No. 20 No. 20

Vestibulocochlear OrganVestibulocochlear Organ

Chapter 2 The Vestibulocochlear Chapter 2 The Vestibulocochlear OrganOrgan

DivisionDivision The The vestibulocochlear organvestibulocochlear organ ( (earear)) is di is di

vided into three parts:vided into three parts: The external ear,The external ear, The middle ear,The middle ear, The internal ear.The internal ear.

FunctionsFunctions The receptor for auditory sensation (spiral The receptor for auditory sensation (spiral

organ or cochlear organ) and organs of organ or cochlear organ) and organs of static balance (vestibular organ) are in the static balance (vestibular organ) are in the internal ear. The internal ear is such an internal ear. The internal ear is such an organ that can receive the stimulation of organ that can receive the stimulation of both sound waves and changes of the both sound waves and changes of the position of the head.position of the head.

The external and the middle ears are a The external and the middle ears are a sound collecting and transmitting sound collecting and transmitting apparatus.apparatus.

Section 1 The External EarSection 1 The External Ear

The external ear consists of:The external ear consists of: the auricle,the auricle, the external acoustic meatusthe external acoustic meatus the tympanic membrane.the tympanic membrane.

ⅠⅠ. The . The AuricleAuricle

It projects from the side of the head.It projects from the side of the head. It anterolateral surface shows irregularly It anterolateral surface shows irregularly

concave, but its posteromedial surface pconcave, but its posteromedial surface presents convex. The orifice of the meaturesents convex. The orifice of the meatus, named the s, named the external acoustic poreexternal acoustic pore, lie, lies in the fossa of the anterolateral surface s in the fossa of the anterolateral surface of the auricle.of the auricle.

ⅡⅡ. The . The External Acoustic MeatusExternal Acoustic Meatus

Morphological characteristicsMorphological characteristics It extends from the external acoustic porIt extends from the external acoustic por

e to the tympanic membrane and is aboe to the tympanic membrane and is about 2.1ut 2.1 ～～ 2.5 cm in length. Its lateral part 2.5 cm in length. Its lateral part is about one-third of the meatus, termed is about one-third of the meatus, termed the the cartilaginous partcartilaginous part, and its medial p, and its medial part is about two-thirds of the meatus, terart is about two-thirds of the meatus, termed the med the bony partbony part..

The meatus passes medially, its lateral part ruThe meatus passes medially, its lateral part runs forwards and upwards, and then backwardns forwards and upwards, and then backwards; and the medial part runs forwards and downs; and the medial part runs forwards and downwards.wards.

In the clinical examination of the meatus, the In the clinical examination of the meatus, the auricle should be drawn upwards, backwards auricle should be drawn upwards, backwards and slightly laterally to render the meatus as sand slightly laterally to render the meatus as straight as possible, so that the tympanic memtraight as possible, so that the tympanic membrane can be viewed.brane can be viewed.

Structural characteristicsStructural characteristics In the subcutaneous tissue of the cartilaIn the subcutaneous tissue of the cartila

ginous part of the meatus there are numginous part of the meatus there are numerous sebaceous and ceruminous glands.erous sebaceous and ceruminous glands. The latter secrete the cerumen (ear wa The latter secrete the cerumen (ear wax).x).

Clinical pointsClinical points The skin of the meatus is thin, and its suThe skin of the meatus is thin, and its su

bcutaneous tissue is scarce but rich in sebcutaneous tissue is scarce but rich in sensory nerve terminals, and is closely adhnsory nerve terminals, and is closely adherent to the cartilaginous and bony parterent to the cartilaginous and bony parts of the meatus, therefore under inflams of the meatus, therefore under inflammatory condition, the meatus is extremematory condition, the meatus is extremely painful.ly painful.

ⅢⅢ. The . The Tympanic MembraneTympanic Membrane

It is oval in form, thin and semi-transparIt is oval in form, thin and semi-transparent, separates the middle ear from the eent, separates the middle ear from the external acoustic meatus. It inclines greatxternal acoustic meatus. It inclines greatly and forms an angle of 45 degrees with ly and forms an angle of 45 degrees with the floor of the meatus, hence the anterthe floor of the meatus, hence the anteroinferior wall of the meatus is longer thaoinferior wall of the meatus is longer than the posterosuperior wall.n the posterosuperior wall.

