External Ear
The external (outer) ear is composed of two parts, the auricle and the external auditory canal (fig. 1-52). The auricle, or pinna, is a cartilaginous structure located on each side of the head. The auricle collects sound waves from the environment, which are then conducted by the external auditory canal to the eardrum. The lining of the external auditory canal contains glands that secrete a wax-like substance called cerumen. Cerumen aids in protecting the eardrum against foreign bodies and microorganisms.

The tympanic membrane, or eardrum, is an oval sheet of fibrous epithelial tissue that stretches across the inner end of the external auditory canal. The eardrum separates the outer and middle ear. The sound waves cause the eardrum to vibrate, and this vibration transfers the sounds from the external environment to the auditory ossicles.

Middle Ear
The middle ear is a cavity in the temporal bone, lined with epithelium. It contains three auditory ossicles-the malleus (hammer), the incus (anvil), and the stapes (stirrup)-which transmit vibrations from the tympanic membrane to the fluid in the inner ear

Figure 1-51.-The vision process.

(fig. 1-52). The malleus is attached to the inner surface of the eardrum and connects with the incus, which in turn connects with the stapes. The base of the stapes is attached to the fenestra ovalis (oval window), the membrane-covered opening of the inner ear. These tiny bones, which span the middle ear, are suspended from bony walls by ligaments. This arrangement provides the mechanical means for transmitting sound vibrations to the inner ear.

The eustachian tube, or auditory tube, connects the middle ear with the pharynx. It is lined with a mucous membrane and is about 36 mm long. Its function is to equalize internal and external air pressure. For example, while riding an elevator in a tall building, you may experience a feeling of pressure in the ear. This condition is usually relieved by swallowing, which opens the eustachian tube and allows the pressurized air to escape and equalize with the area of lower pressure. Divers who ascend too fast to allow pressure to adjust may experience rupture of their eardrums. The eustachian tube can also provide a pathway for infection of the middle ear.

Inner Ear
The inner ear is filled with a fluid called endolymph. Sound vibrations that cause the stapes to move against the oval window create internal ripples that run through the endolymph. These pressurized ripples move to the cochlea, a small snail-shaped structure housing the organ of Corti, the hearing organ (fig. 1-52). The cells protruding from the organ of Corti are stimulated by the ripples to convert these mechanical vibrations into nerve impulses, and these impulses are relayed through the vestibulocochlear (8th cranial) nerve to the auditory area of the cortex in the temporal lobe of the brain. There they are interpreted as the sounds we hear.

Another structure located in the inner ear is composed of the three semicircular canals, situated perpendicular to each other. Movement of the endolymph within the canals, caused by general body movements, stimulates nerve endings, which report these changes in body position to the brain, which in turn uses the information to maintain equilibrium.

The fenestra rotunda (round window) is another membrane-covered opening of the inner ear. It contracts the middle ear and flexes to accommodate the inner ear ripples caused by the stapes.