of
the temporal bone .

The petrosquamous suture line, which does not close
until adult life, can provide a route of access for infection
into the extradural space in children.

Veins from the tympanic cavity running to the superior
petrosal sinus pass through this suture line.

11. THE FLOOR

The floor of the tympanic cavity separates the
hypotympanum from the dome of the jugular bulb.

Its thickness varies according to the height of the jugular
fossa.

Occasionally, the floor is deficient and the jugular bulb is
then covered only by fibrous tissue and a mucous
membrane.

At the junction of the floor and the medial wall of the cavity
there is a small opening that allows the entry of the
tympanic branch of the glossopharyngeal nerve into
the middle ear.

12. THE ANTERIOR WALL

14. THE ANTERIOR WALL

The anterior wall of the tympanic cavity is rather narrow as the
medial and lateral walls converge.

The lower-third consists of a thin plate of bone covering the
carotid artery.

This plate is perforated by the
– superior and inferior caroticotympanic nerves (which
carry sympathetic fibres to the tympanic plexus) and
– tympanic branches of the internal carotid artery.

The middle-third - tympanic orifice of the Eustachian tube.

It is oval and 5 x 2 mm in size. Just above this is a canal
containing the tensor tympani muscle that subsequently runs
along the medial wall of the tympanic cavity enclosed in a thin
bony sheath.

The upper-third is usually pneumatized and may house the
anterior epitympanic sinus, a small niche anterior to the ossicular
heads, which can hide residual cholesteatoma in canal wall up
surgery.

15. THE MEDIAL WALL

17. THE MEDIAL WALL

The medial wall separates the tympanic cavity from the
internal ear.

The promontory is a rounded elevation occupying much
of the central portion of the medial wall.

It covers part of the basal coil of the cochlea and usually
has small grooves on its surface containing the nerves
which form the tympanic plexus.

Sometimes the groove containing the tympanic branch of
the glossopharyngeal nerve may be covered by bone,
thereby forming a small canal.

The promontory gently inclines forwards to merge with the
anterior wall of the tympanic cavity, but is more steeply
sloped posteriorly.

19. 
Behind and above the promontory is the oval window.

It is a kidney-shaped opening that connects the tympanic
cavity with the vestibule, which is closed by the footplate of
the stapes and its surrounding annular ligament.

Its size varies with the size of the footplate, but on average
it is 3.25 mm long and 1.75 mm wide.

The oval window niche can be of varying width
depending on the position of the facial nerve superiorly,
and the prominence of the promontory inferiorly.

21. 
The round window niche lies below and a little behind
the oval window niche from which it is separated by a
posterior extension of the promontory called the
subiculum.

Another ridge of bone, the ponticulus, leaves the
promontory above the subiculum and runs to the pyramid
on the posterior wall of the cavity.

The round window niche is most commonly triangular in
shape, with anterior, posterosuperior and posteroinferior
walls.

The latter two meet posteriorly and lead to the sinus
tympani.

22. 
The round window membrane is usually out of sight,
obscured by the overhanging edge of the promontory
forming the niche and mucosal folds within it.

The membrane is roughly oval in shape, about 2.3 x 1.9
mm in dimension and lies in a plane at right angles to the
plane of the stapes footplate.

It tends to curve towards the scala tympani of the basal
coil of the cochlea, so that it is concave when viewed from
the middle ear.

It appears to be divided into an anterior and posterior
portion by a transverse thickening.

24. 
The facial nerve canal (or Fallopian canal) runs above the
promontory and oval window in an anteroposterior direction.

It has a smooth rounded lateral surface that often has
microdehiscences

When the bone is thin or the nerve exposed by disease,
there are two or three straight blood vessels clearly visible
along this line of nerve.

These are the only straight blood vessels in the middle ear
and indicate that the facial nerve is very close by.

The facial nerve canal is marked anteriorly by the processus
cochleariformis, a curved projection of bone, concave
anteriorly, which houses the tendon of the tensor tympani
muscle as it turns laterally to the handle of the malleus.

Behind the oval window, the facial canal starts to turn
inferiorly as it begins its descent in the posterior wall of the
tympanic cavity.