The thickened margin of the membrane, the fiThe thickened margin of the membrane, the fibrocartilaginous ring, is attached to the tympabrocartilaginous ring, is attached to the tympanic sulcus at the medial end of the external acnic sulcus at the medial end of the external acoustic meatus. In the upper end of the tympanoustic meatus. In the upper end of the tympanic membrane, two bands, the ic membrane, two bands, the anterioranterior and and poposterior mallear foldssterior mallear folds, are prolonged to the lat, are prolonged to the lateral process of the malleus. The small part (1/eral process of the malleus. The small part (1/4) of the membrane above these folds is lax an4) of the membrane above these folds is lax and thin, called the d thin, called the flaccidflaccid partpart, while the remai, while the remainder is tightly stretched, that is the nder is tightly stretched, that is the tense parttense part of the membrane.of the membrane.

In the living body, it is pearly-grey in colour. ThIn the living body, it is pearly-grey in colour. The handle of the malleus is firmly attached to the handle of the malleus is firmly attached to the inner surface of the tympanic membrane as fe inner surface of the tympanic membrane as far as its center, thus the outer surface of the mar as its center, thus the outer surface of the membrane is concave with a central depression, embrane is concave with a central depression, the umbo, formed by the traction of the lower the umbo, formed by the traction of the lower end of the handle of malleus. When the tympaend of the handle of malleus. When the tympanic membrane is examined by otoscope, a brignic membrane is examined by otoscope, a bright area, the ht area, the cone of lightcone of light, anteroinferior to the , anteroinferior to the umbo can be seen.umbo can be seen.

Section 2 The Middle EarSection 2 The Middle Ear

The The middle earmiddle ear lies between the lies between the external and inner ears.external and inner ears.

It includes three parts:It includes three parts: The tympanic cavity,The tympanic cavity, The auditory tube,The auditory tube, The mastoid cells.The mastoid cells.

ⅠⅠ. The . The Tympanic CavityTympanic Cavity It is an irregular air-filled space within the temIt is an irregular air-filled space within the tem

poral bone, and lies between the tympanic meporal bone, and lies between the tympanic membrane and the lateral wall of the inner ear. It mbrane and the lateral wall of the inner ear. It is the principal part of the middle ear, its capais the principal part of the middle ear, its capacity is about 1city is about 1 ～～ 2 cm3. There are auditory oss2 cm3. There are auditory ossicles, ligaments, muscles, vessels and nerves iicles, ligaments, muscles, vessels and nerves inside the tympanic cavity. The tympanic cavitnside the tympanic cavity. The tympanic cavity communicates anteriorly with the nasopharyy communicates anteriorly with the nasopharyns through the auditory tube, and posteriorly ns through the auditory tube, and posteriorly with the mastoid cells through the mastoid anwith the mastoid cells through the mastoid antrum.trum.

ⅠⅠ) The Walls of the Tympanic Cavity) The Walls of the Tympanic Cavity

It possesses six walls.It possesses six walls. 1. The 1. The tegmental walltegmental wall ( (superior wallsuperior wall)) It is a thin plate of compact bone, the tegmen tIt is a thin plate of compact bone, the tegmen t

ympani, it separates the middle cranial fossa frympani, it separates the middle cranial fossa from the tympanic cavity.om the tympanic cavity.

In the first two years of childhood, the unossifiIn the first two years of childhood, the unossified suture of the superior wall may allow the ined suture of the superior wall may allow the infection to spread from the tympanic cavity intfection to spread from the tympanic cavity into the cranial cavity directly.o the cranial cavity directly.

2. The 2. The jugular wall jugular wall ((inferior wallinferior wall)) It consists of the thin plate of bone which sepaIt consists of the thin plate of bone which sepa

rates the tympanic cavity from the jugular fossrates the tympanic cavity from the jugular fossa. The bone of this wall may be deficient, so tha. The bone of this wall may be deficient, so the tympanic cavity is separated from the jugulae tympanic cavity is separated from the jugular vein by mucous membrane and fibrous tissur vein by mucous membrane and fibrous tissue only. This may cause the beginning part of the only. This may cause the beginning part of the jugular vein to project into the tympanic cavie jugular vein to project into the tympanic cavity.ty.

