The ear pdf - Head and Neck

562

CHAPTER 11

The Head and Neck

of strabismus is known as concomitant strabismus and is

common in infancy.

that secrete a yellowish brown wax. The hairs and the wax

The latter are modified sweat glands

ceruminous glands.

sebaceous

hairs

outer third is provided with

the tympanic plate. The meatus is lined by skin, and its

tic cartilage, and the inner two thirds is bone, formed by

The framework of the outer third of the meatus is elas

auricle to the tympanic membrane.

(Figs. 11.27 and 11.28). It conducts sound waves from the

that leads from the auricle to the tympanic membrane

is a curved tube

external auditory meatus

The

intrinsic muscles, which are supplied by the facial nerve.

cartilage covered by skin. It possesses both extrinsic and

collects air vibrations. It consists of a thin plate of elastic

has a characteristic shape (Fig. 11.27A) and

auricle

The

meatus.

The external ear has an auricle and an external auditory

tains the organs of hearing and balance.

panic cavity; and the internal ear, or labyrinth, which con

The ear consists of the external ear; the middle ear, or tym

here, efferent pathways pass to the parasympathetic nucleus

cortex is connected to the eye field of the frontal cortex. From

that occurs when a person suddenly focuses on a near object

part of the nucleus, efferent fibers leave the midbrain in the

both sides via the pretectal nuclei. From the parasympathetic

Pupillary Reflexes

The pupillary reflexes—that is, the reaction of the pupils to

light and accommodation—depend on the integrity of nervous

pathways. In the direct light reflex, the normal pupil reflexly

contracts when a light is shone into the patient’s eye. The ner-

vous impulses pass from the retina along the optic nerve to the

optic chiasma and then along the optic tract. Before reaching

the lateral geniculate body, the fibers concerned with this

reflex leave the tract and pass to the oculomotor nuclei on

oculomotor nerve and reach the ciliary ganglion via the nerve

to the inferior oblique. Postganglionic fibers pass to the con-

strictor pupillae muscles via the short ciliary nerves.

The consensual light reflex is tested by shining the light in

one eye and noting the contraction of the pupil in the opposite

eye. This reflex is possible because the afferent pathway just

described travels to the parasympathetic nuclei of both ocu-

lomotor nerves.

The accommodation reflex is the contraction of the pupil

after having focused on a distant object. The nervous impulses

pass from the retina via the optic nerve, the optic chiasma, the

optic tract, the lateral geniculate body, the optic radiation, and

the cerebral cortex of the occipital lobe of the brain. The visual

of the oculomotor nerve. From there, the efferent impulses

reach the constrictor pupillae via the oculomotor nerve, the

ciliary ganglion, and the short ciliary nerves.

The Ear

-
-

External Ear

and

provide a sticky barrier that prevents the entrance of for

toid, and superficial cervical lymph nodes.

is to the superficial parotid, mas

lymph drainage

The

of the vagus nerve.

from the auriculotemporal nerve and the auricular branch

supply of the lining skin is derived

sensory nerve

The

eign bodies.

concave. Remember that in the adult the external meatus is

Tympanic Membrane Examination

Otoscopic examination of the tympanic membrane is facili-

tated by first straightening the external auditory meatus by

gently pulling the auricle upward and backward in the adult,

and straight backward or backward and downward in the

infant. Normally, the tympanic membrane is pearly gray and

about 1 in. (2.5 cm) long and is narrowest about 0.2 in. (5 mm)

from the tympanic membrane.

C L I N I C A L N O T E S

Middle Ear (Tympanic Cavity)

stapedius muscle.

from whose apex emerges the tendon of the

pyramid,

and 11.30). Below this is a small, hollow, conical projection,

(Figs. 11.29

aditus to the mastoid antrum

lar opening, the

has in its upper part a large, irregu

posterior wall

The

where it forms a shelflike projection.

rates the canals, is prolonged backward on the medial wall,

muscle (Fig. 11.29). The thin, bony septum, which sepa

smaller is the entrance into the canal for the tensor tympani

of these leads into the auditory tube, and the upper and

wall are the openings into two canals. The lower and larger

carotid artery (Fig. 11.30). At the upper part of the anterior

bone that separates the tympanic cavity from the internal

is formed below by a thin plate of

anterior wall

The

vein (Fig. 11.30).

panic cavity from the superior bulb of the internal jugular

be partly replaced by fibrous tissue. It separates the tym

is formed by a thin plate of bone, which may

floor

The

the middle cranial fossa.

from the meninges and the temporal lobe of the brain in

(Figs. 11.29 and 11.30). It separates the tympanic cavity

which is part of the petrous temporal bone

men tympani,

teg

is formed by a thin plate of bone, the

roof

The

wall, lateral wall, and medial wall.

