Chapter 42: External & Middle Ear Anatomy

The temporal bone contains critical passages, including the external acoustic meatus for sound waves and the internal acoustic meatus for the facial and vestibulocochlear nerves. Developmentally, the four temporal components ossify separately, with the inner and middle ears achieving adult size by birth, though certain structures like the mastoid air cells continue expansion, particularly around puberty. The external ear comprises the auricle (pinna) and the external acoustic meatus, functioning to collect and conduct sound towards the tympanic membrane. The auricle, supported by elastic cartilage and covered by thin, adherent skin, features numerous landmarks such as the helix, antihelix, concha, and tragus, and may present congenital variations such as microtia or polyotia. The external acoustic meatus is an S-shaped canal, with its lateral third being cartilaginous and its medial two-thirds osseous, and its medial end is closed by the obliquely positioned tympanic membrane. The middle ear, or tympanic cavity, is an air-filled space within the petrous temporal bone that receives air from the nasopharynx via the pharyngotympanic tube, which also serves to clear mucus and equalize pressure across the tympanic membrane. This crucial cavity contains the three auditory ossicles—the malleus, incus, and stapes—which form an articulated chain that efficiently transfers acoustic energy from the low-amplitude air waves in the meatus to the incompressible fluid (perilymph) of the inner ear, achieving impedance matching by amplifying the force applied to the fenestra vestibuli (oval window). The ossicles are supported by ligaments and intricate synovial joints, including the saddle-shaped incudomalleolar joint and the ball-and-socket incudostapedial joint. The bony boundaries of the middle ear include the tegmen tympani as the roof, the floor often overlying the internal jugular vein superior bulb, and the medial wall featuring the promontory (overlying the cochlea), the fenestra vestibuli, and the fenestra cochleae (round window), which is closed by the secondary tympanic membrane. Posteriorly, the cavity connects through the aditus to the mastoid antrum and mastoid air cells. Two small muscles govern ossicular movement: tensor tympani, supplied by the mandibular division of the trigeminal nerve, tenses the tympanic membrane by pulling the malleus handle medially, while stapedius, supplied by the facial nerve, attaches to the neck of the stapes and acts to dampen excessive vibrations, preventing potential damage. Pathologies such as otosclerosis fix the stapedial footplate, causing conductive hearing loss, often necessitating surgical interventions like stapedectomy or stapedotomy. The complex course of the facial nerve (cranial nerve VII) within the temporal bone is clinically vital, traversing segments including the labyrinthine and tympanic portions within the facial canal before exiting at the stylomastoid foramen, giving off key branches like the chorda tympani and the nerve to stapedius. Vulnerabilities in the nerve's course, especially where the bony canal is sometimes absent (dehiscent), or narrowing at the meatal foramen, are implicated in conditions such as Bell’s palsy. The mucosal lining of the tympanic cavity, which is continuous with the nasopharynx, can be compromised, leading to inflammation (otitis media) or the retention of keratinizing epithelium known as cholesteatoma, which actively erodes bone and potentially damages the inner ear structures or the facial nerve.