3. The3. The carotid wallcarotid wall ( (anterior wallanterior wall)) It is the posterolateral wall of the carotid canal,It is the posterolateral wall of the carotid canal,

close to the internal carotid artery. At the sup close to the internal carotid artery. At the superior part of the anterior wall, there are two paerior part of the anterior wall, there are two parallel canals leading to the tympanic cavity. Thrallel canals leading to the tympanic cavity. The upper is the smaller e upper is the smaller semicannal for tensor tsemicannal for tensor tympaniympani, and the lower is a larger, and the lower is a larger semicanal fsemicanal for auditory tubeor auditory tube, the bony part of auditory tu, the bony part of auditory tube.be.

4. The 4. The mastoid wallmastoid wall ( (posterior wallposterior wall)) It is pierced superiorly by the opening (aIt is pierced superiorly by the opening (a

ditus ) of ditus ) of mastoid antrummastoid antrum. The . The pyramidpyramidal eminenceal eminence is situated below the openi is situated below the opening of mastoid antrum. The cavity of the ng of mastoid antrum. The cavity of the pyramidal eminence contains the stapepyramidal eminence contains the stapedius.dius.

5. The 5. The membranous wallmembranous wall ( (lateral walllateral wall)) It is almost entirely formed by the tympaIt is almost entirely formed by the tympa

nic membrane, only the superior part of nic membrane, only the superior part of this wall is formed by the lateral wall of tthis wall is formed by the lateral wall of the he epitympanic recessepitympanic recess..

6. The labyrinthine wall (medial wall)6. The labyrinthine wall (medial wall) It is the lateral wall of the inner ear. A rounded elevatiIt is the lateral wall of the inner ear. A rounded elevati

on on the middle of this wall is named the on on the middle of this wall is named the promontorpromontoryy. Posterosuperior to the tympanic promontory is the . Posterosuperior to the tympanic promontory is the fenestra vestibulifenestra vestibuli ( (oval windowoval window)) being closed by the being closed by the base of the stapes and annular ligament. The base of the stapes and annular ligament. The fenestra fenestra cochleaecochleae ( (round windowround window)) lies posteroinferior to the t lies posteroinferior to the tympanic promontory, and is closed by the secondary tympanic promontory, and is closed by the secondary tympanic membrane in vivo. The ympanic membrane in vivo. The prominence of facial prominence of facial canalcanal is an arcuate-like ridge formed by the bony can is an arcuate-like ridge formed by the bony canal for the facial nerve, and extends back and down to tal for the facial nerve, and extends back and down to the posterior wall of the tympanic cavity. The facial cahe posterior wall of the tympanic cavity. The facial canal is very thin, or incomplete. In inflammatory conditinal is very thin, or incomplete. In inflammatory condition of the tympanic cavity, the facial nerve may be invoon of the tympanic cavity, the facial nerve may be involved and leading to facial paralysis.lved and leading to facial paralysis.

ⅡⅡ) The structures in the tympanic ) The structures in the tympanic cavitycavity

In the tympanic cavity there are three In the tympanic cavity there are three auditory auditory ossiclesossicles, , two musclestwo muscles, , one nerveone nerve and and airair equal equal to the atmosphere.to the atmosphere.

1. The auditory ossicles and their joints1. The auditory ossicles and their joints The tympanic cavity contains a chain of three The tympanic cavity contains a chain of three

ossicles:ossicles: The The malleusmalleus,, TheThe IncusIncus,, The The stapesstapes..

Laterally, the Laterally, the handle of malleushandle of malleus is attached to the ty is attached to the tympanic membrane, and medially, the base of the stapmpanic membrane, and medially, the base of the stapes is fixed to the circumference of the fencestra vestibes is fixed to the circumference of the fencestra vestibuli, while the incus is placed between the malleus and uli, while the incus is placed between the malleus and the stapes.the stapes.

The three ossicles connencted by joints to form a jointThe three ossicles connencted by joints to form a jointed chain which connects the tympanic membrane wited chain which connects the tympanic membrane with the fenestra vestibuli.h the fenestra vestibuli.

When the tympanic membrane is vibrated by the sounWhen the tympanic membrane is vibrated by the sound wave, the handle of the malleus is moved with it, thed wave, the handle of the malleus is moved with it, then, the incus and the stapes transmit the vibrations to tn, the incus and the stapes transmit the vibrations to the inner ear.he inner ear.