The middle ear has a roof, floor, anterior wall, posterior

nasopharynx and behind with the mastoid antrum.

communicates in front through the auditory tube with the

mately parallel to the plane of the tympanic membrane. It

narrow, oblique, slitlike cavity whose long axis lies approxi

brane (eardrum) to the perilymph of the internal ear. It is a

function is to transmit the vibrations of the tympanic mem

mucous membrane. It contains the auditory ossicles, whose

part of the temporal bone (Fig. 11.28) and is lined with

The middle ear is an air-containing cavity in the petrous

the

Basic Anatomy

563

anterior and posterior malleolar folds

pars flaccida

tragus

helix

lobule

pars tensa

handle of malleus

tympanic membrane

malleus

incus

stapes

semicircular canals

vestibular nerve

cochlear nerve

facial nerve

cochlea

tensor tympani muscle

auditory tube

internal carotid artery

styloid process

facial nerve

promontory

cone of light

umbo

auricle

concha

external auditory meatus

lobule

long process of incus

lateral process of malleus

FIGURE 11.27

be pulled to straighten the external auditory meatus before insertion of the otoscope in the adult.

arrow

Different parts of the auricle of the external ear. The

indicates the direction that the auricle should

B. External and middle

portions of the right ear viewed from in front. The right tympanic membrane as seen through the otoscope.

internal carotid artery

cochlea

internal acoustic meatus

vestibulocochlear nerve

facial nerve

ductus endolymphaticus

jugular foramen sigmoid sinus

mastoid antrum

lateral semicircular canal

external auditory meatus

auricle

incus

malleus

stapes

auditory tube

middle meningeal artery

head

anterior
process

handle

malleus

body

short
process

incus

long process

head

base

stapes

tympanic cavity

promontory

FIGURE 11.28

The auditory ossicles.

Parts of the right ear in relation to the temporal bone viewed from above. B.

564

CHAPTER 11

The Head and Neck

tegmen tympani

head of
malleus

tensor tympani muscle

auditory tube

internal carotid artery

handle of malleus

tympanic membrane

base of stapes

styloid process

chorda tympani

facial nerve

mastoid air
cells

pyramid

stapedius muscle

long process of
incus

posterior ligament

short process of
incus

epitympanic recess

geniculate ganglion

aditus to mastoid antrum

lateral semicircular canal

facial nerve in canal

pyramid

mastoid antrum

mastoid air cells

facial nerve

styloid process

fenestra vestibuli

promontory

fenestra cochleae

anterior

zygomatic arch

processus cochleariformis

tensor tympani muscle

tympanic cavity

anterior

FIGURE 11.29

bony canal.

Medial wall of the right middle ear viewed from the lateral side. Note the position of the facial nerve in its

mastoid antrum.

Lateral wall of the right middle ear viewed from the medial side. Note the position of the ossicles and the

Basic Anatomy

565

greater petrosal nerve

geniculate ganglion

lateral

semicircular canal

sigmoid sinus

mastoid antrum

mastoid air cells

facial nerve

chorda tympani

stapedius

superior bulb of internal jugular vein

tympanic branch of

glossopharyngeal nerve

inferior petrosal sinus

sympathetic plexus

internal carotid

artery

chorda tympani

auditory tube

processus cochleariformis

anterior

tensor tympani

tegmen tympani

periosteal layer of dura

meningeal layer of

dura

arachnoid mater

pia mater

temporal lobe of cerebrum

superior semicircular canal

lateral

posterior

fenestra vestibuli

fenestra cochleae

cochlea

vestibule

groove for facial nerve

ampullae

saccus endolymphaticus

ductus endolymphaticus

duct of cochlea

utricle

semicircular ducts

saccule

FIGURE 11.30

is largely formed by the tympanic

lateral wall

The

labyrinths.

