The external ear collects sound waves and transports them through the external acoustic meatus to the tympanic membrane. The tympanic membrane vibrates, setting three tiny ear ossicles (malleus, incus, and stapes) in the middle ear into motion. The stapes attaches to the lateral wall of the inner ear, where the vibration is transduced into fluid movement. The fluid causes the basilar membrane in the cochlea to vibrate. The vestibulocochlear nerve [cranial nerve (CN) VIII] receives and conducts the impulses to the brain, where there is integration of sound and equilibrium.
The external ear consists of the auricle, or pinna, which lies at the outer end of a short tube called the external acoustic meatus (Figure 19-1A). The auricle funnels sound waves through the external acoustic meatus to the tympanic membrane. The external ear receives general sensory innervation from trigeminal, facial, and vagus nerves, and the great auricular nerve.
A. Coronal section of the temporal bone showing the hearing apparatus. B. Right tympanic membrane viewed through an otoscope. Lateral (C) and medial (D) wall of the middle ear.
The tympanic membrane, or “eardrum,” is a three-layered circular structure (Figure 19-1A–C).
Outer (skin) layer. Composed of modified skin that is continuous with the external acoustic meatus.
Middle (fibrous) layer. Composed of connective tissue through which the chorda tympani nerve (CN VII) passes.
Inner (mucosa) layer. Lined with the mucosa of the middle ear, and receives general sensory innervation via the tympanic plexus (CN IX).
The middle ear is an air-filled chamber that transmits sound waves from air (external ear) to the auditory ossicles (middle ear) and then to the fluid-filled inner ear (Figure 19-1A). The middle ear consists of the tympanic cavity proper, auditory tube, ear ossicles, and branches of CNN VII and IX.
The tympanic cavity proper is the space between the tympanic membrane and the vestibular window. Its mucosa receives general sensory innervation from the tympanic nerve and the tympanic plexus (CN IX) (Figure 19-1A–D). In addition, visceral motor preganglionic parasympathetic fibers from CN IX branch from the tympanic plexus to exit the middle ear as the lesser petrosal nerve en route to innervate the parotid gland.