No. 20
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Vestibulocochlear Organ
Chapter 2 The Vestibulocochlear
Organ
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Division
The vestibulocochlear organ (ear)
is divided into three parts:
The external ear,
The middle ear,
The internal ear.
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Functions
The receptor for auditory sensation (spiral
organ or cochlear organ) and organs of
static balance (vestibular organ) are in the
internal ear. The internal ear is such an
organ that can receive the stimulation of
both sound waves and changes of the
position of the head.
The external and the middle ears are a
sound collecting and transmitting
apparatus.
Section 1 The External Ear
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The external ear consists of:
the auricle,
the external acoustic meatus
the tympanic membrane.
Ⅰ. The Auricle
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It projects from the side of the head.
It anterolateral surface shows
irregularly concave, but its
posteromedial surface presents
convex. The orifice of the meatus,
named the external acoustic pore,
lies in the fossa of the anterolateral
surface of the auricle.
Ⅱ. The External Acoustic Meatus
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Morphological characteristics
It extends from the external acoustic
pore to the tympanic membrane and
is about 2.1～2.5 cm in length. Its
lateral part is about one-third of the
meatus, termed the cartilaginous
part, and its medial part is about
two-thirds of the meatus, termed the
bony part.
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The meatus passes medially, its lateral
part runs forwards and upwards, and then
backwards; and the medial part runs
forwards and downwards.
In the clinical examination of the meatus,
the auricle should be drawn upwards,
backwards and slightly laterally to render
the meatus as straight as possible, so that
the tympanic membrane can be viewed.
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Structural characteristics
In the subcutaneous tissue of the
cartilaginous part of the meatus
there are numerous sebaceous and
ceruminous glands. The latter
secrete the cerumen (ear wax).
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Clinical points
The skin of the meatus is thin, and
its subcutaneous tissue is scarce but
rich in sensory nerve terminals, and
is closely adherent to the
cartilaginous and bony parts of the
meatus, therefore under
inflammatory condition, the meatus
is extremely painful.
Ⅲ. The Tympanic Membrane
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It is oval in form, thin and semitransparent, separates the middle
ear from the external acoustic
meatus. It inclines greatly and forms
an angle of 45 degrees with the floor
of the meatus, hence the
anteroinferior wall of the meatus is
longer than the posterosuperior wall.
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The thickened margin of the membrane,
the fibrocartilaginous ring, is attached to
the tympanic sulcus at the medial end of
the external acoustic meatus. In the upper
end of the tympanic membrane, two
bands, the anterior and posterior
mallear folds, are prolonged to the
lateral process of the malleus. The small
part (1/4) of the membrane above these
folds is lax and thin, called the flaccid
part, while the remainder is tightly
stretched, that is the tense part of the
membrane.
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In the living body, it is pearly-grey in
colour. The handle of the malleus is firmly
attached to the inner surface of the
tympanic membrane as far as its center,
thus the outer surface of the membrane is
concave with a central depression, the
umbo, formed by the traction of the lower
end of the handle of malleus. When the
tympanic membrane is examined by
otoscope, a bright area, the cone of light,
anteroinferior to the umbo can be seen.
Section 2 The Middle Ear
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The middle ear lies between the
external and inner ears.
It includes three parts:
The tympanic cavity,
The auditory tube,
The mastoid cells.
Ⅰ. The Tympanic Cavity
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It is an irregular air-filled space within the
temporal bone, and lies between the
tympanic membrane and the lateral wall
of the inner ear. It is the principal part of
the middle ear, its capacity is about 1～2
cm3. There are auditory ossicles,
ligaments, muscles, vessels and nerves
inside the tympanic cavity. The tympanic
cavity communicates anteriorly with the
nasopharyns through the auditory tube,
and posteriorly with the mastoid cells
through the mastoid antrum.
Ⅰ) The Walls of the Tympanic
Cavity
It possesses six walls.
1. The tegmental wall (superior wall)
 It is a thin plate of compact bone, the
tegmen tympani, it separates the middle
cranial fossa from the tympanic cavity.
 In the first two years of childhood, the
unossified suture of the superior wall may
allow the infection to spread from the
tympanic cavity into the cranial cavity
directly.
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2. The jugular wall (inferior wall)
It consists of the thin plate of bone which
separates the tympanic cavity from the
jugular fossa. The bone of this wall may
be deficient, so the tympanic cavity is
separated from the jugular vein by
mucous membrane and fibrous tissue only.
This may cause the beginning part of the
jugular vein to project into the tympanic
cavity.
3. The carotid wall (anterior wall)
 It is the posterolateral wall of the carotid
canal, close to the internal carotid artery.
