The vertebrate eye is similar in structure and function to a camera. At the back of the eye is a thin, light-sensitive sheet called the retina. The eye also has a lens that focuses the light onto the retina. As the eye looks at a point of light, the light passes through the transparent cornea and then through the lens, both of which bend the light, focusing it onto the retina. The retina is made up of a variety of cell types, including the light-sensitive cells called photoreceptors. There are two types of photoreceptor cells in the retina: rods, which are sensitive to dim light, and cones, which are sensitive to color. Each cone cell is most sensitive to one of three primary colorsâblue, red, or green. The human retina contains mostly rods, but a region in the center of the retina, called the fovea, contains a dense concentration of cones. The fovea is the area of highest visual acuity. When people focus on details, such as a point of light, the eye moves to focus the image on the fovea. As light enters the eye, it must pass through several layers of neurons to reach the photosensitive area of the retina. When light activates a photoreceptor cell, the cells that synapse with the photoreceptor are activated, carrying a signal along the optic nerve to visual centers in the brain. Note that the rods and cones lie at the back of the retina, with their light-sensitive regions pointing away from the source of light.
