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SYSTEMATIC ANATOMY

Chapter 2       Vestibulocochlear Organ

 

The vestibulocochlear organ  (Fig.-2-1) is composed of the auditory apparatus and vestibular apparatus . Both organs buried in the temporal bone are closely associated anatomically. It is divided into three parts: the external, middle and internal ears. The external and the middle ears are a sound collecting and transmitting apparatus . The internal ear is such an organ that can receive the stimuli of both sound waves and changes of the position and movement of the head .

Section 2   External Ear

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

. Auricle

   The auricles  are two rather peculiarly shaped pieces project from the side of the head (Fig.-2-2). Its anterolateral surface shows irregular concave, but its posteromedial surface presents convex. The orifice of the meatusnamed the external acoustic pore , lies in the fossa of the anterolateral surface of the auricle.

The auricle is composed of an irregularly shaped framework of elastic fibrocartilage covered with skin. The skin of the auricle is thin, closely adherent to the cartilage. The auricular lobule ,  flap of skin-covered connective tissure that extends from the lower margin of the auricle.

.  External Acoustic Meatus

The external acoustic meatus  is a curved passageway about 2.5-3.5cm long that extends from the external acoustic pore to the tympanic membrane (Fig.-2-1). The outer third of the meatus is termed the cartilaginous part , and its inner two-thirds is termed the osseous part . It is narrowed at the junction of its osseous and cartilaginous parts. The meatus is not straight but takes a “S” shaped course. Its1ateral part runs forwards and upwards, and then backwards; and the medial part curves forwards and downwards. The external acoustic meatus can be straightened if the auricle should be pulled upwards, backwards, so that the entire external acoustic meatus and the tympanic membrane can be viewed when examine.

The skin, which envelops the auricle, is continued into the external acoustic meatus and covers the outer surface of the tympanic membrane. In the subcutaneous tissue of the cartilaginous part of the meatus there are numerous sebaceous and modified sweat glands (ceruminous glands  . The latter secrete the cerumen  or ear wax. The skin of the meatus is thin, and its subcutaneous tissue is scarce but rich in sensory nerve terminals, and closely adheres to the cartilaginous and bony parts of the meatus, therefore under inflammatory condition, the meatus is extremely painful.

.   Tympanic Membrane

The tympanic membrane  is oval in shape, thin and semitransparent in structure and separates the middle ear from the external acoustic meatus (Fig.-2-1). It inclines greatly and forms an angle of 450 with the floor of the meatus, hence the anterior and inferior walls of the meatus are longer than the posterior and superior walls.

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 part of the tympanic membrane, two folds, the anterior and posterior malleolar folds , are prolonged to the lateral process of the malleus. The small part 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(Fig.-2-3).

    In the living body, the tympanic membrane is pearly gray. 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 , can be seen anteroinferior to the umbo.

   The tympanic membrane is a connective tissue membrane and composed of three layers: an outer cuticular layer, and intermediate fibrous layer and an inner mucous layer. The cuticular layer is continued with the skin of the external acoustic meatus. The fibrous layer lies between the cuticular and mucous layer. The flaccid part of the tympanic membrane is devoid of the fibrous 1ayer. The mucous layer is continued with the mucous membrane of the tympanic cavity.

Section 3  Middle Ear

The middle ear (Fig.-2-1) lies between the external and internal ears, and includes three parts: the tympanic cavitythe auditory tube and the mastoid cells.

   . Tympanic Cavity

   The tympanic cavity  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, and contains the auditory ossicles, ligaments, muscles, vessels and nerves. The tympanic cavity communicates anteriorly with the nasopharynx through the auditory tube , and posteriorly with the mastoid cells through the mastoid antrum  1. Walls of the tympanic cavity

        The tympanic cavity which is a small irregular cavity (Fig.-2-1, 4, 5) possesses six walls:

   1).  Superior wall (tegmental wallis a thin plate of compact bone, the tegmen tympani, which 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.

   2).  Inferior wall (jugular wallis narrower than the tegmental 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).Anterior wall (carotid wall) is close to the internal carotid arteryand there are two parallel canals leading to the tympanic cavity on its superior part. The upper is the smaller semicanal for tensor tympani , and the lower is a larger semicanal for auditory tube , the bony part of the auditory tube.

4).Posterior wall (mastoid wallis deficient above, where there is an aperture, the opening of the opening of mastoid antrum, which leads into the tympanic antrum. The pyramidal eminence  is situated below the opening of mastoid antrum. The cavity of the pyramidal eminence contains the stapedius.

5). Lateral wall (membranous wall ) 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 .

6). Medial wall (labyrinthine wall ) 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 cochlea or the round window1ies 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 canal may be involved, which leads to facial paralysis .