At the superior part of the anterior wall,
there are two parallel canals leading to the
tympanic cavity. The upper is the smaller
semicannal for tensor tympani, and
the lower is a larger semicanal for
auditory tube, the bony part of auditory
tube.
4. The mastoid wall (posterior
wall)
 It is pierced superiorly by the
opening (aditus ) of mastoid
antrum. The pyramidal eminence
is situated below the opening of
mastoid antrum. The cavity of the
pyramidal eminence contains the
stapedius.
5. The membranous wall (lateral
wall)
 It is almost entirely formed by the
tympanic membrane, only the
superior part of this wall is formed by
the lateral wall of the epitympanic
recess.
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6. The labyrinthine wall (medial wall)
It is the lateral wall of the inner ear. A rounded
elevation on the middle of this wall is named the
promontory. Posterosuperior to the tympanic
promontory is the fenestra vestibuli (oval
window) being closed by the base of the stapes
and annular ligament. The fenestra cochleae
(round window) lies posteroinferior to the
tympanic promontory, and is closed by the
secondary tympanic membrane in vivo. The
prominence of facial canal is an arcuate-like
ridge formed by the bony canal for the facial
nerve, and extends back and down to the
posterior wall of the tympanic cavity. The facial
canal is very thin, or incomplete. In inflammatory
condition of the tympanic cavity, the facial nerve
may be involved and leading to facial paralysis.
Ⅱ) The structures in the tympanic
cavity
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In the tympanic cavity there are three
auditory ossicles, two muscles, one nerve
and air equal to the atmosphere.
1. The auditory ossicles and their joints
The tympanic cavity contains a chain of
three ossicles:
The malleus,
The Incus,
The stapes.
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Laterally, the handle of malleus is attached to
the tympanic membrane, and medially, the base
of the stapes is fixed to the circumference of the
fencestra vestibuli, while the incus is placed
between the malleus and the stapes.
The three ossicles connencted by joints to form a
jointed chain which connects the tympanic
membrane with the fenestra vestibuli.
When the tympanic membrane is vibrated by the
sound wave, the handle of the malleus is moved
with it, then, the incus and the stapes transmit
the vibrations to the inner ear.
2. The muscles to motor the auditory
ossicles
 The muscles of the cavity are the tensor
tympani and the stapedius.
 The tensor tympani lies in the semicanal
for tensor tympani, and ends to the
handle of the malleus. It is supplied by the
mandibular nerve of trigeminal nerve.
 The stapedius arises from the pyramidal
eminence of posterior wall of the tympanic
cavity and is inserted into the neck of the
stapes. It is controlled by the facial nerve.
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Under the normal conditions, the tnsor
tympani and the stapedius contract
simultaneously. When the tensor tympani
contracts it pulls the handle of the malleus
medially, tenses the tympanic membrane,
and thus reduces the amplitude of
vibration; whereas the contraction of
stapedius renders base of stapes outwards,
and thus reduces the pressure of sound
wave for the inner ear.
Ⅱ. The Auditory Tube
(Pharyngotympanic tube)
1. Divisions of the auditory tube
 It is the channel through which the
tympanic cavity communicates with
the nasopharynx. It is approximately
3.5～4.0 cm long, and is divided into
the cartilaginous and the bony parts.
The junction of these two parts is
narrowest, 1-2 mm long, termed the
isthmus of auditory tube.
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The bony part:
This part is the posterolateral part of the
tube, about 1/3 of its total length. It
begins in the anterior wall of the tympanic
cavity and passes forward, downward and
inward.
The cartilaginous part:
This part is the anteromedial part of the
tube, about 2/3 of the tube, and opens
into the nasopharynx.
2. Function of the auditory tube
 The function of auditory tube is to
maintain the balance of pressure on both
sides of the tympanic membrane.
 In the normal condition, the pharyngeal
orifice and cartilaginous part is closed;
during the deglutition they are opened and
allow the air to center or leave the
tympanic cavity, this balances the
pressure on both sides of the tympanic
membranes and censures the tympanic
membrane to vibrate freely.
3. Clinical points
 The tube is easily blocked by swelling
of its mucous membrane. When it is
blocked, the residual air in the
tympanic cavity is absorbed,
resulting in the retraction of the
tympanic memrane and interference
with its free movement.
4. Characteristics of children’s
auditory tube
 In childhood, the auditory tube is
shorter and wider than in adult. Its
direction is more horizontal,
therefore, the inflammation of the
pharynx may along the auditory tube
spread into the tympanic cavity and
causes the otitis media.
