Chapter 44: 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).
The external ear, which conducts sound and protects the deeper parts, consists of the auricle (described with the face) and the 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.
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.
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).
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).
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.
The middle ear is supplied by the auriculotemporal (fifth cranial) and tympanic (ninth cranial) nerves and by the auricular branch of the vagus.
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.
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.
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).
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.
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).
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.
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).
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.
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.
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.
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).
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.
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
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.
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.
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).
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.
44-1 What is the structure of the tympanic membrane and where is the membrane incised?
44-2 With what does the tympanic cavity communicate?
44-3 What does the epitympanic recess contain?
44-4 What is the tegmen tympani?
44-5 What closes the fenestra vestibuli, and what closes the fenestra cochleae?
44-6 Of which type are the joints of the middle ear?
44-7 What is the nervus intermedius?
44-8 Which fibers arise in the geniculate ganglion?
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?
44-10 What is the chorda tympani?
44-11 How is the level of a lesion of the facial nerve determined?
44-12 What are the chief signs of facial palsy?
44-13 What is the otic capsule?
44-14 What are the main parts of (a) the osseous and (b) the membranous labyrinth?
44-15 Is the lateral semicircular canal horizontal?
44-16 In which direction does the apex of the cochlea point?
44-17 Which structures separate the scalae from each other and from the cochlear duct?
44-18 Where are the neuro-epithelial areas in the membranous labyrinth?
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.