2. Auditory ossicles

The tympanic cavity contains a chain of three auditory ossicles : the malleus , incus and stapes . Laterally, the hand of malleus is attached to the tympanic membrane, and medially, the base of the stapes is fixed to the circumference of the fenestra vestibuli, while the incus is placed between the malleus and the stapes (Fig.-2-6).

The three ossicles are connected each other by joints to form a chain of bones and to connect 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 stapes transmit the vibration to the inner ear.

3.  Muscles of the tympanic cavity

The muscles of the tympanic cavity are the tensor tympani  and stapedius . The tensor tympani 1ies in the semicanal for tensor tympani and ends the handle of malleus. The stapedius arises from the pyramidal eminence of posterior wall of the tympanic cavity and is inserted into the neck of the stapes. Under the normal conditions, the tensor tympani and the stapedius contract simultaneously. When the tensor tympani contracts it pulls the handle of the malleus medially, and the the tympanic membrane is tensioned, and thus reduces the amplitude of vibration; whereas the contraction of stapedius renders the base of stapes outwards, and thus reduces the pressure of sound wave to the inner ear.

.  Auditory Tube (Eustachian Tube)

The auditory tube (Fig.-2-1) is the channel through which the tympanic cavity communicates with the nasopharynx. It is approximately 3.54.0cm long, and is divided into the cartilaginous and the bony parts. The junction of these two parts is narrowest, termed the isthmus .

The bony part of the tube is the posterolateral part of the tube, and about one-third of its total 1ength. It begins in the anterior wall of the tympanic cavity and passes forward, downward and inward. The anteromedial part of the tube is the cartilaginous part, about two-thirds of the tube, and opens into the nasopharynxIn the normal condition, the pharyngeal orifice  and the cartilaginous part are closed; during the deglutition they are opened and allow the air to enter or leave the tympanic cavity, which balances the pressure on both sides of the tympanic membrane  and ensures the tympanic membrane to vibrate freely.

The auditory tube is easy to be 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 membrane and interference with its free movements.

Contrasted with adult, , the auditory tube is shorter and wider In childhood, and the direction is more horizontal. Therefore, the inflammation of the pharynx may spread along the auditory tube into the tympanic cavity and causes the otitis media .

.  Mastoid Cells

The mastoid cells  are air-filled spaces in the mastoid processes of the temporal bone (Fig.-2-4). It is a series of intercommunicated cavities that anteriorly, through the mastoid antrum , 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(中耳炎) may spread to the mastoid antrum and mastoid cells.

 

Section 4  Internal Ear

 

   The internal (inner) ear  lies medial to the middle ear within the petrous part of the temporal bone (Fig.-2-7) and consists of a series of canals called the bony labyrinth . Inside the bony labyrinth, and following its course, is the membranous labyrinthThe membranous labyrinth is filled with a fluid called endolymph. The membranous and bony labyrinths are separated by a space containing a fluid called perilymph . The endolymph does not communicate with perilymph(Fig.-2-8.

 

                  

. Bony Labyrinth

  The bony labyrinth is a series of intercommunicating canals and cavities in the compact bone, which is lined by periosteum. From before backwards, the bony labyrinth is divided into three parts: the cochlea, the vestibule and the bony semicircular canals. They are different in shape, but communicate with each other (Fig.-2-9).

1. Cochlea  resembles the shell of a snail. It consists of a bony spiral canal , which is coiled two and a half to two and three-quarter turns around a horizontal central pillar, the modiolus . The cochlea is placed anterior to the vestibule. Its apex or cupula points anterolaterally, its basal coil is widest and directed posteromedially towards the fundus of the internal acoustic meatus.

The modiolus is the conical osseous central pillar of the cochlea, through which the vessels and nerves pass. It is about 3 mm long; projects from the central modiolus throughout this bony canal is a spiral bony shelf, the osseous spiral lamina and it divides the cochlear spiral canal into scala vestibuli  and the scala tympani with the basilar membrane of the cochlear duct(Fig.-2-10). The scala vestibuli and the scala tympani pass to the fenestra vestibuli and fenestra cochlea respectively and are filled with perilymph. 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 scala tympani communicate with each other.

 

2.Vestibule  is the central part of the bony labyrinth and is situated medial to the tympanic cavity. It is a somewhat ovoid space. On its lateral wall (namely the medial wall of the tympanic cavity) 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. The medial wall of vestibile corresponds to the fundus of the internal acoustic meatus , through which the peripheral branches of the vestibulocochlear nerve  pass into the membranous labyrinth.

On the posterior wall of the vestibule there are five openings of the semicircular canals. On its anterior wall, a large opening communicates with the scala vestibuli of the cochlea.

3.Bony semicircular canals (Fig.-2-9)are three in number: anterior, posterior and lateral (horizontal) semicircular canal. They are located posterosuperior to the vestibule, and arranged at right angles to each other.

