Describe the anatomy of the external and middle ear.

The External Ear

• The external ear comprises the auricle (pinna) and the external auditory canal, ending at the tympanic membrane.

The Middle Ear

• The middle ear (tympanic cavity) is an air-containing space in the petrous temporal bone, lined by mucosa and connected to the nasopharynx by the eustachian tube and to the mastoid air cells by the aditus ad antrum
• It is described as a biconcave disc or a matchbox on edge, and is divided into three parts by lines drawn through the upper and lower margins of the tympanic membrane: the epitympanum (attic), above; the mesotympanum, opposite; and the hypotympanum, below
• The attic is the site of cholesteatoma, and the protympanum lies anteriorly, leading to the Eustachian tube

• The ossicles — the malleus (head, neck, handle, and the lateral and anterior processes); the incus (body, short process and long process ending in the lenticular process); and the stapes (head, two crura and footplate), which is the smallest bone in the body
• The long process of the incus is the commonest ossicle to necrose, because its blood supply is tenuous and end-arterial — hence incus erosion in chronic otitis media and cholesteatoma
• The intratympanic muscles — tensor tympani, inserted into the malleus and supplied by the mandibular nerve (V3); and stapedius, the smallest skeletal muscle in the body, inserted into the neck of the stapes and supplied by the facial nerve (VII). They contract reflexly to loud sound, stiffening the ossicular chain and attenuating transmission — the basis of the acoustic (stapedial) reflex, and the reason its loss causes hyperacusis in facial palsy
• The chorda tympani crosses the middle ear between the long process of the incus and the handle of the malleus, carrying taste from the anterior two-thirds of the tongue and secretomotor fibres to the submandibular and sublingual glands. It is at risk in middle ear surgery, and its injury causes metallic taste and dryness
• Blood supply — chiefly from branches of the maxillary artery, principally the anterior tympanic, with contributions from the stylomastoid, middle meningeal and ascending pharyngeal arteries
• Nerve supply of the mucosa — the tympanic plexus on the promontory, formed mainly by jacobson’S nerve, the tympanic branch of the glossopharyngeal (IX) — which is why tonsillitis and tonsillectomy cause referred otalgia
• Auricular cartilage has no blood supply of its own, which is the whole reason a haematoma must be drained without delay
• Pull the pinna up and back in an adult and back and down in an infant, whose canal is short, straight and largely cartilaginous
• The isthmus is where foreign bodies impact, at the junction of the cartilaginous and bony parts
• Local anaesthetic cannot be infiltrated in the bony canal.
• The canal cleans itself by outward epithelial migration, which is why cotton buds cause impaction by pushing wax against the flow
• Cerumen is acidic and lysozyme-rich, so removing it predisposes to otitis externa rather than preventing it