The middle ear and its relations. Bony (B) and membranous (C)

membrane (Figs. 11.27 and 11.29).

promontory and the fenestra vestibuli and is known as the

A rounded ridge runs horizontally backward above the

malleus (Fig. 11.29).

laterally to reach its insertion on the handle of the

around which the tendon of the tensor tympani bends

processus cochleariformis,

and forms a pulley, the

sor tympani muscle. Its posterior end is curved upward

and above the fenestra vestibuli. It supports the ten

backward on the medial wall above the promontory

The bony shelf derived from the anterior wall extends

blind end of the scala tympani (see page 569).

On the medial side of this window is the perilymph of the

secondary tympanic membrane.

round and closed by the

which is

fenestra cochleae,

end of the promontory lies the

of the scala vestibuli of the internal ear. Below the posterior

stapes. On the medial side of the window is the perilymph

which is oval shaped and closed by the base of the

tibuli,

fenestra ves

Above and behind the promontory lies the

underlying first turn of the cochlea (Figs. 11.27 and 11.29).

which results from the

promontory,

projection, called the

inner ear. The greater part of the wall shows a rounded

is formed by the lateral wall of the

medial wall

The

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CHAPTER 11

possesses a large body and two processes

The

of the tympanic membrane.

and is attached to the anterior and posterior malleolar folds

cavity by a ligament. The lateral process projects laterally

bone that is connected to the anterior wall of the tympanic

is a spicule of

anterior process

scopic examination. The

It can be seen through the tympanic membrane on oto

attached to the medial surface of the tympanic membrane.

passes downward and backward and is firmly

handle

The

is the constricted part below the head.

neck

the incus. The

is rounded and articulates posteriorly with

head

The

lateral process.

a neck, a long process or handle, an anterior process, and a

is the largest ossicle and possesses a head,

malleus

The

(Figs. 11.28 and 11.29).

The auditory ossicles are the malleus, incus, and stapes

Auditory Ossicles

poral nerve and the auricular branch of the vagus.

and is innervated on its outer surface by the auriculotem

The tympanic membrane is extremely sensitive to pain

membrane by the mucous membrane.

leus is bound down to the inner surface of the tympanic

The handle of the mal

pars tensa.

is tense and is called the

(Fig. 11.27). The remainder of the membrane

pars flaccida

brane that is bounded by the folds is slack and is called the

malleus. The small triangular area on the tympanic mem

pass to the lateral process of the

posterior malleolar folds,

anterior

the sides of the notch, two bands, termed the

is deficient superiorly, which forms a notch. From

sulcus,

tympanic

slotted into a groove in the bone. The groove, or

1 cm in diameter. The circumference is thickened and is

The tympanic membrane is circular and measures about

radiates anteriorly and inferiorly from the umbo.

an otoscope, the concavity produces a “cone of light,” which

the malleus. When the membrane is illuminated through

produced by the tip of the handle of

umbo,

depression, the

cave laterally, and at the depth of the concavity is a small

placed, facing downward, forward, and laterally. It is con

membrane that is pearly gray. The membrane is obliquely

(Fig. 11.27) is a thin, fibrous

tympanic membrane

The

posterior wall, it curves downward behind the pyramid.

On reaching the

prominence of the facial nerve canal.

The Head and Neck

and

incus

(Fig. 11.29).

tympanic membrane can sometimes be recognized on

articulates with the head of the stapes. Its shadow on the

handle of the malleus. Its lower end bends medially and

descends behind and parallel to the

long process

The

head of the malleus.

is rounded and articulates anteriorly with the

body

The

otoscopic examination.

has a head, a neck, two limbs, and a base

stapes

The

the posterior wall of the tympanic cavity by a ligament.

projects backward and is attached to

short process

The

(Fig. 11.28).

of the malleus to the anterior wall of the tympanic cav

runs through the ligament connecting the anterior process

The malleus and incus rotate on an anteroposterior axis that

Movements of the Auditory Ossicles

actions are summarized in Table 11.3.

