the maceration of meatal skin by trapped water. Antibacterial properties Anterior and posterior extrinsic ligaments connect the auricle with the
have been attributed to cerumen but the evidence for this is lacking temporal bone. The anterior ligament extends from the tragus and the
(Campos et al 2000, Pata et al 2003). spine of the helix to the root of the zygomatic process of the temporal
Two types of wax – wet and dry – are recognized. They are genetically bone. The posterior ligament passes from the posterior surface of the
determined. Dry wax is common in East Asians, while the wet type is concha to the lateral surface of the mastoid process. Two main intrinsic
more common in other ethnic groups (Yoshiura et al 2006). Overpro- ligaments connect individual auricular cartilages: a strong fibrous band
duction, accumulation or impaction of wax may completely occlude passes from the tragus to the helix, thereby completing the meatus
the meatus. This may hinder sound from reaching the tympanic mem- anteriorly and forming part of the boundary of the concha; and another
brane and so restrict its natural vibration. Although ceruminous glands band passes between the antihelix and the tail of the helix. Less promi-
and hair follicles are largely limited to the cartilaginous meatus, a few nent bands exist on the cranial aspect of the auricle.
small glands and fine hairs are also present in the roof of the lateral
part of the bony meatus. The warm, humid environment of the rela- Auricular muscles
tively enclosed meatal air aids the mechanical responses of the tym-
panic membrane.
Extrinsic auricular muscles connect the auricle to the skull and scalp,
and move the auricle as a whole. Intrinsic auricular muscles connect
Cartilaginous framework
the different parts of the auricle.
The auricle is a single thin plate of elastic fibrocartilage covered by skin, Extrinsic muscles
its surface moulded by eminences and depressions (Fig. 37.4). It is
The extrinsic auricular muscles are the auriculares anterior, superior and
connected to the surrounding parts by ligaments and muscles, and is
posterior (Fig. 37.5B). The smallest of the three is auricularis anterior,
continuous with the cartilage of the external acoustic meatus. There


a thin fan of pale fibres that arise from the lateral edge of the epicranial
aponeurosis and converge to attach to the spine of the helix. The largest
of the three, auricularis superior, is also thin and fan-shaped, and con-
verges from the epicranial aponeurosis via a thin, flat tendon to attach
to the upper part of the cranial surface of the auricle. The auricularis
A Helix
posterior consists of two or three fleshy fasciculi that arise by short
Vascular aponeurotic fibres from the mastoid part of the temporal bone and
Scapha foramen insert into the ponticulus on the eminentia conchae.
Triangular fossa
Vascular supply The arterial supply of the extrinsic auricular muscles
Temporal bone,
is derived mainly from the posterior auricular artery.
squamous part Crus of helix
Vascular Antihelix Innervation Auriculares anterior and superior are supplied by tem-
foramen
poral branches of the facial nerve, and auricularis posterior is supplied
by the posterior auricular branch of the facial nerve.
Lamina of tragus
Antitragus
Concha Actions In humans, these muscles have very little obvious effect.
Terminal notch Tail of helix Despite the paucity of auricular movement, auditory stimuli may evoke
patterned responses from these small muscles and electromyography
Temporal bone,
can detect the ‘crossed acoustic response’ that can be used to determine
tympanic part Fissura antitragohelicina auditory threshold levels and brainstem latencies, which is elicited by
(antitragohelicine fissure)
this means in investigative clinical neurotology.
Styloid process Isthmus
Intrinsic muscles
B The intrinsic auricular muscles are helicis major and minor, tragicus,
antitragicus, transversus auriculae and obliquus auriculae (Fig. 37.5A).
Helicis major is a narrow vertical band on the anterior margin of the
helix, passing from its spine to its anterior border, where the helix is
Sulcus antihelicis
transversus about to curve back. Helicis minor is an oblique fasciculus covering the
crus of the helix. Tragicus is a short, flattened, vertical band on the


Eminentia Spine of helix lateral aspect of the tragus. Antitragicus passes from the outer part of
conchae the antitragus to the tail of the helix and the antihelix. Transversus
auriculae, located on the cranial aspect of the auricle, consists of scat-
Ponticulus tered fibres – partly tendinous, partly muscular, which extend between
Tail of helix the eminentia conchae and the eminentia scaphae. Obliquus auriculae,
also located on the cranial aspect of the auricle, consists of a few fibres
that extend from the upper and posterior parts of the eminentia conchae
Cartilage of meatus to the eminentia scaphae.
Fig. 37.4 The auricular cartilages of the left ear. A, Lateral surface. Vascular supply The intrinsic auricular muscles are supplied by
B, Medial surface. branches of the posterior auricular and superficial temporal arteries.


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Table 37.1 Common anomalies of the auricle
Anomaly Structural defect Figure
Anotia Complete absence of the external ear that is most likely caused by a developmental disturbance between the
seventh and eighth gestational weeks
Cryptotia (Hidden ear, The upper pole of the pinna is buried beneath the temporal skin
pocket ear)
Microtia Diminutive ear; this is usually an isolated congenital anomaly but is sometimes associated with recognized
syndromes, e.g. fetal alcohol syndrome, maternal diabetic syndrome, thalidomide or isotretinoin exposure
Continued


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Table 37.1 Common anomalies of the auricle—cont’d
Anomaly Structural defect Figure
Polyotia (Mirror ear) Caused by the persistence of pre-auricular tissue that would normally be included in the pinna; the persistent
tissue lies in front of the tragus in the posterior aspect of the cheek
Stahl’s bar (‘Satiro’s ear’, Common congenital deformity of the auricle in which the helix is flattened and the upper crus of the antihelix
‘lop ear’) is duplicated. This produces a ridge of cartilage running from the antihelix to the rim of the helix, causing a
pointing of the ear and a reversal of the normal concavity of the scaphoid fossa. The upper part of the pinna
may flop over
Prominent ears (Bat ear) The antihelical fold is either absent or inadequate


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Table 37.1 Common anomalies of the auricle—cont’d
Anomaly Structural defect Figure
Pre-auricular sinus The result of failure of fusion of the first and second branchial arches. Clinically, pre-auricular sinuses may
become chronically infected and require surgical excision. This may be technically demanding, given the close
proximity to the facial nerve and auricular tubercles around the dorsal end of the first branchial cleft. The
sinuses may be simple pits or complex branching sinuses that occasionally extend deeply towards the external
acoustic meatus so that they lie close to the facial nerve. There is debate as to whether the anomalies are
epithelial inclusions between the hillocks or remnants of the first branchial cleft


External ear
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A into pre-auricular nodes, while the remainder of the pinna drains to
upper deep cervical lymph nodes.
Innervation
Helicis major The sensory innervation of the auricle is complex and not fully deter-
mined. This is perhaps because the external ear represents an area where
skin originally derived from a branchial region meets skin originally
derived from a postbranchial region. The sensory nerves involved are
Helicis minor the great auricular nerve, which supplies most of the cranial surface and
the posterior part of the lateral surface (helix, antihelix, lobule); the
Tragicus lesser occipital nerve, which supplies the upper part of the cranial
Antitragicus surface; the auricular branch of the vagus, which supplies the concavity
of the concha and posterior part of the eminentia; the auriculotemporal
Tail of helix
nerve, which supplies the tragus, crus of the helix and the adjacent part
of the helix; and the facial nerve, which, together with the auricular
branch of the vagus, probably supplies small areas on both aspects of
the auricle, in the depression of the concha and over its eminence. The
details of the cutaneous innervation derived from the facial nerve
require further clarification. It is possible that, as the auricular branch
B of the vagus traverses the temporal bone and crosses the facial canal,
approximately 4 mm above the stylomastoid foramen, it contributes an
ascending branch to the facial nerve and that, in this way, fibres of the
vagus are carried via the facial nerve to the pinna.
Auricularis superior
Obliquus auriculae EXTERNAL ACOUSTIC MEATUS
The external acoustic meatus extends from the concha to the tympanic
membrane; it is approximately 2.5 cm from the floor of the concha and
Transversus auriculae approximately 4 cm from the tragus. It has two structurally different
parts: its lateral third is cartilaginous and its medial two-thirds is
External acoustic meatus
osseous (Figs 37.6–37.8). It forms an S-shaped curve, directed at first
Auricularis posterior medially, anteriorly and slightly up (pars externa), then posteromedi-


ally and up (pars media), and lastly anteromedially and slightly down
(pars interna). It is oval in section; its greatest diameter is obliquely
inclined posteroinferiorly at the external orifice but is nearly horizontal
at its medial end. There are two constrictions: one near the medial end
of the cartilaginous part, and the other, the isthmus, in the osseous part
about 2 cm from the bottom of the concha. The tympanic membrane,
Fig. 37.5 Extrinsic and intrinsic auricular muscles of the left ear. which closes its medial end, is obliquely set, which means that the floor
Auricularis anterior is not illustrated. (With permission from Waschke J, and the anterior wall of the meatus are longer than its roof and pos-
Paulsen F (eds), Sobotta Atlas of Human Anatomy, 15th ed, Elsevier, terior wall.
Urban & Fischer. Copyright 2013.) The lateral, cartilaginous part is approximately 8 mm long. It is
continuous with the auricular cartilage and attached by fibrous tissue
to the circumference of the osseous part. The meatal cartilage is deficient
Innervation The intrinsic auricular muscles on the lateral aspect of posterosuperiorly, and the gap is occupied by a sheet of collagen. Two
the auricle are innervated by the temporal branches of the facial nerve, or three deep fissures (of Santorini) exist in its anterior part. Tumours
and those on the cranial aspect of the auricle are innervated by the of the external acoustic meatus escape the confines of the canal through
posterior auricular branch of the facial nerve. these fissures and spread into the adjacent soft tissues; the extent of
these fissures, which are subject to individual variation, must influence
Actions The intrinsic muscles modify auricular shape minimally, if at outcomes.
all, in most human ears: helicis major can draw the auricle forwards The osseous part is approximately 16 mm long and is narrower than
and upwards. Rare individuals can modify the shape and position of the cartilaginous part. In sagittal section, it is oval or elliptical, and it
their external ears. is directed anteromedially and slightly downwards, with a slight pos-
terosuperior convexity. Its medial end is smaller than the lateral end


Vascular supply and lymphatic drainage and it terminates obliquely. The anterior wall projects medially approxi-
mately 4 mm beyond the posterior and is marked, except above, by a
Arteries The posterior auricular branch of the external carotid artery narrow tympanic sulcus or anulus, to which the perimeter of the tym-
is the dominant blood supply (Imanishi et al 1997). It supplies three panic membrane is attached. Its lateral end is dilated and mostly rough
or four branches to the cranial surface of the auricle; twigs from these for the attachment of the meatal cartilage. The anterior, inferior, and
arteries reach the lateral surface – some through fissures in the cartilage, most of the posterior parts of the osseous meatus are formed by the
others round the margin of the helix. The posterior auricular artery tympanic plate of the temporal bone, which in the fetus is only a tym-
ascends between the parotid gland and the styloid process to the groove panic ring. The posterosuperior region is formed by the squamous part
between the auricular cartilage and mastoid process. The superior aur- of the temporal bone. The outer wall of the meatus is bounded above
icular artery has a constant course and connects the superior temporal by the posterior zygomatic root, below which there may be a suprameatal
artery and the posterior auricular arterial network; this branch can spine.
provide a reliable vascular pedicle for retro-auricular flaps (Moschella
et al 2003). The auricle is also supplied by anterior auricular branches Relations of the meatus The condylar process of the mandible lies
of the superficial temporal artery, which are distributed to its lateral anterior to the meatus and is partially separated from the cartilaginous
surface, and by a branch from the occipital artery. part by a small portion of the parotid gland. A blow on the chin may
cause the condyle to break into the meatus. The middle cranial fossa
Veins Auricular veins correspond to the arteries of the auricle. Arterio- lies above the osseous meatus and the mastoid air cells are posterior to
venous anastomoses are numerous in the skin of the auricle and are it, separated from the meatus only by a thin layer of bone. Its deepest


thought to be important in the regulation of core temperature. part is situated below the epitympanic recess and is anteroinferior to
the mastoid antrum; the lamina of bone that separates it from the
Lymphatic drainage The posterior aspect of the pinna drains to antrum is only 1–2 mm thick and provides the ‘transmeatal approach’
nodes at the mastoid tip. The tragus and upper part of the pinna drain of aural surgery.


