The ear, which houses the peripheral parts of the auditory and vestibular apparatus, is descriptively divided into the external, middle and internal ear. The external ear consists of the auricle or pinna and the external acoustic meatus, at the medial end of which lies the tympanic membrane, separating the external ear from the middle ear. The middle ear or tympanic cavity (tympanum) is a small space in the temporal bone containing the auditory ossicles (malleus, incus and stapes) and air that communicates with the nasopharynx by the auditory tube. By its medial wall the middle ear adjoins the inner ear, which is composed of the osseous labyrinth, another space within the temporal bone, inside which is the membranous labyrinth containing the auditory and vestibular nerve receptors.
External ear
The auricle or pinna has a skeleton of resilient yellow elastic cartilage which is thrown into folds. The folds give the auricle its characteristic shape. The cartilage is covered on both surfaces with adherent hairy skin; it does not extend into the lobule of the ear. The lobule is a tag of skin containing soft fibrofatty tissue; it is easily pierced for earrings. The cartilage of the auricle is prolonged inwards in tubular fashion as the cartilaginous part of the external acoustic meatus, whose attachment to bone stabilizes the auricle in position. Small anterior, superior and posterior auricular muscles attach the auricle to the scalp and skull, and all are supplied by the facial nerve.
The external acoustic meatus is a sinuous tube nearly 3cm in length; it is straightened for introduction of an otoscope by pulling the auricle upwards and backwards. Due to the obliquity of the tympanic membrane at the deep end of the meatus, separating it from the tympanic cavity, its anteroinferior wall is longest and its posterosuperior wall shortest (Fig. 6.68). Its outer third is cartilage, its inner two-thirds bone; in both zones the skin is firmly adherent.
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Figure 6.68 Oblique section through the right external ear, middle ear and pharyngotympanic tube: anterior aspect. |
The bony part is formed by the tympanic part of the temporal bone, C-shaped in cross-section, the gap in the C being applied to the under surface of the squamous and petrous parts. The cartilaginous portion is likewise C-shaped; the gap is filled with fibrous tissue. Hairs and sebaceous glands abound in the skin of the cartilaginous part. Here also are the ceruminous glands, long coiled tubules like modified sweat glands, which secrete a yellowish-brown wax. The meatus is narrowest at the isthmus, a few millimetres from the membrane.
The auricle and external meatus are mainly supplied by the posterior auricular and superficial temporal arteries, with the deeper part of the meatus receiving the deep auricular artery (from the maxillary) which enters the meatus through the squamotympanic fissure. There are corresponding veins.
Lymphatic drainage is to occipital, preauricular and superficial cervical nodes.
The main cutaneous nerves are the great auricular and auriculotemporal nerves, with a small contribution from the vagus. The great auricular supplies the whole of the cranial surface of the auricle (C2, with a little overlap from the lesser occipital at the top) and the lower part of the lateral surface. The auriculotemporal supplies the upper part of the lateral surface and most of the meatal skin. The auricular branch of the vagus (Arnold's nerve) supplies small areas of skin on the cranial auricular surface, posterior wall and floor of the meatus and adjoining part of the tympanic membrane. The facial nerve may also contribute via a communication with the vagus.
Middle ear
The middle ear is an air space in the temporal bone (Fig. 6.68). It contains the three auditory ossicles whose purpose is to transmit sound vibrations from the tympanic membrane in its lateral wall to the inner ear via its medial wall. The cavity of the middle ear, the tympanic cavity or tympanum, is really the intermediate portion of a blind diverticulum from the respiratory mucous membrane of the nasopharynx. From front to back the diverticulum consists of the auditory tube, the tympanic cavity, and the mastoid antrum and air cells.
Tympanic cavity
The tympanic cavity, about 15mm in anteroposterior and vertical diameters, is the shape of a biconcave lens. Its lateral wall is largely occupied by the tympanic membrane, which extends upwards for 10mm from the floor and bulges inwards to within a couple of millimetres of the medial wall. Above the membrane the temporal bone is hollowed out into the epitympanic recess.