The muscles of the ossicles, their nerve supply, and their

stapedius muscles.

tensor tympani

These are the

anular ligament.

of fibrous tissue, the

is attached to the margin of the fenestra vestibuli by a ring

The edge of the base

neck and are attached to the oval

diverge from the

two limbs

of the stapedius muscle. The

is narrow and receives the insertion

neck

of the incus. The

is small and articulates with the long process

head

The

base.

Muscles of the Ossicles

and the

ity, the anterior process of the malleus and the short process

of the incus to the posterior wall of the tympanic cavity.

of the incus, and the ligament connecting the short process

When the tympanic membrane moves

medially

(Fig. 11.31), the handle of the malleus also moves

medially

The head of the malleus and the body of the incus move
laterally. The long process of the incus moves medially

perilymph to increase by a total of 22 to 1.

the base of the stapes, causing the effective pressure on the

tympanic membrane is about 17 times greater than that of

erage increases at a rate of 1.3 to 1. Moreover, the area of the

membrane to the perilymph via the small ossicles, the lev

During passage of the vibrations from the tympanic

fenestra vestibuli.

the base of the stapes is not pulled laterally out of the

articular surfaces between the malleus and incus so that

head of the malleus cause a temporary separation of the

brane moves laterally. Excessive lateral movements of the

The above movements are reversed if the tympanic mem

cochleae at the lower end of the scala tympani (Fig. 11.31).

ing of the secondary tympanic membrane in the fenestra

incompressible, the perilymph causes an outward bulg

cated to the perilymph in the scala vestibuli. Liquid being

ally in the fenestra vestibuli, and the motion is communi

with the stapes. The base of the stapes is pushed medi-

Muscles of the Middle Ear

T A B L E 1 1 . 3

Wall of auditory tube and wall of its

Tensor tympani

Muscle

Origin

Insertion

Nerve Supply

Action

own canal

Handle of malleus

Mandibular division

of trigeminal nerve

Dampens down vibrations of

tympanic membrane

Stapedius

Pyramid (bony projection on posterior

wall of middle ear)

Neck of stapes

Facial nerve

Dampens down vibrations of

stapes

Basic Anatomy

567

malleus

incus

stapes

external auditory

meatus

tympanic membrane

scala vestibuli
filled with perilymph

base of stapes
in fenestra vestibuli
(oval window)

secondary tympanic membrane in
fenestra cochleae (round window)

scala tympani filled with perilymph

spiral
lamina

helicotrema

auricle and external
auditory meatus

tympanic membrane

basilar fibers of basilar membrane

secondary tympanic
membranein fenestra cochleae

scala tympani
filled with perilymph

helicotrema

scala vestibuli filled with perilymph

base of stapes in
fenestra vestibuli

duct of the cochlea
filled with endolymph

FIGURE 11.31

Movement of the

tympani, causing a lateral bulging of the secondary tympanic membrane in the fenestra cochleae.

scala vestibuli. At the apex of the cochlea (the helicotrema), the compression wave in the perilymph passes down the scala

) in the perilymph in the

arrows

The medial movement of the base of the stapes in the fenestra vestibuli causes motion (

ally; the head of the malleus and incus move laterally, and the long process of the incus, with the stapes, moves laterally.

Vibrations of music passing into the external auditory meatus cause the tympanic membrane to move medi-

perilymph (

These are important in understanding the spread of

Relations of the Mastoid Antrum

nicates with the middle ear by the aditus (Fig. 11.29).

petrous part of the temporal bone (Fig. 11.28). It commu

The mastoid antrum lies behind the middle ear in the

Mastoid Antrum

air pressures in the tympanic cavity and the nasal pharynx.

superior constrictor muscle (Fig. 11.80). It serves to equalize

the tube descends, it passes over the upper border of the

third is bony, and its anterior two thirds is cartilaginous. As

panic cavity to the nasal pharynx (Fig. 11.27). Its posterior

The auditory tube connects the anterior wall of the tym

Auditory Tube

) after movement of the base of the stapes. Note the position of the basilar fibers of the basilar membrane.

arrows

infection.

