EAR
The ear—the organ of hearing and equilibrium (balance)—is divided into the external, middle, and internal ear. The external ear and middle ear are mainly concerned with the transfer of sound to the internal ear, which contains the organ for equilibrium as well as for hearing. The tympanic membrane separates the external ear from the middle ear. The pharyngotympanic tube joins the middle ear to the nasopharynx.
EXTERNAL EAR
The external ear is composed of the shell-like auricle (pinna), which collects sound, and the external acoustic meatus (ear canal), which conducts sound to the tympanic membrane.
AURICLE
The auricle (L. auris, ear) is composed of an irregularly shaped plate of elastic cartilage that is covered by thin skin. The auricle has several depressions and elevations. The concha of the auricle is the deepest depression. The elevated margin of the auricle is the helix. The non-cartilaginous lobule (lobe) consists of fibrous tissue, fat, and blood vessels. It is easily pierced for taking small blood samples and inserting earrings. The tragus (G. tragos, goat; alluding to the hairs that tend to grow from this formation, like a goat’s beard) is a tongue-like projection overlapping the opening of the external acoustic meatus.
The arterial supply to the auricle is derived mainly from the posterior auricular and superficial temporal arteries.
The main nerves to the skin of the auricle are the great auricular and auriculotemporal nerves. The great auricular nerve supplies the cranial (medial) surface (commonly called the “back of the ear”) and the posterior part (helix, antihelix, and lobule) of the lateral surface (“front”). The auriculotemporal nerve, a branch of CN V3, supplies the skin of the auricle anterior to the external acoustic meatus. Minor contributions of embryological significance are made to the skin of the concha and its eminence by the vagus and facial nerves.
The lymphatic drainage of the auricle is as follows: the lateral surface of the superior half of the auricle drains to the superficial parotid lymph nodes; the cranial surface of the superior half of the auricle drains to the mastoid lymph nodes and deep cervical lymph nodes; and the remainder of the auricle, including the lobule, drains into the superficial cervical lymph nodes.
EXTERNAL ACOUSTIC MEATUS
The external acoustic meatus is an ear canal that leads inward through the tympanic part of the temporal bone from the auricle to the tympanic membrane, a distance of 2–3 cm in adults. The lateral third of this slightly S-shaped canal is cartilaginous and is lined with skin that is continuous with the auricular skin. The medial two thirds of the meatus is bony and lined with thin skin that is continuous with the external layer of the tympanic membrane. The ceruminous and sebaceous glands in the subcutaneous tissue of the cartilaginous part of the meatus produce cerumen (earwax).
The tympanic membrane, approximately 1 cm in diameter, is a thin, oval semitransparent membrane at the medial end of the external acoustic meatus. This membrane forms a partition between the external acoustic meatus and the tympanic cavity of the middle ear. The tympanic membrane is covered with thin skin externally and mucous membrane of the middle ear internally. Viewed through an otoscope, the tympanic membrane has a concavity toward the external acoustic meatus with a shallow, cone-like central depression, the peak of which is the umbo. The central axis of the tympanic membrane passes perpendicularly through the umbo like the handle of an umbrella, running anteriorly and inferiorly as it runs laterally. Thus, the tympanic membrane is oriented like a mini radar or satellite dish positioned to receive signals coming from the ground in front and to the side of the head.
Superior to the lateral process of the malleus (one of the small ear bones, or auditory ossicles, of the middle ear), the membrane is thin and is called the pars flaccida (flaccid part. It lacks the radial and circular fibers present in the remainder of the membrane, called the pars tensa (tense part). The flaccid part forms the lateral wall of the superior recess of the tympanic cavity.
The tympanic membrane moves in response to air vibrations that pass to it through the external acoustic meatus. Movements of the membrane are transmitted by the auditory ossicles through the middle ear to the internal ear. The external surface of the tympanic membrane is supplied mainly by the auriculotemporal nerve, a branch of CN V3. Some innervation is supplied by a small auricular branch of the vagus (CN X). The internal surface of the tympanic membrane is supplied by the glossopharyngeal nerve (CN IX).