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A
Head of malleus
Anterior semicircular duct
Auricle
Incus
Facial nerve
Cartilage
Stapes
of auricle
Tympanic membrane
Tensor tympani
External
acoustic meatus
Internal carotid artery
Cartilaginous
part of meatus
Osseous part
Pharyngotympanic of meatus
tube
Mastoid process
Lobule of auricle
Tympanic plate Facial nerve
Styloid process
B
Semicircular canals
Malleus
Cochlea Stapes
Fig. 37.6 The external, middle and inner regions of
Incus the left ear. A, Anterior aspect. B, A more detailed
Vestibulocochlear nerve Tympanic membrane view of the relationships of structures in the middle
and inner ear. (B, With permission from Waschke J,
Tympanic cavity
Paulsen F (eds), Sobotta Atlas of Human Anatomy,
Pharyngotympanic tube 15th ed, Elsevier, Urban & Fischer. Copyright 2013.)
Vasculature and lymphatic drainage The arterial supply of the EXTERNAL SURGICAL APPROACHES
external acoustic meatus is derived from the posterior auricular artery, TO THE MIDDLE EAR
the deep auricular branch of the maxillary artery and the auricular
branches of the superficial temporal artery. Associated veins drain into Surgical access to the middle ear can be achieved by a number of
the external jugular and maxillary veins and the pterygoid plexus. The methods. Provided the external acoustic meatus is wide enough, the
lymphatics drain into those associated with the pinna. tympanic membrane can be elevated by incising the skin of the bony
Innervation The sensory innervation of the external acoustic meatus meatus circumferentially, leaving a vascular pedicle superiorly. The
is derived from the auriculotemporal branch of the mandibular nerve canal skin is then elevated from the underlying bone until the fibrous
(see Fig. 32.23), which supplies the anterior and superior walls, and the anulus of the tympanic membrane is visualized. This can then be ele-
auricular branch of the vagus, which supplies the posterior and inferior vated from the tympanic groove and the middle ear mucosa can be


walls. The facial nerve may also contribute via its communication with incised to allow the tympanic membrane to be reflected forwards and
the vagus nerve. upwards. This per-meatal approach is called a tympanotomy and is used


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Internal carotid artery
Pharyngotympanic tube
Epitympanic recess
Groove for inferior
External acoustic meatus petrosal sinus
Anterior semicircular canal First turn of cochlea
Aditus to mastoid antrum Cochlear nerve
Facial nerve
Vestibule
Vestibular nerve
Lateral semicircular canal Aqueduct of vestibule
Posterior semicircular canal
Groove for sigmoid sinus
Mastoid antrum
Fig. 37.7 The left auditory apparatus as if viewed through a semi-transparent temporal bone. Note the first genu in the facial nerve at the site of the
geniculate ganglion.
The postauricular approach involves making an incision approxi-
mately 1 cm behind the postauricular skin crease and deepening the
incision to the periosteum of the mastoid process, dividing the pos-
terior auricular muscles on the way. Grafts can be harvested from the
temporalis fascia. The periosteum is incised and elevated to expose the
bony external acoustic meatus from behind. The skin over the junction
of the bony and cartilaginous meatus is incised to allow the cartilage
of the auricle and meatus to be swung forwards on its blood supply
and so expose the bony meatus and mastoid process. Access can then
be gained by drilling and elevating a tympanomeatal skin flap, as
described for the endaural approach.
Temporalis fascia is the most popular tissue used as a free graft for
repair of the tympanic membrane because it is easily obtainable. In
recent years, tragal perichondrium has become a popular alternative; it
has the additional advantage that the cartilage can also be harvested
and used to reinforce the repair.
More extensive resections of the temporal bone are undertaken using
Fig. 37.8 A bone-windowed, coronal computed tomography (CT) scan of extended pre- or postauricular incisions into the temporal region and
the external ear canal and middle ear showing the curvature of the floor neck. The blood supply of the pinna is sufficient to maintain viability
and roof, and the obliquity of the tympanic membrane (arrow) at the
despite significant elevation and undermining.
medial end of the external acoustic meatus. (Courtesy of Dr Steven
Connor.)
MIDDLE EAR


for stapedectomy, ossiculoplasty, myringoplasty and the removal of
small middle-ear tumours. The middle ear is an irregular, laterally compressed space in the petrous
If the external acoustic meatus is too narrow to allow adequate visu- part of the temporal bone. It is lined with mucous membrane and filled
alization of the middle ear, or if access is required to the mastoid aditus with air, which reaches it from the nasopharynx via the pharyngotym-
and antrum, it is necessary to displace the superficial soft tissues. There panic tube (see Figs 37.6–37.7; Fig. 37.9). The middle ear contains
are two main external approaches to the middle ear: the endaural three small bones – the malleus, incus and stapes, collectively called
approach and the postauricular approach. the auditory ossicles – which form an articulated chain connecting the
The endaural approach involves making an incision in the notch lateral and medial walls of the cavity, and which transmit the vibrations
between the tragus and the helix. This is carried down to expose the of the tympanic membrane across the cavity to the cochlea.
lower margin of temporalis muscle and the bone of the external acous- The essential function of the middle ear is to transfer energy effi-
tic meatus. The cartilaginous meatus is separated from the bony meatus ciently from relatively weak vibrations in the elastic, compressible air
and reflected laterally as a conchomeatal flap. The bony meatus can in the external acoustic meatus to the incompressible fluid around the
then be widened by drilling away bone (canalplasty). This gives more delicate receptors in the cochlea. Mechanical coupling between the two
space to manipulate the delicate structures of the middle ear, as well as systems must match their resistance to deformation or ‘flow’, i.e. their
improving subsequent visualization of the tympanic membrane when impedance, as closely as possible. Aerial waves of low amplitude and
the incision has healed. low force per unit area arrive at the tympanic membrane, which has


External and middle ear
631.e1
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C D
Tensor Malleus Incudo- Incus
tympani stapedial joint
E
Incudo-
stapedial joint
Fig. 37.9, cont’d C, D, E, Coronal CT scans showing details of the ossicular mass. C, Malleus and tensor tympani. D and E, Incus and incudostapedial
joint.


ExTERnAl And middlE EAR
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bounded laterally by the tympanic membrane and medially by the
lateral wall of the internal ear, the promontory. It communicates pos-
teriorly with the mastoid antrum and the mastoid air cells, and anter-
iorly with the nasopharynx via the pharyngotympanic tube (see Figs
37.6–37.7).
The tympanic cavity and mastoid antrum, auditory ossicles and
structures of the internal ear are all almost fully developed at birth and
subsequently alter little; almost all of the volume changes are due to
expansion of the epitympanic space (Osborn et al 2011). In the fetus,
the cavity contains a gelatinous tissue that has practically disappeared
by birth, when it is filled by a fluid that is absorbed when air enters via
the pharyngotympanic tube. The tympanic cavity is a common site of
infection in childhood.
BOUNDARIES OF THE TYMPANIC CAVITY
The tympanic cavity has a roof, a floor and lateral, medial, posterior
and anterior walls.
Roof
A thin plate of compact bone, the tegmen tympani, separates the cranial
and tympanic cavities, and forms much of the anterior surface of the
A
petrous temporal bone. It is prolonged posteriorly as the roof of the
mastoid antrum and anteriorly it covers the canal for tensor tympani.
In youth, the unossified petrosquamosal suture may allow the spread
of infection from the tympanic cavity to the meninges. In adults, veins
from the tympanic cavity traverse this suture to reach the superior pet-
rosal or petrosquamous sinus and thus may also transmit infection to
these structures through a process of thrombophlebitis.
Longitudinal fractures of the middle cranial fossa almost always
involve the tympanic roof, accompanied by dislocation of the ossicular
chain, rupture of the tympanic membrane, or a fractured roof of the
osseous external acoustic meatus, which can be seen as a notch on
otoscopy. Such injuries usually cause bleeding from the ear, with
escape of cerebrospinal fluid if the dura mater has been torn (CSF
otorrhoea).
Floor


The floor of the tympanic cavity is a narrow, thin, convex plate of
bone that separates the cavity from the superior bulb of the internal
jugular vein. The bone may be patchily deficient, in which case the
tympanic cavity and the vein are separated only by mucous mem-
brane and fibrous tissue. Alternatively, the floor is sometimes thick
and may contain some accessory mastoid air cells. A small aperture
for the tympanic branch of the glossopharyngeal nerve lies near the
medial wall.
B
Lateral wall
Fig. 37.9 A, An axial CT scan at the level of the ossicular mass (white
arrow) in the epitympanum, showing the mastoid antrum and its
relationship to the lateral (horizontal) semicircular canal (black arrow). The lateral wall consists mainly of the tympanic membrane but also
B, An axial CT scan at the level of the entrance of the pharyngotympanic contains the ring of bone to which the membrane is attached (see Fig.
tube, showing its relationship to the internal carotid artery (black arrow). 37.15A). The lateral epitympanic bony wall is wedge-shaped in section
(A, B, Courtesy of Dr Steven Connor.) Continued online and its sharp inferior portion is known as the outer attic wall or scutum.
This part is easily eroded or blunted by cholesteatoma, a feature easily
15–20 times the area of the stapedial footplate that contacts the peri- detected on computed tomography (CT) scans (see Fig. 37.8). There is
lymph in the inner ear; the force per unit area generated by the footplate a deficiency or notch in the upper part of this ring, close to which are
is increased by a similar amount, while the amplitude of vibration is the small openings of the anterior and posterior canaliculi for the
almost unchanged. chorda tympani and the petrotympanic fissure. The posterior canalicu-
Protective mechanisms incorporated into the design of the middle lus for the chorda tympani is situated in the angle between the posterior
ear include the presence of the pharyngotympanic tube (to equalize and lateral walls of the tympanic cavity just behind the tympanic mem-
pressure on both sides of the delicate tympanic membrane); the shape brane, at a variable position approximately level with the upper end of


