Sense Organs

Light first passes through the transparent cornea, which does most of the bending of the rays. The coloured iris adjusts the size of the central hole, the pupil, to control how much light enters. The light then crosses the lens, whose shape is changed by the ciliary muscles to focus the image. The image lands, upside-down, on the retina, the light-sensitive inner layer. The retina contains two kinds of cells: rod cells for dim light and cones for bright light and colour. The optic nerve then carries the impulses to the brain, which turns the picture the right way up.

Accommodation is the eye's ability to focus on near and far objects by changing the curvature of the lens. For a near object the ciliary muscles contract, the lens becomes more convex (fatter); for a distant object the muscles relax and the lens flattens. Two special regions sit on the retina. The yellow spot (fovea) has the densest cone cells and gives the sharpest, most colourful vision. The blind spot is where the optic nerve exits; it has no light-sensitive cells, so any image falling there is not seen.

In myopia (short-sight) the eyeball is too long or the lens too curved, so distant objects focus in front of the retina; a concave (diverging) lens corrects it. In hypermetropia (long-sight) the eyeball is too short or the lens too flat, so near objects focus behind the retina; a convex (converging) lens corrects it. Presbyopia is age-related stiffening of the lens, losing accommodation, corrected with bifocals. Astigmatism is an unevenly curved cornea, corrected with a cylindrical lens.

The outer ear (pinna and auditory canal) gathers sound waves onto the ear drum, which vibrates. In the middle ear three tiny bones, the ossicles malleus, incus and stapes, pass on and amplify the vibration to the oval window. In the fluid-filled inner ear the spiral cochlea contains sensory hair cells that turn the vibrations into nerve impulses, sent along the auditory nerve to the brain. The three semicircular canals do not hear at all; they detect movement of the head and keep us balanced. The Eustachian tube links the middle ear to the throat to equalise air pressure.