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    A natural or synthetic substance derived from vitamin A.


Not human-induced or manmade.

Any of many organic substances that are vital in small amounts to the normal functioning of the body. Vitamins are found in food, produced by the body, and manufactured synthetically; along with minerals, they are known as micronutrients.


Reticuloendothelial system
A network of phagocytic cells.

Retin-a is a prescription or over-the-counter drug which is (or once was) approved in the United States and possibly in other countries. Active ingredient(s): tretinoin.

Retin-a micro
Retin-a micro is a prescription or over-the-counter drug which is (or once was) approved in the United States and possibly in other countries. Active ingredient(s): tretinoin.

A membrane lining the inside of the back of the eye that contains light-sensitive nerve cells that convert focused light into nerve impulses, making vision possible.

Light-absorbing portion of rhodopsin. The absorption of light causes retinal to change from 11-cis to 11-trans configuration

Retinal detachment
A retinal detachment occurs when the retina, that part of the eye that contains the photoreceptors, detaches from the underlying layers of cells, called the choroid. A retinal detachment my be the result of injury to the eye such as blunt trauma (remember Sugar Ray, the boxer) or it may result from other things like high myopia or age-related macular degeneration. In certain diseases, retinal breaks and tears occur and these may lead to a retinal detachment. Early warning signs include bright dots or lights or some of your side vision may appear dark. A retinal detachment may be treatable if detected early, so see your Ophthalmologist.

Retinal layers
Retinal layers are: Outer nuclear layer - Contains photoreceptors; Inner nuclear layer - Contains bipolar, horizontal and amacrine cells; Ganglion cell layer - Contains ganglion cells; Outer plexiform layer - Contains processes of receptor, bipolar and horizontal cells; Inner plexiform layer - Contains processes of bipolar, amacrine and ganglion cells.

Retinex Theory
Theory of lightness and color perception. Argues that the color of an object is not determined by the composition of the light coming from the object. The color of a unit area is determined by a trio of numbers each computed on a single waveband to give the relationship for the waveband between the unit area and the rest of the unit areas in a visual scene.

Inflammation of the retina.

Retinitis Pigmentosa (RP)

The most common cancer in the eye occurring in early childhood. A parent or doctor may first suspect a problem by detecting whiteness in the normally dark pupil. Occasionally it leads to a wandering eye (strabismus). It does not spread from one eye to the other but about 25% have a tumor in each eye. Immediate medical treatment is necessary. Sometimes the eye(s) must be removed to prevent spreading of the tumor into the brain.

Degeneration of the retina.

Retinotectal pathway
Pathway from the retina to superior colliculi.

Retinotopic map
A preservation of the spatial relationships of the photoreceptors in the retina in a higher brain representation.

Retisert is a prescription or over-the-counter drug which is (or once was) approved in the United States and possibly in other countries. Active ingredient(s): fluocinolone acetonide.


Rheumatic Fever
A serious inflammatory condition which follows Streptococcal pharyngitis. Acute rheumatic fever can destroy the heart valves or leave them vulnerable to scarring, thickening, calcification, and fusion of the leaflets later in life.

A cytoplasmic cellular structure, composed of ribonucleic acid and protein, that functions in the synthesis of protein. Ribosomes interact with messenger RNA and transfer RNA to join together amino acid units into a polypeptide chain according to the sequence determined by the genetic code.

Radioisotope scan
Uses radioactive substances introduced into the body to create an image of the functioning adrenal gland.

Risk factors
Traits that make it more likely that a person will develop an illness.

Restless Leg Syndrome (RLS)
Wittmaack-Ekbom or restless legs syndrome is a disorder of the nervous system that affects sensation and movement in the legs and causes the limbs to feel uncomfortable. Relief can be temporarily gained by moving the legs which cause sufferers to seem fidgety and restless.


Rohypnol (the trade name of flunitrazepam) is a sedative that was made in the early 1970's by Roche and was used in hospitals only for deep sedation. It was first released on the market in 1975 in tablet form in Europe. The tablet was then released in the early 1980's to other countries. It came in 5 mg, 2 mg, and 1 mg sizes, but due to its strength the higher doses were taken off the market and now it only comes in one strength: 1 mg. It is the strongest sleeping tablet to ever be made.

Radiation Oncology
The study and treatment of cancers using radiation (x-rays, gamma rays or electrons).

Radiation Therapy
The use of x-rays, gamma rays, or electrons to treat cancers.

Range of motion
the extent that a joint will move from full extension to full flexion.

Resting tremor
A tremor of a limb that increases when the limb is at rest.

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