The tympanic membrane is a thin fibrous structure covered externally with a thin layer of stratified squamous epithelium and internally with low columnar epithelium. The framework consists of collagen fibres. The membrane is circular, 1cm in diameter, and lies obliquely at 55° with the external acoustic meatus, facing downwards, forwards and laterally (Fig. 6.68). It is concave towards the meatus. At the depth of the concavity is a small depression, the umbo. When the drum is illuminated for inspection, the concavity of the membrane produces a ‘cone of light’ radiating from the umbo over the anteroinferior quadrant. The handle of the malleus is firmly attached to the inner surface of the membrane. From the lateral process of the malleus two thickened fibrous folds (mallear folds) diverge up to the margins of the tympanic bone; between them the small upper segment of the membrane is lax (pars flaccida, Shrapnell's membrane). This part and the neck of the malleus are crossed internally by the chorda tympani. The rest of the membrane, the main part, is the pars tensa. It is held tense by the inward pull of the tensor tympani muscle. Its tension is affected by difference of pressure in the tympanic cavity and external meatus in cases of auditory tube obstruction. The tympanic membrane is thickened at its circumference and slotted into a groove in the tympanic plate.
The tympanic membrane is supplied by the deep auricular artery (maxillary) on the meatal side, and on the mucosal side the stylomastoid artery (posterior auricular) forms a circular anastomosis with the anterior tympanic branch of the maxillary round the margin of the membrane.
On the meatal surface the tympanic membrane is supplied by the auriculotemporal nerve, supplemented by the vagus. The tympanic branch of the glossopharyngeal nerve, via the tympanic plexus, supplies the mucosal surface.
The medial wall of the tympanic cavity (which is also the lateral wall of the internal ear) has as its most prominent feature the promontory (Fig. 6.69), due to the first turn of the cochlea and indented with fine grooves by the tympanic plexus. Above it is a horizontal ridge for the canal for the facial nerve, and immediately above that is the (horizontal) bulge due to the lateral semicircular canal. Above and behind the promontory is the oval window (fenestra vestibuli), closed in life by the foot-piece of the stapes. Below and behind the promontory is the round window (fenestra cochleae), closed in life by the fibrous secondary tympanic membrane.
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Figure 6.69 Medial wall of the right middle ear. |
The roof of the tympanum is the tegmen tympani, a laminar projection of petrous bone that roofs in also the canal for the tensor tympani and the tympanic (mastoid) antrum. Above it the temporal lobe lies in the middle cranial fossa (Fig. 6.69).
The floor is a thin plate of bone above the jugular fossa. At the anterior end is the internal opening of the tympanic canaliculus, where the tympanic branch of the glossopharyngeal nerve enters, from the jugular fossa, the external opening of the canaliculus being on the ridge of bone between the fossa and the carotid canal (Fig. 6.19).
The anterior wall is shortened by approximation of roof and floor. It is perforated by the openings of two canals: the lower and larger of these is the bony part of the auditory tube, the upper and smaller is the canal for the tensor tympani muscle (Figs 6.68 and 6.69). The lower part of this wall forms the posterior wall of the carotid canal and is perforated by tympanic branches of the internal carotid artery and sympathetic fibres from the internal carotid plexus.
The posterior wall is deficient above, where there is an aperture, the aditus, which leads back into the mastoid antrum. The ridge for the canal for the facial nerve and the bulge due to the lateral semicircular canal continue backwards along the medial wall of the aditus. Below the aditus a hollow cone, the pyramid, projects into the tympanic cavity (Fig. 6.69); its apex is perforated by the tendon of stapedius. Close to the posterior margin of the tympanic membrane is the tiny posterior canaliculus for the chorda tympani.
The auditory ossicles form by synovial joints a bony chain for transmission of vibrations from the tympanic membrane to the internal ear. The malleus and incus are developed from the proximal end of the first arch cartilage (see Fig. 1.20, p. 25), the stapes comes from the second arch cartilage (see Fig. 1.21, p. 25).
The malleus is shaped like a round-headed club. There is a constriction, the neck, between head and handle. The convex head lies in the epitympanic recess (Fig. 6.68). Its posterior surface has an articular facet for the incus. The narrow neck lies against the pars flaccida of the tympanic membrane. The chorda tympani crosses medial to the neck. The handle projects somewhat backwards down to the umbo; its upper end has a projection, the lateral process. The two form a lateral concavity moulded to the medial convexity of the tympanic membrane; the periosteum of lateral process and handle is firmly fixed to the fibrous layer of the membrane. The mallear folds are attached to the apex of the lateral process. The tiny anterior process is directed forwards from just below the neck; it is embedded in the fibres of the anterior ligament, which passes through the petrotympanic fissure to the spine of the sphenoid; like the sphenomandibular ligament it is derived from the perichondrium of the first arch cartilage.