2. The muscles to motor the auditory ossicles2. The muscles to motor the auditory ossicles The muscles of the cavity are the tensor tympaThe muscles of the cavity are the tensor tympa

ni and the stapedius.ni and the stapedius. The The tensor tympanitensor tympani lies in the semicanal for t lies in the semicanal for t

ensor tympani, and ends to the handle of the ensor tympani, and ends to the handle of the malleus. It is supplied by the mandibular nervmalleus. It is supplied by the mandibular nerve of trigeminal nerve.e of trigeminal nerve.

TheThe stapediusstapedius arises from the pyramidal emin arises from the pyramidal eminence of posterior wall of the tympanic cavity aence of posterior wall of the tympanic cavity and is inserted into the neck of the stapes. It is cnd is inserted into the neck of the stapes. It is controlled by the facial nerve.ontrolled by the facial nerve.

Under the normal conditions, the tnsor tympaUnder the normal conditions, the tnsor tympani and the stapedius contract simultaneously. ni and the stapedius contract simultaneously. When the tensor tympani contracts it pulls the When the tensor tympani contracts it pulls the handle of the malleus medially, tenses the tymhandle of the malleus medially, tenses the tympanic membrane, and thus reduces the amplitpanic membrane, and thus reduces the amplitude of vibration; whereas the contraction of stude of vibration; whereas the contraction of stapedius renders base of stapes outwards, and apedius renders base of stapes outwards, and thus reduces the pressure of sound wave for ththus reduces the pressure of sound wave for the inner ear.e inner ear.

ⅡⅡ. The . The Auditory TubeAuditory Tube (Pharyngot (Pharyngotympanic tube)ympanic tube)

1. Divisions of the auditory tube1. Divisions of the auditory tube It is the channel through which the tympIt is the channel through which the tymp

anic cavity communicates with the nasoanic cavity communicates with the nasopharynx. It is approximately 3.5pharynx. It is approximately 3.5 ～～ 4.0 c4.0 cm long, and is divided into the cartilaginm long, and is divided into the cartilaginous and the bony parts. The junction of tous and the bony parts. The junction of these two parts is narrowest, 1-2 mm lonhese two parts is narrowest, 1-2 mm long, termed theg, termed the isthmus of auditory tubeisthmus of auditory tube..

The bony part:The bony part: This part is the posterolateral part of the tube, This part is the posterolateral part of the tube,

about 1/3 of its total length. It begins in the anabout 1/3 of its total length. It begins in the anterior wall of the tympanic cavity and passes fterior wall of the tympanic cavity and passes forward, downward and inward.orward, downward and inward.

The cartilaginous part:The cartilaginous part: This part is the anteromedial part of the tube, This part is the anteromedial part of the tube,

about 2/3 of the tube, and opens into the nasoabout 2/3 of the tube, and opens into the nasopharynx.pharynx.

2. Function of the auditory tube2. Function of the auditory tube The function of auditory tube is to The function of auditory tube is to

maintain the balance of pressure on both maintain the balance of pressure on both sides of the tympanic membrane.sides of the tympanic membrane.

In the normal condition, the pharyngeal In the normal condition, the pharyngeal orifice and cartilaginous part is closed; orifice and cartilaginous part is closed; during the deglutition they are opened and during the deglutition they are opened and allow the air to center or leave the allow the air to center or leave the tympanic cavity, this balances the tympanic cavity, this balances the pressure on both sides of the tympanic pressure on both sides of the tympanic membranes and censures the tympanic membranes and censures the tympanic membrane to vibrate freely.membrane to vibrate freely.

3. Clinical points3. Clinical points The tube is easily blocked by swelling of The tube is easily blocked by swelling of

its mucous membrane. When it is blockeits mucous membrane. When it is blocked, the residual air in the tympanic cavity d, the residual air in the tympanic cavity is absorbed, resulting in the retraction ois absorbed, resulting in the retraction of the tympanic memrane and interferencf the tympanic memrane and interference with its free movement.e with its free movement.

4. Characteristics of children’s auditory 4. Characteristics of children’s auditory tubetube

In childhood, the auditory tube is shorteIn childhood, the auditory tube is shorter and wider than in adult. Its direction is r and wider than in adult. Its direction is more horizontal, therefore, the inflammmore horizontal, therefore, the inflammation of the pharynx may along the auditation of the pharynx may along the auditory tube spread into the tympanic cavity ory tube spread into the tympanic cavity and causes the otitis media.and causes the otitis media.

