Structure of the external ear
The external ear is the auricle (pinna) and the external acoustic meatus. The auricle is a plate of elastic cartilage covered by skin, except the lobule, which is fat and fibrous tissue with no cartilage.
- Helix: the curled outer rim. Its crus of helix is the front end that sweeps into the concha above the meatus.
- Antihelix: the curved ridge inside the helix. Above, it splits into superior and inferior crura, and the triangular fossa is the hollow between them.
- Scaphoid fossa: the groove between helix and antihelix.
- Concha: the deep bowl leading into the meatus, divided by the crus of helix into the cymba above and the cavum below.
- Tragus and antitragus: the flap in front of the meatus and the small prominence opposite it; the intertragic notch lies between them.
The external acoustic meatus runs about 2.5 cm from the concha to the drum. Its lateral third is cartilaginous, carries hairs and ceruminous glands, and moves with the auricle; its medial two-thirds is bony. The canal curves, so the auricle is pulled up and back in an adult to straighten it for otoscopy.
Tympanic membrane
The tympanic membrane (eardrum) is a thin oval membrane about 1 cm across that closes the medial end of the meatus. It faces laterally, forwards and downwards, so its upper and posterior parts sit nearer the examiner. It has three layers: skin outside, a fibrous middle layer and mucosa inside.
- Pars tensa: the large, taut lower part with a full fibrous layer. Its thickened rim is the fibrocartilaginous annulus, seated in the tympanic sulcus of the temporal bone.
- Pars flaccida: the small, lax part above the anterior and posterior malleolar folds, lacking organised fibres. It overlies the attic (epitympanic recess).
- Handle (manubrium) of malleus: embedded in the drum, running down and back from the lateral process to the umbo.
- Lateral process of malleus: the small knob at the top of the handle, pushing the drum outwards.
- Umbo: the deepest point of the drum, at the tip of the handle.
- Cone of light: the reflection of the otoscope light in the anteroinferior quadrant, fanning away from the umbo.
A thin drum lets the long process of the incus show through posterosuperiorly, behind and parallel to the handle, and the chorda tympani can be seen crossing its upper part.
Middle ear, ossicles and chorda tympani
The middle ear is an air-filled cavity in the petrous temporal bone that transmits vibration from the drum to the inner ear through three ossicles. The malleus is fixed to the drum, the incus links it to the stapes, and the stapes footplate sits in the oval window. The large area of the drum compared with the footplate, plus the lever of the chain, overcomes the loss of energy that occurs when sound passes from air into fluid.
Two muscles damp the chain. Tensor tympani attaches to the handle of the malleus and is supplied by the mandibular nerve (V3). Stapedius attaches to the neck of the stapes and is supplied by the facial nerve (CN VII); its paralysis causes hyperacusis.
The chorda tympani leaves the facial nerve in the facial canal just above the stylomastoid foramen, enters the middle ear through its posterior wall, and crosses the upper drum between its fibrous and mucous layers. It passes medial to the handle of the malleus and lateral to the long process of the incus, then leaves through the petrotympanic fissure to join the lingual nerve. It carries taste from the anterior two-thirds of the tongue and secretomotor fibres to the submandibular and sublingual glands.
The auditory (pharyngotympanic) tube runs from the anterior wall of the middle ear to the nasopharynx and equalises pressure across the drum.
Blood supply and innervation
The auricle is supplied by the posterior auricular and superficial temporal arteries, with a contribution from the occipital artery. The outer surface of the drum is supplied by the deep auricular branch of the maxillary artery, and its inner surface by the anterior tympanic branch of the maxillary artery and the stylomastoid branch of the posterior auricular artery.
Sensation comes from several nerves, which explains referred ear pain.
| Area | Nerve |
|---|---|
| Lower auricle and most of its cranial surface | Great auricular (C2, C3) |
| Upper cranial surface | Lesser occipital (C2) |
| Tragus, upper lateral auricle, anterior meatus and drum | Auriculotemporal (V3) |
| Concha, posterior meatus and drum | Auricular branch of vagus, with a small facial nerve contribution |
| Inner surface of drum, middle ear | Tympanic branch of glossopharyngeal |
Clinical relevance
Most ear conditions examined in surgery trainees change the appearance of the drum or depend on the nerve supply of the canal.
- Acute otitis media: a red, bulging drum with the cone of light and malleus landmarks lost.
- Cholesteatoma: usually starts as a retraction pocket in the pars flaccida and erodes the ossicles, the facial canal or the tegmen.
- Myringotomy and grommets: placed in the inferior half of the pars tensa, away from the ossicles and chorda tympani posterosuperiorly.
- Auricular haematoma: blood lifts the perichondrium off the cartilage, which depends on it for nutrition; without drainage the cartilage dies and a cauliflower ear results.
- Ear canal cough reflex: touching the posterior canal stimulates the auricular branch of the vagus and can provoke coughing.
- Referred otalgia: pain from the teeth, tonsil, pharynx or larynx is felt in the ear through V3, IX and X.
On the specimen
The station shows an auricle and an otoscopic or dissected view of the drum. Name the auricle parts by their relation to the meatus, and the drum parts by their relation to the handle of the malleus.
- Crus of helix or crus of antihelix: the crus of helix runs horizontally into the concha just above the meatus; the crura of antihelix are higher, bordering the triangular fossa.
- Tragus or antitragus: the tragus is in front of the meatus; the intertragic notch is the gap below it.
- Lateral process or umbo: the lateral process is the bright knob at the top of the handle; the umbo is at its lower end in the centre of the drum.
- Long process of incus or handle of malleus: the incus is behind the handle, fainter and shorter, in the posterosuperior quadrant.
- Pars flaccida or pars tensa: the flaccida is the small area above the lateral process.
Side the drum from the cone of light: it points anteroinferiorly, at about 5 o'clock in a right ear and 7 o'clock in a left.