The anterior semicircular canal  lies in a vertical plane across the long axis of the petrous part of the tem`poral bone deep to the arcuate eminence; the posterior semicircular canal  lies in a vertical plane parallel to the long axis of petrous part of temporal bone; and the lateral (horizontal) semicircular canal  is nearly horizontal. Each canal has two bony crura, one of which is dilated, named the ampullary bony crus ; the other crura of the anterior and posterior canals open together the vestibule by one common bony crus , while the another crus of the lateral canal opens into the vestibule separately.

. Membranous Labyrinth

The membranous labyrinth  is a series of intercommunicating membranous canals and sacs which lying within the bony labyrinth (Fig.-2-8,11). It is similar to bony labyrinth in shape. The membranous labyrinth is lined with epithelium. The spiral organ and vestibular organs are situated in its walls.

The membranous labyrinth, from before backwards, includes: the cochlear duct, the utricle and saccule, and the membranous semicircular ducts. The various parts of the membranous labyrinth form a closed system of communicating channels and contain endolymph.

1.Cochlear duct  is shaped like a somewhat triangular tube and is spiral for two and a half to two and three-quarters turns around the central modiolus within the cochlear spiral canal. The cochlear duct extends from the vestibule to the summit of the cochlea.

A transverse section through the cochlea shows that the cochlear spiral canal is divided into three separated channels, namely the scala tympani, scala vestibuli and cochlear duct. The cochlear duct is triangular on the transverse section, and has three walls (Fig.-2-10). Its roof (superior wall) is the vestibular membrane  that separates the cochlear duct from the scala vestibuli; its lateral wall termed external wall of cochlear duct  is formed by the thickened endosteum 1ining the bony canal of the cochlea and is concerned with the production of the endolymphits floor is formed by the basilar membrane (spiral membrane) and the outer part of the osseous spiral lamina, and 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 of the duct, through the ductus reunions, communicates with the saccule.

The spiral organ is situated on the basilar membrane. It is the receptor for auditory sensation and consists of a number of hair and support cells (Fig.-2-12.

The scala tympani separated from the scala vestibuli by the cochlear duct, and only on the apex of the cochlea, the two scalae communicate with each other through the helicotrema.

2. Utricle and saccule  lie within the vestibule(Fig.-2-11). The utricle, an elongated sac, lies in the posterosuperior part of the vestibule. The saccule, a globular vesicle, 1ies in the anteroinferior part of the vestibule and its lower end communicates with the cochlear duct through the ductus reunions . Both of utricle and saccule contain endolymph. On the posterior wa1l of the utricle there are five openings of the membranous semicircular ducts. The small utriculosaccular duct  joins the utricle to the saccule and continues with the endo1ymphatic duct , which traverses the vestibular aqueduct  and ends as a blind dilatation, the endolymphatic sac which lies behin the internal acoustic pore.

The macula sacculi  and macula utriculi  lie respectively on the wall of saccule and utricle. The maculae are the organs of static balance. They may be stimulated not 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 .

3. Semicircular ducts  lie within the bony semicircular canal, and are similar to them in shape, but are approximately one-third of the diameter of them (Fig.5-2-8). The semicircular ducts also are three in number. Each one has an ampulla , which lies within the corresponding bony ampulla. On the wall of membranous ampullae  there are the ampullary crests , which are the organs of kinetic balance, and may be stimulated by the movements of angular acceleration of the head. The membranous semicircular ducts open by five openings into the utricle.

. Conduction of Sound

There are three routes to conduct the sound waves :

1. First pathway: The sound wave    the external acoustic meatus    tympanic membrane   chain of the auditory ossicles     fenestra vestibuli the perilymph within scala vestibuli the endo1ymph within cochlear duct the  spiral organ nerve impulse

helicotrema the perilymph within the scala tympani second tympanic membrane

2. Second pathway: The sound wavethe external acoustic meatus air in the tympanic cavity the second tympanic membrane on the fenestra cochleathe perilymph within the scala tympani the endo1ymph within the cochlear duct the spiral organ nerve impulse

3. Third pathway: The sound wave skull the bony 1abyrinth the perilymph within scals vestibuli and scala tympani the endo1ymph within the cochlear duct the spiral organ nerve impulse

   The first and second pathways conduct through air (aerial conduction) . The third pathway is the bone (or cranial) conduction . In the normal conduction, the sound waves are conducted mainly through the first pathway. When the auditory ossicles of the patient are abnormal in function, the sound waves are transmitted through the second pathway, but the auditory is much more decreased.

   .  Internal Acoustic Meatus

   The internal acoustic meatus  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 vessels of the labyrinth enter or leave the internal ear.

                     

Lu Guangming , Gu Xiaosong:Nantong Medical College