{{short description|Colored part of an eye}} {{About|the part of the eye|other uses|Iris (disambiguation)}} {{Infobox anatomy | Name = Iris | Latin = iris | Image = Human eye close up, anterior view.jpg | Caption = The iris in humans is the [[eye color|colored]] (typically brown, blue, or green) area, with the [[pupil]] (the circular black spot) in its center, and surrounded by the white [[sclera]]. | Width = | Image2 = Schematic diagram of the human eye en.svg | Caption2 = Schematic diagram of the human eye (iris labeled at upper right) | part_of = Front of [[eye]] | system = [[Visual system]] | Precursor = [[Mesoderm]] and [[ectoderm|neural ectoderm]] | Artery = [[Long posterior ciliary arteries]] | Vein = | Nerve = [[Long ciliary nerves]], [[short ciliary nerves]] | Lymph = }} The '''iris''' ({{plural form}}: '''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 (optics)|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 (mythology)|Iris]], goddess of the rainbow in the ''[[Iliad]]'',<ref name="Liddel">{{cite web|url=http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0073%3Aentry%3Di%29%3Dris |title=ἶρις |last1=Liddell |first1=Henry George|last2=Scott|first2=Robert |work=A Greek-English Lexicon |publisher=Perseus Digital Library |date= 1940 }}</ref> due to the many colors the human iris can take.<ref>{{Cite OED | iris | ref=none }}</ref>
==Structure== The iris consists of two layers: the front [[pigment]]ed [[Wikt:fibrovascular|fibrovascular]] layer known as a [[stroma of iris|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. [[File:Pupillary light reflex.jpg|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 [[epithelium|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.<ref name="iris">"eye, human." Encyclopædia Britannica from [[Encyclopædia Britannica 2006 Ultimate Reference Suite DVD]]</ref> 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. [[File:Iris regions.jpg|370px|right]] The iris is divided into two major regions: # The '''pupillary zone''' is the inner region whose edge forms the boundary of the pupil. # The '''ciliary zone''' is the rest of the iris that extends to its origin at the ciliary body. The '''{{Anchor|Collarette}}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.<ref name="iris"/> It is typically defined as the region where the sphincter muscle and dilator muscle overlap. Radial ridges extend from the periphery to the pupillary zone, to supply the iris with blood vessels. The root of the iris is the thinnest and most peripheral.<ref name="iris1">Gold, Daniel H; Lewis, Richard; "Clinical Eye Atlas," pp. 396–397</ref>
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.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|page= 462|isbn= 0-03-910284-X}}</ref>
===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.{{Citation needed|date=November 2010}} *'''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.<ref name="iris1"/>
===Back=== *The '''radial contraction folds of Schwalbe''' are a series of very fine radial folds in the pupillary portion of the iris extending from the pupillary margin to the collarette. They are associated with the scalloped appearance of the pupillary ruff. *The '''structural folds of Schwalbe''' are radial folds extending from the border of the ciliary and pupillary zones that are much broader and more widely spaced, continuous with the "valleys" between the ciliary processes. *Some of the '''circular contraction folds''' are a fine series of ridges that run near the pupillary margin and vary in thickness of the iris pigment epithelium; others are in ciliary portion of iris.<ref name="iris1"/>
===Microanatomy=== [[File:Auge Iris.jpg|thumb|Light micrograph of the iris near to the pupil. ''M. sph.'' sphincter muscle, ''L'' lens]] [[File:Doppler holography of Olivier Martinache's myosis.gif|thumb|Constriction of the pupil ([[miosis]]) observed by [[laser Doppler imaging]] reveals radial vessels of the iris.]] [[File:Voluntary pupil dilation.gif|thumb|A human eye demonstrating its owner's rare ability to voluntarily dilate and constrict his pupil on command, via voluntary control of his iris muscles.]] [[File:SD OCT - Anterior Chamber Angle Cross-Section (with viewfinder).png|thumb|Anterior chamber cross-section imaged by an [[SD-OCT]].]] From anterior (front) to posterior (back), the layers of the iris are: *Anterior limiting layer *[[Stroma of iris]] *[[Iris sphincter muscle]] *[[Iris dilator muscle]] (myoepithelium) *Anterior pigment epithelium *Posterior pigment epithelium
===Development=== The stroma and the anterior border layer of the iris are derived from the [[neural crest]], and behind the stroma of the iris, the sphincter pupillae and dilator pupillae muscles, as well as the iris epithelium, develop from optic cup neuroectoderm.