of the articulations between the ossicles; and the reflex contractions of the handle of the malleus. This variable position or entrance into the
stapedius and tensor tympani in response to sounds of fairly high tympanic cavity should be kept in mind when mobilizing the chorda
intensity (preventing damage caused by sudden or excessive excursions tympani in stapes surgery. It may not reflect inferiorly to the extent
of the ossicles). desired and can be easily damaged in the process. Within the mastoid,
The space within the middle ear can be subdivided into three parts. the chorda tympani passes into a minute canal that descends in front
These are the mesotympanum or tympanic cavity proper, which is of the facial canal and ends in it about 6 mm above the stylomastoid
opposite the tympanic membrane; the epitympanum or attic, which is foramen. This canal and the chorda tympani should not be mistaken
above the level of the membrane, and contains the head of the malleus for the main trunk of the facial nerve when skeletonizing the Fallopian
and the body and short process of the incus; and the hypotympanum, canal during surgery. The canaliculus transmits the chorda tympani and
which is in the floor of the cavity between the jugular bulb and the a branch of the stylomastoid artery to the tympanic cavity. The chorda
lower margin of the tympanic membrane. The vertical and anteropos- tympani leaves the tympanic cavity through the anterior canaliculus,
terior diameters of the mesotympanum and hypotympanum are each which opens at the medial end of the petrotympanic fissure.
approximately 15 mm; the transverse diameter is 6 mm superiorly and The petrotympanic fissure is a mere slit approximately 2 mm in
4 mm inferiorly, narrowing to 2 mm opposite the umbo. The cavity is length, which opens just above and in front of the ring of bone to which


middle ear
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trally. Ultrastructurally, the filaments are 10 nm in diameter and are
linked at 25 nm intervals. They have a distinctive amino acid compos-
ition, and may consist of a protein peculiar to the tympanic membrane.
Small groups of collagen fibrils appear at 11 weeks in utero, interspersed
with small bundles of elastin microfibrils. Older specimens contain
more typically cross-banded collagen fibrils and an amorphous elastin
component. The fibrous stratum is replaced by loose connective tissue
in the pars flaccida.
The mucous stratum is a part of the mucosa of the tympanic cavity
and is thickest near the upper part of the membrane. It consists of a
single layer of very flat cells, with overlapping interdigitating boundaries
and desmosomes and tight junctions between adjacent cells. The cyto-
plasm contains only a few organelles; the luminal surfaces of these
apparently metabolically inert cells have a few irregular microvilli and
are covered by an amorphous electron-dense material. There are no
ciliated columnar cells.
Innervation
The tympanic membrane is mainly innervated by the auriculotemporal
nerve and appears to perceive only pain. There is a minor, inconstant
and overlapping sensory supply from the facial, glossopharyngeal and
vagus nerves.
The auricular branch of the vagus arises from the superior vagal
ganglion and is joined soon after by a ramus from the inferior ganglion
of the glossopharyngeal nerve. It passes behind the internal jugular vein
and enters the mastoid canaliculus on the lateral wall of the jugular
fossa. It traverses the temporal bone and crosses the facial canal about
4 mm above the stylomastoid foramen. At this point, it supplies an
Cone of light Handle of malleus Position of umbo
ascending branch to the facial nerve. Fibres of the nervus intermedius
Fig. 37.10 An auroscopic view of the left tympanic membrane. Note that may pass to the auricular branch of the vagus here, which may explain
a bright cone of light is seen in the anteroinferior quadrant of the the cutaneous vesiculation that sometimes accompanies geniculate


membrane when it is illuminated. (Courtesy of Mr Simon A Hickey.) herpes. The auricular branch then traverses the tympanomastoid fissure
and divides into two rami. One ramus joins the posterior auricular
nerve and the other is distributed to the skin of part of the cranial
the tympanic membrane is attached. It contains the anterior process surface of the auricle, the posterior wall and floor of the external acous-
and anterior ligament of the malleus, and transmits the anterior tym- tic meatus, and to the adjoining part of the outer surface of the tym-
panic branch of the maxillary artery to the tympanic cavity. panic membrane. The auricular branch therefore contains somatic
afferent nerve fibres, which probably terminate in the spinal trigeminal
Tympanic membrane nucleus. Stimulation of the vagus nerve, e.g. in syringing the ear, can
cause a reflex bradycardic reaction on heart rate. From a practical point
of view, the tympanic membrane can be anaesthetized for minor pro-
The tympanic membrane separates the tympanic cavity from the exter-
cedures like myringotomy by the application of topical anaesthetic
nal acoustic meatus (Fig. 37.10; see Figs 37.6, 37.15). It is thin, semi-
agents. For more complicated surgical procedures, e.g. tympanoplasty,
transparent and almost oval, though somewhat broader above than
it is possible to provide sufficient anaesthesia by local infiltration at the
below. It lies obliquely, at an angle of approximately 55° with the
level of the bony and cartilaginous external acoustic meatus.
meatal floor in adults. Its longest, anteroinferior diameter is 9–10 mm,
and its shortest is 8–9 mm. Most of its circumference is a thickened Otitis media
fibrocartilaginous ring or anulus, which is attached to the tympanic It is assumed that acute otitis media usually arises as a result of ascend-
sulcus at the medial end of the meatus. The anulus contains radially ing infection from the nasopharynx via the pharyngotympanic tube to
orientated smooth muscle cells in several locations that possibly play the middle ear cleft. From there, it may extend to the mastoid aditus


a role in controlling blood flow or maintaining tension (Henson et al and antrum. Swelling secondary to the infection may result in the
2005). The sulcus is deficient superiorly, i.e. it is notched. Two bands, closure of both exits from the middle ear, i.e. the pharyngotympanic
the anterior and posterior malleolar folds, pass from the ends of this tube and the aditus, with subsequent accumulation of pus under pres-
notch to the lateral process of the malleus. The small triangular part of sure, which causes lateral bulging and inflammation of the tympanic
the membrane, the pars flaccida, lies above these folds and is lax and membrane. The latter may burst, releasing mucopurulent discharge into
thin. The major part of the tympanic membrane, the pars tensa, is taut. the external acoustic meatus, which results in a release of the pressure
The handle of the malleus is firmly attached to the internal surface of in the middle ear and a diminution in the levels of pain. After a brief
the tympanic membrane as far as its centre, which projects towards the period, the discharge dries up and, for the most part, the resultant
tympanic cavity. The inner surface of the membrane is thus convex and perforation of the tympanic membrane heals. Normal ventilation and
the point of greatest convexity is termed the umbo. Although the mem- drainage of mucus from the middle ear are restored once the swelling
brane as a whole is convex on its inner surface, its radiating fibres are and function of the pharyngotympanic tube resolve. On occasion, the
curved with their concavities directed inwards. process will fail to produce a perforation of the tympanic membrane
and the inflammatory exudates will not drain. The immune defence
Microstructure
system sterilizes the exudates of organisms, resulting in a sterile mucoid
Histologically, the tympanic membrane is composed of an outer cuticu-
effusion, otitis media with effusion or glue ear (see below). The effusion
lar layer, an intermediate fibrous layer and an inner mucous layer.
may cause protracted deafness because its relatively incompressible
The cuticular stratum is continuous with the thin skin of the meatus.
nature prevents free vibration of the tympanic membrane (Bluestone


It is keratinized, stratified squamous in type, devoid of dermal papillae
and Klein 2002).
and hairless. Its subepithelial tissue is vascularized and may develop a
Myringoplasty
few peripheral papillae. Ultrastructurally, it is typically 10 cells thick and
has two zones: a superficial layer of non-nucleated squames, and a deep Persistent perforation of the tympanic membrane caused by infection
zone that resembles the epidermal prickle cell layer (stratum spino- or trauma leads to hearing impairment and predisposes to continuing
sum). There are numerous desmosomes between cells, the deepest of infection as a result of contamination with organisms from the external
which lie on a continuous basal lamina, but lack epithelial pegs and acoustic meatus. This condition is known as chronic suppurative otitis
hemidesmosomes. The cells of this stratum have a propensity for lateral media of the tubotympanic type. Myringoplasty is a surgical procedure
migration and differentiation not shared with any other stratified squa- that uses a connective tissue scaffold or graft to support healing of the
mous epithelia in the body. perforation. The most common technique involves the elevation of the
The fibrous stratum consists of an external layer of radiating fibres tympanic anulus and the placement of a piece of fibrous connective
that diverge from the handle of the malleus, and a deep layer of circular tissue, e.g. part of the fibrous deep fascia that invests the lateral surface
fibres, which are plentiful peripherally but sparse and scattered cen- of temporalis or the perichondrium of the tragal cartilage, on to the


ExTERnAl And middlE EAR
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4
nOiTCES
A Fig. 37.11 The medial wall
Mastoid antrum of the left tympanic cavity,
anterolateral aspect. A, The
Facial canal lateral wall and adjacent parts
of the anterior and superior
Processus cochleariformis
walls have been removed; the
Trigeminal facial canal and carotid canal
impression have been opened. B, A section
Mastoid cells along the axis of the petrous
part of the temporal bone. (With
permission from Waschke J,
Paulsen F (eds), Sobotta Atlas
Carotid canal of Human Anatomy, 15th ed,
Elsevier, Urban & Fischer.
Copyright 2013.)
Promontory
Round window Stylomastoid foramen
B
Prominence of lateral semicircular canal
Pyramidal eminence
Prominence of facial canal
Fenestra vestibuli
Facial canal
Canal for tensor tympani
Trigeminal impression
Carotid canal
Septum of Mastoid wall
musculotubal canal (posterior wall)
Tympanic opening of
pharyngotympanic tube Fossa incudis
Groove of promontory Posterior sinus
Promontory External acoustic
meatus
Tympanic cells
Sinus tympani
Fossa for fenestra cochleae
Subiculum of promontory
Tympanic groove
Styloid process Mastoid process
undersurface of the tympanic membrane to close the perforation. The of bone, the ponticulus, leaves the promontory above the subiculum
healed edges of the perforation are stripped of epithelium to encourage and runs to the pyramid on the posterior wall of the cavity. The fenestra
healing and scar formation. The fibrous tissue supports the healing cochleae lies completely under the overhanging edge of the promontory
tympanic membrane and may, in part, be incorporated into the repair. in a deep hollow or niche, and is placed very obliquely. In dried speci-
Once the perforation is healed, the vibratory function of the tympanic mens, it opens anterosuperiorly from the tympanic cavity into the scala
membrane is usually restored to normal. tympani of the cochlea, but in life it is closed by the secondary tympanic


