What is the Iris? Where is the Iris Layer in the Eye?

The main duty of the iris is to perfectly adjust the amount of light entering the inside of the eye (the retina) by dilating and constricting the "pupil," which is the black hole in the middle of it. In a very brightly lit environment, the iris contracts to constrict the pupil (restricting incoming light); in the dark, it relaxes to dilate the pupil, allowing more light to be gathered.

There are color pigments called "melanin" on the front surface of the iris. If the melanin pigment in the iris is very dense, the eyes appear dark brown or black. If the pigment amount is very low, the eyes appear blue as a result of light reflection (refraction); if it is at a medium level, they appear green or hazel. That is, blue-eyed people do not have a blue pigment; they merely have an absence of pigment.

The inflammation of the iris layer is called Iritis (or anterior uveitis). It manifests with severe redness in the eye, extreme sensitivity to light (photophobia), throbbing pain, and blurred vision. It is a serious condition mostly associated with immune system diseases (rheumatism, etc.) and requires emergency treatment.