ⅢⅢ. The Mastoid Antrum and Mastoi. The Mastoid Antrum and Mastoid Cellsd Cells

They are air-filled spaces in the mastoid proceThey are air-filled spaces in the mastoid process of the temporal bone. They are a series of inss of the temporal bone. They are a series of intercommunicated cavities, and anteriorly, thrtercommunicated cavities, and anteriorly, through the mastoid antrum they communicate ough the mastoid antrum they communicate with the tympanic cavity. Since the mucous mwith the tympanic cavity. Since the mucous membrane of the mastoid air cells is continuous embrane of the mastoid air cells is continuous with that of the mastoid antrum and tympanic with that of the mastoid antrum and tympanic cavity, the otitis media may spread to the mascavity, the otitis media may spread to the mastoid antrum and the mastoid cells.toid antrum and the mastoid cells.

Section 3 The Internal EarSection 3 The Internal Ear The inner (internal) ear lies in the petrous part of the tThe inner (internal) ear lies in the petrous part of the t

emporal bone.emporal bone. It consits of two parts: the bony labyrinth and the meIt consits of two parts: the bony labyrinth and the me

mbranous labyrinth.mbranous labyrinth. The former is composed of the compact bone, and the The former is composed of the compact bone, and the

latter, a series of communicating membranous sacs alatter, a series of communicating membranous sacs and ducts, is contained within the bony labyrinth.nd ducts, is contained within the bony labyrinth.

The The membranous labyrinthmembranous labyrinth is filled with is filled with endolymphendolymph,, and the space between the membranous and bony la and the space between the membranous and bony labyrinth is filled with byrinth is filled with perilymphperilymph. The endolymph does . The endolymph does not communicates with the perilymph.not communicates with the perilymph.

ⅠⅠ. The . The Bony LabyrinthBony Labyrinth

From before backwards, the bony From before backwards, the bony labyrinth is divided into three parts:labyrinth is divided into three parts:

The The cochleacochlea,, The The vestibule,vestibule, The The bony semicircular canalsbony semicircular canals.. They are various with each other in They are various with each other in

shape, but communicate with each shape, but communicate with each other.other.

ⅠⅠ)The )The VestibuleVestibule

It is the central part of the bony It is the central part of the bony labyrinth, and is situated medial to labyrinth, and is situated medial to the tympanic cavity. It is a somewhat the tympanic cavity. It is a somewhat ovoid space. There are three ovoid space. There are three foremen on the posterior part of the foremen on the posterior part of the vestibule communicating with three vestibule communicating with three bony semicircular canals. It appears bony semicircular canals. It appears four walls:four walls:

1. Lateral wall1. Lateral wall It is the medial wall of the tympanic cavity.It is the medial wall of the tympanic cavity. Fenestra vestibuli Fenestra vestibuli andand fenestra cochleae: fenestra cochleae: On its lateral wall there are two openings: the On its lateral wall there are two openings: the ff

enestra vestibulienestra vestibuli communicates with the tym communicates with the tympanic cavity and is closed by the base of the stpanic cavity and is closed by the base of the stapes with its annular ligament in vivo; the apes with its annular ligament in vivo; the fenfenestra cochleaeestra cochleae is closed by the secondary tym is closed by the secondary tympanic membrane.panic membrane.

2. Medial wall2. Medial wall It is the posterior part of the fundus of the internal acoIt is the posterior part of the fundus of the internal aco

ustic meatus, through which the peripheral branches ustic meatus, through which the peripheral branches of the vestibulocochlear nerve pass into the membranof the vestibulocochlear nerve pass into the membranous labyrinth.ous labyrinth.

3. Anterior wall3. Anterior wall On this wall there is the inlet of cochlear spiral canal cOn this wall there is the inlet of cochlear spiral canal c

ommunicating with the scala vestibuli of the cochlea.ommunicating with the scala vestibuli of the cochlea. 4. Posterior wall4. Posterior wall On the posterior wall of the vestibule there are the five On the posterior wall of the vestibule there are the five

openings of the semicircular canals. openings of the semicircular canals.

ⅡⅡ) The ) The CochleaCochlea The cochlea is placed anterior to the vestibule, The cochlea is placed anterior to the vestibule,

resembling the shell of a snail. It is composed resembling the shell of a snail. It is composed of of modiolus modiolus andand cochlear spiral canalcochlear spiral canal windin winding spirally for 2.5 turns around the central modig spirally for 2.5 turns around the central modiolus.olus.