membrane. This is somewhat concave towards the tympanic cavity and
Medial wall convex towards the cochlea, and is bent so that its posterosuperior third
forms an angle with its anteroinferior two-thirds. The membrane is
composed of an external layer derived from the tympanic mucosa; an
The medial wall of the tympanic cavity is also the lateral boundary of
internal layer, derived from the cochlear lining membrane; and an
the internal ear. Its features are the promontory, fenestra vestibuli
intermediate, fibrous layer.
(fenestra ovalis, oval window), fenestra cochleae (fenestra rotunda,
The prominence of the facial nerve canal indicates the position of
round window) and the facial prominence (Fig. 37.11).
the upper part of the bony facial canal (Fallopian canal), which con-
The promontory is a rounded prominence furrowed by small grooves
tains the facial nerve. The canal crosses the medial tympanic wall from
that lodge the nerves of the tympanic plexus. It lies over the lateral
the cochleariform process anteriorly, runs just above the fenestra vesti-
projection of the basal turn of the cochlea. A minute spicule of bone
buli, and then curves down into the posterior wall of the cavity. Its
frequently connects the promontory to the pyramidal eminence of the
lateral wall may be partly deficient.
posterior wall. The apex of the cochlea lies near the medial wall of
the tympanic cavity, anterior to the promontory. A depression behind
Posterior wall
the promontory is known as the sinus tympani.
The fenestra vestibuli is a kidney-shaped opening situated above and
behind the promontory, and leading from the tympanic cavity to the The posterior wall of the tympanic cavity is wider above than below. Its
vestibule of the inner ear. Its long diameter is horizontal and its convex main features are the aditus to the mastoid antrum, the pyramid and
border is directed upwards. It is occupied by the base of the stapes, the the fossa incudis (see Fig. 37.11).
footplate; the circumference of the footplate is attached to the margin The aditus to the mastoid antrum is a large irregular aperture that


of the fenestra by an anular ligament. leads back from the epitympanic recess into the upper part of the
The fenestra cochleae is situated below and a little behind the mastoid antrum. A rounded eminence on the medial wall of the aditus,
fenestra vestibuli, from which it is separated by a posterior extension above and behind the prominence of the facial nerve canal, corresponds
of the promontory, called the subiculum. Occasionally, another ridge to the position of the lateral semicircular canal.


middle ear
635
73
RETPAHC
The pyramidal eminence is situated just behind the fenestra vestibuli nerve. Some investigators maintain that these are not continuous with
and in front of the vertical part of the facial nerve canal. It is hollow the mastoid cells but grow independently from the tympanic cavity. All
and contains the stapedius muscle. Its summit projects towards the of these extensions of the mastoid air cells are pathologically important
fenestra vestibuli and is pierced by a small aperture that transits the since infection may spread to the structures around them. (For further
tendon of stapedius. The cavity in the pyramidal eminence is prolonged reading on the mastoid air cells and other pneumatized regions of the
down and back in front of the facial nerve canal; it communicates with temporal bone, see Allam (1969).)
the canal by an aperture through which a small branch of the facial
nerve passes to stapedius. Innervation The mastoid air cells are innervated by a meningeal
The fossa incudis is a small depression in the lower and posterior branch of the mandibular division of the trigeminal nerve.
part of the epitympanic recess. It contains the short process of the incus,
Mastoiditis
which is fixed to the fossa by ligamentous fibres.
Mastoiditis is a potentially dangerous, life-threatening condition that
Mastoid antrum develops as a result of the spread of bacterial infection from the tym-
The mastoid antrum is an air sinus in the petrous part of the temporal panic cavity via the aditus to the mastoid antrum and associated
bone. Its topographical relations are of considerable surgical impor- mastoid air cells. Occasionally, the infection may spread through the
tance. The aditus to the mastoid antrum, which leads back from the tegmen tympani to the dura mater of the middle cranial fossae, to cause
epitympanic recess, opens in the upper part of its anterior wall. The meningitis or a temporal lobe abscess. Similar spread may be seen into
lateral semicircular canal lies medial to the aditus. The descending part the posterior cranial fossa and cerebellum.
of the facial nerve canal is anteroinferior. The medial wall is related to Infection may spread laterally through the cortical bone of the lateral


the posterior semicircular canal. The sigmoid sinus lies some distance aspect of the mastoid process to form a subperiosteal postauricular
posteriorly; the distance can be extremely variable and is dependent on abscess (Bezold’s abscess), or through the cortical bone of the tip of the
the degree of pneumatization of the mastoid. The roof is formed by the mastoid process to the attachment of the posterior belly of digastric and
tegmen tympani, and so the antrum lies below the middle cranial fossa sternocleidomastoid, which stimulates painful muscular contraction
and the temporal lobe of the brain. The floor has several openings that and torticollis.
communicate with the mastoid air cells. The lateral wall, which offers
Anterior wall
the usual surgical approach to the cavity, is formed by the postmeatal
process of the squamous part of the temporal bone. This is only 2 mm
thick at birth but increases at an average rate of 1 mm a year, attaining The inferior, larger area of the anterior wall of the tympanic cavity is
a final thickness of 12–15 mm. In adults, the lateral wall of the antrum narrowed by the approximation of the medial and lateral walls of the
corresponds to the suprameatal triangle (Macewen’s triangle) on the cavity (see Fig. 37.6). It is a thin lamina and forms the posterior wall
outer surface of the skull. This is palpable through the cymba conchae. of the carotid canal. It is perforated by the superior and inferior carotico-
The superior side of the triangle, the supramastoid crest, is level with tympanic nerves and the tympanic branch or branches of the internal
the floor of the middle cranial fossa; the anteroinferior side, which carotid. The canals for tensor tympani and the osseous part of the
forms the posterosuperior margin of the external acoustic meatus, indi- pharyngotympanic tube open above it, the canal for tensor tympani
cates approximately the position of the descending part of the facial being superior to that for the pharyngotympanic tube. Both canals
nerve canal; and the posterior side, formed by a posterior vertical incline downwards and anteromedially, to open in the angle between


tangent to the posterior margin of the external acoustic meatus, is ante- the squamous and petrous parts of the temporal bone, and are sepa-
rior to the sigmoid sinus. rated by a thin, osseous septum. The canal for tensor tympani and the
The adult capacity of the mastoid antrum is variable, but on average bony septum runs posterolaterally on the medial tympanic wall, and
is 1 ml, with a general diameter of 10 mm. Unlike the other air sinuses ends immediately above the fenestra vestibuli. Here, the posterior end
in the skull, it is present at birth, and indeed is then almost adult in of the septum is curved laterally to form a pulley, the processus coch-
size, although it is at a higher level relative to the external acoustic leariformis (cochleariform process), which is a surgical landmark for
meatus than it is in adults. In the very young, the thinness of the lateral the identification of the geniculate ganglion of the facial nerve. The
antral wall and the absence or underdevelopment of the mastoid tendon of tensor tympani turns laterally over the pulley before attach-
process mean that the stylomastoid foramen and emerging facial nerve ing to the upper part of the handle of the malleus.
are very superficially situated.
Pharyngotympanic tube blockage in children
Mastoid air cells
Though the mastoid antrum is well developed at birth, the mastoid air The pharyngotympanic tube serves to ventilate the middle ear, exchang-
cells are merely minute antral diverticula at this stage. As the mastoid ing nasopharyngeal air with the air in the middle ear, which has been
develops in the second year, the air cells gradually extend into it and altered in its composition via transmucosal gas exchange with the
by the fourth year they are well formed, although their greatest growth haemoglobin in the blood vessels of the mucosa. The tube also carries
occurs at puberty. They vary considerably in number, form and size. mucus from the middle ear cleft to the nasopharynx as a result of ciliary
Usually, they interconnect and are lined by a mucosa with squamous transport.


non-ciliated epithelium, continuous with that in the mastoid antrum In children, the pharyngotympanic tube is relatively narrow. It is
and tympanic cavity. They may fill the mastoid process, even to its tip, prone to obstruction when the mucosa swells in response to infection
and some may be separated from the sigmoid sinus and posterior or allergic challenge; obstruction results in a relative vacuum being
cranial fossa only by extremely thin bone, which is occasionally defi- created in the middle ear secondary to transmucosal gas exchange, and
cient (see Fig. 37.11). Some may lie superficial to, or even behind, the this in turn promotes mucosal secretion and the formation of a middle
sigmoid sinus, and others may be present in the posterior wall of the ear effusion. Because of the collapsibility of the pharyngotympanic
descending part of the facial nerve canal. Those in the squamous part tube, the vacuum thus created can overcome the distending effect of the
of the temporal bone may be separated from deeper cells in the petrous muscles of the tube and ‘lock’ the tube shut. The resultant persistent
part by a plate of bone in the line of the squamomastoid suture (Körn- middle ear effusion, otitis media with effusion (glue ear), can cause
er’s septum). Sometimes, they extend only minimally into the mastoid hearing loss by splinting the tympanic membrane and impeding its
process, in which case the process consists largely of dense bone or vibration. It can also provide an ideal environment for the proliferation
trabecular bone containing bone marrow. Varieties of the mastoid of bacteria, with the result that an acute otitis media may develop (see
process are recognized. The three types most commonly described are above). It is possible to relieve the vacuum and unlock the tube, and
pneumatized (with many air cells); sclerotic or diploic (with few or no then remove the effusion by myringotomy, i.e. by surgically creating a
air cells); and mixed (contain both air cells and bone marrow). hole in the tympanic membrane. This hole will generally heal rapidly
The mastoid process may sometimes have no air cells at all. Alter- and it is common practice to insert a flanged ventilation tube (a


natively, air cells may extend beyond the mastoid process into the grommet or tympanostomy tube) to keep the hole open. Migration of
squamous part of the temporal bone above the supramastoid crest; into the outer squamous layer of the tympanic membrane eventually dis-
the posterior root of the zygomatic process of the temporal bone; into places the tube and the myringotomy heals.
the osseous roof of the external acoustic meatus just below the middle
cranial fossa; or into the floor of the tympanic cavity very close to the
superior jugular bulb. Rarely, a few may excavate the jugular process of AUDITORY OSSICLES
the occipital bone. An important group may extend medially into the
petrous part of the temporal bone, even to its apex, and are related A chain of three mobile ossicles – the malleus, incus and stapes –
to the pharyngotympanic tube, carotid canal, labyrinth and abducens transfers sound waves across the tympanic cavity from the tympanic


ExTERnAl And middlE EAR
636
4
nOiTCES
Malleus Fig. 37.12 The left Incus
Head
auditory ossicles.
The incus is shaped less like an anvil, from which it is named, than a
Facet for premolar tooth with its two diverging roots. It has a body and two
incus processes (see Fig. 37.12). The body is somewhat cubical but laterally
compressed. On its anterior surface, it has a saddle-shaped facet for
articulation with the head of the malleus. The long process, rather more
Lateral Neck
process Anterior than half the length of the handle of the malleus, descends almost verti-
process cally, behind and parallel to the handle. Its lower end bends medially
and ends in a rounded lenticular process, the medial surface of which
Handle is covered with cartilage and articulates with the head of the stapes. The
short process, somewhat conical, projects backwards and is attached by
ligamentous fibres to the fossa incudis in the lower and posterior part
Posterior aspect Medial aspect
of the epitympanic recess. The presence of a single nutrient vessel within
the long process of the incus and the absence of any collateral circula-
tion render this segment of the bone susceptible to aseptic necrosis
Incus
following middle ear infections.
Facet for
malleus Ossification
The incus has a cartilaginous precursor continuous with the dorsal
extremity of Meckel’s cartilage. Ossification often spreads from a single
Short limb centre in the upper part of its long process in the fourth fetal month;
Long limb
the lenticular process may have a separate centre.
Body
Lenticular
process Stapes
Medial aspect Anterior aspect
The stapes is also known as the stirrup. It has a head, neck, two limbs
(processes or crura) and a base (footplate) (see Fig. 37.12). The head
Stapes Head (caput) is directed laterally and has a small cartilaginous facet for
articulation with the lenticular process of the incus. The neck is the
Neck constricted part supporting the head, and the tendon of stapedius is
attached to its posterior surface. The processes diverge from the neck