25. 
The region above the level of the facial nerve canal forms
the medial wall of the epitympanum.

The dome of the lateral semicircular canal is the major
feature of the posterior portion of the epitympanum, lying
posterior and extending a little lateral to the facial canal.

In well - aerated mastoid bones, the labyrinthine bone over
the superior semicircular canal may be prominent, running
at right angles to the lateral canal and joining it anteriorly at
a swelling which houses the ampullae of the two canals.

In front and a little below this, above the processus
cochleariformis, may be a slight swelling corresponding to
the geniculate ganglion, with the bony canal of the
greater superficial petrosal nerve running for a short
distance anteriorly.

26. THE POSTERIOR WALL

28. THE POSTERIOR WALL

The posterior wall is wider above than below.

Upper part a large irregular opening - the aditus ad
antrum, that leads back from the posterior epitympanum
into the mastoid antrum.

Below the aditus is a small depression, the fossa incudis,
which houses the short process of the incus and its
suspensory ligament.

Below the fossa incudis and medial to the opening of the
chorda tympani nerve is the pyramid, a small hollow
conical projection with its apex pointing anteriorly.

This houses the stapedius muscle and tendon, which
inserts into the posterior aspect of the head of stapes.

The canal within the pyramid curves downwards and
backwards to join the descending portion of the facial
nerve canal.

31. 
The facial recess is a groove which lies between the
pyramid with facial nerve, and the annulus of the tympanic
membrane .

This is shallower lower down where the facial nerve canal
forms only a slight prominence on the posterior wall.

The facial recess is, therefore, bounded
– medially by the facial nerve and
– laterally by the tympanic annulus,
– with the chorda tympani nerve running obliquely
through the wall between the two.

The chorda always runs medial to the tympanic
membrane.

The angle between the facial nerve and the chorda allows
a posterior tympanotomy, allowing access to the middle
ear from the mastoid without disruptiong the tympanic
membrane.

34. 
The sinus tympani is a posterior extension of the
mesotympanum and lies deep to both the promontory and
the facial nerve.

This extension of air cells into the posterior wall can be
extensive, and is probably the most inaccessible site in
the middle ear and mastoid.

The sinus can extend as far as 9 mm into the mastoid
bone when measured from the tip of the pyramid.

The medial wall of the sinus tympani becomes continuous
with the posterior portion of the medial wall of the tympanic
cavity where it is related to the oval and round window
niches and the subiculum of the promontory.

On rare occasions it can communicate with the mastoid air
cells.

35. 
Cholesteatoma which has extended to the sinus tympani
from the mesotympanum is extremely difficult to eradicate.

The worst region for access is above the pyramid,
posterior to an intact stapes and medial to the facial
nerve.

A retrofacial approach to this region via the mastoid is not
possible because the posterior semicircular canal blocks
access.

37. THE CONTENTS OF THE TYMPANIC
CAVITY

The tympanic cavity contains the
– ossicles,
– two muscles,
– the chorda tympani and
– the tympanic plexus.

The ossicles are the malleus, incus and stapes that form a
semi-rigid bony chain for conducting sound.

The malleus is the most lateral and is attached to the
tympanic membrane, whereas the stapes is attached to
the oval window.

40. THE MALLEUS (the hammer)

The malleus is the largest of the three ossicles, measuring
up to 9 mm in length.

It comprises a head, neck and handle or manubrium.

The head lies in the epitympanum and is suspended by
the superior ligament, which runs upward to the tegmen
tympani.

The head of the malleus has a saddle-shaped facet on its
posteromedial surface to articulate with the body of the
incus by a synovial joint.

Below the neck of the malleus, the bone broadens and
gives rise to the lateral process, the anterior process and
the handle.

The lateral process is a prominent landmark on the
tympanic membrane and receives the anterior and
posterior malleolar folds from the tympanic annulus.

41. 
The chorda tympani crosses the upper part of the
malleus handle on its medial surface above the insertion of
the tendon of tensor tympani, but below the neck of the
malleus itself.

The neck of the malleus connects the handle with the head
and amputation of the head by cutting through the neck
leaves both chorda tympani and tensor tympani intact.

A slender anterior ligament arises from the anterior
process to insert into the petrotympanic fissure.

The handle runs downwards, medially and slightly
backwards between the mucosal and fibrous layers of the
tympanic membrane.