Its apex or cupula of cochlea points anterolateIts apex or cupula of cochlea points anterolaterally, its base is directed posteromedially towarally, its base is directed posteromedially towards the bottom of the internal acoustic meatus.rds the bottom of the internal acoustic meatus.

The The modiolusmodiolus is the conical osseous central pi is the conical osseous central pillar of the cochlea, through which the vessels allar of the cochlea, through which the vessels and nerves pass.nd nerves pass.

The The osseous spiral laminaosseous spiral lamina of modiolus projec of modiolus projects from the modiolus into the spiral canal, and ts from the modiolus into the spiral canal, and divides the cochlear canal into divides the cochlear canal into scala vestibuliscala vestibuli and the and the scala tympaniscala tympani with the basilar memb with the basilar membrane of the cochlear duct. The scala vestibuli arane of the cochlear duct. The scala vestibuli and the scala tympani pass to the fenestra vestind the scala tympani pass to the fenestra vestibuli and the fenestra cochleae respectively anbuli and the fenestra cochleae respectively and are filled with perilymph.d are filled with perilymph.

The width of lamina of modiolus gradually decThe width of lamina of modiolus gradually decreases from the basal to the apical coil of the creases from the basal to the apical coil of the cochlea, and near the summit of the cochlea thochlea, and near the summit of the cochlea the lamina ends in a hook-shaped process, the e lamina ends in a hook-shaped process, the hhamulus of spiral laminaamulus of spiral lamina..

The hamulus and the modiolus form the The hamulus and the modiolus form the helichelicotremaotrema, through which the scala vestibuli and , through which the scala vestibuli and the scala tympani communicate with each oththe scala tympani communicate with each other.er.

ⅢⅢ) The ) The Semicircular CanalsSemicircular Canals They are three in number, anterior (superior), They are three in number, anterior (superior),

posterior and lateral. They are situated posterposterior and lateral. They are situated posterosuperior to the vestibule.osuperior to the vestibule.

1. The anterior semicircular canal1. The anterior semicircular canal The The anterior semicircular canalanterior semicircular canal lies in a verti lies in a verti

cal plane across the long axis of the petrous pacal plane across the long axis of the petrous part of the temporal bone deep to the arcuate ert of the temporal bone deep to the arcuate eminence.minence.

2. The lateral semkicircular canal2. The lateral semkicircular canal The The lateral semicircular canallateral semicircular canal is nearly horiz is nearly horiz

ontal.ontal.

3. The posterior semicircular canal3. The posterior semicircular canal The The posterior semicircular canalposterior semicircular canal lies in a vert lies in a vert

ical plane parallel to the long axis of petrous pical plane parallel to the long axis of petrous part of temporal bone.art of temporal bone.

Each canal has two crura, one of which is dilatEach canal has two crura, one of which is dilated, named the ed, named the bony ampullabony ampulla..

The other crura of the anterior and posterior cThe other crura of the anterior and posterior canals open together into the vestibule by one anals open together into the vestibule by one common bony crus. While the another crus of tcommon bony crus. While the another crus of the lateral canal opens into the vestibule separhe lateral canal opens into the vestibule separately.ately.

ⅡⅡ. The . The Membranous LabyrinthMembranous Labyrinth

It is a series of membranous canals It is a series of membranous canals and sacs which lie within the bony and sacs which lie within the bony labyrinth. It is similar to bony labyrinth. It is similar to bony labyrinth in shape but is smaller. The labyrinth in shape but is smaller. The membranous labyrinth is lined with membranous labyrinth is lined with epithelium. The spiral organ and epithelium. The spiral organ and vestibular organs are situated in its vestibular organs are situated in its walls.walls.

Composition:Composition: The membranous labyrinth, from before backwards, iThe membranous labyrinth, from before backwards, i

ncludes: ncludes: thethe utricle and sacculeutricle and saccule, in the vestibule,, in the vestibule, thethe semicircular ductssemicircular ducts, in the bony semicircular cana, in the bony semicircular cana

ls,ls, the the cochlear ductcochlear duct, in the cochlear spiral canal of coch, in the cochlear spiral canal of coch

lea.lea. The various parts of the membranous labyrinth form a The various parts of the membranous labyrinth form a

closed system of channels which communicate freely closed system of channels which communicate freely with one another and is filled with the endolymph. with one another and is filled with the endolymph.

