The external ear is composed of the auricle or pinna (Fig. 13-1B) and the external auditory canal (Fig. 13-1A). Modified sweat glands in the external ear canal secrete cerumen, a wax-like substance that keeps the tympanic membrane (TM) soft and has bacteriostatic properties, a defense against foreign bodies. The TM or eardrum, a translucent, pearly gray, concave membrane, seen in Figure 13-1C, serves as a partition stretched across the inner end of the auditory canal, separating it from the middle ear.
The middle ear (a small, air-filled chamber in the temporal bone) contains three auditory ossicles: the malleus, the incus, and the stapes. These tiny bones are responsible for transmitting sound waves from the eardrum to the inner ear through the oval window. Air pressure is equalized on both sides of the TM by means of the eustachian tube, which connects the middle ear to the nasopharynx. The distinct landmarks of the TM (Fig. 13-1C) include the following:
Cone of light reflection of the otoscope light due to the concave membrane
Pars flaccid, top portion of the membrane that appears to be less taut
Pars tensa, the bottom of the membrane that appears to be taut
The middle ear is separated from the inner ear (Fig. 13-1A) by a bony partition containing two openings, the round and oval windows. The inner ear, or labyrinth, is fluid filled and is made up of the bony labyrinth and an inner membranous labyrinth. The bony labyrinth has three parts: the cochlea, the vestibule, and the semicircular canals.
The ears serve as sensory organs for hearing. Sound vibrations traveling through air are collected by and funneled through the external ear and cause the eardrum to vibrate. Sound waves are then transmitted through auditory ossicles as the vibration of the eardrum causes the malleus, the incus, and then the stapes to vibrate. As the stapes vibrates at the oval window, the sound waves are passed to the fluid in the inner ear. The movement of this fluid stimulates the hair cells of the spiral organ of Corti and initiates the nerve impulses that travel to the brain by way of the acoustic nerve.
Sounds waves are transmitted through:
The external and middle ear. This is referred to as conductive hearing. Conductive hearing loss is related to dysfunction of the external or middle ear (e.g., impacted earwax, otitis media, foreign object, perforated eardrum, middle ear drainage, otosclerosis).
The inner ear. This is referred to as sensorineural hearing (or perceptive hearing). Sensorineural hearing loss is related to dysfunction of the inner ear (i.e., organ of Corti, cranial nerve VIII, temporal lobe of the brain).
the skull bones, which serve to augment usual sound waves through air, bone, and, finally, fluid. This pathway is less efficient than the conductive or sensorineural pathway.