Retina
On the inside back wall of the eyeball is a thin layer of tissue called the retina. This term comes from a Latin word meaning "net." It's an apt name because your retina consists of millions of light-sensitive cells and nerve cells that capture the images focused onto them by your cornea and lens.
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The light-sensitive cells (sometimes called photoreceptors) are either rods or cones. There are about 20 rod cells for every cone cell. Rod cells allow you to see in very dim light or off to the side while looking ahead (peripheral, or side, vision), but they can't distinguish colors. Cone cells distinguish color exquisitely but require more light to function. This is why it can be hard to see color in the evening or in dim light. (Hence the saying, "At night all cats are gray.") Cone cells are concentrated in the center of your retina and allow you to see sharp detail when you're looking straight ahead at a well-lit object.
Light striking the rods and cones triggers a chemical reaction. This in turn generates electrical impulses that are relayed through the optic nerve to the visual cortex, the seeing portion of your brain. The image that your retina receives is upside-down. It's also reversed, similar to how you see a reversed image of yourself when you look in a mirror. These effects are caused by the convex shape of both the cornea and the lens. Your brain reinterprets this information, allowing you to see the images in their correct orientation. The brain must also merge the image from both eyes to produce a clear picture.
The outer part of your retina is nourished mainly by the choroid, a layer of arteries and veins sandwiched between the retina and the sclera. The inner part of the retina receives its nutrition from retinal blood vessels.
Macula and fovea
At the center of your retina is the macula, which is densely packed with cone cells. This dark reddish patch is the part of your retina that provides your central, or straight-ahead, vision and allows you to see fine detail. It's used for reading and other close-up work. Within the macula is a small depression called the fovea, which contains only cone cells and provides your sharpest vision.
Outside the macula your retina contains primarily rod cells, which can't process images as sharply as the cone cells in the macula but are responsible for peripheral and night vision.
Optic nerve
The visual information gathered by your retina is carried to the visual cortex of your brain by a bundle of over 1 million nerve fibers. This communication cable between your eyes and your brain is called the optic nerve. The brain instantly decodes the visual impulses, coordinating signals from both eyes to produce a three, dimensional image.
A yellowish circle visible on the retina is where the optic nerve forms at the back of the eye. This location is called the optic disk.
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Muscles of the eyeball
Each eyeball has six muscles attached to the sclera, allowing you to move both eyes and track an object without necessarily turning your head. These eye muscles, working individually or together, allow you to shift your visual field left, right, up, down and diagonally. Your brain coordinates these eye movements, so the eyes move in unison when tracking an object. To find out more, you can check out Is It Possible To Improve Your Eyesight Naturally.