Year 1 · Senses, behaviour and coordination · Section 5.3

Hearing and sight

The structure of the ear

Each inner ear contains structures for hearing and balance. Sound consists of vibrations travelling through a medium such as air. The pinna, or outer ear, collects sound and directs it through the ear canal to the eardrum, which vibrates.

Glands in the ear canal make earwax, helping protect and lubricate the canal and trap particles.

Behind the eardrum lies the air-filled middle-ear cavity, containing three small bones: the hammer, anvil and stirrup. These ossicles transmit vibrations to the oval window, a membrane-covered opening beside the cochlea.

The cochlea is filled with fluid. Vibrations move the fluid and stimulate sensory hair cells, leading to impulses carried by the auditory nerve to the brain.

The Eustachian tube connects the middle ear to the throat. It opens during swallowing or yawning, helping equalise air pressure on both sides of the eardrum.

The structure of the eye

Several structures protect the eye and keep it moist. Eyebrows divert sweat, eyelashes help keep particles out, and tear glands produce fluid that prevents drying and washes away dirt.

The sclera is the tough, white outer covering. Eye muscles attached to it move the eye. At the front, the transparent cornea admits and bends light. The choroid, beneath the sclera, contains blood vessels supplying much of the eye.

The coloured iris controls the size of the pupil, the opening through which light enters. Behind the pupil, the lens helps focus light onto the retina.

The retina contains light-sensitive cells. Their signals lead to impulses carried along the optic nerve to the brain.

The macula is the central region used for detailed vision; its centre, the fovea, gives the sharpest vision. The blind spot, where the optic nerve leaves the eye, has no light-sensitive cells.

The vitreous humour is a clear, jelly-like material helping maintain the eye's shape and support the retina.

The pupil reflex

The pupil reflex regulates the amount of light entering the eye. In bright light, the pupil becomes smaller; in dim light, it becomes larger.

How the eye works

Light enters through the cornea and pupil and is focused onto the retina. The lens changes shape, becoming more rounded or flatter, to focus on objects at different distances.

The image formed on the retina is inverted. The brain interprets the incoming signals so that you perceive the world in its usual orientation.

Short-sightedness and long-sightedness

Short-sightedness (myopia): Distant objects appear blurred because light focuses in front of the retina, often because the eyeball is too long or the eye bends light too strongly. Concave lenses correct the focus.

Long-sightedness (hyperopia): Light would focus behind the retina when the eye is relaxed, often because the eyeball is too short or the eye does not bend light enough. Nearby objects may be blurred. Convex lenses help correct the focus. Young people can sometimes compensate by changing the shape of their own lens.