MIDDLE EAR
The tympanic cavity or cavity of the middle ear is the narrow air-filled chamber in the petrous part of the temporal bone. The cavity has two parts: the tympanic cavity proper, the space directly internal to the tympanic membrane, and the epitympanic recess, the space superior to the membrane. The tympanic cavity is connected anteromedially with the nasopharynx by the pharyngotympanic tube and posterosuperiorly with the mastoid cells through the mastoid antrum. The tympanic cavity is lined with mucous membrane that is continuous with the lining of the pharyngotympanic tube, mastoid cells, and mastoid antrum.
The contents of the middle ear are the:
- Auditory ossicles (malleus, incus, and stapes).
- Stapedius and tensor tympani muscles.
- Chorda tympani nerve, a branch of CN VII
- Tympanic plexus of nerves.
WALLS OF TYMPANIC CAVITY
The middle ear is shaped like a lozenge or narrow box with concave sides. It has six walls.
- The tegmental wall (roof) is formed by a thin plate of bone, the tegmen tympani, which separates the tympanic cavity from the dura mater on the floor of the middle cranial fossa.
- The jugular wall (floor) is formed by a layer of bone that separates the tympanic cavity from the superior bulb of the internal jugular vein.
- The membranous (lateral) wall is formed almost entirely by the peaked convexity of the tympanic membrane; superiorly it is formed by the lateral bony wall of the epitympanic recess. The handle of the malleus is attached to the tympanic membrane, and its head extends into the epitympanic recess.
- The labyrinthine (medial) wall (medial wall) separates the tympanic cavity from the internal ear. It also features the promontory of the labyrinthine wall, formed by the initial part (basal turn) of the cochlea, and the oval and round windows, which, in a dry cranium, communicate with the internal ear.
- The mastoid wall (posterior wall) features an opening in its superior part, the aditus (L. access) to the mastoid antrum, connecting the tympanic cavity to the mastoid cells; the canal for the facial nerve descends between the posterior wall and the antrum, medial to the aditus.
- The anterior carotid wall separates the tympanic cavity from the carotid canal; superiorly, it has the opening of the pharyngotympanic tube and the canal for the tensor tympani.
The mastoid antrum is a cavity in the mastoid process of the temporal bone. The antrum (L. from G., cave), like the tympanic cavity, is separated from the middle cranial fossa by a thin plate of the temporal bone, called the tegmen tympani. This structure forms the tegmental wall (roof) for the ear cavities and is also part of the floor of the lateral part of the middle cranial fossa. The mastoid antrum is the common cavity into which the mastoid cells open. The antrum and mastoid cells are lined by mucous membrane that is continuous with the lining of the middle ear. Antero-inferiorly, the antrum is related to the canal for the facial nerve.
PHARYNGOTYMPANIC TUBE
The pharyngotympanic tube (auditory tube) connects the tympanic cavity to the nasopharynx, where it opens posterior to the inferior nasal meatus. The posterolateral third of the tube is bony, and the remainder is cartilaginous. The pharyngotympanic tube is lined by mucous membrane that is continuous posteriorly with that of the tympanic cavity and anteriorly with that of the nasopharynx.
The function of the pharyngotympanic tube is to equalize pressure in the middle ear with the atmospheric pressure, thereby allowing free movement of the tympanic membrane. By allowing air to enter and leave the tympanic cavity, this tube balances the pressure on both sides of the membrane. Because the walls of the cartilaginous part of the tube are normally in apposition, the tube must be actively opened. The tube is opened by the expanding girth of the belly of the levator veli palatini as it contracts longitudinally, pushing against one wall while the tensor veli palatini pulls on the other. Because these are muscles of the soft palate, equalizing pressure (“popping the eardrums”) is commonly associated with activities such as yawning and swallowing.