The incus has a relatively large body and two slender processes or limbs. The body is rounded and laterally compressed. It lies in the epitympanic recess and articulates anteriorly with the head of the malleus (Fig. 6.68). The short limb projects backwards to lie in a shallow fossa in the posterior wall just below the aditus. The long limb projects down into the cavity of the middle ear, just behind and parallel with the handle of the malleus. Its tip hooks medially and is bulbous—the lentiform nodule—for articulation with the stapes.
The stapes has a small head showing a concave facet for articulation with the lentiform nodule. A narrower neck diverges into slender anterior and posterior limbs, which are attached to the base (or footpiece) like a rider's stirrup. This is attached to the oval window by an annular ligament.
The tensor tympani arises from and occupies the canal above the bony part of the auditory tube. The slender muscle ends in a round tendon which passes across the cavity of the middle ear and is inserted into the handle of the malleus. Its nerve supply is from the mandibular nerve via its branch to the medial pterygoid (see p. 364). Contraction of the muscle draws the handle of the malleus inwards, making the drum more highly concave and therefore more tense.
The stapedius arises from the interior of the hollow pyramid. Its tendon emerges from the apex of the pyramid and is inserted into the back of the neck of the stapes. The muscle is supplied from the facial nerve by a branch given off in the facial (stylomastoid) canal. Its action is to retract the neck of the stapes, thus tilting the footpiece in the oval window. Paralysis of the stapedius causes an abnormally increased power of hearing (hyperacusis).
Mastoid antrum and air cells
The mastoid (tympanic) antrum lies behind the epitympanic recess in the petrous part of the temporal bone. It is connected to the recess by the aditus. Its size is very variable; it may be up to 1cm in diameter. When large it is covered by a thin layer of bone, when small by a thick layer. Its lateral wall corresponds to the suprameatal triangle at the posterosuperior margin of the external acoustic meatus (see p. 507) and the antrum lies about 15mm deep to the surface of the bone here. It is roofed by the tegmen tympani.
The mastoid antrum is present at birth and is then almost adult size. During the first year mastoid air cells, lined with adherent mucoperiosteum, burrow out from the mastoid antrum into the thin plate of bone at the bottom of the groove for the sigmoid sinus. Later they pneumatize the mastoid process for a variable distance, even to the tip. They may be separated from the sigmoid sinus and posterior cranial fossa by extremely thin bone.
The thin mucous membrane of the middle ear, continuous with that of the auditory tube and mastoid antrum, adheres to all the structures enumerated above: the walls; ossicles; ligaments; and muscles. The lining epithelium is columnar and ciliated, but squamous and non-ciliated in the antrum and air cells.
The anterior tympanic from the maxillary, the stylomastoid from the posterior auricular (or occipital), and tympanic branches from the internal carotid, middle meningeal and ascending pharyngeal arteries supply the middle ear. Venous drainage is to the pterygoid plexus and superior petrosal sinus. Thrombophlebitis from suppuration in the middle ear may lead to meningitis, and by retrograde venous spread to a cerebral abscess in the temporal lobe. Veins from the mastoid antrum communicate via the mastoid emissary vein with the posterior auricular vein and the sigmoid sinus. Spreading infection from the mastoid antrum and air cells can lead to sigmoid sinus thrombosis, meningitis and a cerebellar abscess.
Lymphatic drainage from the middle ear is to preauricular, retropharyngeal and upper deep cervical nodes.
The mucous membrane of the middle ear is supplied by branches of the tympanic plexus. This is mainly formed by the tympanic branch of the glossopharyngeal nerve (Jacobson's nerve), which forms a fine plexiform network on the promontory (Fig. 6.69). It is joined by sympathetic fibres from the internal carotid plexus which enter the tympanic cavity through the wall of the carotid canal.