== Function == [[File:Iris structure.png|thumb|Structure of the iris and surrounding parts showing the dilator and sphincter muscles (''dilator pupillae'' and ''sphincter pupillae'').]] The iris controls the size of the [[pupil]] by means of contracting the [[iris sphincter muscle]] and/or the [[iris dilator muscle]]. The size of the pupils is dependent on many factors (including light, emotional state, cognitive load, arousal, stimulation), and can range from less than 2 mm in diameter, to as large as 9 mm in diameter. However, there is considerable variation in maximal pupil diameter by individual humans, and decreases with age.<ref>{{cite web |title=Aging Eyes and Pupil Size |url=http://amateurastronomy.org/Events/EH361.html |url-status=dead |archive-url=https://web.archive.org/web/20131023134125/http://amateurastronomy.org/Events/EH361.html |archive-date=2013-10-23 |access-date=2013-08-28 |publisher=Amateurastronomy.org}}</ref><ref>{{cite journal |last1=Winn |first1=B. |last2=Whitaker |first2=D. |last3=Elliott |first3=D. B. |last4=Phillips |first4=N. J. |date=March 1994 |title=Factors Affecting Light-Adapted Pupil Size in Normal Human Subjects |url=http://www.iovs.org/content/35/3/1132.full.pdf |journal=Investigative Ophthalmology & Visual Science |volume=35 |issue=3 |pages=1132–1137 |pmid=8125724 |access-date=2013-08-28}}</ref> The irises also contract the pupils when [[Accommodation reflex|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.<ref>{{Cite journal |last1=Eberhardt |first1=Lisa V. |last2=Grön |first2=Georg |last3=Ulrich |first3=Martin |last4=Huckauf |first4=Anke |last5=Strauch |first5=Christoph |date=2021-10-01 |title=Direct voluntary control of pupil constriction and dilation: Exploratory evidence from pupillometry, optometry, skin conductance, perception, and functional MRI |url=https://dspace.library.uu.nl/bitstream/handle/1874/412825/1_s2.0_S0167876021008448_main.pdf?sequence=1&isAllowed=y |journal=International Journal of Psychophysiology |volume=168 |pages=33–42 |doi=10.1016/j.ijpsycho.2021.08.001 |pmid=34391820 |issn=0167-8760}}</ref> However, there is no clear purpose or advantage to this.
==Eye color== {{main|Eye color}} {{human-centric}} [[File:Eye colors map of Europe.png|thumb|Human eye pigmentation in Europe]] [[Image:Ageev iris.jpg|thumb|right|Among human phenotypes, blue-green-gray eyes are a relatively rare{{Citation needed|date=May 2023}} eye color and the exact color is often perceived to vary according to its surroundings.]] The iris is usually strongly [[pigment]]ed, 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]],<ref name="iris" /> or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, the only pigment that contributes substantially to normal iris color is the dark pigment [[melanin]]. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of an organism. Structurally, this huge molecule is only slightly different from its equivalent found in [[skin]] and [[hair]]. Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue- and green-eyed people. The [[limbal ring]] appears as a dark ring encircling the iris on some individuals, but is a result of the optical properties of the region between the [[cornea]] and [[sclera]], not of pigments in the iris.
===Genetic and physical factors determining iris color=== [[File:Human eye with limbal ring, anterior view.jpg|thumb|A light brown iris with prominent limbal ring. Light brown irises contain pheomelanin.]] Iris color is a highly complex phenomenon consisting of the combined effects of texture, pigmentation, fibrous tissue, and blood vessels within the iris [[stroma of iris|stroma]], which together make up an individual's [[epigenetic]] constitution in this context.<ref name="iris1"/> An organism's "eye color" is actually the color of one's iris, the [[cornea]] being transparent and the white [[sclera]] entirely outside the area of interest.
