RETINA

layer of nervous tissue, covering the back two-thirds of the eyeball, in which stimulation by light occurs, initiating the sensation of vision. The rest of the eyeball serves essentially as a supporting shell for this light-sensitive tissue, providing it with nutrition and focusing light onto the appropriate portion of the retina. The retina is actually an extension of the brain, forming embryonically from brain tissue and connected to the brain proper by the optic nerve. The retina is a complex tissue consisting of several layers, only one of which contains light-sensitive cells; light must pass through the covering layers to reach this layer. The light-sensitive cells are of two types, rods and cones, which are differentiated structurally by their distinctive shapes and functionally by their sensitivity to different kinds of light. Rods predominate in nocturnal animals and are most sensitive to reduced light intensities; in humans they provide night vision and aid in visual orientation. Cones are more prominent in animals that are active during the day, including humans, and provide detailed vision, as for reading, and colour perception. In general, the more cones per unit area of retina, the finer the detail that can be discriminated by that area. Rods are fairly well distributed over the entire retina, but cones tend to concentrate in two sites: the fovea centralis, a pit at the rear of the retina, which contains no rods and has the densest concentration of cones in the eye, and the surrounding macula lutea, a circular patch of yellow-pigmented tissue about one centimetre in diameter. When light enters the eye, it passes through the lens and is refracted, focusing an image onto the retina. Light-sensitive chemicals in the rods and cones react to specific wavelengths of light and trigger nerve impulses. Complex interconnections (synapses) among retinal nerves assemble these impulses into a coherent pattern, which, in turn, is carried through the optic nerve to the visual centres of the brain, where they are interpreted.