The Ear
The ear contains receptors which are sensitive to sound vibrations in the air between the frequencies of 30 per second and 20,000 per second.
Structure. The OUTER EAR is a tube opening on the side of the head and leading inwards to the ear-drum. At the outside end there may be an extension of skin and cartilage. the pinna, which in some mammals helps to concentrate and direct the vibrations into the ear and assists in judging the direction from which the sound came. A membrane of skin and fine fibres is stretched across the innermost end of the outer ear, closing it off completely. This is called the ear-drum.
The MIDDLE EAR is an air-filled cavity in the skull. It communicates with the back of the mouth cavity through a narrow tube, the Eustachian tube. Three small bones, or ossicles, in the middle ear link the ear-drum to a small opening in the skull, the oval window, which leads to the inner ear.
The INNER EAR is filled with a fluid and consists mainly of a coiled tube, the cochlea, with sensory endings in it. It is here that the sound vibrations are converted to nervous impulses.
Hearing. Vibrations in the air that constitute sound waves enter the outer ear and set the ear-drum vibrating. This vibra- tion is transmitted through the three ossicles which act as levers and cause the innermost of them, the stapes, to vibrate against the oval window. The fact that the ear-drum is greater than the oval window, together with the action of the levers of the ossicles, result in an increase of about 22 times in the force of the vibrations that reach the inner ear.
The oscillations of the stapes set the fluids of the inner ear vibrating, particularly in the cochlea. A membrane consisting of transverse fibres runs the length of the cochlea; in the first part of this membrane the fibres are short, and in the last part longer. Although details of the mechanism are not yet fully worked out it is thought that the short fibres in the first part of the cochlea respond to high-frequency vibrations and the long fibres in the last part to low-frequency vibrations, with a continuous range of intermediate fibres which respond to other stimu- frequencies. When the transverse fibres vibrate they stimu- this late the sensory cells resting upon them. According to this theory the pitch of a note could be determined by the brain, is owing to the fact that only a particular group of fibres is stimulated by a certain frequency and will send impulses through the auditory nerve to the brain, while the others will be unaffected.
Eustachian tubes. Air pressure in the middle ear is usually the same as atmospheric pressure. If changes take place in the pressure outside the ear-drum, for example when gaining height rapidly in an aircraft or even in a lift, the pressure is equalized by the Eustachian tube opening and admitting more air to, or releasing excess air from, the middle ear. Normally the Eustachian tubes are closed, and are opened only when one is swallowing or yawning, when a "popping sound may be heard in the ears.
Sense of direction of sound. With two ears the sound from a single source will be heard more loudly in one ear than the other, and very slightly earlier. The fact that the two ears are stimulated to different extents enables animals to estimate the direction from which the sound comes. Most mammals can also move their ear pinnace to a favourable position for receiving the sound and so obtain a more accurate bearing.
A source of sound which is equidistant from both ears is difficult to locate because it can be below eye-level, directly in front, directly above or behind the head and still stimulate both ears equally. In such a situation dogs will cock their heads on one side, resulting in one ear being stimulated more than the other.
A dog can locate the position of a sound in one out of thirty two positions all round it, while humans are accurate in the perception of one out of only eight possible sources. Cats have been shown capable of distinguishing the position of two sounds only half a metre apart and eighteen metres away.
Semicircular canals and sense of balance.
The utriculus, sacculus and semicircular canals are organs of balance and posture. In the fluid-filled cavities of the utriculus and sacculus are gelatinous plates containing chalky granules, called otoliths, which lie above sensitive patches of the lining. Sensory fibres from these patches are embedded in the otoliths
and when the head is tilted, the otoliths pull on the fibres. The nervous impulses fired off from these organs as a result of such stimulation reach the brain and set off a reflex tending to return the body to its normal posture.
The semicircular canals contain a fluid and in the ampullae are sense organs which respond to movements of the When the head rotates, the fluid tends to remain stationary for a time and is thought to displace the cupula in the ampulla. The three canals are in planes at right angles to each other and are stimulated by rotation in their respective planes.
The utricles are regarded as responding to the tilting movements of the head or body while the semicircular canals are stimulated by accelerations of a rotary kind in their particular plane. If the utricles and semicircular canals did not function properly, animals would keep falling over unless they relied on their eyes. Without the semicircular canals one could probably stand upright if quite stationary, but it would be very difficult to maintain balance while moving or changing direction.
Experiences of vertigo during night flying show that we normally rely on information from the eyes and other sense organs, as well as from the utriculus and semicircular canals, for accurate sensations of position and balance.