Posterior process Anterior process
and are connected at their ends by a flattened oval plate, the base, which
Base (footplate) forms the footplate of the stapes. The footplate is attached to the margin
of the fenestra vestibuli by a ring of fibres (the anular ligament). The
Superior aspect Basal aspect
anterior process is shorter, thinner and less curved than the posterior.
Ossification
membrane to the fenestra vestibuli. The malleus is attached to the
tympanic membrane, and the base (footplate) of the stapes is attached The stapes is preformed in the perforated dorsal moiety of the hyoid
to the rim of the fenestra vestibuli. The incus is suspended between arch cartilage of the fetus. Ossification starts from a single endochon-
them and articulates with both bones. dral centre, which appears in the base in the fourth month in utero and
then gradually spreads through the limbs of the stapes to reach the
Malleus head.
At birth, the auditory ossicles have achieved their full adult size and
configuration. They increase in density during the first years of life as
The malleus is the largest of the ossicles and is shaped somewhat like
marrow cavities are replaced with endosteal bone and fine trabeculae
a mallet (Fig. 37.12). It is 8–9 mm long and has a head, neck, handle
(Isaacson 2014).
(manubrium) and anterior and lateral processes. The head is the large
upper end of the bone and is situated in the epitympanic recess. It is
Ossicular ligaments
ovoid in shape, articulates posteriorly with the incus, and is covered
elsewhere by mucosa. The cartilaginous articular facet for the incus is
narrowed near its middle and consists of a larger upper part and a The ossicles are connected to the tympanic walls by ligaments (see Fig.
smaller lower part, orientated almost at right angles to each other. 37.6B): three for the malleus and one each for the incus and stapes.
Opposite the constriction, the lower margin of the facet projects in the Some are mere mucosal folds that carry blood vessels and nerves to and


form of a process, the spur of the malleus. The neck is the narrowed from the ossicles and their articulations, and others contain a central,
part below the head, and inferior to this is an enlargement from which strong band of collagen fibres.
the anterior and lateral processes project. The anterior ligament of the malleus stretches from the neck of the
The handle of the malleus is connected by its lateral margin to the malleus, just above the anterior process, to the anterior wall of the
tympanic membrane (see Figs 37.6, 37.10, 37.15A). It is directed down- tympanic cavity near the petrotympanic fissure. Some of its collagen
wards, medially and backwards. It decreases in size towards its free end, fibres traverse this fissure to reach the spine of the sphenoid, and others
which is curved slightly forwards and is flattened transversely. Near the continue into the sphenomandibular ligament. The latter, like the an-
upper end of its medial surface there is a slight projection to which the terior malleolar ligament, is derived from the perichondrial sheath of
tendon of tensor tympani is attached. The anterior process is a delicate Meckel’s cartilage. The anterior malleolar ligament may contain muscle
bony spicule, directed forwards from the enlargement below the neck, fibres, called laxator tympani or musculus externus mallei. The lateral
and connected to the petrotympanic fissure by ligamentous fibres. In ligament of the malleus is a triangular band that stretches from the
fetal life, it is the longest process of the malleus and is continuous in posterior part of the border of the tympanic incisure to the head of
front with Meckel’s cartilage. The lateral process is a conical projection the malleus. The superior ligament of the malleus connects the head of
from the root of the handle of the malleus. It is directed laterally, and the malleus to the roof of the epitympanic recess.
is attached to the upper part of the tympanic membrane and, via the The posterior ligament of the incus connects the end of its short
anterior and posterior malleolar folds, to the sides of the notch in the process to the fossa incudis. The superior ligament of the incus is little


upper part of the tympanic sulcus. more than a mucosal fold passing from the body of the incus to the
roof of the epitympanic recess.
Ossification The vestibular surface and rim of the stapedial base are covered with
The cartilaginous precursor of the malleus originates as part of the hyaline cartilage. The cartilage encircling the base is attached to the
dorsal end of Meckel’s cartilage. With the exception of its anterior margin of the fenestra vestibuli by a ring of elastic fibres, the anular
process, the malleus ossifies from a single endochondral centre that ligament of the base of the stapes. The posterior part of this ligament
appears near the future neck of the bone in the fourth month in utero. is much narrower than the anterior part; it acts as a kind of hinge on
The anterior process ossifies separately in dense connective tissue and which the stapedial base moves when stapedius contracts and during
joins the rest of the bone at about the sixth month in utero. acoustic oscillation.


middle ear
637
73
RETPAHC
Ossicular articulations Tensor tympani
Tensor tympani is a long, slender muscle that occupies the bony canal
The ossicular articulations are typical synovial joints. The incudomall- above the osseous part of the pharyngotympanic tube, from which it is
eolar joint is saddle-shaped and the incudostapedial joint is a ball and separated by a thin bony septum (see Fig. 37.6). It arises from the
socket articulation. The articular surfaces are covered with articular cartilaginous part of the pharyngotympanic tube and the adjoining
cartilage, and each joint is enveloped by a capsule rich in elastic tissue region of the greater wing of the sphenoid, as well as from its own canal.
and lined by synovial membrane. Versican, fibronectin, elastin and It passes back within its canal, and ends in a slim tendon which bends
hyaluronan have been co-localized in the attachments of tensor tympani laterally round the pulley-like processus cochleariformis and attaches
and stapedius to the malleus and stapes, respectively, and in the anular to the handle of the malleus, near its root (see Fig. 37.14).
ligament of the incudostapedial joint in histological sections of adult
middle ears (Kawase et al 2012). Co-localization between elastic fibres Vascular supply Tensor tympani receives its arterial blood supply
and hyaluronan in middle ear entheses looks to be a postnatal matu- from the superior tympanic branch of the middle meningeal artery.
rational event, possibly an adaptation to acoustic oscillation producing
multidirectional mechanical stress (Takanashi et al 2013). Innervation Tensor tympani is innervated by a branch of the nerve to
medial pterygoid (a ramus of the mandibular division of the trigeminal
Movements of the auditory ossicles
nerve), which traverses the otic ganglion without interruption to reach
The handle of the malleus faithfully follows all movements of the tym- the muscle.
panic membrane. The malleus and incus rotate together around an axis


that runs from the short process and posterior ligament of the incus to Actions Tensor tympani draws the handle of the malleus medially,
the anterior ligament of the malleus. When the tympanic membrane
and so tenses the tympanic membrane and helps to damp sound vibra-
and handle of the malleus move inwards (medially), the long process
tions; its action also pushes the base of the stapes more tightly into the
of the incus moves in the same direction and pushes the stapedial
fenestra vestibuli.
footplate towards the labyrinth and the perilymph contained within the
labyrinth. The movement of the perilymph causes a compensatory Stapedius
outward bulging of the secondary tympanic membrane. These events
Stapedius arises from the wall of a conical cavity in the pyramidal emi-
are reversed when the tympanic membrane moves outwards. If the nence on the posterior wall of the tympanic cavity, and from its continu-
movement of the tympanic membrane is considerable, however, the
ation anterior to the descending part of the facial nerve canal. Its minute
incus does not follow the full outward excursion of the malleus and
tendon emerges from the orifice at the apex of the pyramid and passes
merely glides on it at the incudomalleolar joint, so preventing a disloca-
forwards to attach to the posterior surface of the neck of the stapes. The
tion of the footplate of the stapes from the fenestra vestibuli. When the
muscle is of an asymmetric bipennate form. It contains numerous small
handle of the malleus is carried medially, the spur at the lower margin
motor units, each of only 6–9 muscle fibres; a few neuromuscular
of the head of the malleus locks the incudomalleolar joint, and this
spindles exist near the myotendinous junction.
necessitates an inward movement of the long process of the incus. The
joint is unlocked again when the handle of the malleus is carried out-
Vascular supply Stapedius receives its arterial blood supply from
wards. The three bones together act as a bent lever so that the stapedial
branches of the posterior auricular, anterior tympanic and middle
footplate does not move in the fenestra vestibuli like a piston, but rocks


meningeal arteries.
on a fulcrum at its anteroinferior border, where the anular ligament is
thick. The rocking movement around a vertical axis, which is like a
Innervation Stapedius is supplied by a branch of the facial nerve that
swinging door, is said to happen only at moderate intensities of sound.
is given off in the facial canal.
With loud, low-pitched sounds, the axis becomes horizontal, and the
upper and lower margins of the stapedial footplate oscillate in opposite
Actions Stapedius helps to damp down excessive sound vibrations. It
directions around this central axis, thus preventing excessive displace-
opposes the action of tensor tympani (which pushes the stapes more
ment of the perilymph.
tightly into the fenestra vestibuli). Paralysis of stapedius results in
Otosclerosis, stapedectomy and hyperacusis.
stapedotomy
Stapedial and tensor tympani reflexes When noises are loud,
Otosclerosis is a localized disease of the bone derived from the embry-
and immediately before speaking, a reflex contraction of stapedius and
onic otic capsule in which lamellar bone is replaced by woven bone of
tensor tympani takes place that helps damp down the movement of the
greater thickness and vascularity. It may develop spontaneously but is
ossicular chain before vibrations reach the internal ear. The afferent
often seen in pedigrees; the inherited form is associated with mutations
pathways involve the auditory component of cranial nerve VIII, and
of the RELN gene. The position of the focus of new bone formation
higher centres prior to speech. The efferent pathway involves the facial
determines its effect on the function of the ear. When new bone devel-
nerve (stapedius) and the mandibular nerve (tensor tympani).
ops around the footplate of the stapes, it may fix the footplate to the
margin of the fenestra vestibuli and prevent it from moving. This
impedes the passage of vibrations of the tympanic membrane passing
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE
through the ossicular chain to the inner ear, producing a conductive
hearing loss. Otosclerosis affecting other parts of the otic capsule is


thought to cause a sensorineural element to the overall hearing loss. A number of arteries supply the walls and contents of the tympanic
Stapedectomy is a surgical procedure designed to bypass the fixation cavity. The deep auricular, anterior tympanic and stylomastoid arteries
of the stapes footplate caused by otosclerosis or congenital fixation. The are larger than the others.
tympanic membrane is temporarily elevated for access to the middle The deep auricular branch of the first part of the maxillary artery
ear and, under microscopic control, the incudostapedial joint is disar- often arises with the anterior tympanic artery. It ascends in the parotid
ticulated. The limbs of the stapes and stapedius are then both divided gland behind the temporomandibular joint, pierces the cartilaginous
and the superstructure of the stapes removed. A small hole (stapedot- or bony wall of the external acoustic meatus and supplies its cuticular
omy) is then made in the fixed footplate of the stapes using a microdrill, lining, the exterior of the tympanic membrane and the temporoman-
reamer or laser to expose the fluids of the inner ear. A small graft of dibular joint.
connective tissue is used to seal the hole with a flexible membrane. A The anterior tympanic branch of the first part of the maxillary artery
piston, usually made of Teflon or titanium incorporating a wire made ascends behind the temporomandibular joint and enters the tympanic
of stainless steel, platinum or titanium, is crimped on to the long cavity through the petrotympanic fissure. It ramifies on the interior of
process of the incus and placed in the perforation in the stapes foot- the tympanic membrane, and forms a vascular circle around it with the
plate. The tympanic membrane is then returned. The connection posterior tympanic branch of the stylomastoid artery. It also anasto-
between the tympanic membrane and the inner ear is thus reconstituted moses with twigs of the artery of the pterygoid canal and caroticotym-
and hearing restored. panic branches of the internal carotid artery in the mucosa of the
tympanic cavity.
The stylomastoid branch of the occipital or posterior auricular arter-