42. 
The handle is very closely attached to the membrane at its
lower end, there is a fine web of mucosa separating the
membrane from the handle in the upper portion before it
becomes adherent again at the lateral process.

This can be opened surgically to create a slit without
perforating the membrane to allow a prosthesis to be
crimped around the malleus handle in certain types of
ossicular reconstruction.

On the deep, medial surface of the handle, near its upper
end, is a small projection into which the tendon of the
tensor tympani muscle inserts.

43. THE INCUS (the anvil)

The incus articulates with the malleus and has a body and
two processes.

The body lies in the epitympanum and has a cartilage-
covered facet corresponding to that on the malleus. The body
of the incus is suspended by the superior incudal ligament
that is attached to the tegmen tympani.

The short process projects backwards from the body to lie
in the fossa incudis to which it is attached by a short
suspensory ligament.

The long process descends into the mesotympanum behind
and medial to the handle of the malleus, and at its tip is a
small medially directed lentiular process.

It has been called the fourth ossicle because of its
incomplete fusion with the tip of the long process, giving the
appearance of a separate bone or at least a sesamoid bone.

The lenticular process articulates with the head of the stapes.

44. THE STAPES (the stirrup)

The stapes is shaped like a stirrup and consists of a head,
neck, the anterior and posterior crura and a footplate.

The head points laterally and has a small cartilage-
covered depression for a synovial articulation with the
lenticular process of the incus.

The stapedius tendon inserts into the posterior part of the
neck and upper portion of the posterior crus.

The two crura arise from the broader lower part of the neck
and the anterior crus is thinner and less curved than the
posterior one.

Both are hollowed out on their concave surfaces, which
gives an optimum combination of strength and lightness.

The two crura join the footplate, which usually has a
convex superior margin, an almost straight inferior margin
and curved anterior and posterior ends.

45. 
The average dimensions of the footplate are 3 mm long-
and 1.4 mm wide, and it lies in the oval window where it is
attached to the bony margins by the annular ligament.

The long axis of the footplate is almost horizontal, with the
posterior end being slightly lower than the anterior.

46. THE STAPEDIUS MUSCLE

The stapedius arises from the walls of the conical cavity
within the pyramid as well as from the downward curved
continuation of this canal in front of the descending portion
of the facial nerve.

A slender tendon emerges from the apex of the pyramid
and inserts into the stapes.

The muscle is supplied by a small branch of the facial
nerve.

47. THE TENSOR TYMPANI MUSCLE

It arises from the walls of the bony canal lying above the
Eustachian tube. Parts of it also arise from the cartilaginous
portion of the Eustachian tube and the greater wing of the
sphenoid.

The muscle then passes backwards into the tympanic cavity
where it lies on the medial wall, a little below the level of the
facial nerve.

The bony covering of the canal is often deficient in its
tympanic segment where the muscle is replaced by a slender
tendon.

This enters the processus cochleariformis where it is held
down by a transverse tendon as it turns through a right angle
to pass laterally and insert into the medial aspect of the
upper end of the malleus handle.

It is supplied by mandibular nerve from its branch, the
medial pterygoid nerve.

48. THE CHORDA TYMPANI NERVE

It enters the tympanic cavity from the posterior
canaliculus at the junction of the lateral and posterior
walls.

It runs across the medial surface of the tympanic
membrane between the mucosal and fibrous layers.

Then passes medial to the upper portion of the handle of
the malleus above the tendon of tensor tympani.

Continues forwards and leaves by way of the anterior
canaliculus, which subsequently joins the petrotympanic
fissure.

49. THE TYMPANIC PLEXUS

It is formed by the
– tympanic branch of the glossopharyngeal nerve
(Jacobson's nerve) and
– caroticotympanic nerves, which arise from the
sympathetic plexus around the internal carotid
artery.

The nerves form a plexus on the promontory and provide
the branches to the mucous membrane lining the tympanic
cavity, Eustachian tube and mastoid antrum and air cells.

The plexus also provides branches to join the greater
superficial petrosal nerve and the lesser superficial
petrosal nerve that contains all the parasympathetic fibres
of the glossopharyngeal nerve.

