Ear and Temporal Bone

Auricle (pinna):

• The visible, external cartilaginous structure, elastic cartilage covered by skin, funnelling sound waves into the external acoustic meatus.
• Sensory innervation is complex, drawn from multiple sources: great auricular nerve (cervical plexus, C2-C3, majority of the auricle), auriculotemporal nerve (CN V3, anterosuperior part), and small contributions from CN VII and CN X (posterior/concha region), this mixed innervation explains referred otalgia (ear pain) from pathology in structures sharing these same nerves elsewhere (e.g., dental pain via V3, or pharyngeal/laryngeal pathology via CN X).

External acoustic meatus:

• A curved canal (an initial outward/upward/backward curve, then an inward/downward/forward curve, clinically relevant to otoscopy technique, where the auricle is pulled up and back in adults to straighten this curve for a clear view of the tympanic membrane, versus down and back in young children, whose canal anatomy differs) extending from the auricle to the tympanic membrane.
• Lateral third: cartilaginous; medial two-thirds, bony (within the temporal bone's tympanic part).
• Lined by skin containing ceruminous (wax-producing) and sebaceous glands in the cartilaginous portion.

Tympanic membrane:

• A thin, semi-transparent membrane separating the external and middle ear, obliquely oriented (facing anteroinferolaterally).

Pars tensa:

• The larger, taut, inferior portion of the membrane, containing a fibrous middle layer (giving it its tension/rigidity), the main vibrating portion responsible for sound transmission.

Pars flaccida:

• The smaller, lax, superior portion, lacking the fibrous middle layer: more susceptible to retraction and a common site of origin for cholesteatoma (an abnormal keratinising squamous epithelial growth within the middle ear, see Applied Anatomy).

Umbo:

• The most depressed, central point of the tympanic membrane, corresponding to the tip of the malleus's attachment, visible as a landmark on otoscopy.

Handle of malleus:

• Visible through the translucent membrane on otoscopy, running from the umbo superiorly, a key otoscopic landmark for orientation and for assessing normal middle ear anatomy/pathology.

Sensory innervation:

• Outer surface: Auriculotemporal nerve (CN V3) anterosuperiorly, and the auricular branch of the vagus nerve (CN X) posteroinferiorly.
• Inner surface: Tympanic branch of the glossopharyngeal nerve (CN IX), via the tympanic plexus.
• This mixed sensory supply (from three different cranial nerves across the two surfaces) is a frequently tested detail, and explains why myringotomy (tympanic membrane incision) can sometimes trigger a vagal reflex (via the CN X contribution).

• An air-filled cavity within the petrous temporal bone, between the tympanic membrane and the inner ear, containing the ossicular chain.

Walls:

• Roof (tegmen tympani): A thin bony plate separating the middle ear from the middle cranial fossa, its thinness is clinically significant, as middle ear infection can potentially spread intracranially through this roof.
• Floor (jugular wall): Overlies the jugular bulb (origin of the internal jugular vein), separated by a thin bony plate.
• Medial wall (labyrinthine wall): Separates the middle ear from the inner ear, featuring the promontory (a bulge overlying the cochlea's basal turn), the oval window (fenestra vestibuli, where the stapes footplate sits), and the round window (fenestra cochleae, covered by the secondary tympanic membrane), also crossed by the facial nerve's tympanic segment, running within a bony canal along this wall, making it vulnerable to injury during middle ear surgery if the canal's bony covering is dehiscent (a relatively common anatomical variant).
• Lateral wall (membranous wall): Formed predominantly by the tympanic membrane itself.
• Anterior wall (carotid wall): Separates the middle ear from the carotid canal (internal carotid artery), and contains the opening of the auditory (pharyngotympanic/Eustachian) tube.
• Posterior wall (mastoid wall): Contains the opening to the mastoid antrum (the aditus ad antrum), the passage connecting the middle ear to the mastoid air cell system.

Malleus:

• The most lateral ossicle, with its handle embedded in the tympanic membrane (at the umbo), its head articulating with the incus; the tensor tympani muscle inserts on its handle's upper part.