Since the middle ear and the external ear are supplied by branches of the trigeminal, glossopharyngeal and vagus nerves, pain in the ear (otalgia) may be referred from other areas supplied by these nerves, especially the pharynx, larynx, posterior part of tongue and teeth.
The plexus gives off the lesser petrosal nerve. This contains preganglionic parasympathetic fibres from the inferior salivary nucleus, destined to supply the parotid gland via the otic ganglion. The fibres enter the plexus with the glossopharyngeal tympanic branch. The nerve leaves the middle ear through a canaliculus in the anterior wall above the auditory tube and emerges in the middle cranial fossa through a small hiatus lateral to that for the greater petrosal nerve (see Fig. 8.4, p. 507); it then passes through the foramen ovale to reach the otic ganglion (Figs 6.21 and 6.27).
Facial nerve and the ear
The facial nerve itself is not within the middle ear cavity but passes through the petrous bone from the internal acoustic meatus to the stylomastoid foramen in three directions, laterally, posteriorly and downwards, in that order. First the main trunk of the nerve runs laterally from the internal acoustic meatus with the nervus intermedius, which contains the parasympathetic fibres for the pterygopalatine and submandibular ganglia (see Fig. 1.15, p. 21), and also taste fibres from the anterior part of the tongue and the soft palate. The two parts of the facial nerve here lie above the vestibule, with the cochlea in front and the semicircular canals behind. The nervus intermedius now joins the main nerve at the geniculate ganglion. The greater petrosal nerve passes forwards from the ganglion through a canal in the petrous bone and emerges from a hiatus to lie on a groove on the floor of the middle cranial fossa (Figs 6.101and 8.4, p. 507), which it leaves through the foramen lacerum to become part of the nerve of the pterygoid canal (see p. 370).
The facial nerve now passes backwards from the ganglion in the canal which raises the ridge on the medial wall of the tympanic cavity above the promontory and below the prominence of the lateral semicircular canal (Fig. 6.69). Finally the nerve passes downwards medial to the aditus to the antrum and emerges from the stylomastoid foramen. The nerve to stapedius and the chorda tympani leave this part of the nerve in the middle ear.
The chorda tympani is a mixed visceral nerve, containing taste fibres from the tongue (cell bodies in the geniculate ganglion) and secretomotor fibres for the salivary glands of the floor of the mouth (cell bodies in the superior salivary nucleus in the pons). At about 6mm above the stylomastoid foramen the chorda tympani leaves the facial nerve in the facial canal and pierces the posterior wall of the tympanic cavity (Fig. 6.27). It runs forward over the pars flaccida of the tympanic membrane and the neck of the malleus, lying just beneath the mucous membrane (Fig. 6.21). It passes out of the front of the middle ear and emerges from the medial end of the petrotympanic fissure (Fig. 6.35 and see p. 507), grooves the medial side of the spine of the sphenoid, and joins the lingual nerve 2cm below the base of the skull.
Auditory tube
The auditory tube (pharyngotympanic tube, Eustachian tube) connects the nasopharynx with the middle ear. Over 3cm long, it slopes from the middle ear forwards and medially at 45° and downwards at 30°. Like the external acoustic meatus it has bony and cartilaginous parts, but the proportions are reversed.
The bony part, over 1cm long, tapers down from the anterior wall of the middle ear to its orifice where it perforates the petrous part of the temporal bone. This is the narrowest part of the tube, the isthmus; it lies posteromedial to the spine of the sphenoid and lateral to the carotid canal. It is lined with adherent mucoperiosteum, which is surfaced by ciliated columnar epithelium and, as in the middle ear, has no glands.
The cartilaginous part, over 2cm long, joins the bony orifice at the isthmus and is lodged in the groove between the greater wing of the sphenoid and the apex of the petrous part of the temporal bone (Fig. 6.35). It is made of elastic cartilage, which in transverse section resembles an inverted J (long limb medial) open inferolaterally where it is closed by fibrous tissue. It enlarges from the isthmus like a trumpet, with its open end expanded, particularly the long posterior limb which forms the tubal elevation in the lateral wall of the nasopharynx (Fig. 6.39). The mucosa is lined by ciliated columnar cells and has mucous glands. The cilia beat towards the nasopharynx, thus protecting the middle ear from airborne particles, including bacteria.