50. THE MUCOSA OF THE TYMPANIC CAVITY

Mucus-secreting respiratory mucosa bearing cilia.

Three distinct mucocilary pathways can be identified
– epitympanic,
– promontorial and
– hypotympanic, (largest).

Each of these pathways coalesces at the tympanic orifice
of the Eustachian tube.

The mucous membrane lines the bony walls of the
tympanic cavity, and extends to cover the ossicles and
their supporting ligaments.

56. 
It also covers the tendons of the two middle ear muscles
and carry their blood supply.

These folds separate the middle ear space into
compartments.

The only route for ventilation of the epitympanic space
from the mesotympanum is via two small openings
between the various mucosal folds - the anterior and
posterior isthmus tympani.

Prussak's space is found between the pars flaccida and
the neck of the malleus, bounded by the lateral malleolar
fold.

This space can play an important role in the retention of
keratin and subsequent development of cholesteatoma.

57. THE BLOOD SUPPLY OF THE TYMPANIC CAVITY

Arise from both the internal and external carotid system.

The overlap is extensive and great variability is present.

Supply is from the anterior tympanic, stylomastoid,
maxillary, posterior auricular, middle meningeal, ascending
pharyngeal, artery of pterygoid canal and internal carotid
arteries.

The anterior tympanic and stylomastoid arteries are
the biggest.

Anterior tympanic artery br. of Maxillary Artery supplies
Tympanic membrane; malleus and incus; anterior part of
tympanic cavity.

Stylomastoid artery br. of Posterior Auricular artery
supplies Posterior part of tympanic cavity; stapedius
muscle and Mastoid air cells.

58. THE EUSTACHIAN TUBE

60. THE EUSTACHIAN TUBE

It is a dynamic channel that links the middle ear with the
nasopharynx.

Length = 36 mm (reached by the age of 7).

It runs downwards from the middle ear at 45° and is turned
forwards and medially.

Consists of two unequal cones, connected at their apices.

The lateral third is bony and arises from the anterior wall of
the tympanic cavity.

Medial two-thirds cartilaginous part.

Its narrowest portion is called the isthmus, where the
diameter is only 0.5 mm or less.

61. 
It is lined with respiratory mucosa containing goblet cells
and mucous glands, having ciliated epithelium on its floor.

At its nasopharyngeal end, the mucosa is truly respiratory;
but in passing along the tube towards the middle ear, the
number of goblet cells and glands decreases, and the
ciliary carpet becomes less profuse.

It runs through the squamous and petrous portions of the
temporal bone, gradually tapering to the isthmus.

A thin plate of bone forms the roof, separating the tube
from the tensor tympani muscle above.

The carotid canal lies medially and can impinge on the
bony Eustachian tube.

62. 
The cartilaginous part of the tube is around 24 mm long
and consists of a fibrocartilaginous skeleton to which
attached the peritubal muscles.

At its upper border, the cartilage is bent over to resemble
an inverted J, forming a longer medial cartilaginous
lamina and shorter lateral cartilaginous lamina.

The cartilage is fixed to the base of the skull in a groove
between the petrous part of the temporal bone and the
greater wing of the sphenoid, which terminates near the
root of the medial pterygoid plate.

Thus, the back (posteromedial) wall is composed of
cartilage and the front (anterolateral) wall comprises
cartilage and fibrous tissue.

The apex of the cartilage is attached to the isthmus of the
bony portion, while the wider medial end protrudes into the
nasopharynx, lying directly under the mucosa to form the
torus tubarius.

63. 
In the nasopharynx, the tube opens 1-1.25 cm behind and
below the posterior end of the interior turbinate.

The opening is triangular in shape and is surrounded
above and behind by the torus.

The salpingopharyngeal fold stretches from the lower
part of the torus downwards to the wall of the pharynx.

The levator palati, as it enters the soft palate, results in a
small swelling immediately below the opening of the tube.

Behind the torus is the pharyngeal recess or fossa of
Rosenmuller.

Lymphoid tissue is present around the tubal orifice and in
the fossa of Rosenmuller, and may be prominent in
childhood.

65. MUSCLES ATTACHED TO THE
EUSTACHIAN TUBE

The tensor palati muscle arises from the bony wall and
from along the whole length of the lateral cartilaginous
lamina that forms the upper portion of the front wall of the
cartilaginous tube.

