Download External Acoustic Meatus.

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2016 / 4 / 4
The Ear
The ears are concerned with equilibration as well as hearing. Adjectives relating to
the ear. Each ear comprises three parts: external, middle, and internal .
External ear
The external ear, which conducts sound and protects the deeper parts, consists of
the auricle and the external acoustic meatus.
External Acoustic Meatus.
The external meatus, about 25 mm long, extends from the concha to the tympanic
membrane. Its lateral part is cartilaginous and continuous with the auricle; the
longer medial part is bony. The cartilaginous part of the meatus is slightly concave
anteriorly, and a speculum may be inserted more readily when the auricle is pulled
backward and upward. The meatus is lined by the skin of the auricle, which
presents hairs, and sebaceous and ceruminous glands.
Sensory Innervation and Blood Supply.
The external ear is supplied by the auriculotemporal (fifth cranial) nerve (and
probably by contributions from cranial nerves VII, IX, and X) and the great
auricular nerve. The posterior auricular and superficial temporal arteries of the
external carotid artery provide the blood supply.
Tympanic Membrane.
The ear drum about 1 cm in diameter, separates the external meatus from the
tympanic cavity. Its fibrous basis is attached to the tympanic plate of the temporal
bone and is covered laterally by epidermis and medially by the mucous membrane
of the middle ear. The larger portion of the membrane is its tense part; the
anterosuperior comer, or flaccid part, is bounded by anterior and posterior mallear
folds. The tympanic membrane is set very obliquely . Its lateral surface is concave,
and its deepest point is the umbo. The handle and lateral process of the malleus are
attached to the medial surface of the tympanic membrane.
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Middle ear
The middle ear consists largely of an air space in the temporal bone. This tympanic
cavity contains the auditory ossicles and communicates with (1) the mastoid air
cells and mastoid antrum by means of the aditus and (2) the nasopharynx by means
of the auditory tube
Boundaries
The lateral wall is the tympanic membrane. However, a portion of the tympanic
cavity, the epitympanic recess, forms an attic above the level of the tympanic
membrane and communicates with the aditus.
The epitympanic recess contains the head of the malleus and the body and
short crus of the incus. The roof is formed by a portion of the petrous part of the
temporal bone known as the tegmen tympani, which separates the middle ear from
the middle cranial fossa. The floor is the jugular fossa, in which is lodged the
superior bulb of the internal jugular vein. The anterior wall presents the semicanal
for the tensor tympani muscle, the opening of the auditory tube, and the carotid
canal, in which is lodged the internal carotid artery. The posterior wall presents the
aditus, which leads to the mastoid antrum, and the pyramidal eminence, which
contains the stapedius muscle. The mastoid portion of the temporal bone,
especially the mastoid process, is usually hollowed out by air cells, which
communicate with each other and with the mastoid antrum and are lined by
mucoperiosteum.
The medial wall presents (1) the prominence of the lateral semicircular canal
and the prominence of the facial nerve canal; (2) the oval window (fenestra
vestibuli), closed by the base of the stapes, and the cochleariform process; (3) the
promontory, formed by the basal turn of the cochlea; and (4) the round window
(fenestra cochleae), closed by mucous membrane. The tympanic plexus, situated
on the promontory, is formed chiefly by the tympanic nerve (from cranial nerve
IX), which gives sensory fibers to the middle ear and secretomotor fibers to the
parotid gland
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Auditory Ossicles, Joints, and Muscles.
The ossicles are the malleus (hammer), incus (anvil), and stapes. The malleus
presents a head and neck; a handle (manubrium) and lateral process, embedded in
the tympanic membrane; and an anterior process, attached to the petrotympanic
fissure. The incus presents a body, short crus, and long crus. The stapes presents a
head, anterior and posterior crura, and a base or footplate, which is attached by an
annular ligament to the margin of the oval window. The incudomallear and
incudostapedial joints are saddle and ball-and-socket synovial joints, respectively.
The tensor tympani arises from the cartilaginous part of the auditory tube, enters a
semicanal, turns laterally around the cochleariform process, and is inserted on the
handle of the malleus. Supplied by the mandibular nerve and tympanic plexus, the
tensor draws the handle of the malleus medially, thereby tensing the tympanic
membrane. The stapedius arises within the pyramidal eminence and is inserted on
the neck of the stapes. Supplied by the facial nerve, the stapedius draws the stapes
laterally. Both the tensor and the stapedius attenuate sound transmission through
the middle ear.
Sensory Innervation.
The middle ear is supplied by the auriculotemporal (fifth cranial) and tympanic
(ninth cranial) nerves and by the auricular branch of the vagus.
Functional Considerations.
