Chapter XXI

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Just as the eye converts light rays into nerve impulses that are interpreted as images by the brain, the auditory mechanisms of the human body convert sound waves into nerve impulses that are perceived by the brain as sounds. The ear is divided into three parts: outer, middle, and inner. The visible part of the outer ear is called the “” (Lat. “little ear, ear flap”), or the “pinna” (Lat. “”), and the marginal ridge around its outer rim is called the “helix” (Grk. “”). Anterior to the opening of the ear is the cartilaginous projection known as the “tragus,” from the Greek word for “,” since it is often hairy (the plural tragi may refer to the hairs themselves on the auricle). Also part of the outer ear is the external acoustic (Lat. “pathway”), or ear canal, which contains (Lat. “wax”) a yellow substance that lubricates and protects the canal.

At the end of the canal is the (“drum-like”) membrane, or ear drum, which separates the outer ear from the middle ear. Connected to this membrane are three ossicles, or tiny bones: the , , and (Latin for “hammer,” “anvil,” and “stirrup,” the objects to which these respective ossicles bear some resemblance) all serve to amplify the sound waves transmitted by the tympanic membrane. The middle ear is also connected to the throat through the tube, which helps to regularize pressure in the ear and also allows for the draining of mucus. This canal is named after the 16th-century Italian anatomist who discovered, described, and named in his own honor an unrivalled number of parts of the human body, including a valve in the inferior vena cava, the medulla in the kidneys, a projection in the middle ear known as the tuber, and a catheter that provides air to the aforementioned tube in the ear.

The transition between the middle ear and inner ear is the window, a small opening that is in direct contact with the stapes. The inner ear contains the (Lat. “snail” or “snail-shell”), which contains fluid called “perilymph.” When sound vibrations are transmitted to this fluid, they are converted to nerve impulses, which travel along the auditory nerve to the brain, which interprets the signals as sound. The inner ear also contains additional structures that help with balance and the sensation of movement: the (Lat. “little leather bag”), (Lat. “little sac”), and semicircular canals.