The ostium (opening) of the tube is attached to the back of the medial pterygoid plate just below the skull base. The tubal elevation is made more prominent, especially in the young, by lymphoid follicles in the mucous membrane (tubal tonsil). The posterior limb is elongated by the vertical salpingopharyngeal fold, draped over salpingopharyngeus.
The pharyngobasilar fascia is attached to the lower part of the tube; lateral to this the tensor palati arises outside the pharynx, and medial to this the levator palati arises inside the pharynx. Both are attached in part to the tube and contract during swallowing which opens the tube and allows equalization of air pressure on the two sides of the tympanic membrane. Air is slowly lost from the middle ear and mastoid cavities by absorption into the capillaries thereof.
The blood supply of the tube is from the ascending pharyngeal and middle meningeal arteries. Its veins drain into the pharyngeal plexus. The lymphatic drainage is to retropharyngeal lymph nodes. The nerve supply is by the pharyngeal branch of the pterygopalatine ganglion (maxillary nerve) and the tympanic plexus (glossopharyngeal nerve).
Internal ear
The internal ear is buried in the petrous part of the temporal bone and is practically full adult size at birth. It consists of a complex series of connected cavities, the osseous labyrinth, within which lies a correspondingly complex fluid-filled sac, the membranous labyrinth. The fluid it contains is endolymph and, because the membranous labyrinth is smaller than the osseous, its walls are not all pressed tightly against the bone but are mostly separated from it by another fluid, perilymph. The endolymph and perilymph do not communicate with one another.
The parts of the osseous labyrinth, in order from front to back, are the cochlear canal (or cochlea), the vestibule, and the semicircular canals (Figs 6.70 and 6.71; these illustrations depict casts of the bony cavity). The parts of the membranous labyrinth (Fig. 6.72) are the cochlear duct (within the cochlear canal and concerned with hearing), the utricle and saccule (within the vestibule and concerned with static balance), and the semicircular ducts (within the semicircular canals and concerned with kinetic balance).
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Figure 6.70 Left osseous labyrinth in the temporal bone, from above and behind. The cochlea is at the front, the vestibule in the middle and the semicircular canals at the back. |
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Figure 6.71 Left osseous labyrinth, from the lateral side. |
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Figure 6.72 Left membranous labyrinth, from the medial side. The stippled part represents the osseous labyrinth. |
Osseous labyrinth
The cavity of the osseous labyrinth is lined by endosteum and opens into the medial wall of the middle ear through the oval window (closed in life by the footpiece of the stapes) and the round window (closed in life by the secondary tympanic membrane) (Figs 6.71 and 6.69). It also opens into the posterior cranial fossa through the aqueduct of the vestibule (Fig. 6.105), closed in life by the endolymphatic duct, and through the aqueduct of the cochlea (Fig. 6.72) through which perilymph is believed to drain into the cerebrospinal fluid. The source of perilymph is uncertain; it may be derived from cerebro-spinal fluid or as an ultrafiltrate from perilymphatic blood capillaries.
The cochlea is a conical snail-shaped cavity in the petrous bone. It consists of two and three-quarter spiral turns of a tapering canal. The bony canal is of greatest calibre at the basal turn; this part projects laterally, producing the promontory on the medial wall of the middle ear (Fig. 6.69).
The axial conical bony stem around which the canal spirals is the modiolus. The base of the modiolus lies at the fundus of the internal acoustic meatus and its apex lies across the long axis of the petrous bone, pointing towards the middle ear. The apex of the modiolus is overlaid by the blind extremity of the apical turn of the cochlea.
From the modiolus a spiral shelf of bone projects into the canal, like a thread projecting from a screw. This is the spiral lamina. Its projection is widest in the basal and narrowest in the apical turn. The membranous cochlear duct (Figs 6.72 and 6.73) is attached to the spiral lamina and to the outer bony wall of the canal.
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Figure 6.73 Transverse section through a single turn of the cochlea. |
The bony canal of the cochlea is thus partitioned by the spiral lamina and the membranous cochlear duct, which contains endolymph. The canal on the apical side of the partition is the scala vestibuli, that on the basal side the scala tympani (Fig. 6.73); they contain perilymph. They communicate with each other around the blind apical extremity of the cochlear duct.