The arteries of the pharyngotympanic tube are derived from the ascending pharyngeal artery, a branch of the external carotid artery, and the middle meningeal artery and artery of the pterygoid canal, ranches of the maxillary artery.
The veins of the pharyngotympanic tube drain into the pterygoid venous plexus. Lymphatic drainage of the tube is to the deep cervical lymph nodes.
The nerves of the pharyngotympanic tube arise from the tympanic plexus, which is formed by fibers of the glossopharyngeal nerve (CN IX). Anteriorly, the tube also receives fibers from the pterygopalatine ganglion.
AUDITORY OSSICLES
The auditory ossicles form a mobile chain of small bones across the tympanic cavity from the tympanic membrane to the oval window (L. fenestra vestibuli), an oval opening on the labyrinthine wall of the tympanic cavity leading to the vestibule of the bony labyrinth. These ossicles are the first bones to be fully ossified during development and are essentially mature at birth. The bone from which they are formed is exceptionally dense (hard). The ossicles are covered with the mucous membrane lining the tympanic cavity; but unlike other bones, they lack a surrounding layer of osteogenic periosteum.
MALLEUS.
The malleus (L. a hammer) attaches to the tympanic membrane. The rounded superior head of the malleus lies in the epitympanic recess. The neck of the malleus lies against the flaccid part of the tympanic membrane, and the handle of the malleus is embedded in the tympanic membrane, with its tip at the umbo; thus, the malleus moves with the membrane. The head of the malleus articulates with the incus; the tendon of the tensor tympani inserts into its handle near the neck. The chorda tympani crosses the medial surface of the neck of the malleus. The malleus functions as a lever, with the longer of its two processes and its handle attached to the tympanic membrane.
Incus. The incus (L. an anvil) is located between the malleus and the stapes and articulates with them. It has a body and two limbs. Its large body lies in the epitympanic recess, where it articulates with the head of the malleus. The long limb lies parallel to the handle of the malleus, and its interior end articulates with the stapes by way of the lenticular process, a medially directed projection. The short limb is connected by a ligament to the posterior wall of the tympanic cavity.
STAPES.
The stapes (L. stirrup) is the smallest ossicle. It has a head, two limbs, and a base. Its head, directed laterally, articulates with the incus. The base (footplate) of the stapes fits into the oval window on the medial wall of the tympanic cavity. The oval base is attached to the margins of the oval window. The base of the stapes is considerably smaller than the tympanic membrane; as a result, the vibratory force of the stapes is increased approximately 10 times over that of the tympanic membrane. Consequently, the auditory ossicles increase the force but decrease the amplitude of the vibrations transmitted from the tympanic membrane through the ossicles to the internal ear.
MUSCLES ASSOCIATED WITH AUDITORY OSSICLES.
Two muscles dampen or resist movements of the auditory ossicles; one also dampens movements (vibration) of the tympanic membrane. The tensor tympani is a short muscle that arises from the superior surface of the cartilaginous part of the pharyngotympanic tube, the greater wing of the sphenoid, and the petrous part of the temporal bone. The muscle inserts into the handle of the malleus. The tensor tympani pulls the handle medially, tensing the tympanic membrane and reducing the amplitude of its oscillations. This action tends to prevent damage to the internal ear when one is exposed to loud sounds. The tensor tympani is supplied by the mandibular nerve (CN V3).
The stapedius is a tiny muscle inside the pyramidal eminence (pyramid), a hollow, cone-shaped prominence on the posterior wall of the tympanic cavity. Its tendon enters the tympanic cavity by emerging from a pinpoint foramen in the apex of the eminence and inserts on the neck of the stapes. The stapedius pulls the stapes posteriorly and tilts its base in the oval window, thereby tightening the anular ligament and reducing the oscillatory range. It also prevents excessive movement of the stapes. The nerve to the stapedius arises from the facial nerve (CN VII).
