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Chapter 44: The ear

The ear

The ears are concerned with equilibration as well as hearing. Adjectives relating to the ear are aura(l), and auditory (L.) and otic and acoustic (Gk). Each ear comprises three parts: external, middle, and internal (fig. 44-1).

External ear

The external ear, which conducts sound and protects the deeper parts, consists of the auricle (described with the face) 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 (fig. 44-2), 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 (fig. 44-1). 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. Incisions through the membrane are made in its postero-inferior quadrant to avoid the ossicles and chorda tympani. The membrane can be examined in vivo by a speculum (fig. 53-11A). The tympanic membrane is highly sensitive (cranial nerves V and X laterally and IX medially).

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 (figs. 44-3 and 44-4). The tympanic cavity and auditory tube develop as a recess of the embryonic pharynx. Mucous membrane covers the structures in the tympanic cavity.

Boundaries (fig. 44-5).

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 (fig. 44-5) 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 (see fig. 48-9).

Auditory Ossicles, Joints, and Muscles.

The ossicles (fig. 44-6) are the malleus (hammer), incus (anvil), and stapes (stirrup). 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.

Facial nerve

The facial, or seventh cranial, nerve (fig. 44-7) 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 (see fig. 44-9), 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 (see fig. 44-5). 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 (see fig. 47-5A). 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 (table 44-1).

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 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 (see fig. 47-4D).

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 (fig. 44-8). 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. The chief components of the internal ear are summarized in table 44-2.

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 (fig. 44-9).

Semicircular canals.

The anterior, posterior, and lateral semicircular canals are at right angles one to another (see fig. 44-4). 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 (see fig. 44-4). 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 (see fig. 44-8).

Cochlea.

The cochlea (fig. 44-10), 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 (see fig. 44-9). A bony core, the modiolus, transmits the cochlear nerve and contains the spiral ganglion, the sensory ganglion for this nerve (fig. 44-10). 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 helicotrema; see fig. 44-1); 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 (see fig. 44-8) 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 (fig. 44-11) 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 (fig. 44-11), 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 (see fig. 44-8). The duct ends in the endolymphatic sac (fig. 44-11), under cover of the dura on the petrous part of the temporal bone.

Cochlear duct.

The cochlear duct winds from the saccule to the apex of the cochlea, where it ends blindly (see fig. 44-1) and extends from the osseous spiral lamina to the wall of the cochlea (see fig. 44-10). 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 (see fig. 44-1). 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; see fig. 43-3) 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 (see fig. 44-10). The vestibular and cochlear parts of the eighth cranial nerve have to be tested separately. Deafness may be caused by (1) a lesion of the cochlea or cochlear nerve fibers (neural deafness) or (2) a disease or blockage of the middle or external ear (conductive deafness).

Additional readings

Anson, B. J., and Donaldson, J. A., Surgical Anatomy of the Temporal Bone, 3rd ed., W. B. Saunders Company, Philadelphia, 1981. An atlas of detailed illustrations and photomicrographs.

Vidic, B., and O'Rahilly, R., An Atlas of the Anatomy of the Ear, W. B. Saunders Company, Philadelphia. 1971. Color slides and key drawings.

Wolff, D., Bellucci, R. J., and Eggston, A. A., Surgical and Microscopic Anatomy of the Temporal Bone. Hafner, New York, 1971. Photomicrographs of serial sections.

Questions

44-1 What is the structure of the tympanic membrane and where is the membrane incised?

44-1 The tympanic membrane is a sandwich composed of epidermis, a fibrous core, and mucosa (on the middle ear side). The membrane is incised (myringotomy) in the postero-inferior quadrant of the pars tensa to avoid the ossicles and chorda tympani. The incision would allow the exit of fluid or pus from the middle ear (in otitis media).

44-2 With what does the tympanic cavity communicate?

44-2 The tympanic cavity communicates with (1) the mastoid air cells and mastoid antrum by way of the aditus and (2) the nasopharynx by way of the auditory tube (see fig. 44-3).

44-3 What does the epitympanic recess contain?

44-3 The epitympanic recess, an attic above the level of the typmanic membrane, contains the head of the malleus and the body and short crus of the incus (see fig. 44-4).

44-4 What is the tegmen tympani?

44-4 The tegmen (L., covering; cf. integument) tympani is the portion of the petrous part of the temporal bone that covers the middle ear and separates it from the middle cranial fossa.

