EYE
The human eye. The retina of the eye contains about 137 million light-sensitive cells in an area of about 650 sq mm/1 sq in. There are 130 million rod cells for black and white vision and 7 million cone cells for colour vision. The optic nerve contains about 1 million nerve fibres. The focusing muscles of the eye adjust about 100,000 times a day. To exercise the leg muscles to the same extent would need an 80 km/50 mi walk.
Organ of vision. In the human eye, the light is focused by the combined action of the curved cornea, the internal fluids, and the lens. The insect eye is compound – made up of many separate facets, known as ommatidia, each of which collects light and directs it separately to a receptor to build up an image. Many invertebrates have much simpler eyes, with no lenses. Among molluscs, cephalopods have complex eyes similar to those of vertebrates. The mantis shrimp's eyes contain ten colour pigments with which to perceive colour; some flies and fish have five, while the human eye has only three.
The human eye is a roughly spherical structure contained in a bony socket. Light enters it through the cornea, a transparent region at the front of the tough outer sclera and passes through the circular opening (pupil) in the iris (the coloured part of the eye). The muscular iris controls the size of the pupil and hence the amount of light entering the eye. Light then passes through the lens, which is held in position by suspensory ligaments and ciliary muscles. The ciliary muscles act on the lens (the rounded transparent structure behind the iris) to change its shape, allowing images of objects nearby and at a distance to be focused on the retina at the back of the eye. The retina is packed with light-sensitive cells (rods and cones). Rods work well in conditions of low light but are unable to sense colour. Cones work well in bright light and are responsible for colour vision. The part of the retina on which lies the precise point at which the image is focused contains mainly cones. The rods and cones in the retina send impulses to the brain along sensory neurons in the optic nerve.