the antrum communicates with the mastoid air cells

is perforated with holes, through which

Inferior wall

fossa and the temporal lobe of the brain (Fig. 11.30).

pani, which is related to the meninges of the middle cranial

is the thin plate of bone, the tegmen tym

Superior wall

(Fig. 11.30).

is related to the posterior semicircular canal

Medial wall

suprameatal triangle (see page 663).

is (1.5 cm) thick and forms the floor of the

Lateral wall

venous sinus and the cerebellum (Fig. 11.30).

separates the antrum from the sigmoid

Posterior wall

the aditus to the mastoid antrum (Fig. 11.30).

is related to the middle ear and contains

Anterior wall

(Fig. 11.30).

promontory.

The nerve then bends sharply backward above the

(Figs. 11.29 and 11.30).

geniculate ganglion

the sensory

wall of the middle ear. Here, the nerve expands to form

vestibule of the internal ear until it reaches the medial

canal (Fig. 11.28). The nerve runs laterally above the

meatus (see page 612), the facial nerve enters the facial

612. On reaching the bottom of the internal acoustic

The entire course of the facial nerve is described on page

Facial Nerve

lined with mucous membrane.

with the antrum and the middle ear (Fig. 11.30). They are

cating cavities within the process that are continuous above

year of life. The mastoid air cells are a series of communi

The mastoid process begins to develop during the second

Mastoid Air Cells

568

CHAPTER 11

otic ganglion.

leaves the skull through the foramen ovale and joins the

the parotid gland via the otic ganglion (see page 631). It

which sends secretomotor fibers to

lesser petrosal nerve,

supplies the lining of the middle ear and gives off the

The tympanic plexus

tympanic plexus.

which form the

promontory (Fig. 11.30). Here it splits into branches,

passes through the floor of the middle ear and onto the

nerve, just below the jugular foramen (see page 614). It

The tympanic nerve arises from the glossopharyngeal

Tympanic Nerve

to the submandibular and sublingual salivary glands

reach the submandibular ganglion and are there relayed

Preganglionic parasympathetic secretomotor fibers

ganglion.

the peripheral processes of the cells in the geniculate

lae) and the floor of the mouth. The taste fibers are

anterior two thirds of the tongue (not the vallate papil

from the mucous membrane covering the

Taste fibers

The chorda tympani contains

page 613).

infratemporal fossa, where it joins the lingual nerve (see

dle ear through the petrotympanic fissure and enters the

of the tympanic membrane. The nerve leaves the mid

between the mucous membrane and the fibrous layers

dle of the malleus (Fig. 11.29). It lies in the interval

tympanic membrane and crosses the root of the han

tympanic membrane. It then runs forward over the

the middle ear close to the posterior border of the

above the stylomastoid foramen (Fig. 11.29). It enters

arises from the facial nerve just

chorda tympani

The

(Fig. 11.30). It supplies the muscle within the pyramid.

as it descends in the facial canal behind the pyramid

arises from the facial nerve

nerve to the stapedius

The

ganglion.

gopalatine fossa, where it ends in the pterygopalatine

This passes forward and enters the ptery

goid canal.

nerve of the ptery

internal carotid artery and forms the

deep petrosal nerve from the sympathetic plexus on the

part of the temporal bone and is eventually joined by the

The nerve emerges on the superior surface of the petrous

from the mucous membrane of the palate.

of the nose and palate. It also contains many taste fibers

palatine nerves to the glands of the mucous membrane

other postganglionic fibers pass through the nasal and

the zygomatic and lacrimal nerves to the lacrimal gland;

pterygopalatine ganglion and are there relayed through

preganglionic parasympathetic fibers that pass to the

at the geniculate ganglion (Fig. 11.30). It contains

arises from the facial nerve

greater petrosal nerve

The

Facial Nerve

Important Branches of the Intrapetrous Part of the

emerges through the stylomastoid foramen into the neck.

wall of the middle ear, behind the pyramid, and finally

mastoid antrum (Fig. 11.30). It descends in the posterior

curves downward on the medial side of the aditus of the

On arriving at the posterior wall of the middle ear, it

The Head and Neck

■

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■

that

of the infection into the mastoid antrum and the mastoid air

Infections and Otitis Media

Pathogenic organisms can gain entrance to the middle ear by

ascending through the auditory tube from the nasal part of the

pharynx. Acute infection of the middle ear (otitis media) pro-

duces bulging and redness of the tympanic membrane.

