Retina - definition, function and diseases

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Retina - the innermost layer of the EYE. The retina receives light images and converts them to NERVE impulses the OPTIC NERVE conveys to the BRAIN. The retina is two tissue-thin layers that together are less than 1⁄2 millimeter in thickness. The outer pigment layer provides a completely light-absorbing, nonreflective lining. The inner sensory layer contains the photoreceptors (rods and cones) responsible for vision. Rods detect only shades of gray though can register images of very low intensity. Cones detect color and detail.

Laid out flat, like a disk, the retina measures just under 2 inches in diameter. The primary work of vision takes place in an area about the size of a postage stamp called the macula. Most of the retina’s 120 million rods and 6 million cones reside in the macula. A section of the central retina no larger than a pencil eraser, the macula, contains almost no rods and an abundance of cones and handles detail vision. A pencil-point depression within the macula, the fovea, has the highest concentration of cones.

The optic nerve enters the retina somewhat to the NOSE-side at the back of the eye, along with the ARTERY and VEIN that manage the retina’s BLOOD supply. Vitreous humor, a gelatinous substance, fills the inner eye and holds pressure against the retina, keeping it smoothly and tightly adhered to the choroid.

The ophthalmologist can examine the surface of the retina using OPHTHALMOSCOPY. Under illumination the retina appears reddish orange. The optic nerve disk appears as a pale, pinkish circle. The macula, of similar size, appears as a darker and less distinct circular area with a depression, the fovea, in its center.

Conditions that can Affect the Retina
AGE-RELATED MACULAR DEGENERATION (ARMD) COLOR DEFICIENCY
COLOR DEFICIENCY NIGHT BLINDNESS
RETINAL DETACHMENT RETINITIS PIGMENTOSA
RETINOBLASTOMA RETINOPATHY
traumatic injury  

For further discussion of the retina within the context of eye structure and function please see the overview section “The Eyes.”

See also ELECTRORETINOGRAPHY; FLASHES; LENS; VISION IMPAIRMENT; VITREOUS DETACHMENT.

Resource: Facts On File Encyclopedia Of Health And Medicine

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