Introduction
The cornea and lens focus light onto the retina, the transparent, light-sensitive membrane on the inner surface of the back of the eye. The central area of the retina, called the macula, primarily contains a high density of color-sensitive photoreceptor cells. These cells, called cones, produce the sharpest visual images and are responsible for central vision. The peripheral area of the retina, which surrounds the macula, contains photoreceptor cells called rods, which respond to lower lighting levels but are not color sensitive. The rods are responsible for peripheral vision and night vision.
 See the figure Viewing the Retina.
The optic nerve carries signals generated by the photoreceptors (cones and rods). Each photoreceptor sends a tiny branch to join the optic nerve. The optic nerve extends into the brain and connects to neurons that carry signals to the vision center of the brain, where they are interpreted as visual images.
The optic nerve and the retina have a rich supply of blood vessels that carry blood and oxygen. Part of this supply of blood vessels comes from the choroid, which is the layer of blood vessels that lies between the retina and the outer white coat of the eye (the sclera). The central retinal artery (the other major source of blood to the retina) reaches the retina near the optic nerve and then branches out within the retina.
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