The basal turn of the cochlea sees the termination of the spiral lamina. Here the scala tympani is sealed off into a blind end. There are two holes in this cul-de-sac. One leads laterally into the middle ear—the round window—which is closed in life by the secondary tympanic membrane. The other is the beginning of a canal, the aqueduct of the cochlea (perilymphatic duct), which leads down through the substance of the petrous bone and opens into the cochlear canaliculus, below the internal acoustic meatus, in the anterior compartment of the jugular foramen. The aqueduct of the cochlea is patent in life. The arachnoid mater is attached to the margin of its opening, so that perilymph draining down the aqueduct is received into the cerebrospinal fluid in the subarachnoid space.
The modiolus is perforated spirally at its base in the internal acoustic meatus by the branches of the cochlear nerve (Fig. 6.108). These run into the modiolus and fan out spirally towards the base of the spiral lamina. Here the spiral ganglion containing their bipolar cell bodies lies in the spiral canal at the base of the spiral lamina of the modiolus (Fig. 6.73). This is the counterpart of the posterior root ganglion of a spinal nerve (i.e. it contains the cell bodies of the first neuron of a sensory pathway). The spiral ganglion (cochlear nerve) connects the sound receptors in the spiral organ (see below) with the cochlear nuclei in the brainstem.
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Figure 6.108 Left internal acoustic meatus with the osseous labyrinth. |
The vestibule is a hollow in the petrous bone which contains the membranous saccule and utricle (Figs 6.70 and 6.72). The scala vestibuli of the cochlea opens into the front of the vestibule and the five orifices of the semicircular canals open posteriorly.
The medial wall abuts on the internal acoustic meatus and is perforated by minute foramina for the branches of the vestibular nerve to the saccule, utricle and semicircular canals.
The lateral wall of the vestibule abuts on the middle ear behind the promontory. Here is the opening of the oval window (Fig. 6.71), closed in life by the foot-piece of the stapes and its annular ligament.
The semicircular canals lie in three planes at right angles to each other (Fig. 6.71). Each is about two-thirds of a circle; in length along the curve they measure about 20mm. Their calibre is 1mm except at one end, where each is dilated as the ampulla to a calibre of 2mm.
The anterior (superior) semicircular canal is placed in a vertical plane across the long axis of the petrous bone, convexity upwards, ampulla laterally (Fig. 6.70). Its convexity produces the arcuate eminence on the upper surface of the petrous bone in the middle cranial fossa (see p. 448). It lies highest of the three canals.
The posterior semicircular canal is placed in a vertical plane in the long axis of the petrous bone, convexity backwards, ampulla below. The ampulla is innervated separately by a branch of the vestibular nerve which pierces the foramen singulare in the internal acoustic meatus (Fig. 6.108).
The lateral semicircular canal is placed 30° off the horizontal plane, convexity backwards and laterally, ampulla anteriorly. The ampulla bulges the medial wall of the aditus and epitympanic recess above the facial canal. The lateral semicircular canal lies horizontal if the head nods 30° forwards. The canal opens by each end separately into the back of the vestibule. The anterior and posterior canals open separately at their ampullated ends, but their non-ampullated ends fuse into a common canal. Thus only five openings connect the three canals with the cavity of the vestibule.
The anterior and posterior canals, lying across and along the axis of the petrous bone, are each at 45° with the sagittal plane. Thus the posterior canal of one side lies parallel with the anterior canal of the opposite side.
Membranous labyrinth
The membranous labyrinth is a reduced replica of the hollow bony labyrinth (Fig. 6.72). It consists of one continuous closed cavity containing endolymph. The membranous covering consists of three layers. The outer fibrous layer is vascular and in places adherent to the endosteum of the bony labyrinth forming the stria vascularis which produces endolymph. The intermediate layer is homogeneous like a basal lamina and the inner epithelial layer is elaborated in three places into receptors of sound, static balance and kinetic balance, supplied by the cochlear (hearing) and vestibular (balance) divisions of the eighth nerve.