44-5 What closes the fenestra vestibuli, and what closes the fenestra cochleae?

44-5 The oval window (fenestra vestibuli) is closed by the base of the stapes. The round window (fenestra cochleae) is closed by the secondary tympanic membrane (the lateral layer of which is tympanic mucosa). The wall of the labyrinth is subject to bone disease (otosclerosis), and invasion of the oval window causes fixation of the stapes. To establish an alternative sound pathway, an artificial window can be made surgically (fenestration) in the lateral semicircular canal.

44-6 Of which type are the joints of the middle ear?

44-6 The incudomallear and incudostapedial joints (see fig. 44-6) are saddle and ball-and-socket synovial joints, respectively. The stapediovestibular junction at the fenestra vestibuli is either a fibrous or a synovial joint.

44-7 What is the nervus intermedius?

44-7 The nervus intermedius is the smaller part of the facial nerve, intermediate in position between cranial nerve VIII and the larger part of VII. The nervus intermedius joins the remainder (motor root) of VII in the internal acoustic meatus. It contains fibers transmitted by the chorda tympani (taste fibers for the anterior two-thirds of the tongue, secretomotor fibers for salivary glands, and some pain fibers) and also fibers for the lacrimal gland (see fig. 44-7).

44-8 Which fibers arise in the geniculate ganglion?

44-8 The geniculate ganglion contains the cells of origin of the taste fibers in the facial nerve (see fig. 44-7). Similarly, the trigeminal ganglion contains the cells of origin of most of the sensory fibers in the trigeminal nerve, and a comparable afferent significance applies to the ganglia of cranial nerves VIII, IX, and X. Hence these sensory ganglia differ functionally from the parasympathetic ganglia associated with the cranial nerves: the ciliary, pterygopalatine, otic, and submandibular ganglia (see table 43-3).

44-9 Which are the main directions taken by the facial nerve (a) in approaching the geniculate ganglion, (b) after it leaves the ganglion, and (c) in approaching the stylomastoid foramen?

44-9 The facial nerve (see fig. 44-9) runs (a) laterally to the geniculate ganglion, (b) posteriorward after it leaves the ganglion, and (c) inferiorward to the stylomastoid foramen. It then proceeds anteriorward in the parotid gland (see fig. 47-5).

44-10 What is the chorda tympani?

44-10 The chorda tympani, not recognized as a nerve by Falloppio (1561), is a branch of the facial nerve that crosses the tympanic membrane and joins the lingual nerve to gain the tongue. The chorda contains (1) taste fibers from the anterior two thirds of the tongue and (2) secretory fibers, which synapse in the submandibular ganglion, the postganglionic fibers supplying the submandibular gland (as discovered by Claude Bernard, 1851). The fibers of the chorda tympani run in the nervus intermedius (see fig. 44-7).

44-11 How is the level of a lesion of the facial nerve determined?

44-11 The level of a lesion of the facial nerve is inferred from effects that depend on whether specific branches are intact. In supranuclear lesions (i.e., above the facial nucleus in the pons), movements of the upper third of the face is spared whereas the lower two thirds are paralyzed. In other words, a lesion of the upper motor neuron affects the voluntary movements in only the lower part of the face. The reason is that the neurons innervating the frontalis, orbicularis, and corrugator muscles receive command signas from both cerebral coticies (i.e., a bilateral central innervation). In lesions of the nerve itself, all the facial muscles are involved, and taste and salivary secretion (chorda tympani) may be affected, depending on how far back along the course of the nerve it was damaged. Interference with the innervation of the stapedius may cause excessive sensitivity to sounds. If the lesion is in the parotid gland or face, complete or partial facial paralysis may result depending on the branches involved. The common type of unilateral facial paralysis is a lesion of uncertain origin in the facial canal near the stylomastoid foramen: Bell's palsy (1821).

44-12 What are the chief signs of facial palsy?

44-12 Important signs of facial paralysis are inability to wrinkle the forehead and shut the eyes tightly (in lesions of the facial nucleus or nerve - so-called infranuclear lesions) and inability to smile and show the teeth on one side. When an attempt is made to display the teeth, the angle of the mouth is drawn up on only the normal side and the mouth characteristically appears triangular (see fig. 47-4D)

44-13 What is the otic capsule?

44-13 The otic capsule is a layer of dense bone around the perilymphatic space in the petrous part of the temporal bone. In the adult, the capsule is an integral part of the petrous portion of the bone and no longer maintains the anatomical individuality it possessed in the fetus.