Ⅲ. The Mastoid Antrum and
Mastoid Cells
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They are air-filled spaces in the mastoid
process of the temporal bone. They are a
series of intercommunicated cavities, and
anteriorly, through the mastoid antrum
they communicate with the tympanic
cavity. Since the mucous membrane of the
mastoid air cells is continuous with that of
the mastoid antrum and tympanic cavity,
the otitis media may spread to the
mastoid antrum and the mastoid cells.
Section 3 The Internal Ear
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The inner (internal) ear lies in the petrous part of
the temporal bone.
It consits of two parts: the bony labyrinth and
the membranous labyrinth.
The former is composed of the compact bone,
and the latter, a series of communicating
membranous sacs and ducts, is contained within
the bony labyrinth.
The membranous labyrinth is filled with
endolymph, and the space between the
membranous and bony labyrinth is filled with
perilymph. The endolymph does not
communicates with the perilymph.
Ⅰ. The Bony Labyrinth
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From before backwards, the bony
labyrinth is divided into three parts:
The cochlea,
The vestibule,
The bony semicircular canals.
They are various with each other in
shape, but communicate with each
other.
Ⅰ)The Vestibule
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It is the central part of the bony
labyrinth, and is situated medial to
the tympanic cavity. It is a somewhat
ovoid space. There are three
foremen on the posterior part of the
vestibule communicating with three
bony semicircular canals. It appears
four walls:
1. Lateral wall
 It is the medial wall of the tympanic cavity.
 Fenestra vestibuli and fenestra cochleae:
 On its lateral wall there are two openings:
the fenestra vestibuli communicates
with the tympanic cavity and is closed by
the base of the stapes with its annular
ligament in vivo; the fenestra cochleae
is closed by the secondary tympanic
membrane.
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2. Medial wall
It is the posterior part of the fundus of the
internal acoustic meatus, through which the
peripheral branches of the vestibulocochlear
nerve pass into the membranous labyrinth.
3. Anterior wall
On this wall there is the inlet of cochlear spiral
canal communicating with the scala vestibuli of
the cochlea.
4. Posterior wall
On the posterior wall of the vestibule there are
the five openings of the semicircular canals.
Ⅱ) The Cochlea
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The cochlea is placed anterior to the
vestibule, resembling the shell of a snail.
It is composed of modiolus and cochlear
spiral canal winding spirally for 2.5 turns
around the central modiolus.
Its apex or cupula of cochlea points
anterolaterally, its base is directed
posteromedially towards the bottom of the
internal acoustic meatus.
The modiolus is the conical osseous
central pillar of the cochlea, through which
the vessels and nerves pass.
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The osseous spiral lamina of modiolus
projects from the modiolus into the spiral
canal, and divides the cochlear canal into
scala vestibuli and the scala tympani
with the basilar membrane of the cochlear
duct. The scala vestibuli and the scala
tympani pass to the fenestra vestibuli and
the fenestra cochleae respectively and are
filled with perilymph.
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The width of lamina of modiolus gradually
decreases from the basal to the apical coil
of the cochlea, and near the summit of the
cochlea the lamina ends in a hook-shaped
process, the hamulus of spiral lamina.
The hamulus and the modiolus form the
helicotrema, through which the scala
vestibuli and the scala tympani
communicate with each other.
Ⅲ) The Semicircular Canals
They are three in number, anterior
(superior), posterior and lateral. They are
situated posterosuperior to the vestibule.
1. The anterior semicircular canal
 The anterior semicircular canal lies in a
vertical plane across the long axis of the
petrous part of the temporal bone deep to
the arcuate eminence.
2. The lateral semkicircular canal
 The lateral semicircular canal is nearly
horizontal.
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3. The posterior semicircular canal
 The posterior semicircular canal lies in
a vertical plane parallel to the long axis of
petrous part of temporal bone.
 Each canal has two crura, one of which is
dilated, named the bony ampulla.
 The other crura of the anterior and
posterior canals open together into the
vestibule by one common bony crus. While
the another crus of the lateral canal opens
into the vestibule separately.
Ⅱ. The Membranous Labyrinth
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It is a series of membranous canals
and sacs which lie within the bony
labyrinth. It is similar to bony
labyrinth in shape but is smaller. The
membranous labyrinth is lined with
epithelium. The spiral organ and
vestibular organs are situated in its
walls.
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Composition:
The membranous labyrinth, from before
backwards, includes:
the utricle and saccule, in the vestibule,
the semicircular ducts, in the bony semicircular
canals,
the cochlear duct, in the cochlear spiral canal of
cochlea.