MUSCLES ies supplies the posterior part of the tympanic cavity and mastoid air
cells. It also enters the stylomastoid foramen to supply the facial nerve
There are two intratympanic muscles: tensor tympani and stapedius and semicircular canals. In the young, its posterior tympanic branch
(Anderson 1976). forms a circular anastomosis with the anterior tympanic artery.


ExTERnAl And middlE EAR
638
4
nOiTCES
The smaller arteries supplying the tympanic cavity include the pet- inferior ganglion of the glossopharyngeal nerve, and reaches the tym-
rosal branch of the middle meningeal artery, which enters through the panic cavity via the tympanic canaliculus for the tympanic nerve. The
hiatus for the greater petrosal nerve; the superior tympanic branch of superior and inferior caroticotympanic nerves are postganglionic sym-
the middle meningeal artery, which traverses the canal for tensor pathetic fibres that are derived from the carotid sympathetic plexus and
tympani; an inferior tympanic branch from the ascending pharyngeal traverse the wall of the carotid canal to join the plexus.
artery, which traverses the tympanic canaliculus together with the tym- The tympanic plexus supplies branches to the mucosa of the tym-
panic branch of the glossopharyngeal nerve, to supply the medial wall panic cavity, pharyngotympanic tube and mastoid air cells. It sends a
of the tympanic cavity; a branch from the artery of the pterygoid canal, branch to the greater petrosal nerve via an opening anterior to the
which accompanies the pharyngotympanic tube; and a tympanic fenestra vestibuli. The lesser petrosal nerve, which may be regarded as
branch or branches from the internal carotid artery, which is given off the continuation of the tympanic branch of the glossopharyngeal nerve,
in the carotid canal and perforates the thin anterior wall of the tympanic traverses the tympanic plexus. It occupies a small canal below that for
cavity. the tensor tympani. It runs past, and receives a connecting branch from,
The mastoid air cells and dura mater are also supplied by a mastoid the geniculate ganglion of the facial nerve. The lesser petrosal nerve
branch from the occipital artery. It is small in size and sometimes emerges from the anterior surface of the temporal bone via a small
absent. When present, it enters the cranial cavity via the mastoid opening lateral to the hiatus for the greater petrosal nerve and then
foramen near the occipitomastoid suture. traverses either the foramen ovale or the sphenopetrosal fissure or the


In early fetal life, a stapedial artery traverses the stapes. innominate canal (of Arnold) to join the otic ganglion. Postganglionic
The veins from the tympanic cavity terminate in the pterygoid secretomotor fibres leave this ganglion in the auriculotemporal nerve
venous plexus and the superior petrosal sinus. A small group of veins to supply the parotid gland. In former times, the operation of tympanic
runs medially from the mucosa of the mastoid antrum through the arch neurectomy was undertaken to reduce salivation in patients who
formed by the superior (anterior) semicircular canal, and emerges on drooled and to diminish facial sweating in those with Frey’s syndrome
to the posterior surface of the petrous temporal bone at the subarcuate following parotidectomy; less invasive and more effective procedures
fossa. These veins drain into the superior petrosal sinus and are the are employed nowadays.
remains of the large subarcuate veins of childhood. They represent a
potential route for the spread of infection from the mastoid antrum to
Facial nerve
the meninges.
Lymphatic vessels of the tympanic and antral mucosae drain to the
parotid or upper deep cervical lymph nodes. Vessels draining the tym- The facial nerve consists of intracranial (cisternal), intratemporal and
panic end of the pharyngotympanic tube probably end in the deep extratemporal portions. The intratemporal portion is further divided
cervical nodes. into meatal, labyrinthine, tympanic (horizontal) and mastoid (vertical)
segments (Fig. 37.14). The cisternal portion emerges from the ventro-
lateral aspect of the caudal border of the pons. The meatal portion
INNERVATION enters the porus acusticus of the internal auditory meatus accompanied
by the nervus intermedius, the vestibulocochlear nerve and the labyrin-
The tympanic cavity contains the tympanic plexus and the facial nerve. thine vessels (see Fig. 37.7). In this part of its course, the nerve lacks a
Branches from the plexus and the facial nerve supply structures within fibrous sheath or endoneurium and is surrounded by a thin layer of


the tympanic cavity but also leave the cavity to supply structures on the arachnoid. The motor root, which supplies the muscles of the face, and
face. the nervus intermedius, which contains sensory fibres concerned with
the perception of taste and parasympathetic (secretomotor) fibres to
Tympanic plexus various glands, are still separate components; they usually merge within
the meatus. The labyrinthine segment runs from the fundus of the
The nerves that constitute the tympanic plexus ramify on the surface of internal acoustic meatus to the geniculate ganglion, where the nerve
the promontory on the medial wall of the tympanic cavity. They are makes the first bend or genu (see Figs 37.7, 37.13). The labyrinthine
derived from the tympanic branch of the glossopharyngeal nerve and segment is the shortest and narrowest part of the facial nerve; it lacks
the caroticotympanic nerves (Fig. 37.13). The former arises from the anastomosing arterial cascades, which renders it susceptible to vascular
compression. The tympanic part initially curves around the oval window
niche, then lies just anterior and inferior to the lateral semicircular canal
and bends again at the second genu to become the vertical or mastoid
Motor root part. The mastoid part is the longest of the petrous segments and runs
from the pyramidal process to the stylomastoid foramen. The tympanic
and mastoid segments of the facial nerve are supplied by the facial arch,
Sensory root
formed by the petrosal branch of the middle meningeal artery and the
Geniculate ganglion of facial nerve
stylomastoid branch of the posterior auricular artery. The extratemporal
Greater petrosal nerve Facial nerve course of the facial nerve is described on page 502.
Nerve of The branches that arise from the facial nerve within the temporal
pterygoid canal
bone can be divided into those that come from the geniculate ganglion
and those that arise within the facial canal.
Internal carotid Tympanic plexus
The main branch from the geniculate ganglion is the greater (super-
artery
Deep Nerve to stapedius ficial) petrosal nerve, a branch of the nervus intermedius. The greater


petrosal petrosal nerve passes anteriorly, receives a branch from the tympanic
nerve plexus and traverses a hiatus on the anterior surface of the petrous part
Caroticotympanic nerve(s) Middle of the temporal bone. It enters the middle cranial fossa, runs forwards
meningeal
Lesser petrosal nerve Chorda tympani in a groove on the bone above the lesser petrosal nerve, and then passes
plexus
beneath the trigeminal ganglion to reach the foramen lacerum. Here it
Otic ganglion
is joined by the deep petrosal nerve from the internal carotid sympa-
Nerve to medial To auricular thetic plexus, to become the nerve of the pterygoid canal (Vidian
pterygoid ramus of vagus nerve). The greater petrosal nerve contains parasympathetic fibres des-
tined for the pterygopalatine ganglion, and taste fibres from the palate.
Anterior division of T bry am np ca hnic Glossopharyngeal The geniculate ganglion also communicates with the lesser petrosal
mandibular nerve nerve nerve.
Auriculo- Posterior The nerve to stapedius arises from the facial nerve in the facial nerve
auricular
Inferior alveolar nerve temporal nerve nerve canal behind the pyramidal eminence of the posterior wall of the tym-
panic cavity and passes forwards through a small canal to reach the
Lingual nerve muscle.
Fig. 37.13 The intracranial branches of the facial and glossopharyngeal The chorda tympani (see Fig. 37.13; Fig. 37.15) leaves the facial
nerves and the connections of the tympanic plexus and the otic ganglion. nerve some 6 mm above the stylomastoid foramen and runs antero-
The course of the taste fibres from the mucous membrane of the palate superiorly in a canal to enter the tympanic cavity via the posterior
and from the anterior presulcal part of the tongue is represented by the canaliculus. It then curves anteriorly in the substance of the tympanic
blue lines. membrane between its mucous and fibrous layers (see Fig. 37.15A), and


middle ear
639
73
RETPAHC
Incus Malleus Meatal foramen
Tympanic segment Internal auditory artery
Stapes Labyrinthine segment
Geniculate ganglion
First genu
Lateral semicircular
canal
Supratubal recess
Greater petrosal nerve
Second genu
Stapedius Cochleariform
process
Mastoid segment
Round window
Stylomastoid artery
Facial nerve
Digastric ridge Styloid process
Posterior belly,
digastric muscle
Fig. 37.14 The anatomy of the infratemporal portion of the facial nerve and associated middle ear structures. The nerve is vulnerable to injury at several
sites. Perigeniculate region: nerve compression and ischaemia are most likely to occur in the narrow meatal and labyrinthine segments, which are the
vascular watershed areas of branches of the external carotid artery and the posterior circulation; the first genu is tethered by the greater petrosal nerve,
increasing susceptibility to shearing injuries; and the geniculate ganglion is susceptible to injury during surgical dissection in the supratubal recess of the
anterior epitympanum. Tympanic segment: the nerve is most frequently dehiscent above the oval window; the distal tympanic segment and the second
genu are both susceptible to injury in cholesteatoma surgery as a result of pathological dehiscence or distorted anatomy and failure to identify important
surgical landmarks. Mastoid segment: the facial nerve is positioned lateral to the tympanic anulus and just distal to the stylomastoid foramen in the lower
portion of its vertical course and is therefore susceptible to injury during surgery involving the external acoustic meatus. (Redrawn from Francis HW 2010
Anatomy of the temporal bone, external ear, and middle ear. In: Flint PW, Haughey BH, Lund VJ, et al (eds) Cummings Otolaryngology: Head & Neck
Surgery, 5th ed. Mosby. Ch 127, 1821–1830.)
crosses medial to the upper part of the handle of the malleus to the flaccid paralysis of the ipsilateral muscles of facial expression; decreased
anterior wall of the tympanic cavity, where it enters the anterior cana- lacrimation in the ipsilateral eye (which is controlled by neurones in