INTERNAL EAR
The internal ear contains the vestibulocochlear organ concerned with the reception of sound and the maintenance of balance. Buried in the petrous part of the temporal bone, the internal ear consists of the sacs and ducts of the membranous labyrinth. The membranous labyrinth, containing endolymph, is suspended within the perilymph-filled bony labyrinth, either by delicate filaments similar to the filaments of arachnoid mater that traverse the subarachnoid space or by the substantial spiral ligament. It does not float. These fluids are involved in stimulating the end organs for balance and hearing, respectively.
BONY LABYRINTH
The bony labyrinth is a series of cavities (cochlea, vestibule, and semicircular canals) contained within the otic capsule of the petrous part of the temporal bone. The otic capsule is made of bone that is denser than the remainder of the petrous temporal bone and can be isolated (carved) from it using a dental drill. The otic capsule is often erroneously illustrated and identified as being the bony labyrinth. However, the bony labyrinth is the fluid-filled space, which is surrounded by the otic capsule, and is most accurately represented by a cast of the otic capsule after removal of the surrounding bone.
Cochlea. The cochlea is the shell-shaped part of the bony labyrinth that contains the cochlear duct, the part of the internal ear concerned with hearing. The spiral canal of the cochlea begins at the vestibule and makes 2.5 turns around a bony core, the modiolus, the cone-shaped core of spongy bone about which the spiral canal of the cochlea turns. The modiolus contains canals for blood vessels and for distribution of the branches of the cochlear nerve. The apex of the cone-shaped modiolus, like the axis of the tympanic membrane, is directed laterally, anteriorly, and inferiorly. The large basal turn of the cochlea produces the promontory of the labyrinthine wall of the tympanic cavity. At the basal turn, the bony labyrinth communicates with the subarachnoid space superior to the jugular foramen through the cochlear aqueduct. It also features the round window (L. fenestra cochleae), closed by the secondary tympanic membrane.
VESTIBULE OF BONY LABYRINTH.
The vestibule of the bony labyrinth is a small oval chamber (approximately 5 mm long) that contains the utricle and saccule and parts of the balancing apparatus (vestibular labyrinth). The vestibule features the oval window on its lateral wall, occupied by the base of the stapes. The vestibule is continuous with the bony cochlea anteriorly, the semicircular canals posteriorly, and the posterior cranial fossa by the vestibular aqueduct.
The aqueduct extends to the posterior surface of the petrous part of the temporal bone, where it opens posterolateral to the internal acoustic meatus. The vestibular aqueduct transmits the endolymphatic duct and two small blood vessels.
SEMICIRCULAR CANALS.
The semicircular canals (anterior, posterior, and lateral) communicate with the vestibule of the bony labyrinth. The canals lie posterosuperior to the vestibule into which they open; they are set at right angles to each other. The canals occupy three planes in space. Each semicircular canal forms approximately two thirds of a circle, and is approximately 1.5 mm in diameter, except at one end where there is a swelling, the bony ampulla. The canals have only five openings into the vestibule because the anterior and posterior canals have one limb common to both. Lodged within the canals are the semicircular ducts.
MEMBRANOUS LABYRINTH
The membranous labyrinth consists of a series of communicating sacs and ducts that are suspended in the bony labyrinth. The labyrinth contains endolymph, a watery fluid similar in composition to intracellular fluid, thus differing in composition from the surrounding perilymph (which is like extracellular fluid) that fills the remainder of the bony labyrinth. The membranous labyrinth—composed of two divisions, the vestibular labyrinth and the cochlear labyrinth—consists of more parts than does the bony labyrinth:
- Vestibular labyrinth: utricle and saccule, two small communicating sacs in the vestibule of the bony labyrinth.
- Three semicircular ducts in the semicircular canals.
- Cochlear labyrinth: cochlear duct in the cochlea.
