Glossary · Sinh học
Retina
The light-sensitive layer of tissue at the back of the eye. Its photoreceptor cells turn light into electrical signals, which travel along the optic nerve to the brain.
The eye does not send pictures to the brain. What leaves the retina is electrical impulses, and an image appears only after the brain reconstructs one from them.
The retina holds two kinds of photoreceptor, differently distributed. Rods are strongly photosensitive and sit around the outer edges of the retina, carrying vision in dim light and at the periphery; they detect only shades of grey, not colour. Cones are less sensitive and so need bright light, cluster near the centre of the retina, and are what give us colour; the fovea, the region responsible for acute vision, has a high density of cones.
Because the retina is only a thin layer of tissue lining the back of the eyeball, it can pull away from its place. Retinal detachment is an ophthalmic emergency: left untreated it can cause permanent loss of sight.