From these broad origins the muscle descends, converges
to a short tendon that turns medially around the pterygoid
hamulus and then spreads out within the soft palate to
meet fibres from the other side in a midline raphe.

The tensor palati separates the tube from the otic
ganglion, the mandibular nerve and its branches, the
chorda tympani nerve and the middle meningeal artery.

It is supplied by the Mandibular Nerve.

66. 
Salpingopharyngeus is attached to the inferior part of the
cartilage of the tube near its pharyngeal opening, and it
descends to blend with the palatopharyngeus.

Levator palati arises from the lower surface of the cartilaginous
tube and from the lower surface of the petrous bone, and from
fascia forming the upper part of the carotid sheath.

It first lies inferior to the tube, then crosses to the medial side
and spreads out into the soft palate.

Salpingopharyngeus and the levator palati are supplied from the
pharyngeal plexus.

The ascending pharyngeal and middle meningeal arteries
supply the Eustachian tube.

The veins drain into the pharyngeal plexus and the lymphatics
pass to the retropharyngeal nodes.

The nerve supply arises from the pharyngeal branch of the
sphenopalatine ganglion (Vb) for the ostium, the nervus
spinosus (Vc) for the cartilaginous portion and from the
tympanic plexus (IX) for the bony part.

67. THE MASTOID AIR CELL SYSTEM

69. 
The extent of pneumatization of the temporal bone varies
according to heredity, environment, nutrition, infection, and
eustachian tube function.

There are five recognized regions of pneumatization: the
middle ear, mastoid, perilabyrinthine, petrous apex,
and accessory.

The mastoid region is subdivided into the mastoid antrum,
central mastoid, and peripheral mastoid.

The bony labyrinth divides the perilabyrinthine region into
supralabyrinthine and infralabyrinthine areas.

The apical area and the peritubal area comprise the
petrous apex region.

The accessory region encompasses the zygomatic,
squamous, occipital, and styloid areas.

70. 
There are five recognized air cell tracts.

The posterosuperior tract runs at the juncture of the
posterior and middle fossa aspects of the temporal bone.

The posteromedial cell tract parallels and runs inferior to
the posterosuperior tract.

The subarcuate tract passes through the arch of the
superior semicircular canal.

The perilabyrinthine tracts run superior and inferior to
the bony labyrinth, whereas the peritubal tract surrounds
the eustachian tube.

The anterior petrous apex is pneumatized in only 10 to
15% of specimens. Most often, it is diploic; in a small
percentage of cases, it is sclerotic.

71. 
The mastoid antrum is an air-filled sinus in the petrous
part of temporal bone.

It communicates with the middle ear by the aditus.

Antrum is well developed at birth.

Volume = 2 ml (adult).

The roof of the mastoid antrum and mastoid air cell space
form the floor of the middle cranial fossa.

The medial wall relates to the posterior semicircular canal.

More deeply and inferiorly is the dura of the posterior
cranial fossa and the endolymphatic sac.

73. 
Posterior to the endolymphatic system is the sigmoid
sinus, which curves downwards only to turn sharply
upwards to pass medial to the facial nerve and then
becomes the dome of the jugular bulb in the middle ear
space.

The posterior belly of the digastric muscle forms a groove
in the base of the mastoid bone.

The digastric ridge inside the mastoid lies lateral to the
sigmoid sinus and the facial nerve and is a useful
landmark for finding the nerve.

The periosteum of the digastric groove continues anteriorly
and part of it becomes the endosteum of the stylomastoid
foramen and subsequently of the facial nerve canal.

74. 
MacEwen's triangle is a direct lateral relation to the
mastoid antrum and is formed by
– a posterior prolongation of the line of the zygomatic
arch and
– a tangent to this, that passes through the posterior
border of the external auditory meatus.

In most of the population, the mastoid air cell system is
fairly extensive with air cells.

Alternatively, the mastoid antrum may be the only airfilled
space in the mastoid process when the name acellular or
sclerotic is applied. This condition occurs in 20 percent of
adult temporal bones and is seen in individuals with
chronic ear disease.

Normally lining of the mastoid is a flattened, nonciliated
epithelium without goblet cells or mucus glands.