Sound waves set the tympanic membrane in motion. These vibrations are
converted into intensified movements of the stapes by the lever-like action of the
auditory ossicles. Movements at the oval window are accompanied by
compensatory movements at the round window. Sound vibrations are transmitted
to the inner ear by (1) the auditory ossicles and the oval window, (2) air in the
tympanic cavity and the round window, and (3) bone conduction through the skull.
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Facial nerve
The facial, or seventh cranial, nerve is described here because of its close
relationship to the middle ear. The facial nerve comprises a larger part, which
supplies the muscles of facial expression, and a smaller part termed the nervus
intermedius, which contains taste fibers for the anterior two thirds of the tongue,
secretomotor fibers for the lacrimal and salivary glands, and some pain fibers. The
two parts leave the brain at the caudal border of the pons (cerebellopontine angle)
and enter (with the eighth cranial nerve) the internal acoustic meatus. The facial
nerve traverses the facial canal in the temporal bone. Superior to the promontory
on the medial wall of the middle ear, the nerve expands to form the geniculate
ganglion, which contains the sensory ganglion cells of origin of its taste fibers. The
nerve turns sharply posteriorward and then sweeps inferiorward immediatly
posterior to the middle ear . It emerges from the skull at the stylomastoid foramen.
Finally, the facial nerve enters the parotid gland, forms the parotid plexus, and
gives terminal branches to the facial muscles .The facial nerve traverses in
succession the (1) posterior cranial fossa, (2) internal acoustic meatus, (3) facial
canal, and (4) parotid gland and face.
Branches
The facial nerve gives a number of communicating branches. The chief named
branches arise in the facial canal, just after exiting from the base of the skull, and
in the parotid gland.
In the facial canal, the geniculate ganglion gives rise to the greater petrosal
nerve. This branch passes anteromedially to join the deep petrosal nerve (from the
carotid sympathetic plexus) and form the nerve of the pterygoid canal, which
reaches the pterygopalatine ganglion. The greater petrosal nerve contains
secretomotor fibers for the lacrimal and nasal glands and a number of afferent
fibers of uncertain distribution and function.
In the facial canal, the facial nerve gives off the nerve to the stapedius muscle
and the chorda tympani. The chorda tympani enters the tympanic cavity, passes
medial to the tympanic membrane and the handle of the malleus, and again enters
the temporal bone. It leaves the skull through the petrotympanic fissure to reach
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the infratemporal fossa where it joins the lingual nerve, with which it is distributed
to the anterior two thirds of the side and dorsum of the tongue. The chorda contains
(1) taste fibers from the anterior two thirds of the tongue and from the soft palate
and (2) preganglionic secretory fibers, which synapse in the submandibular
ganglion, the postganglionic fibers supplying the submandibular, sublingual, and
lingual glands.
Immediately inferior to the base of the skull, the facial nerve gives off
muscular branches to the stylohyoid muscle and to the posterior belly of the
digastric muscle. It also gives rise to the posterior auricular nerve, which supplies
motor fibers to the auricular and occipitalis muscles and sensory fibers to the
auricle.
Within the parotid gland, the facial nerve forms the parotid plexus, from which
terminal branches radiate in the face to supply the muscles of facial expression,
including the platysma. The branches are usually classified as temporal, zygomatic,
buccal, marginal, and cervical. They contain afferent (probably proprioceptive or
pain or both) as well as motor fibers.
Examination.
For examination of the facial muscles, the subject is asked to show the teeth, puff
out the cheeks, whistle, frown, and close the eyes tightly. The level of a lesion of
the facial nerve is inferred from effects that depend on whether specific branches
are intact. Involvement of the facial nerve in the facial canal (as in Bell's palsy)
results in ipsilateral drooping of the mouth and difficulty with eye closure
Internal ear
The internal ear, situated within the petrous part of the temporal bone, consists of a
complex series of fluid-filled spaces, the membranous labyrinth, lodged within a
similarly complex cavity, the bony (osseous) labyrinth . The cochlea is the
essential organ of hearing. Other portions (the utricle and semicircular ducts) of the
internal ear constitute the vestibular apparatus, although equilibrium is also
maintained by vision and proprioceptive impulses.
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Osseous Labyrinth.
The osseous labyrinth comprises a layer of dense bone (otic capsule) in the petrous
part of the temporal bone and the enclosed perilymphatic space, which contains a
fluid very similar to extracellular fluid, the perilymph. The perilymphatic space
consists of a series of continuous cavities: semicircular canals, vestibule, and
cochlea
Semicircular canals.
The anterior, posterior, and lateral semicircular canals are at right angles one to
another .The lateral canal is not quite horizontal and the other cans are positioned
such that the anterior canal on one side is in the same plane as the posterior canal
of the other side.
Vestibule.
