: irides or irises) is a thin, annular structure in the eye in most mammals and birds that is responsible for controlling the diameter and size of the pupil, and thus the amount of light reaching the retina.

The high pigment content blocks light from passing through the iris to the retina, restricting it to the pupil.

The iris along with the anterior ciliary body provide a secondary pathway for aqueous humour to drain from the eye.

Many fish have neither, and, as a result, their irises are unable to dilate and contract, so that the pupil always remains of a fixed size.

The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, and blue.

The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of an organism.

Most human irises also show a condensation of the brownish stromal melanin in the thin anterior border layer, which by its position has an overt influence on the overall color.

Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color (see heterochromia, below).

[citation needed] The optical mechanisms by which the nonpigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature.

Interference is recognised by characteristic dependence of color on the angle of view, as seen in eyespots of some butterfly wings, although the chemical components remain the same.

White babies are usually born blue-eyed since no pigment is in the stroma, and their eyes appear blue due to scattering and selective absorption from the posterior epithelium.

Uncommon in humans, it is often an indicator of ocular disease, such as chronic iritis or diffuse iris melanoma, but may also occur as a normal variant.

Siberian Husky dogs show heterochromia,[12][better source needed] possibly analogous to the genetically determined Waardenburg syndrome of humans.

[13] Iridology (also known as iridodiagnosis) is an alternative medicine technique whose proponents believe that patterns, colors, and other characteristics of the iris can be examined to determine information about a patient's systemic health.