Eye

The retina is the innermost layer of the eye. The retina is thin, delicate, extremely complex sensory tissue composed of layers of light sensitive nerve cells. The retina begins at the ciliary body and encircles the entire posterior portion of the eye. Photoreceptor cells in the rods and cones, convert light first to chemical energy and then electrical energy. Rods function in dim light, allowing limited nocturnal (night) vision: it is with rods that we see the stars. Rods cannot detect color, but they are the first receptors to detect movement. There are about 126 million rods in each eye and about six million cones. Cones provide acute vision, function best in bright light, and allow color vision. Cones are most heavily concentrated in the central fovea, a tiny hollow in the posterior part of the retina and the point of most acute vision. Dense fields of both rods and cones are found in a circular belt surrounding the fovea, the macula lutea. Continuing outward from this belt, the cone density decreases and the ratio of rods to cones increases. Both rods and cones disappear completely at the edges of the retina.