Ear

I

Introduction

Ear, organ of hearing and balance. It is composed of three divisions—external, middle, and internal—the greater part of which is enclosed within the temporal bone.

II

Structure

The outer ear is that portion of the hearing apparatus lateral to the eardrum, or tympanic membrane, which separates the outer ear from the middle ear. It comprises the auricle, or pinna (the external flap of the ear), and the external auditory canal, which is 3 cm (1.25 in) in length.

The middle ear, on the inner side of the eardrum, embodies the mechanism for the conduction of sound waves to the internal ear. It is a narrow passage, or cleft, that extends vertically for about 15 mm (0.6 in) and for about the same distance horizontally. The middle ear is in direct communication with the back of the nose and throat by way of the eustachian tube, which allows for passage of air into and out of the middle ear. Traversing the middle ear is a chain of three small, movable bones called the ossicles: the malleus, or hammer handle; the incus, or anvil; and the stapes, or stirrup. The ossicles connect the eardrum acoustically to the fluid-filled internal ear.

The internal ear, or labyrinth, is the part of the temporal bone containing the organs of hearing and balance to which the filaments of the auditory nerve (see Nervous System) are distributed. It is separated from the middle ear by the fenestra ovalis, or oval window. The internal ear consists of membranous canals housed in a dense portion of the temporal bone and is divided into the cochlea (Greek, “snail shell”), the vestibule, and three semicircular canals. All these canals communicate with one another and are filled with a gelatinous fluid called endolymph.

III

Hearing

Sound waves, which are actually changes in air pressure, are carried through the external auditory canal to the eardrum, causing it to vibrate. These vibrations are communicated by the ossicular chain in the middle ear through the oval window to the fluid in the inner ear. The movement of the endolymph stimulates the movement of a set of fine hair-like projections called hair cells as the cochlea vibrates. Collectively these projections are called the organ of Corti. The hair cells transmit signals directly to the auditory nerve, which carries information to the brain. The overall pattern of response of the hair cells to vibrations of the cochlea encodes information about sound in a way that is interpretable by the brain's auditory centres.

The range of hearing, like that of vision, varies in different people. The maximum range of human hearing includes sound frequencies from about 16 to 28,000 cycles per second. The least noticeable change in tone that can be picked up by the ear varies with pitch and loudness. A change of vibration frequency (pitch) corresponding to about 0.03 per cent of the original frequency can be detected by the most sensitive human ears in the range between 500 and 8,000 vibrations per second. The ear is less sensitive to frequency changes for sounds of low frequency or low intensity.

The sensitivity of the ear to sound intensity (loudness) also varies with frequency. Sensitivity to change in loudness is greatest between 1,000 to 3,000 cycles, where a change of one decibel can be detected—and becomes less when sound-intensity levels are lowered.

The variation in the sensitivity of the ear to loud sounds causes several important phenomena. Extremely loud tones produce in the ear entirely different tones that are not present in the original tone. These subjective tones are probably caused by imperfections in the natural function of the middle ear. The harshness in tonality caused by greatly increasing sound intensities, as when a radio volume control is adjusted to produce excessively loud sounds, results from subjective tones produced in the ear. The loudness of a pure tone also affects its pitch. High tones may increase as much as a whole musical-scale note; low tones tend to become lower as sound intensity increases. This effect is noticeable only for pure tones. Because most musical tones are complex, hearing is usually not affected to an appreciable degree by this phenomenon. In sound masking, the production in the ear of harmonics of lower-pitched sounds may deafen the ear to the perception of higher-pitched sounds. Masking is what makes it necessary to raise one's voice in order to be heard in a noisy place. See Deafness.

IV

Equilibrium

The semicircular canals and the vestibule are concerned with the sense of equilibrium, or balance. Hairs in these canals, similar to those that form the organ of Corti, respond to changes in the position of the head.

The three semicircular canals extend from the vestibule approximately at right angles to each other, providing sensory organs to record movements of the head in each of the three planes of space: up and down, forwards and backwards, and to the left or right. Lying over the hair cells in the vestibule are crystals of calcium carbonate, known technically as otoliths and popularly as ear sand. When the head is tilted, the otoliths shift, and the hairs beneath respond to the change in pressure. The eyes and certain sensory cells in the skin and internal tissues also help to maintain equilibrium, but when the labyrinth of the ear is damaged or destroyed, disturbances of equilibrium invariably follow. With eyes closed, a person with a disease or disturbance of the internal ear may be unable to stand without swaying or falling.