liculus. It exits the skull at the petrotympanic fissure, and its further the greater petrosal nerve); and hyperacusis or decreased tolerance of
course is described on page 552. The chorda tympani contains para- loud noises in the ipsilateral ear as a result of paralysis of stapedius. Its
sympathetic fibres that supply the submandibular and sublingual sali- cause remains the subject of speculation. Magnetic resonance imaging
vary glands via the submandibular ganglion and taste fibres from the studies suggest that there are inflammatory changes in labyrinthine and
anter ior two-thirds of the tongue. perigeniculate segments of the facial nerve. In the vast majority of cases,
spontaneous full recovery takes place after a few weeks.
Dehiscences of the facial nerve canal
The facial nerve may be somewhat variable in its anatomical course
through the temporal bone (Proctor and Nager 1982). It may split into TYMPANIC MUCOSA
two or three strands, starting at the geniculate ganglion, and then
make its way across the promontory to the stylomastoid foramen, or The mucosa of the tympanic cavity is pale, thin and slightly vascular. It
pass a few millimetres posteriorly to its second genu, before it turns is continuous with that of the pharynx, via the pharyngotympanic tube,
inferiorly posterior to the fossa incudis, a position where it is particu- and covers the ossicles, muscles and nerves in the cavity to form the
larly vulnerable during surgical exploration of the mastoid antrum. inner layer of the tympanic membrane and the outer layer of the sec-
The more proximal the division into strands, the more bizarre is the ondary tympanic membrane. It also spreads into the mastoid antrum
subsequent course. More distal bifurcations pass either side of the and air cells. The middle ear mucosa is a mucus-secreting respiratory
fenestra vestibuli. It may be dehiscent, particularly in its second part, mucosa bearing cilia on its surface. The precise distribution of the
when it occasionally overhangs the stapes, or run inferior to the stape- mucociliary epithelium varies in normal middle ears, being more wide-


dial superstructure, a position that renders it vulnerable during surgery spread in the young. Three distinct mucociliary pathways have been
to the stapes (Barnes et al 2001). The motor fibres to the face may be identified – epitympanic, promontorial and hypotympanic – the latter
carried through the chorda tympani, which is then enlarged. When being the largest. Each of these pathways coalesces at the tympanic
this is the case, the distal facial nerve dwindles to a fibrous strand in a orifice of the pharyngotympanic tube (Gleeson et al 1991).
narrowed stylomastoid foramen. In chronic bone disease in the tym- The mucosa forms several vascular folds that extend from the tym-
panic cavity, the facial nerve may be exposed in its canal. Inflammation panic walls to the ossicles: one descends from the roof of the cavity to
may lead to facial paralysis of the infranuclear or lower motor neurone the head of the malleus and the upper margin of the body of the incus,
type. and a second surrounds the stapedius. Other folds invest the chorda
tympani nerve and tensor tympani. The folds separate off saccular
Bell’s palsy recesses that give the interior of the tympanic cavity a somewhat
Bell’s palsy is the eponym given to an idiopathic lower motor neurone honeycombed appearance; these mucosal folds have been described in
facial palsy. It may be complete or partial, and is characterized by a greater detail by Proctor (1964). Of note, the superior recess of the


ExTERnAl And middlE EAR
640
4
nOiTCES
A Handle of malleus Chorda tympani
Tegmen tympani
Epitympanic recess
Tensor tympani
Pharyngotympanic tube,
osseous part
Head of malleus
Chorda tympani
Incus
Mastoid Aditus to Tympanic membrane
antrum mastoid antrum
Bristle passed into
facial nerve canal Styloid process B
through stylomastoid foramen
Tympanic Tympanic Long process of incus
anulus membrane
Fig. 37.15 The chorda tympani nerve. A, An oblique vertical section through the left temporal bone, to show the roof and lateral wall of the middle ear,
the chorda tympani and the mastoid antrum. B, The chorda tympani crossing the tympanic membrane, auroscopic view. (B, Courtesy of Mr Simon A
Hickey.)
tympanic membrane, Prussak’s space, lies between the neck of the and carrying infection to the soft tissues. Cholesteatoma can therefore
malleus and the pars flaccida, bounded by the lateral malleolar fold. cause deafness through damage to the ossicles and inner ear; problems
This space can play an important role in the retention of keratin and with balance through damage to the vestibule and semicircular canals;
subsequent development of cholesteatoma. facial palsy through ischaemia and necrosis of the facial nerve; and
intracranial sepsis. Treatment involves microsurgical dissection of the
Cholesteatoma
invading sac of epithelium with, wherever possible, preservation of the
Cholesteatoma is the name given to keratinizing squamous epithelium delicate structures just listed.
within the middle ear. There is debate as to how such epithelium comes
to be in the middle ear. Theories include development from embryo-
logical cell rests, metaplasia from inflamed mucoperiosteum, and aber- Bonus e-book images and table
rant migration of squamous epithelium either through a perforation in
the tympanic membrane (usually in the pars flaccida or posterosuperior


pars tensa) or within an area of tympanic membrane atelectasis where Fig. 37.9 C, D, E, Coronal CT scans showing details of the
the tympanic membrane becomes adherent to the medial wall of the ossicular mass. C, Malleus and tensor tympani. D, Incus. E, Incus
tympanic cavity. It is likely that all of these processes may be implicated and incudostapedial joint.
at some time. A feature of cholesteatoma that remains poorly under-
stood is its ability to erode bone, by activating osteoclasts, thus allowing Table 37.1 Common anomalies of the pinna.
the epithelium to proliferate and invade, destroying the temporal bone
KEY REFERENCES
Allam AF 1969 Pneumatization of the temporal bone. Ann Otol Rhinol Proctor B, Nager GT 1982 The facial canal: normal anatomy, variations and
Laryngol 78:49–64. anomalies. Ann Otol Rhinol Laryngol 91:33–61.
Anderson SD 1976 The intratympanic muscles. In: Hinchcliffe R (ed) A detailed anatomical description, emphasising the relations of the facial
Scientific Foundations of Otolaryngology. London: Heinemann; pp. canal to adjacent structures and variations in the course of the canal.
257–80.


External and middle ear
640.e1
73
RETPAHC
REFERENCES
Allam AF 1969 Pneumatization of the temporal bone. Ann Otol Rhinol Moschella F, Cordova A, Pirrello R et al 2003 The supra-auricular arterial
Laryngol 78:49–64. network: anatomical bases for the use of superior pedicle retro-auricular
Anderson SD 1976 The intratympanic muscles. In: Hinchcliffe R (ed) skin flaps. Surg Radiol Anat 25:3–4.
Scientific Foundations of Otolaryngology. London: Heinemann; Osborn AJ, Oghalai JS, Vrabec JT 2011 Middle ear volume as an adjunct
pp. 257–80. measure in congenital aural atresia. Int J Pediatr Otorhinolaryngol 75:
Barnes G, Liang J, Hall S et al 2001 Development of the Fallopian canal in 910–14.
humans – a morphologic and radiologic study. Otol Neurotol 22: Pata YS, Ozturk C, Akbas Y et al 2003 Has cerumen a protective role in
931–7. recurrent external otitis? Am J Otolaryngol 24:209–12.
Bluestone C, Klein J 2002 Otitis media, atelectasis and Eustachian tube Porter CJW, Tan ST 2005 Congenital auricular anomalies: topographic
dysfunction. In: Bluestone CD, Stool SE (eds) Pediatric Otolaryngology, anatomy, embryology, classification and treatment strategies. Plast
vol. 1, 4th ed. Philadelphia: Elsevier, Saunders; pp. 474–86. Reconstr Surg 115:1701–12.
Campos A, Betancor L, Arias A et al 2000 Influence of human wet cerumen Proctor B 1964 The development of the middle ear spaces and their surgical
on the growth of common and pathogenic bacteria of the ear. J Laryngol significance. J Otolaryngol 78:631–49.
Otol 114:925–9. Proctor B, Nager GT 1982 The facial canal: normal anatomy, variations and


Gleeson M, Felix H, Neivergelt J 1991 Quantitative and qualitative analysis anomalies. Ann Otol Rhinol Laryngol 91(suppl 93):33–61.
of the human middle ear mucosa. In: Sade J (ed) The Eustachian Tube, Purkait R 2013 Progression of growth in the external ear from birth to
Basic Aspects. Amsterdam: Kugler & Ghedini; pp. 125–31. maturity: a 2-year follow-up study in India. Aesthetic Plast Surg
Henson MM, Madden VJ, Rask-Andersen H et al 2005 Smooth muscle in 37:605–16.
the annulus fibrosus of the tympanic membrane in bats, rodents, insec- Simms DL, Neely JG 1989 Growth of the lateral surface of the temporal
tivores and humans. Hear Res 200:29–37. bone in children. Laryngoscope 99:795–9.
Imanishi N, Nakajima H, Aiso S 1997 Arterial anatomy of the ear. Okajimas Takanashi Y, Shibata S, Katori Y et al 2013 Fetal development of the
Folia Anat Jpn 73:313–23. elastic-fiber-mediated enthesis in the human middle ear. Ann Anat
Isaacson G 2014 Endoscopic anatomy of the pediatric middle ear. Otolaryn- 195:441–8.
gol Head Neck Surg 150:6–15. Yoshiura K, Kinoshita A, Ishida T et al 2006 A SNP in the ABCC11 gene is
Kawase T, Shibata S, Katori Y et al 2012 Elastic fiber-mediated enthesis in the determinant of human earwax type. Nat Genet 38:324–30.
the human middle ear. J Anat 221:331–40.
RECOMMENDED READING
Anson BJ, Donaldson JA 1976 The Surgical Anatomy of the Temporal Bone Wright A 1997 Anatomy and ultrastructure of the human ear. In: Kerr GA
and Ear. Philadelphia: Saunders. (ed) Scott Brown’s Otolaryngology, vol. 1, 6th ed. London: Butterworth–


Phelps PD, Lloyd GAS 1990 Diagnostic Imaging of the Ear, 2nd ed. Berlin: Heinemann; pp. 1–50.
Springer.