Complications of Otitis Media

Inadequate treatment of otitis media can result in the spread

cells (acute mastoiditis). Acute mastoiditis may be followed

by the further spread of the organisms beyond the confines

of the middle ear. The meninges and the temporal lobe of the

brain lie superiorly. A spread of the infection in this direction

could produce a meningitis and a cerebral abscess in the tem-

poral lobe. Beyond the medial wall of the middle ear lie the

facial nerve and the internal ear. A spread of the infection in

this direction can cause a facial nerve palsy and labyrinthi-
tis with vertigo. The posterior wall of the mastoid antrum is

related to the sigmoid venous sinus. If the infection spreads

in this direction, a thrombosis in the sigmoid sinus may well

take place. These various complications emphasize the impor-

tance of knowing the anatomy of this region.

C L I N I C A L N O T E S

The Internal Ear, or Labyrinth

rior canal is also vertical but is placed parallel with the long

right angles to the long axis of the petrous bone. The poste

The superior semicircular canal is vertical and placed at

(Fig. 11.30).

semicircular ducts

canals are the

which is common to two of the canals. Lodged within the

The canals open into the vestibule by five orifices, one of

ampulla.

Each canal has a swelling at one end called the

—open into the posterior part of the vestibule.

lateral

semicircular canals—superior, posterior,

The three

of the membranous labyrinth (Fig. 11.30).

utricle

Lodged within the vestibule are the

panic membrane.

secondary tym

which is closed by the

fenestra cochleae,

closed by the base of the stapes and its anular ligament, and

which is

fenestra vestibuli,

canals. In its lateral wall are the

posterior to the cochlea and anterior to the semicircular

the central part of the bony labyrinth, lies

vestibule,

The

in which is suspended the membranous labyrinth.

lymph,

peri

are lined by endosteum and contain a clear fluid, the

are cavities situated in the substance of dense bone. They

the semicircular canals, and the cochlea (Fig. 11.30). These

The bony labyrinth consists of three parts: the vestibule,

Bony Labyrinth

bony labyrinth.

series of membranous sacs and ducts contained within the

the bone, and the membranous labyrinth, comprising a

the bony labyrinth, comprising a series of cavities within

bone, medial to the middle ear (Fig. 11.28). It consists of

The labyrinth is situated in the petrous part of the temporal

the

sac-

cule and

and

Basic Anatomy

forms the spiral organ of Corti and contains

membrane

The highly specialized epithelium that lies on the

ductus reuniens.

and is connected to the saccule by the

is triangular in cross section

duct of the cochlea

The

semicircular ducts.

is detected in the sensory receptors in the ampullae of the

to that of the walls of the semicircular ducts. This change

semicircular ducts changes its speed of movement relative

the head accelerates or decelerates, the endolymph in the

begins or ceases to move, or whenever a movement of

so that all three planes are represented. Whenever the head

figuration. They are arranged at right angles to each other

diameter than the semicircular canals, have the same con

although much smaller in

semicircular ducts,

The

tation of the head to gravity or other acceleration forces.

cialized sensory receptors, which are sensitive to the orien

Located on the walls of the utricle and saccule are spe

face of the petrous part of the temporal bone.

(Fig. 11.30). This lies beneath the dura on the posterior sur

saccus endolymphaticus

end in a small blind pouch, the

being joined by the ductus utriculosaccularis, passes on to

as described previously. The ductus endolymphaticus, after

is globular and is connected to the utricle,

saccule

The

ductus utriculosaccularis.

lymphaticus by the

indirectly connected to the saccule and the ductus endo

is the larger of the two vestibular sacs. It is

utricle

The

All these structures freely communicate with one another.

the duct of the cochlea, which lies within the bony cochlea.

lar ducts, which lie within the bony semicircular canals; and

which are lodged in the bony vestibule; the three semicircu

rounded by perilymph. It consists of the utricle and saccule,

labyrinth (Fig. 11.30). It is filled with endolymph and sur

The membranous labyrinth is lodged within the bony

Membranous Labyrinth

fenestra cochleae.