Incus:

• The middle ossicle, with a body articulating with the malleus's head and a long process (crus) articulating with the stapes; its short process is a key surgical landmark, attaching near the aditus ad antrum.

Stapes:

• The most medial ossicle (and the smallest bone in the body), with a head articulating with the incus, two crura, and a footplate that sits within the oval window, transmitting vibrations directly to the inner ear's perilymph.

• Together the three ossicles form a mechanical lever chain, amplifying sound pressure from the relatively large tympanic membrane to the much smaller oval window (the impedance-matching function of the middle ear, essential for efficient transmission of airborne sound vibrations into the fluid-filled inner ear).

Tensor tympani:

• Origin: Cartilaginous part of the auditory tube and the adjacent bony canal.
• Insertion: Handle of the malleus.
• Innervation: A branch of the mandibular division of the trigeminal nerve (CN V3), reflecting its first pharyngeal arch developmental origin, shared with the other muscles of mastication.
• Action: Dampens excessive ossicular movement/tympanic membrane tension in response to loud sounds, protecting the inner ear (part of the acoustic reflex, alongside stapedius).

Stapedius:

• Origin: Within the pyramidal eminence, a small bony projection on the posterior middle ear wall.
• Insertion: Neck of the stapes.
• Innervation: Facial nerve (CN VII), via the nerve to stapedius, given off within the facial canal, reflecting its second pharyngeal arch origin.
• Action: Dampens excessive stapes movement in response to loud sounds (the other component of the acoustic reflex); its paralysis (e.g., in facial nerve palsy affecting this branch) causes hyperacusis (increased sensitivity/discomfort to normal sounds, given the loss of this protective dampening function).

Course:

• Connects the middle ear (anterior wall) to the nasopharynx, running an oblique course, downward, forward, and medially.

Cartilaginous portion:

• The medial (nasopharyngeal) two-thirds, normally collapsed at rest, opened actively by the tensor veli palatini muscle during swallowing/yawning (equalising middle ear pressure with atmospheric pressure).

Bony portion:

• The lateral (middle ear) one-third, within the temporal bone, always patent (not subject to active opening/closing like the cartilaginous portion).

Relation to nasopharynx:

• Opens into the lateral nasopharyngeal wall, with the surrounding mucosal elevation termed the tubal tonsil (part of Waldeyer's ring) and the pharyngeal recess (fossa of Rosenmüller) lying just posterior to the tubal opening, a clinically important site for nasopharyngeal carcinoma to arise, given its relatively hidden position, often not readily visible on routine examination.
• This tube's role in pressure equalisation explains the "popping" sensation with altitude changes, and its relatively horizontal, shorter, wider configuration in young children (compared to adults) explains their increased susceptibility to otitis media, given easier retrograde spread of nasopharyngeal secretions/pathogens up into the middle ear.

Mastoid antrum:

• A air-filled cavity within the mastoid part of the temporal bone, communicating with the middle ear via the aditus ad antrum (posterior middle ear wall) and with the mastoid air cells.

Mastoid air cells:

• A honeycomb-like network of interconnected air spaces within the mastoid process, continuous with the mastoid antrum/middle ear, their extent and pneumatisation is variable between individuals, and this interconnected air cell system provides the anatomical pathway for middle ear infection to spread posteriorly into the mastoid (mastoiditis: see Applied Anatomy).

Bony labyrinth:

• A series of cavities within the dense petrous temporal bone, comprising the cochlea (anteriorly, hearing), vestibule (centrally, containing the utricle/saccule), and semicircular canals (posteriorly, three canals oriented in orthogonal planes (balance/rotational movement detection)) filled with perilymph.

Membranous labyrinth:

• A series of membranous ducts/sacs suspended within the bony labyrinth (cochlear duct within the cochlea, utricle and saccule within the vestibule, semicircular ducts within the semicircular canals), filled with endolymph, a distinct fluid with a different ionic composition from perilymph, critical for normal sensory transduction (disorders of endolymph volume/pressure regulation, e.g., Ménière's disease, cause characteristic vertigo/hearing symptoms).