CHAPTER
38
Inner ear
The inner ear contains the organ of hearing and the organs of balance. A
Ampulla of anterior
All are located within the labyrinth, a series of interlinked cavities in semicircular canal
the petrous temporal bone containing interconnected membranous
Ampulla of lateral
sacs and ducts. All spaces within the labyrinth are filled with fluid. The semicircular canal Crus commune
different sacs contain sensory epithelia consisting of supporting cells
Cupula of cochlea (common limb)
and mechanosensory cells, the hair cells that underlie acoustico-lateralis
sensory systems in all vertebrates. In humans, there are six such mech-
anosensory epithelia: the organ of Corti within the cochlea (the hearing
organ); the utricle and saccule (static balance organs); and the cristae
of the semicircular canals (dynamic balance organs). Whilst sharing
the same basic structure, hair cells and the accessory systems that sur-
round them show specific adaptations to each of the different sensory
modalities.
The disarticulated temporal bone is described in detail in Chapter
37. The internal acoustic meatus (internal auditory canal) and bony
and membranous labyrinths are described here.
OSSEOUS (BONY) LABYRINTH
Vestibule Oval window Round window Ampulla of posterior
semicircular canal
The bony labyrinth consists of the vestibule (sacculus and utriculus/ B Anterior semicircular canal
saccule and utricle), semicircular canals and cochlea, which are all cavi- Opening of crus commune Crus commune
ties lined by periosteum and which contain the membranous labyrinth Elliptical recess
(Fig. 38.1). The bone is denser and harder than that of the other parts
of the petrous bone, and it is therefore possible, particularly in young Vestibular crest Posterior
skulls, to dissect the bony labyrinth out from the petrous temporal semicircular canal
bone.
Spherical recess
The osseous and membranous labyrinths are filled with fluid (Fig.
38.2). The gap between the internal wall of the osseous labyrinth and
the external surface of the membranous labyrinth is filled with peril-
ymph, a clear fluid with an ionic composition similar to that of other


extracellular fluids, i.e. low in potassium ions and high in sodium and
calcium. The membranous labyrinth contains endolymph, a fluid with
an ionic composition more like that of cytosol, i.e. high in potassium
ions and low in sodium and calcium. Moreover, the endolymphatic
compartment has an electrical potential that is approximately 80 mV
more positive than the perilymphatic compartment (the endolym-
phatic potential). These differences in ionic composition and potential,
maintained by homeostatic tissues in the walls of the labyrinth, are Lateral semicircular canal
essential to maximize the sensitivity of the mechanosensory hair cells
Osseous Scala vestibuli Round window Orifice of vestibular aqueduct
that convert the vibrations set up in the inner ear fluids by head or spiral lamina
Scala tympani Orifice of Cochlear recess
sound movements into electrical signals that are transmitted via the
cochlear recess
vestibulocochlear nerve to the vestibular and cochlear nuclei, respec-
Anterior
tively, in the brainstem.
semicircular duct
C Helicotrema Ampulla of anterior
semicircular duct Crus commune
VESTIBULE Utricle Ampulla of lateral
Hamulus semicircular duct
The vestibule is the central part of the bony labyrinth and lies medial Saccule P seo mst ie cr irio cr ular
to the tympanic cavity, posterior to the cochlea and anterior to the duct
semicircular canals (see Fig. 38.1). It is somewhat ovoid in shape but
flattened transversely, and (on average) measures 5 mm from front to
back and vertically, and 3 mm across. In its lateral wall is the opening
of the oval window (fenestra vestibuli), into which the base of the Cochlear duct
stapes inserts, and to which the base of the stapes is attached by an
anular ligament. Anteriorly, on the medial wall, is a small spherical Vestibule
recess that contains the saccule; it is perforated by several minute holes,
Oval window
the macula cribrosa media, which transmit fine branches of the vestibu-
lar nerve to the saccule. Behind the recess is an oblique vestibular crest, Ductus reuniens L sea mte ir ca irl cular duct


the anterior end of which forms the vestibular pyramid. This crest
Round window
Ampulla of posterior
divides below to enclose a small depression, the cochlear recess, which semicircular duct
Endolymphatic duct
is perforated by vestibulocochlear fascicles as they pass to the vestibular
end of the cochlear duct. Posterosuperior to the vestibular crest, in the Endolymphatic sac Utriculosaccular duct
roof and medial wall of the vestibule, is the elliptical recess (see Fig. Fig. 38.1 The left bony labyrinth. A, Lateral aspect. B, Interior. C, The
38.1B), which contains the utricle. The pyramid and adjoining part of membranous labyrinth (blue) projected on to the bony labyrinth. 641


Inner ear
642
4
nOITCeS
Interglobular spaces Semicircular duct Tunica propria canals in 10 human skulls led Blanks et al (1975) to suggest that
containing cartilage cells (containing endolymph) the planes of the three ipsilateral canals are not completely perpen-
dicular to each other. The angles were measured as: lateral/anterior
111.76 ± 7.55°, anterior/posterior 86.16 ± 4.72°, posterior/lateral
95.75 ± 4.66°. The planes of similarly orientated canals of the two sides
also showed some departure from being parallel: left anterior/right
posterior 24.50 ± 7.19°, left posterior/right anterior 23.73 ± 6.71°, left
lateral/right lateral 19.82 ± 14.93°. The same observers (Curthoys et al
1977) also measured the dimensions of the canals. The mean radii of
the osseous canals were found to be as follows: lateral 3.25 mm, ante-
rior 3.74 mm, posterior 3.79 mm. The diameters of the osseous canals
are 1 mm (minor axis) and 1.4 mm (major axis). The membranous
ducts within them are much smaller, but are also elliptical in transverse
section, and have major and minor axes of 0.23 and 0.46 mm (see Fig.
38.2). Representative means for ampullary dimensions are as follows:
length 1.94 mm, height 1.55 mm. Phylogenetic studies suggest that the
arc sizes of the semicircular canals in humans and other primates may
be functionally linked to sensory control of body movements. The
angulation and dimensions of the canals may be related to locomotor
behaviour and possibly to agility, or more specifically to the frequency
spectra of natural head movements (see review by Spoor and Zonneveld
(1998)).
COCHLEA
Semicircular canal The cochlea (from the Greek cochlos for snail) is the most anterior part
(containing perilymph) Endosteum Blood vessels of the labyrinth, lying in front of the vestibule (see Figs 38.1 and 38.9A).


Fig. 38.2 A transverse section through the left posterior semicircular It is 5 mm from base to apex, and 9 mm across its base. Its apex, or
canal and duct. cupula, points towards the anterosuperior area of the medial wall of
the tympanic cavity (see Fig. 38.9A). Its base faces the bottom of the
internal acoustic meatus and is perforated by numerous apertures for
the cochlear nerve. The cochlea has a conical central bony core, the
the elliptical recess are perforated by a number of holes, the macula modiolus, and a spiral canal runs around it. A delicate osseous spiral
cribrosa superior; those in the former transmit nerves to the utricle and lamina (or ledge) projects from the modiolus, partially dividing the
those in the latter transmit nerves to the ampullae of the anterior and canal (see Fig. 38.9B). Within this bony spiral lies the membranous
lateral semicircular canals (see Fig. 38.1B). The region of the pyramid cochlear duct, attached to the modiolus at one edge and to the outer
and elliptical recess corresponds to the superior vestibular area in the cochlear wall by its other edge. There are therefore three longitudinal
internal acoustic meatus (see Fig. 38.3). The vestibular aqueduct opens channels within the cochlea. The middle canal (the cochlear duct or
below the elliptical recess. It reaches the posterior surface of the petrous scala media) is blind and ends at the apex of the cochlea; its flanking
bone and contains one or more small veins and part of the membra- channels communicate with each other at the modiolar apex at a
nous labyrinth, the endolymphatic duct (see Fig. 38.1C). In the poste- narrow slit, the helicotrema (see Fig. 38.1C). Two elastic membranes
rior part of the vestibule are the five openings of the semicircular canals; form the upper and lower bounds of the scala media. One is Reissner’s
in its anterior wall is an elliptical opening that leads into the scala membrane, the thin vestibular membrane that separates the scala media
vestibuli of the cochlea. from the scala vestibuli. The other is the basilar membrane, which


forms the partition between the scala media and the scala tympani. The
organ of Corti, the sensory epithelium of hearing, sits on the inner
SEMICIRCULAR CANALS surface of the basilar membrane. At the base of the scala vestibuli is the
oval window (fenestra vestibuli), which leads on to the vestibular cavity
The three semicircular canals – anterior (superior), posterior and lateral but is sealed by the footplate of the stapes. The scala tympani is sepa-
(horizontal) – are located posterosuperior to the vestibule (see Fig. rated from the tympanic cavity by the secondary tympanic membrane
38.1). They are compressed from side to side and each forms approxi- at the round window (fenestra cochleae). The central cochlear core, the
mately two-thirds of a circle. They are unequal in length but similar in modiolus, has a broad base near the lateral end of the internal acoustic
diameter along their lengths, except where they bear a terminal swell- meatus, where it corresponds to the spiral tract (tractus spiralis forami-
ing, an ampulla, which is almost twice the diameter of the canal. nosus). There are several openings in this area for the fascicles of the
The anterior semicircular canal is 15–20 mm long. It is vertical in cochlear nerve: those for the first 11 turns run through the small holes
2
orientation and lies transverse to the long axis of the petrous temporal of the spiral tract, and those for the apical turn run through the hole
bone under the anterior surface of its arcuate eminence. The eminence that forms the centre of the tract. Canals from the spiral tract go through
may not accurately coincide with this semicircular canal but may the modiolus and open in a spiral sequence into the base of the osseous
instead be adapted to the occipitotemporal sulcus on the inferior spiral lamina. Here the small canals enlarge and fuse to form Rosenthal’s
surface of the temporal lobe of the brain. The ampulla at the anterior canal, a spiral canal in the modiolus that follows the course of the
end of the canal opens into the upper and lateral part of the vestibule. osseous spiral lamina and contains the spiral ganglion (see Fig. 38.9B).


Its other end unites with the upper end of the posterior canal to form The main tract continues through the centre of the modiolus to the
the crus commune (common limb), which is 4 mm long, and opens cochlear apex.
into the medial part of the vestibule. The osseous cochlear canal spirals for about 23 turns around the
4
The posterior semicircular canal is also vertical but curves backwards modiolus and is 35 mm long. At its first turn, the canal bulges towards
almost parallel with the posterior surface of the petrous bone. It is the tympanic cavity, where it underlies the promontory. At the base of
18–22 mm long and its ampulla opens low in the vestibule, below the the cochlea, the canal is 3 mm in diameter but it becomes progressively
cochlear recess where the macula cribrosa inferior transmits nerves to reduced in diameter as it spirals apically to end at the cupula. In addi-
it. Its upper end joins the crus commune. tion to the round and oval windows, which are the two main openings
The lateral canal is 12–15 mm long and its arch runs horizontally at its base, the canal has a third, smaller opening for the cochlear aque-
backwards and laterally. Its anterior ampulla opens into the upper and duct or canaliculus. The latter is a minute funnel-shaped canal that runs
lateral angle of the vestibule, above the oval window and just below the to the inferior surface of the petrous temporal bone; it transmits a small
ampulla of the anterior canal; its posterior end opens below the opening vein to the inferior petrosal sinus (see p. 437) and connects the sub-
of the crus commune. arachnoid space to the scala tympani.
The two lateral semicircular canals of the two ears are often described The osseous or primary spiral lamina is a ledge that projects from
as being in the same plane and the anterior canal of one side as being the modiolus into the osseous canal like the thread of a screw (see Fig.
almost parallel with the opposite posterior canal. However, measure- 38.9B). It is attached to the inner edge of the basilar membrane and


ments of the angular relations of the planes of the semicircular osseous ends in a hook-shaped hamulus at the cochlear apex, partly bounding