The cochlear duct is the spiral anterior part of the membranous labyrinth which contains the sound receptors. It is attached to the apical surface of the spiral lamina and to the outer bony walls of the cochlea. It commences at a blind extremity at the apex of the cochlea and after two and three-quarter spiral turns ends in a bulbous extremity in the basal turn of the cochlea. The minute ductus reuniens connects this extremity with the saccule (Fig. 6.72).
Two membranes enclose the duct of the cochlea. It is triangular in cross-section (Fig. 6.73). One side of the triangle is formed by the basilar membrane which extends in the line of the spiral lamina to the outer bony wall of the cochlea. Throughout its length it supports the spiral organ.
Another side is formed by the delicate vestibular (Reissner's) membrane which passes obliquely across the cochlea on the apical side of the basilar membrane. Connecting the two membranes, and completing the triangle, is the endosteum of the outer wall of the cochlea, which is thickened here to form the spiral ligament.
In the spiral organ (of Corti) the sensory hair cells, the receptors for hearing, are lodged on supporting cells. The ‘hairs’ are modified microvilli (stereocilia) and are overlaid by the tectorial membrane, a sheet of keratin-like protein that projects from beneath the inner attachment of the vestibular membrane (Fig. 6.73). The hair cells are supplied by the dendrites of the spiral ganglion.
The mechanism of the organ of Corti is not fully understood. It is probable that sound vibrations are communicated from the perilymph to the endolymph through the delicate vestibular membrane and cause appropriate fibres in the basilar membrane to bulge. The overlying hair cells are thus stimulated.
The saccule is a fibrous sac that lies in the lower front part of the vestibule, connected to the basal part of the cochlear duct by the very small ductus reuniens. The utricle is a similiar sac in the upper and back part of the vestibule and receives the five openings of the three semicircular ducts (Fig. 6.72). The two sacs lie with their adjacent walls in contact; a small duct leads from each and they unite in a Y-shaped manner to form the endolymphatic duct. This lies in the aqueduct of the vestibule and projects as a blind diverticulum, the endolymphatic sac, beneath the dura mater of the posterior cranial fossa. Endolymph is absorbed by the epithelium of the sac.
The medial wall of the saccule and the floor of the utricle are thickened to form the maculae, the areas that contain the sensory receptors for static balance. As in the cochlear duct, they are called hair cells (with stereocilia), and are here overlaid by the gelatinous otolithic (statoconial) membrane which contains particles of calcium carbonate (otoliths or statoconia).
The semicircular ducts are only a quarter the calibre of the bony canals except at the ampullae, which they almost fill. Each membranous duct is adherent by its convexity to the wall of the bony canal in which it lies. The ducts open into five orifices in the back of the utricle inside the elliptical recess.
A transverse crest on the medial surface of each ampulla forms the crista, containing the sensory receptors (hair cells) for kinetic balance, here overlaid by the gelatinous cupula.
The hair cells of the maculae of the utricle and saccule, and the cristae of the semicircular ducts, are stimulated by movements of the otolithic membranes or cupulae respectively in response to head movements, e.g. changes in position, speed of change, etc. They are supplied by the vestibular nerve.
Blood supply of the labyrinth
The labyrinthine artery (from the basilar or its anterior inferior cerebellar branch) divides in the internal acoustic meatus into branches which accompany the cochlear and vestibular nerves to the labyrinth. Branches of the stylomastoid artery assist. The veins unite to form a labyrinthine vein which leaves the internal acoustic meatus and joins the inferior petrosal sinus. Various irregular veins penetrate the petrous bone independently to open into the superior petrosal sinus. A small vein lies in each aqueduct; that in the aqueduct of the cochlea joins the inferior petrosal sinus, that in the aqueduct of the vestibule joins the superior petrosal sinus.
Distribution of the eighth nerve
The cochlear nerve enters the front of the inferior part of the fundus of the internal aconstic meatus in spiral fashion to reach the organ of hearing (Fig. 6.108). The lower division of the vestibular nerve supplies the macula of the saccule and, through the foramen singulare, the ampulla of the posterior semicircular duct. The upper division of the vestibular nerve supplies the macula of the utricle and the ampullae of the anterior and lateral semicircular ducts. The cell bodies of the cochlear fibres lie in the spiral ganglion in the base of the spiral lamina. The cell bodies of the vestibular fibres lie in the vestibular ganglion in the depths of the internal acoustic meatus.