44-14 What are the main parts of (a) the osseous and (b) the membranous labyrinth?

44-14 The main parts of the osseous labyrinth are the semicircular canals, vestibule, and cochlea. The membranous labyrinth comprises the semicircular ducts, utricle and saccule, and cochlear duct (see table 44-2). The cochlea is the essential organ of hearing. The vestibular apparatus, concerned with equilibration, includes the utricle and semicircular ducts.

44-15 Is the lateral semicircular canal horizontal?

44-15 The lateral semicircular canal becomes horizontal only when the head is flexed about 30 degrees. A paper model of the three canals is simple to construct (J. D. Lithgow, J. Laryngol., 35:81, 1920).

44-16 In which direction does the apex of the cochlea point?

44-16 The apex of the cochlea points anterolaterally, whereas that of the petrous part of the temporal bone is directed anteromedially (see fig. 44-9).

44-17 Which structures separate the scalae from each other and from the cochlear duct?

44-17 In a section through the cochlea, the scala vestibuli is seen to be separated from the scala tympani by the osseous spiral lamina and the cochlear duct. The cochlear duct is separated from the scala vestibuli and scala tympani by the vestibular (Reissner, 1851) and basilar membranes, respectively (see fig. 44-10D).

44-18 Where are the neuro-epithelial areas in the membranous labyrinth?

44-18 The neuro-epithelial areas in the membranous labyrinth are in the ampullae of the semicircular ducts (ampullary cristae), in the utricle and saccule (maculae), and in the cochlear duct (spiral organ). This makes a total of six receptive areas in each ear (see fig. 44-11). The spiral organ (Corti, 1851) is the organ of hearing.

Figure legends

Figure 44-1 Basic structure of the ear. The base of the stapes closes the oval window (fenestra vestibuli), and the secondary tympanic membrane closes the round window (fenestra cochleae).

Figure 44-2 Right tympanic membrane, lateral aspect. The interrupted lines delimit the quadrants. The umbo is approximately opposite the promontory of the middle ear. The head of the malleus and the body and short crus of the incus are in the epitympanic recess, above the level of the tympanic membrane. Cf. fig. 53-11A.

Figure 44-3 The tympanic cavity and communicating air spaces. Right lateral aspect.

Figure 44-4 Drawing of the ear by Max Brodel (with lettering modified), showing the osseous labyrinth, cranial nerves VII and VIII, and the auditory tube.

Figure 44-5 Highly schematic view of the medial wall of the right tympanic cavity, lateral aspect. The tympanic plexus is shown on the promontory. (After Maisonnet and Coudane.)

Figure 44-6 Ossicles of the right ear, medial aspect. A, Processes and articulations: 1, incudomallear; 2, incudostapedial. B, Diagram of the lever-like action of the ossicles. For simplification, movements (arrows) are shown as though they take place in one plane. The fixed pivots (in black) are anchored to the bony walls of the tympanic cavity. (A is based on Bassett, B on Guelke and Keen.)

Figure 44-7 The facial nerve and its components.

Figure 44-8 Osseous and membranous labyrinths, showing the basic arrangement and terminology of the internal ear. The membranous labyrinth and endolymph are shown in blue. That the aqueduct of the cochlea communicates with the subarachnoid space is questionable.

Figure 44-9 Right temporal bone viewed from above. The osseous labyrinth, tympanic cavity, and air spaces are shown as if the surrounding bone were transparent.

Figure 44-10 The cochlea, lateral aspect. A, Modiolus. B, Modiolus with the addition of the osseous spiral lamina. C, The cochlea, showing the basilar membrane, cochlear duct, and osseous labyrinth. D, Section through the cochlea, showing the scala vestibuli, scala tympani, cochlear duct, and spiral ganglion. An arrow is placed in the helicotrema. (After Wolff et al.) 1 to 3, basal, middle, and apical coils, respectively, of cochlea; 8, cochlear nerve; B.M., basilar membrane; CO., cochlear duct; I.A.M., internal acoustic meatus.

Figure 44-11 Membranous labyrinth. Note the ampullae of the semicircular ducts. The black lines and areas in the ampullae, utricle, saccule, and cochlear duct represent the neuro-epithelium: the ampullary crests, maculae, and spiral organ, respectively. Two cross sections are included. C.D., cochlear duct; S.C.C, semicircular canal; S.C.D., semicircular duct; S.T., scala tympani; S.V., scala vestibuli.

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