For Patients - What is the Retina?

"What is the Retina?"

ret·i·na [rétt'n ] (plural ret·i·nas, ret·i·nae [rétt'nae]) noun a light-sensitive membrane in the back of the eye containing rods and cones that receive an image from the lens and send it to the brain through the optic nerve.

Light rays reflected from any object we look at enter the eye and are focused by the eye's optical structures: cornea, iris, pupil, and lens. The final destination of the light rays is the retina, a layer of nerve tissue that lines two-thirds of the back of the eye. In the center of the retina is the macula, an area that is only 1.5 mm (0.06 in) in diameter. The macula is responsible for the clearest, most detailed vision.

The retina is made up of two types of cells: cones and rods. Cones are nerve cells that are sensitive to light, detail, and color. Millions of cone cells are packed into the macula, aiding it in providing the visual detail needed to scan the letters on an eye chart, see a street sign, or read the words in a newspaper.

Cones also produce the sensation we call color. Cones contain three different pigments, which respond either to blue, red, or green wavelengths of light. Cones mix the color signals to produce the variety of colors we see. If a person is missing one or more of the pigments, that person is said to be color-blind and has difficulty distinguishing between certain colors, such as red from green.

Rods are designed for night vision and the detection of motion and objects. They also provide peripheral vision, but they do not see as acutely as cones. Rods are insensitive to color. When a person passes from a brightly lit place to one that is dimly illuminated, such as entering a movie theater during the day, the interior seems very dark. After some minutes this impression passes and vision becomes more distinct. In this period of adaptation to the dark the eye becomes almost entirely dependent on the rods for vision, which operate best at very low light levels. Since the rods do not distinguish color, vision in dim light is almost colorless.

Light rays that reflect from the upper half of any object we look at are focused on the lower half of the retina. Rays from the lower half of the same object are focused on the upper half of the retina. This would seem to give us an upside-down picture of the world. Fortunately, these signals are rearranged when the brain processes them into an image that is right side up.

"Vision," Microsoft® Encarta® Online Encyclopedia 2001
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