The spiral ligament, a spiral thickening of the periosteal lining of the cochlear canal, secures the cochlear duct to the spiral canal of the cochlea. The remainder of the membranous labyrinth is suspended by delicate filaments that traverse the perilymph.
The semicircular ducts open into the utricle through five openings, reflective of the way the surrounding semicircular canals open into the vestibule. The utricle communicates with the saccule through the utriculosaccular duct, from which the endolymphatic duct arises. The saccule is continuous with the cochlear duct through the ductus reuniens, a uniting duct. The utricle and saccule have specialized areas of sensory epithelium called maculae. The macula of the utricle (L. macula utriculi) is in the floor of the utricle, parallel with the base of the cranium, whereas the macula of the saccule (L. macula sacculi) is vertically placed on the medial wall of the saccule.
The hair cells in the maculae are innervated by fibers of the vestibular division of the vestibulocochlear nerve. The primary sensory neurons are in the vestibular ganglia, which are in the internal acoustic meatus.
The endolymphatic duct traverses the vestibular aqueduct and emerges through the bone of the posterior cranial fossa, where it expands into a blind pouch called the endolymphatic sac. The endolymphatic sac is located under the dura mater on the posterior surface of the petrous part of the temporal bone. The sac is a storage reservoir for excess endolymph, formed by the blood capillaries in the membranous labyrinth.
SEMICIRCULAR DUCTS.
Each semicircular duct has an ampulla at one end containing a sensory area, the ampullary crest (L. crista ampullari). The crests are sensors for recording movements of the endolymph in the ampulla resulting from rotation of the head in the plane of the duct. The hair cells of the crests, like those of the maculae, stimulate primary sensory neurons, whose cell bodies are in the vestibular ganglia.
COCHLEAR DUCT.
The cochlear duct is a spiral tube, closed at one end and triangular in cross section. The duct is firmly suspended across the cochlear canal between the spiral ligament on the external wall of the cochlear canal and the osseous spiral lamina of the modiolus. Spanning the spiral canal in this manner, the endolymph-filled cochlear duct divides the perilymph-filled spiral canal into two channels that are continuous at the apex of the cochlea at the helicotrema, a semilunar communication at the apex of the cochlea.
Waves of hydraulic pressure created in the perilymph of the vestibule by the vibrations of the base of the stapes ascend to the apex of the cochlea by one channel, the scala vestibuli. The pressure waves then pass through the helicotrema and descend back to the basal turn of the cochlea by the other channel, the scala tympani. Here, the pressure waves again become vibrations, this time of the secondary tympanic membrane in the round window, and the energy initially received by the (primary) tympanic membrane is finally dissipated into the air of the tympanic cavity.
The roof of the cochlear duct is formed by the vestibular membrane. The floor of the duct is also formed by part of the duct, the basilar membrane, plus the outer edge of the osseous spiral lamina. The receptor of auditory stimuli is the spiral organ (of Corti), situated on the basilar membrane. It is overlaid by the gelatinous tectorial membrane.
The spiral organ contains hair cells, the tips of which are embedded in the tectorial membrane. The organ is stimulated to respond by deformation of the cochlear duct induced by the hydraulic pressure waves in the perilymph, which ascend and descend in the surrounding scalae vestibuli and tympani.
INTERNAL ACOUSTIC MEATUS
The internal acoustic meatus is a narrow canal that runs laterally for approximately 1 cm within the petrous part of the temporal bone. The internal acoustic meatus opening is in the posteromedial part of this bone, in line with the external acoustic meatus. The internal acoustic meatus is closed laterally by a thin, perforated plate of bone that separates it from the internal ear. Through this plate pass the facial nerve (CN VI), the vestibulocochlear nerve (CN VIII) and its divisions, and blood vessels. The vestibulocochlear nerve divides near the lateral end of the internal acoustic meatus into two parts: a cochlear nerve and a vestibular nerve.
Clinically oriented anatomy 7th ed.