middle ear by the secondary tympanic membrane at the

The perilymph in the scala tympani is separated from the

stapes and the anular ligament at the fenestra vestibuli.

tibuli is separated from the middle ear by the base of the

below. The perilymph within the scala ves

scala tympani

above and the

scala vestibuli

the cochlear canal into the

the spiral lamina to the outer bony wall, thus dividing

stretches from the free edge of

basilar membrane

it. The

jects into the interior of the canal and partially divides

winds around the modiolus and pro

spiral lamina,

rated by branches of the cochlear nerve. A spiral ledge,

the bottom of the internal acoustic meatus. It is perfo

The modiolus has a broad base, which is situated at

wall of the middle ear.

lea is responsible for the promontory seen on the medial

base faces posteromedially. The first basal turn of the coch

structure is conical. The apex faces anterolaterally and the

successive turn is of decreasing radius so that the whole

low bony tube makes two and one half spiral turns. Each

around which a hol

modiolus,

sists of a central pillar, the

anterior part of the vestibule (Fig. 11.30). Basically, it con

resembles a snail shell. It opens into the

cochlea

The

the mastoid antrum, above the facial nerve canal.

zontal position, and it lies in the medial wall of the aditus to

axis of the petrous bone. The lateral canal is set in a hori

569

-
-

the

-
-

basilar

the sensory receptors for hearing. For a detailed

ion

descript

of the spiral organ, a textbook of histology should be
consulted.

sion on the base, on either side of the symphysis menti

is a small, roughened depres

digastric fossa

The

The lower border of the body of the mandible is called

roots of the teeth.

in the adult, it contains 16 sockets for the

alveolar part;

The upper border of the body of the mandible is called

part of the mylohyoid line (Fig. 11.32).

for the sublingual gland, lies above the anterior

gual fossa,

sublin

below the posterior part of the mylohyoid line. The

superficial part of the submandibular salivary gland, lies

for the

submandibular fossa,

the third molar tooth. The

the area of the mental spines to an area below and behind

as an oblique ridge that runs backward and laterally from

can be seen

mylohyoid line

muscles below (Fig. 11.31). The

gin to the genioglossus muscles above and the geniohyoid

these give ori

mental spines;

the median plane are seen the

On the medial surface of the body of the mandible in

alveolar nerve and vessels.

lar tooth; it transmits the terminal branches of the inferior

can be seen below the second premo

mental foramen

The

symphysis menti.

two halves during development at the

midline, has a faint ridge indicating the line of fusion of the

on its external surface in the

body of the mandible,

The

(Fig. 11.32).

angle of the mandible

on each side at the

The body of the mandible meets the ramus

rami.

a pair of

body

The mandible consists of a horseshoe-shaped

mandibular joint.

of the face, and it articulates with the skull at the temporo

The mandible or lower jaw is the largest and strongest bone

organ of Corti.

spiral

branches of this nerve pass from the ganglion to the

modiolus in the base of the spiral lamina. The peripheral

that is lodged in a canal winding around the

ganglion

spiral

glion of this nerve takes the form of an elongated

foramina at the base of the modiolus. The sensory gan

divides into branches, which enter

cochlear nerve

The

the saccule, and the ampullae of the semicircular ducts.

the membranous labyrinth, where they supply the utricle,

end of the internal acoustic meatus and gain entrance to

The branches of the nerve then pierce the lateral

ganglion.

vestibular

is expanded to form the

vestibular nerve

The

lear portions (Fig. 11.28).

(see page 613), the nerve divides into vestibular and coch

On reaching the bottom of the internal acoustic meatus

Vestibulocochlear Nerve

The Mandible

and

the

the base.

(Fig. 11.32). It is in these fossae that the anterior bellies of

On the lateral surface of the ramus are markings for the

(Fig. 11.32).

mandibular notch

the two processes are separated by the

head;

process,

condyloid

and a posterior

coronoid process

an anterior

is vertically placed and has

ramus of the mandible

The

the digastric muscles are attached.

attachment of the masseter muscle. On the medial

e is

surfac

for the inferior alveolar nerve and

mandibular foramen

the