ⅠⅠ) The Utricle and Saccule) The Utricle and Saccule

1. The utricle1. The utricle The The utricleutricle It is an elongated sac, lies in the po It is an elongated sac, lies in the po

sterosuperior part of the vestibule.sterosuperior part of the vestibule. On the posterior wall of the utricle there are fivOn the posterior wall of the utricle there are fiv

e openings of the semicircular ducts. Forward, e openings of the semicircular ducts. Forward, it communicates with the saccule and it communicates with the saccule and endolyendolymphatic ductmphatic duct. The endolymphatic duct traver. The endolymphatic duct traverses the vestibular aqueduct and ends as a blinses the vestibular aqueduct and ends as a blind dilatation, the d dilatation, the endolymphatic sacendolymphatic sac..

The The maculautriculimaculautriculi lie on the base of the uppelie on the base of the upper end and anterior wall of the utricle. They are r end and anterior wall of the utricle. They are position receptors (static balance). They may position receptors (static balance). They may be stimulated only by the changes of the positibe stimulated only by the changes of the position of the head, but also may be stimulated by on of the head, but also may be stimulated by the linear movements on acceleration or decelthe linear movements on acceleration or deceleration of the head. The nerve impulses transveration of the head. The nerve impulses transverse through the utricular branch of the vestiberse through the utricular branch of the vestibulocochlear nerve.ulocochlear nerve.

2. The saccule2. The saccule The The sacculesaccule is a globular vesicle, lies in the ant is a globular vesicle, lies in the ant

eroinferior part of the vestibule and its lower eeroinferior part of the vestibule and its lower end communicates with the cochlear duct thrond communicates with the cochlear duct through the ugh the ductus reunionsductus reunions. Backward, it comm. Backward, it communicates with the utricle and endolymphatic sunicates with the utricle and endolymphatic sac respectively through ac respectively through utriculosaccular ductutriculosaccular duct and and endolymphatic ductendolymphatic duct..

The The macula sacculimacula sacculi lie on the anterosup lie on the anterosuperior wall of the saccule. They also may erior wall of the saccule. They also may be stimulated by the changes of the statibe stimulated by the changes of the static position of the head, and the linear moc position of the head, and the linear movements on acceleration or deceleration vements on acceleration or deceleration of the head. The nerve impulses transverof the head. The nerve impulses transverse through the saccular branch of the vese through the saccular branch of the vestibular nerve.stibular nerve.

ⅡⅡ) The Semicircular Ducts) The Semicircular Ducts

They lie within the bony semicircular caThey lie within the bony semicircular canal, and are similar to them in shape, bunal, and are similar to them in shape, but are approximately 1/4-1/3 of the diamet are approximately 1/4-1/3 of the diameter of them. The semicircular ducts also ter of them. The semicircular ducts also are three in number. Each one has a are three in number. Each one has a memembranous ampullambranous ampulla which lies within th which lies within the corresponding bony ampulla.e corresponding bony ampulla.

On the wall of membranous ampullae thOn the wall of membranous ampullae there are the ere are the ampullary crests (crista amampullary crests (crista ampullaris)pullaris), which are the organs of positi, which are the organs of position receptors (kinetic balance), and may on receptors (kinetic balance), and may be stimulated by the movements of angbe stimulated by the movements of angular acceleration of the head.ular acceleration of the head.

The semicircular ducts open by five opeThe semicircular ducts open by five openings into the utricle.nings into the utricle.

ⅢⅢ) The Cochlear Duct) The Cochlear Duct It is a spirally arranged canal that makes about 2.5 turIt is a spirally arranged canal that makes about 2.5 tur

ns, and lies in the bony canal of the cochlea, between ns, and lies in the bony canal of the cochlea, between the osseous spiral lamina and the lateral wall of cochlthe osseous spiral lamina and the lateral wall of cochlear spiral canal. The cochlear duct extends from the vear spiral canal. The cochlear duct extends from the vestibule communicating with saccule to the summit of estibule communicating with saccule to the summit of the cochlea (blind end).the cochlea (blind end).

A transverse section through the cochlea shows that cA transverse section through the cochlea shows that cochlear spiral canal is divided into three separated chochlear spiral canal is divided into three separated channels, namely the annels, namely the scala tympaniscala tympani, , scala vestibuliscala vestibuli and t and the he cochlear ductcochlear duct..