The various parts of the membranous labyrinth
form a closed system of channels which
communicate freely with one another and is filled
with the endolymph.
Ⅰ) The Utricle and Saccule
1. The utricle
 The utricle It is an elongated sac, lies in
the posterosuperior part of the vestibule.
 On the posterior wall of the utricle there
are five openings of the semicircular ducts.
Forward, it communicates with the saccule
and endolymphatic duct. The
endolymphatic duct traverses the
vestibular aqueduct and ends as a blind
dilatation, the endolymphatic sac.
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The maculautriculi lie on the base of the
upper end and anterior wall of the utricle.
They are position receptors (static
balance). They may be stimulated only by
the changes of the position of the head,
but also may be stimulated by the linear
movements on acceleration or
deceleration of the head. The nerve
impulses transverse through the utricular
branch of the vestibulocochlear nerve.
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2. The saccule
The saccule is a globular vesicle, lies in
the anteroinferior part of the vestibule
and its lower end communicates with the
cochlear duct through the ductus reunions.
Backward, it communicates with the utricle
and endolymphatic sac respectively through
utriculosaccular duct and endolymphatic
duct.
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The macula sacculi lie on the
anterosuperior wall of the saccule.
They also may be stimulated by the
changes of the static position of the
head, and the linear movements on
acceleration or deceleration of the
head. The nerve impulses transverse
through the saccular branch of the
vestibular nerve.
Ⅱ) The Semicircular Ducts
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They lie within the bony semicircular
canal, and are similar to them in
shape, but are approximately 1/41/3 of the diameter of them. The
semicircular ducts also are three in
number. Each one has a
membranous ampulla which lies
within the corresponding bony
ampulla.
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On the wall of membranous ampullae
there are the ampullary crests
(crista ampullaris), which are the
organs of position receptors (kinetic
balance), and may be stimulated by
the movements of angular
acceleration of the head.
The semicircular ducts open by five
openings into the utricle.
Ⅲ) The Cochlear Duct
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It is a spirally arranged canal that makes about
2.5 turns, and lies in the bony canal of the
cochlea, between the osseous spiral lamina and
the lateral wall of cochlear spiral canal. The
cochlear duct extends from the vestibule
communicating with saccule to the summit of the
cochlea (blind end).
A transverse section through the cochlea shows
that cochlear spiral canal is divided into three
separated channels, namely the scala tympani,
scala vestibuli and the cochlear duct.
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The cochlear duct:
It is triangular on the transverse section, and has
three walls.
Its superior wall is the vestibular wall
(vestibular membrane) that separates the
cochlear duct from the scala vestibuli;
Its lateral wall is formed by the thickend
endosteum lining the bony canal of the cochlea
and is concerned with the production of the
endolymph;
Its inferior wall tympanic wall (membranous
spiral lamina, or basilar membrane) separates
the cochlear duct from the scala tympani.
The cochlear duct ends in its upper blind
extremity, and is attached to the apex of the
cochlea. The lower end, through the ductus
reunions, communicates with the saccule.
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The spiral organ (Corti organ):
It is situated on the basilar
membrane. It is the receptor for
auditory sensation and consists of a
number of hair and support cells.
Ⅲ. The Conduction of Sound:
There are two routs to conduct the sound waves:
Ⅰ) The aerial conduction
1. In the normal condition
 In the normal condition, the sound waves are
conducted mainly through the following pathway.
 The sound waves→the external acoustic
meatus→ tympanic membrane→chain of the
auditory ossicles→fenestra vestibuli → the
perilymph within the scala vestibuli→the
endolymph within the cochlear duct→the spiral
organ.
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 helicotrema→the perilymph within the scala
tympani
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2. In the abnormal consition
 When the tympanic membrane and
auditory ossicles are abnormal in
function, the sound waves are
transmitted through the following
pathway, but the audition is much
more decreased.
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The sound waves→the external
acoustic meatus→air in the tympanic
cavity→the second tympanic
membrane on the fenestra
cochleae→the perilymph within the
tympanic scala→the endolymph
within the cochlear duct→the spiral
organ.
Ⅱ) The bone (or cranial)
conduction
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The sound waves→skull→the bony
labyrinth→the perilymph within scala
vestibuli and scala tympani→the
endolymph within the cochlear
duct→the spiral rogan.
Ⅳ. The Internal Acoustic Meatus
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It is a short canal within the petrous part
of the temporal bone. The opening of the
meatus (internal acoustic pore) locates at
the center of the posterior surface of the
petrous part.
Through the fundus of meatus, the facial,
the vestibulocochlear nerves and the
vessels of the labyrinth enter or leave the
internal ear.