AN40.1-5 | Organs of hearing and equilibrium — Glossary

External ear composed of elastic cartilage covered by skin; the lobule has no cartilage and is the site of ear piercing

S-shaped canal (2.5 cm) from the concha to the tympanic membrane; outer 1/3 cartilaginous with ceruminous glands, inner 2/3 bony

Thin membrane separating external from middle ear; pars tensa (3 layers, taut) and pars flaccida (2 layers, lax — site of cholesteatoma)

Triangular light reflex in the anteroinferior quadrant of the tympanic membrane on otoscopy; a key normal landmark

Three tiny bones (malleus, incus, stapes) transmitting vibration from the tympanic membrane to the oval window with 22:1 amplification

Smallest bone in the body; its footplate fits in the oval window; immobilised in otosclerosis causing conductive hearing loss

Middle ear muscle supplied by CN V3; pulls the malleus handle medially to tense the tympanic membrane

Smallest skeletal muscle, supplied by CN VII; dampens stapes oscillation; paralysis in Bell's palsy causes hyperacusis

Tube connecting the middle ear to the nasopharynx; shorter, wider, and more horizontal in children, predisposing to otitis media

Thin bony roof of the middle ear separating it from the temporal lobe and middle cranial fossa; route for intracranial spread of infection

Spiral canal of 2.5 turns containing the Organ of Corti on the basilar membrane; tonotopic: base = high frequency, apex = low frequency

Sensory organ of hearing on the basilar membrane containing inner and outer hair cells covered by the tectorial membrane

Three perpendicular canals (anterior, posterior, lateral) detecting rotational acceleration; each has an ampulla with crista ampullaris

Membranous sac in the vestibule with macula detecting linear horizontal acceleration; contains otoconia on the otolithic membrane

Membranous sac in the vestibule with macula detecting vertical linear acceleration and gravity

Auricular branch of the vagus nerve (CN X) supplying the posterior EAM; explains the ear-cough reflex and referred otalgia from vagal territory

Keratinized epithelial cyst from pars flaccida retraction; erodes bone via collagenases causing hearing loss, facial palsy, and intracranial complications

Incision in the tympanic membrane to drain middle ear fluid; performed in the anteroinferior quadrant to avoid the ossicles, facial nerve, and chorda tympani

Middle ear infection; common in children due to short horizontal Eustachian tube; complications include mastoiditis, meningitis, and facial palsy

Necrotising Pseudomonas infection of the EAM in diabetics spreading to skull base osteomyelitis; not a malignancy but aggressive infection

Abnormal perception of sounds as excessively loud; caused by stapedius paralysis in Bell's palsy (CN VII palsy)

Fluid filling the bony labyrinth (scala vestibuli and scala tympani); similar to CSF, high sodium, low potassium

Fluid filling the membranous labyrinth (scala media, utricle, saccule, semicircular ducts); high potassium, low sodium, similar to intracellular fluid