# Ear Anatomy > FMGE Anatomy ear notes: external canal nerve supply, ossicles with stapedius hyperacusis, Eustachian tube, facial nerve canal and mastoid antrum. - Canonical URL: https://prepelephant.com/topics/fmge/anatomy/ear-anatomy-fmge - Exam / course: FMGE · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Ear Anatomy", PrepElephant, https://prepelephant.com/topics/fmge/anatomy/ear-anatomy-fmge ## Direct answer The smallest bone in the body sits in the middle ear — the stapes, whose footplate is ankylosed in otosclerosis to produce conductive deafness — and the smallest skeletal muscle, stapedius, is paralysed by a facial nerve lesion to produce hyperacusis. The external acoustic meatus is about 2.5 cm long, S-shaped, and pulled straight in adults by retracting the pinna upward and backward. Through the middle ear's medial wall runs the facial nerve in a canal that is sometimes congenitally dehiscent — why otitis media can present with a facial palsy. ## What you must remember - The external canal's outer third is cartilaginous with hairs and ceruminous (modified sweat) glands; the inner two-thirds is bony with thin adherent skin, so furuncles here are exquisitely painful and malignant otitis externa (Pseudomonas in diabetics) erodes the skull base. - Sensation of the meatus and pinna: auriculotemporal nerve (V3) in front, great auricular (C2–C3) over the lower pinna, and the auricular branch of the vagus (Arnold's nerve) in the concha — its stimulation provokes the cough reflex exams ask about. - The tympanic membrane's landmarks — handle of the malleus, the light cone anteroinferiorly, and the pars flaccida (Shrapnell's membrane) above — guide myringotomy in the safe posteroinferior quadrant; the ossicular chain malleus–incus–stapes beyond it transmits sound to the oval window, with tensor tympani (V3) tensing the drum and stapedius (VII) dampening the stapes, whose paralysis makes sounds uncomfortably loud. - The middle ear's roof, the tegmen tympani, is thin bone against the middle cranial fossa — otitic infection here causes meningitis or temporal lobe abscess; the floor borders the jugular bulb; the medial wall carries the promontory with the oval window above and round window below. - The facial nerve traverses the temporal bone through internal acoustic meatus, labyrinthine, tympanic (horizontal, above the oval window) and mastoid segments, giving the nerve to stapedius and chorda tympani before exiting at the stylomastoid foramen; the canal is dehiscent in a significant minority, exposing the nerve to infection. - The Eustachian tube, about 36 mm in adults and opened by tensor veli palatini on swallowing, opens into the nasopharynx 1.25 cm behind the inferior turbinate; in children it is shorter (about 18 mm), wider and more horizontal — the reason for frequent acute otitis media. - The mastoid antrum is surface-marked by MacEwen's (suprameatal) triangle; mastoiditis erodes the tip to present as Bezold's abscess in the neck. The inner ear holds the cochlea with the organ of Corti, the utricle and saccule (otolith organs of linear acceleration), and three semicircular canals with ampullary cristae; endolymph is potassium-rich, perilymph sodium-rich. - The internal acoustic meatus carries the facial and vestibulocochlear nerves together, so a vestibular schwannoma first deafens, then traps the fifth and seventh nerves, then presses the brainstem. ## Otitis media with facial palsy, worked through A child with acute ear pain develops a sagging posterosuperior tympanic membrane and, two days later, cannot close the right eye or smile on the right. The anatomy explains the sequence. Suppurative infection reaches the facial nerve in its horizontal tympanic segment above the oval window, where the canal is thin or dehiscent; the nerve swells within it, and a lower motor neuron palsy appears — the whole half of the face including the forehead, with hyperacusis if the lesion is proximal to the nerve to stapedius. The bulging membrane demands myringotomy and antibiotics, and the palsy usually recovers. Reason onward with the same map: erosion of the tegmen tympani would give meningism and temporal lobe abscess; spread to the mastoid antrum, a protruding ear with Bezold's abscess; through the round window, vertigo and sensorineural loss — one cavity, four exits, four syndromes. ## Where students slip The ear's nerve supply is memorised piecemeal and misassigned: in front of the meatus is V3 (auriculotemporal), the lower pinna C2–C3 (great auricular), the concha the vagus (Arnold's nerve) — forgetting Arnold's nerve costs the cough-on-manipulation stems. The second confusion is the two middle-ear muscles: stapedius is facial, tensor tympani is mandibular. Students also forget that the child's Eustachian tube is shorter, wider and more horizontal, the three-word answer to why children otitis-media while adults mostly do not. ## Frequently asked questions ### Which is the smallest bone and which the smallest skeletal muscle in the body? The stapes in the middle ear; and stapedius, supplied by the facial nerve, whose paralysis causes hyperacusis. ### Why does otitis media cause facial palsy in some children? The facial nerve's horizontal tympanic segment runs in a thin or congenitally dehiscent canal above the oval window, exposed to middle-ear infection. ### How does the child's Eustachian tube differ from the adult's? It is about half as long (18 mm), wider and more horizontal, letting nasopharyngeal infection reach the middle ear easily. ### What is MacEwen's triangle? The suprameatal triangle behind and above the external meatus — the surface marking of the mastoid antrum. ### Which nerve supplies the concha and posterior meatal wall, and what reflex follows? The auricular branch of the vagus (Arnold's nerve); stimulation provokes a cough reflex. ### Where does the Eustachian tube open in the nasopharynx? About 1.25 cm behind the posterior end of the inferior turbinate, at the torus tubarius, opened by tensor veli palatini.