To most people the ‘ear’ is just a convoluted flap of cartilage and skin sticking out of the side of the head, with a small but mysterious hole at its centre. It does not look a very sophisticated piece of sensory equipment. But behind this flap, encased within the bones of the skull, are the organs of hearing and balance -both marvels of intricate and miniature mechanics. Of all our senses, hearing is perhaps second in importance only to sight, and loss of balance would render us unable to move unaided. Knowledge of these minute structures has largely been-unravelled only in the last decade, with the advent of new microscopic techniques (particularly the ‘three-dimensional’ images of scanning electron microscopy). The structure and functions of the ear fall conveniently into three physical and anatomical areas: the outer ear, middle ear and inner ear.
The outer ear is the visible part of the ear, the flap or auricle, and the external auditory canal. The auricle (also called the pinna) collects and funnels sound waves from the air into the external canal. There are some small muscles attached to the auricle which some people can use to wiggle their ears. These muscles are all that is left, in evolutionary terms, of the efficient musculature used by many other animals to swivel and direct their sonar-like ears towards the source of a sound.
The auditory canal is about 25mm long in an adult, and curves slightly upwards and then forwards until it reaches the outer surface of the eardrum. Doctors straighten the canal, in order to see more clearly inside the ear and examine the eardrum, by pulling the auricle up and back.
The outer third of the auditory canal is formed from cartilage lined with skin that bears hairs and modified sweat glands. These sweat glands produce wax, which traps airborne particles to prevent them accumulating. The inner two-thirds of the canal is formed from bone with a thin, delicate skin lining.
The middle ear starts at the eardrum, or tympanic membrane. It consists of a cavity in the temporal bone, part of the skull. The eardrum is a thin circular membrane about 8mm in diameter. Airborne sound waves cause it to vibrate and these vibrations are transmitted across the middle ear by a chain of three very small bones called the auditory ossicles. The first of these, the malleus (hammer), is embedded in the eardrum and thus vibrates with it. The malleus joins to the second bone, the incus (anvil), suspended by ligaments from the roof of the middle ear. The incus in turn is joined to the third bone, the stapes (stirrup). The footplate (base) of the stapes fits into a small oval window in the inner wall of the middle ear, through which the sound waves pass to the fluid chambers of the inner ear. Below the oval window is the small round window closed by a thin membrane. In order that the eardrum may vibrate freely, the middle ear normally contains air at atmospheric pressure. Air pressure is dependent on air passing along the Eustachian tube, which opens from the front wall of the middle ear and passes forwards and inwards to the back of the nasal cavity. From the rear wall of the middle ear extends a series of cavities andthat make up the air of the mastoid bone .
Inside the inner ear, embedded in the bone of the base of the skull, are the sensory mechanisms for hearing and balance. They consist of labyrinth-like, bony structures filled with a fluid called perilymph. Suspended inside this bony labyrinth is a membranous labyrinth, a delicate system of tubes and sacs which contain a different fluid called endolymph. The bony cavity is fully formed (at adult size) in a newborn baby, and is the only part of the body that does not have to grow during childhood. There are three sections of the inner ear: the semicircular canals; the vestibule; and the cochlea. The semicircular canals lie in three planes. Two of them are vertical, one arching forwards and one sideways, whereas the third lies horizontally. The back end of each canal widens into a duct called an ampulla, which contains a layer of specialized hair-bearing cells called the crista. The ends of the hairs are embedded in a small gelatinous mass, the cupola. Movements of the endolymph fluid in the canal cause the hairs to bend, much as submerged seaweed bends to and fro with the motion of waves. The moving hairs stimulate nerve cells at their bases. These nerve cells send impulses to the brain which interprets them to give the sensation of movement.
The vestibule contains two pockets or sacs, the utricle and the saccule. The ends of the semicircular canals open into the utricle, which is connected by a small duct to the saccule and from there to the cochlea. The utricle and saccule each contain a patch of sensory tissue called the macula, which differs from crista in that its gelatinous mass contains small chalky particles called otoliths. The macula of the utricle is horizontal whereas that of the saccule is vertical. Changes of posture and the pull of gravity on the otoliths stimulates the nerve cells at the base of the hairs, to give us a sense of position and speed of movement. The cochlea is a spiral coil similar in shape to a snail’s shell, with two and a half turns. Within the spiral of the cochlea lies a strip of sensory tissue called the basilar membrane, which contains more hair cells. Sound waves, transmitted through the fluid perilymph, cause the basilar membrane to vibrate, and the vibrations are converted by the hair cells into nerve impulses. These travel along the auditory nerve to the brain to give the perception of sound.