Cochlea:

• A spiral, snail-shell-shaped structure (approximately 2.5 turns), containing the organ of Corti within the cochlear duct, the sensory receptor apparatus for hearing, converting mechanical sound vibrations (transmitted via the oval window/perilymph) into neural signals.

Vestibule:

• The central chamber of the bony labyrinth, containing the utricle and saccule, which detect linear acceleration/head position relative to gravity (via otolith organs).

Semicircular canals:

• Three canals (anterior/superior, posterior, lateral/horizontal), oriented in three roughly orthogonal planes, each containing a semicircular duct with an ampulla housing the sensory crista, detecting angular/rotational head movement.

• Within the temporal bone, the nerve travels through the internal acoustic meatus alongside CN VII, dividing into cochlear (hearing) and vestibular (balance) divisions before reaching their respective sensory organs.

Facial canal:

• A tortuous bony canal within the petrous temporal bone, through which the facial nerve travels after entering via the internal acoustic meatus, before exiting via the stylomastoid foramen, its course includes several notable bends (genu) and passes directly across the medial wall of the middle ear, making segments of it vulnerable during middle ear/mastoid surgery.

Geniculate ganglion:

• Located at the first bend (genu) of the facial canal, containing the cell bodies of the nerve's special sensory (taste) fibres, the site of origin for the greater petrosal nerve.

Greater petrosal nerve:

• Arises at the geniculate ganglion, carrying preganglionic parasympathetic fibres, exiting the temporal bone to run across the middle cranial fossa floor, eventually joining the deep petrosal nerve (sympathetic) to form the nerve of the pterygoid canal, reaching the pterygopalatine ganglion: full detail in Section 14.

Nerve to stapedius:

• Arises within the facial canal's descending (mastoid) segment, supplying stapedius.

Chorda tympani:

• Arises within the facial canal's descending segment (below the nerve to stapedius), crosses the middle ear cavity medial to the tympanic membrane's upper part (visible on careful otoscopy, and vulnerable during middle ear surgery/tympanic membrane perforation repair), exits via the petrotympanic fissure, then joins the lingual nerve (CN V3), carrying taste (anterior two-thirds of tongue) and parasympathetic fibres (to the submandibular ganglion, supplying submandibular/sublingual glands).

Stylomastoid foramen:

• The facial nerve's exit point from the temporal bone, between the styloid and mastoid processes, at which point it becomes purely a motor nerve (all sensory/parasympathetic branches having already been given off intratemporally) before entering the parotid gland.

Otitis media spread:

• Middle ear infection can spread via several anatomically predictable routes: posteriorly into the mastoid air cells (mastoiditis), superiorly through the thin tegmen tympani into the middle cranial fossa (intracranial abscess/meningitis), or medially to affect the facial nerve (given its exposed course along the medial wall) or the inner ear (labyrinthitis, if the oval/round windows are breached).

Mastoiditis:

• Spread of middle ear infection into the mastoid air cell system via the aditus ad antrum, causing pain/swelling/erythema over the mastoid process, and potentially progressing to intracranial complications (given the mastoid's proximity to the sigmoid sinus and posterior cranial fossa) if untreated.

Facial nerve injury:

• Can occur at any point along its intratemporal course, commonly related to temporal bone fracture, cholesteatoma erosion, or iatrogenic injury during middle ear/mastoid surgery, given the nerve's close, sometimes dehiscent relationship to the middle ear's medial wall.

Conductive versus sensorineural anatomical basis:

• Conductive hearing loss results from pathology affecting sound transmission through the external or middle ear (e.g., cerumen impaction, tympanic membrane perforation, ossicular chain disruption/otosclerosis), sound fails to reach the inner ear efficiently, but the inner ear/neural pathway itself remains functional.
• Sensorineural hearing loss results from pathology affecting the inner ear (cochlea) or the vestibulocochlear nerve/central auditory pathway: the sound-conducting mechanism is intact, but the sensory transduction or neural transmission itself is impaired.
• This fundamental anatomical distinction (conducting apparatus vs sensory/neural apparatus) underlies the clinical tuning fork tests (Weber and Rinne) used to differentiate the two patterns at the bedside.