The iris (: irides or irises) is a thin, ring-shaped 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. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm. Eye color is defined by the iris.

Etymology

The word "iris" is derived from "ἶρις", the Greek word for "rainbow", as well as Iris, goddess of the rainbow in the Iliad, due to the many colors the human iris can take.

Structure

The iris consists of two layers: the front pigmented fibrovascular layer known as a stroma and, behind the stroma, pigmented epithelial cells.

The stroma is connected to a sphincter muscle (sphincter pupillae), which contracts the pupil in a circular motion, and a set of dilator muscles (dilator pupillae), which pull the iris radially to enlarge the pupil, pulling it in folds.

thumb|360x360px|The iris (brown coloured portion of the eye) controls the size of the pupil by contracting the sphincter pupillae and dilator pupillae muscles.

The sphincter pupillae is the opposing muscle of the dilator pupillae. The pupil's diameter, and thus the inner border of the iris, changes size when constricting or dilating. The outer border of the iris does not change size. The constricting muscle is located on the inner border.

The back surface is covered by a heavily pigmented epithelial layer that is two cells thick (the iris pigment epithelium), but the front surface has no epithelium. This anterior surface projects as the dilator muscles. The high pigment content blocks light from passing through the iris to the retina, restricting it to the pupil. The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body. The iris and ciliary body together are known as the anterior uvea. Just in front of the root of the iris is the region referred to as the trabecular meshwork, through which the aqueous humour constantly drains out of the eye, with the result that diseases of the iris often have important effects on intraocular pressure and indirectly on vision. The iris along with the anterior ciliary body provide a secondary pathway for aqueous humour to drain from the eye.

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The iris is divided into two major regions:

  1. The pupillary zone is the inner region whose edge forms the boundary of the pupil.
  2. The ciliary zone is the rest of the iris that extends to its origin at the ciliary body.

The collarette is the thickest region of the iris, separating the pupillary portion from the ciliary portion. The collarette is a vestige of the coating of the embryonic pupil.

The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in reptiles (including birds). 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.

Front

  • The crypts of Fuchs are a series of openings located on either side of the collarette that allow the stroma and deeper iris tissues to be bathed in aqueous humor. Collagen trabeculae that surround the border of the crypts can be seen in blue irises.
  • The midway between the collarette and the origin of the iris: These folds result from changes in the surface of the iris as it dilates.
  • Crypts on the base of the iris are additional openings that can be observed close to the outermost part of the ciliary portion of the iris. The irises also contract the pupils when accommodation is initiated, to increase the depth of field.

Very few humans possess the ability to exert direct voluntary control over their iris muscles, which grants them the ability to dilate and constrict their pupils on command. However, there is no clear purpose or advantage to this.

Eye color

thumb|Human eye pigmentation in Europe

thumb|right|Among human phenotypes, blue-green-gray eyes are a relatively rare eye color and the exact color is often perceived to vary according to its surroundings.

The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, and blue. Occasionally, the color of the iris is due to a lack of pigmentation, as in the pinkish-white of oculocutaneous albinism,

All the contributing factors towards eye color and its variation are not fully understood. Autosomal recessive/dominant traits in iris color are inherent in other species, but coloration can follow a different pattern.

Different colors in the two eyes

thumb|right|240px|Example of [[heterochromia – one eye of the subject is brown, the other hazel.]]

Heterochromia (also known as a heterochromia iridis or heterochromia iridum) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). 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. Sectors or patches of strikingly different colors in the same iris are less common. Anastasius the First was dubbed dikoros (having two irises) for his patent heterochromia since his right iris had a darker color than the left one.

In contrast, heterochromia and variegated iris patterns are common in veterinary practice. Siberian Husky dogs show heterochromia, possibly analogous to the genetically determined Waardenburg syndrome of humans. Some white cat fancies (e.g., white Turkish Angora or white Turkish Van cats) may show striking heterochromia, with the most common pattern being one uniformly blue, the other copper, orange, yellow, or green.

Clinical significance

  • Angle closure glaucoma
  • Aniridia
  • Anisocoria
  • Horner's syndrome
  • Iridocyclitis
  • Iridoplegia
  • Iritis
  • Miosis/Mydriasis
  • Synechia
  • Third nerve palsy

Alternative medicine

Iridology

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. Practitioners match their observations to "iris charts", which divide the iris into zones corresponding to specific parts of the human body. Iridologists see the eyes as "windows" into the body's state of health.

Iridology is not supported by quality research studies, and is considered pseudoscience.

Graphics

<gallery widths="240" heights="200">

<!-- Please restrict images: Should we say one side view and one front view? -->

Image:Gray878.png|Iris, front view

File:Fluorescein angiograpy of iris reveals radial layout of blood vessels.jpg|Fluorescein angiograpy of the iris reveals a radial layout of blood vessels.

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See also

  • Blood–ocular barrier
  • Coloboma
  • Gonioscopy
  • Human eye
  • Iris recognition

References

  • Detailed photographs of human irides
  • – "Sagittal Section Through the Eyeball"