The Ear: Some Applied Anatomy

THE PINNA The external ear or pinna, is composed of cartilage with closely adherent perichondrium and skin. It is developed from six tubercles of the first branchial arch. Fistulae and accessory auricles result from failure of fusion of these tubercles. THE EXTERNAL AUDITORY MEATUS The external auditory meatus is about 25 mm in length, has a skeleton of cartilage in its outer third (where it contains hairs and ceruminous glands) and has bone in its inner two-thirds. The skin of the inner part is exceedingly thin, adherent and sensitive. At the medial end of the meatus there is the antero-inferior recess, in which wax, debris or foreign bodies may lodge. THE TYMPANIC MEMBRANE (Fig. 1.1) The tympanic membrane is composed of three layer—skin, fibrous tissue and mucosa.The normal appearance of the membrane is pearly and opaque, with a well-defined light reflex due to its concave shape. THE TYMPANIC CAVITY Medial to the tympanic membrane, the tympanic cavity is an air-containing space 15 mm high and 15 mm antero-posteriorly, although only 2mm deep in parts. The middle ear contains the ossicular chain of malleus, incus and stapes (Fig. 1.2) and its medial wall is crowded with structures closely related to one another: the facial nerve, the round and oval windows, the lateral semicircular canal and basal turn of the cochlea.The major reason for having an air-containing middle ear is to reduce the acoustic impedance that would be caused if a sound wave in air were to be applied directly to the cochlear fluids.Without this impedance matching, 99% of the sound energy would simply be reflected at an air/fluid interface. THE EUSTACHIAN TUBE The Eustachian tube connects the middle-ear cleft with the nasopharynx and is responsible for the aeration of the middle ear. The tube is morehorizontal in the infant than in the adult and secretions or vomit may enter the tympanic cavity more easily in the supine position. The tube is normally closed and is opened by the palatal muscles on swallowing.This is impaired by the presence of a palatal cleft. THE FACIAL NERVE The facial nerve is embedded in bone in its petrous part but exits at the stylomastoid foramen (Fig. 1.3). In infants, the mastoid process is undevelopedand the nerve very superficial.

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