The cochlear ductThe cochlear duct:: It is triangular on the transverse section, and has three waIt is triangular on the transverse section, and has three wa

lls.lls. Its Its superior wallsuperior wall is the is the vestibular wall (vestibular membvestibular wall (vestibular memb

rane)rane) that separates the cochlear duct from the scala vest that separates the cochlear duct from the scala vestibuli;ibuli;

Its Its lateral walllateral wall is formed by the thickend endosteum linin is formed by the thickend endosteum lining the bony canal of the cochlea and is concerned with the g the bony canal of the cochlea and is concerned with the production of the endolymph;production of the endolymph;

Its Its inferior wallinferior wall tympanic wall ( tympanic wall (membranous spiral lamimembranous spiral laminana, or, or basilar membranebasilar membrane) separates the cochlear duct fro) separates the cochlear duct from the scala tympani.m the scala tympani.

The cochlear duct ends in its upper blind extremity, and is The cochlear duct ends in its upper blind extremity, and is attached to the apex of the cochlea. The lower end, throuattached to the apex of the cochlea. The lower end, through the ductus reunions, communicates with the saccule.gh the ductus reunions, communicates with the saccule.

The spiral organ (Corti organ):The spiral organ (Corti organ): It is situated on the basilar membrane. It It is situated on the basilar membrane. It

is the receptor for auditory sensation anis the receptor for auditory sensation and consists of a number of hair and suppod consists of a number of hair and support cells.rt cells.

ⅢⅢ. The Conduction of Sound: . The Conduction of Sound: There are two routs to conduct the sound waves:There are two routs to conduct the sound waves: Ⅰ Ⅰ) The aerial conduction) The aerial conduction 1. In the normal condition1. In the normal condition In the normal condition, the sound waves are conductIn the normal condition, the sound waves are conduct

ed mainly through the following pathway.ed mainly through the following pathway. The sound waves→the external acoustic meatus→ tyThe sound waves→the external acoustic meatus→ ty

mpanic membrane→chain of the auditory ossicles→fempanic membrane→chain of the auditory ossicles→fenestra vestibuli → the perilymph within the scala vesnestra vestibuli → the perilymph within the scala vestibuli→the endolymph within the cochlear duct→the stibuli→the endolymph within the cochlear duct→the spiral organ.piral organ.

↓ ↑ ↓ ↑ helicotrema→the perilymph within the scala tympanihelicotrema→the perilymph within the scala tympani

2. In the abnormal consition2. In the abnormal consition When the tympanic membrane and audiWhen the tympanic membrane and audi

tory ossicles are abnormal in function, ttory ossicles are abnormal in function, the sound waves are transmitted through he sound waves are transmitted through the following pathway, but the audition ithe following pathway, but the audition is much more decreased.s much more decreased.

The sound waves→the external acoustic The sound waves→the external acoustic meatus→air in the tympanic cavity→the meatus→air in the tympanic cavity→the second tympanic membrane on the fenesecond tympanic membrane on the fenestra cochleae→the perilymph within the stra cochleae→the perilymph within the tympanic scala→the endolymph within ttympanic scala→the endolymph within the cochlear duct→the spiral organ.he cochlear duct→the spiral organ.

ⅡⅡ) The bone (or cranial) conduction) The bone (or cranial) conduction

The sound waves→skull→the bony labyThe sound waves→skull→the bony labyrinth→the perilymph within scala vestibrinth→the perilymph within scala vestibuli and scala tympani→the endolymph uli and scala tympani→the endolymph within the cochlear duct→the spiral rogwithin the cochlear duct→the spiral rogan. an.

ⅣⅣ. The Internal Acoustic Meatus. The Internal Acoustic Meatus

It is a short canal within the petrous part of the It is a short canal within the petrous part of the temporal bone. The opening of the meatus (inttemporal bone. The opening of the meatus (internal acoustic pore) locates at the center of thernal acoustic pore) locates at the center of the posterior surface of the petrous part.e posterior surface of the petrous part.

Through the fundus of meatus, the facial, the vThrough the fundus of meatus, the facial, the vestibulocochlear nerves and the vessels of the estibulocochlear nerves and the vessels of the labyrinth enter or leave the internal ear.labyrinth enter or leave the internal ear.