Hearing and the Ear
Ear, the organ in which the sensations of hearing and head motion are initiated. These two functions take place in an intricate mechanism encased in the hardest bone of the body, the temporal bone, situated between the external ear and the center of the skull. The outer ear is constructed for receiving sound waves and transmitting them to these internal mechanisms. In the dense bone of the internal ear are two of the body's most delicate structures: the cochlea, the organ of hearing; and the labyrinth, which is one of the sources of nerve stimuli in the control of body balance. The present article concerns the ear in human beings. For discussions of ears and hearing in other animals, the reader should consult Bird, Insects, Mammals, and articles on individual kinds of animals.
ANATOMY OF THE EAR
There are three anatomical divisions of the ear: the external, the middle, and the internal or inner ear.
External Ear
The projecting portion of the external ear, known as the auricle, has a framework made up of a semi-rigid supporting tissue called cartilage. The opening of the external auditory canal is located at the lower forward margin of the auricle. The external auditory canal extends inward and slightly forward.
Cerumen
The waxlike material formed from the secretions of the sebaceous and apocrine glands in the external ear is called cerumen. Its function is to protect the skin of the external canal from bacterial infection and trap foreign bodies such as insects that may enter the ear. They are later ejected with the wax. The amount of wax produced varies in different individuals. Impacted cerumen may cause conductive deafness.
Middle Ear
A thin, flat membrane called the tympanic membrane or eardrum, stretches across the inner end of the external auditory canal. Beyond the eardrum is a flattened, rectangular, air-filled space called the middle ear cavity or tympanic cavity. Three tiny bones, called the ossicles, traverse this cavity as a chain of levers transmitting vibrations of the eardrum to the inner ear. Because of their shape, the ossicles are designated as the malleus of hammer, the incus or anvil, and the stapes or stirrup.
The malleus is fastened to the center of the tympanic membrane. It is anchored to the walls of the cavity by tendons and is connected to the incus which, in turn, is fastened to the stapes. The footplate of the stapes, which is shaped like the footplate of an ordinary stirrup, fits into the oval window, an opening in the bony wall of the inner ear. Tiny muscles regulate sound transmission by tensing the bones.
A membrane vibrates best when air pressures are equal on its two sides. The tympanic membrane has this advantage since the tympanic cavity is supplied with the air from the nose and throat by way of the Eustachian tube, which enters into its lower forward angle. This tube is opened by swallowing or yawning, which permits air to enter the tympanic cave and maintain atmospheric pressure.
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