The term vestibule is restricted here to the middle part of the bony labyrinth,
immediately medial to the tympanic cavity .It contains the utricle and saccule of
the membranous labyrinth. The oval window, situated between the vestibule and
the tympanic cavity, is closed by the footplate (base) of the stapes. The aqueduct of
the vestibule transmits the endolymphatic duct .
Cochlea.
The cochlea named for its resemblance to the shell of a snail, is a helical tube of
about 2.5 turns. Its base lies against the lateral end of the internal acoustic meatus,
its basal coil forms the promontory of the middle ear, and its apex is directed
anterolaterally. A bony core, the modiolus, transmits the cochlear nerve and
contains the spiral ganglion, the sensory ganglion for this nerve .A winding shelf,
the osseous spiral lamina, projects from the modiolus like the flange of a screw.
The cochlear duct extends from this lamina to the wall of the cochlea so that the
space in the cochlea is divided into three ducts: a scala vestibuli anteriorly and a
scala tympani posteriorly, separated by the cochlear duct (that is part of the
membranous labyrinth). The scala vestibuli begins in the vestibule and passes to
the apex of the cochlea, where the two scalae communicate with each other (at the
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helicotrema; the scala tympani returns to end blindly near the round window,
which is closed by the secondary tympanic membrane.
Perilymphatic duct.
The perilymphatic duct, or aqueduct of the cochlea, is situated in a bony channel,
the cochlear canaliculus, and is frequently said to connect the scala tympani with
the subarachnoid space with the reult that the perilymph is compositionally similar
to C.S.F. and that meningitis can produce damage to the inner ear.
Membranous Labyrinth.
The membranous labyrinth lies within the bony labyrinth and contains a different
fluid, endolymph. The membranous labyrinth consists of a series of continuous
cavities: semicircular ducts, utricle and saccule, and cochlear duct.
Semicircular ducts.
The anterior, posterior, and lateral semicircular ducts are situated eccentrically in
the corresponding canals. One end of each duct is enlarged as an ampulla, each of
which possesses a neuroepithelial ampullary crest, which is stimulated by relative
movement of the endolymph particulalry during angular head movements (i.e.,
spinning, pitching or tumbling).
Utricle and saccule.
The utricle and saccule lie in the vestibule and communicate with each other by
utricular and saccular ducts. The utricle has five openings for the semicircular
ducts: the anterior and posterior semicircular ducts have one opening in common.
The saccule is joined to the cochlear duct by the ductus reuniens. The utricle and
saccule each present a neuro-epithelial macula, which is stimulated by gravity and
by linear acceleration. The endolymphatic duct arises from the utricular and
saccular ducts and is transmitted by the aqueduct of the vestibule The duct ends in
the endolymphatic sac under cover of the dura on the petrous part of the temporal
bone.
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Cochlear duct.
The cochlear duct winds from the saccule to the apex of the cochlea, where it ends
blindly and extends from the osseous spiral lamina to the wall of the cochlea (see
Its anterior and posterior walls are the vestibular and basilar membranes,
respectively. The spiral organ, the organ of hearing, lies against the basilar
membrane and includes neuro-epithelial hair cells attached to a gelatinous mass,
the tectorial membrane. The spiral ligament comprises the third side of the
triangular-shaped duct. There are a series of blood vessels in this ligament (the
stria vascularis) that are responsible for production of endolymph.
Functional Considerations.
The functional details of the internal ear are incompletely understood. Movement
of the stapes in the oval window results in compensatory movement of the
secondary tympanic membrane in the fenestra cochleae .Vibrations that are
generated in the perilymph will move the membranes that bound the cochlear duct
and displace the hair cells of the spiral organ in relation to the tectorial membrane.
Most of the energy transmitted to the inner ear is absorbed by the basilar
membrane, the motion pattern of which is different depending on the frequency of
the sound. Low-frequency sounds cause maximum displacement in the basilar
membrane located in the apex of the cochlea, whereas high-frequency sounds
maximally vibrate the membranes of the basal portion of the cochlea.
Vestibulocochlear nerve
The vestibulocochlear, or eighth cranial, nerve contains afferent fibers from the
internal ear. It leaves the brain at the lower border of the pons (cerebellopontine
angle; and enters (with cranial nerve VII) the internal acoustic meatus. The
vestibular part, concerned with equilibration, is distributed to the maculae of the
utricle and saccule and to the ampullary crests of the semicircular ducts. The
vestibular fibers arise in bipolar cells in the vestibular ganglion in the internal
acoustic meatus. The cochlear part of the eighth cranial nerve, concerned with
hearing, is distributed to the hair cells of the spiral organ. The cochlear fibers arise
in bipolar cells in the spiral ganglion in the modiolus. The vestibular and cochlear
parts of the eighth cranial nerve have to be tested separately.
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