Melanin is yellowish to dark hazel in the stromal pigment cells, and black in the [[iris pigment epithelium]], which lies in a thin but very opaque layer across the back of the iris. 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.<ref name="iris1"/> The degree of dispersion of the melanin, which is in subcellular bundles called [[melanosomes]], has some influence on the observed color, but melanosomes in the iris of humans and other vertebrates are not mobile, and the degree of pigment dispersion cannot be reversed. Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color (see [[heterochromia]], below). Colors other than brown or black are due to selective reflection and absorption from the other stromal components. Sometimes, [[lipofuscin]], a yellow "wear and tear" pigment, also enters into the visible eye color, especially in aged or diseased green eyes.{{citation needed|date=November 2022}}
The optical mechanisms by which the nonpigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature. Simple selective absorption and reflection by biological molecules ([[hemoglobin]] in the blood vessels, [[collagen]] in the vessel and stroma) is the most important element. [[Rayleigh scattering]] and [[Tyndall effect|Tyndall scattering]], (which also happen in the sky) and [[diffraction]] also occur. [[Raman scattering]], and constructive [[Interference (wave propagation)|interference]], as in the feathers of birds, do not contribute to the color of the eye, but interference phenomena are important in the brilliantly colored iris pigment cells ([[iridophore]]s) in many animals. Interference effects can occur at both molecular and light-microscopic scales, and are often associated (in melanin-bearing cells) with quasicrystalline formations, which enhance the optical effects. Interference is recognised by characteristic dependence of color on the angle of view, as seen in [[eyespot (mimicry)|eyespot]]s of some [[butterfly]] [[wing]]s, 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. If melanin is deposited substantially, brown or black color is seen; if not, they will remain blue or gray.<ref name="eb">"Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 175 Encyclopædia Britannica, 1987</ref>
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=== {{main|Heterochromia}} [[Image:Heterochromia.jpg|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 I of the Byzantine Empire|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.<ref>{{Cite journal|last=Baldwin|first=Barry|title=Physical Descriptions of Byzantine Emperors|date=1981|journal=Byzantion|volume=51|issue=1|pages=8–21|jstor=44170668|issn=0378-2506}}</ref><ref>{{Cite journal|last1=Fronimopoulos|first1=John|last2=Lascaratos|first2=John|date=1992-03-01|title=Some Byzantine chroniclers and historians on ophthalmological topics|journal=Documenta Ophthalmologica|language=en|volume=81|issue=1|pages=121–132|doi=10.1007/BF00155022|pmid=1473460|s2cid=26240821|issn=1573-2622}}</ref>
In contrast, heterochromia and variegated iris patterns are common in veterinary practice. [[Siberian Husky]] dogs show heterochromia,<ref name="hc">{{Cite web |url=http://www.environmentalgraffiti.com/featured/heterochromia-animals/18455 |title = Heterochromia in Animals | website = Environmental Graffiti | last = Fabricius | first = Karl |access-date=2010-10-27 |archive-url=https://web.archive.org/web/20100923005934/http://www.environmentalgraffiti.com/featured/heterochromia-animals/18455 |archive-date=2010-09-23 |url-status=dead }}</ref>{{better source needed|reason=Original source lists three Wikipedia pages as sources, thus a possible source of [[citogenesis]]|date=March 2021}} 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.<ref name="hc"/> Striking variation within the same iris is also common in some animals, and is the norm in some species. Several herding breeds, particularly those with a [[Merle (coat color in dogs)|blue merle]] coat color (such as [[Australian Shepherd]]s and [[Border Collie]]s) may show well-defined blue areas within a brown iris, as well as separate blue and darker eyes.{{Citation needed|date=November 2010}} Some horses (usually within the white, spotted, palomino, or cremello groups of breeds) may show amber, brown, white and blue all within the same eye, without any sign of eye disease.{{Citation needed|date=November 2010}}
One eye with a white or bluish-white iris is also known as a "walleye".<ref>"walleye", def. 1a, Merriam-Webster Dictionary</ref>
==Clinical significance== *[[Angle closure glaucoma]] *[[Aniridia]] *[[Anisocoria]] *[[Horner's syndrome]] *[[Iridocyclitis]] *[[Iridoplegia]] *[[Iritis]] *[[Miosis]]/[[Mydriasis]] *[[Synechia (eye)|Synechia]] *[[Third nerve palsy]]
==Alternative medicine== ===Iridology=== {{main|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 [[:wikt:systemic|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.<ref>{{cite web|last1=Novella|first1=Steven|title=Iridology|url=https://sciencebasedmedicine.org/iridology/|website=Science-Based Medicine|access-date=20 August 2017|archive-url=https://web.archive.org/web/20170701050230/https://sciencebasedmedicine.org/iridology/|archive-date=1 July 2017|url-status=dead}}</ref>
Iridology is not supported by quality research studies,<ref name="Ernst">{{cite journal |author=Ernst E |title=Iridology: not useful and potentially harmful |journal=Arch. Ophthalmol. |volume=118 |issue=1 |pages=120–1 |date=January 2000 |pmid=10636425 |doi=10.1001/archopht.118.1.120|doi-access= }}</ref> and is considered pseudoscience.<ref name=nonsense>{{cite web|url=http://www.quackwatch.com/01QuackeryRelatedTopics/iridology.html|title=Iridology Is Nonsense|date=9 November 2015|access-date=6 August 2023|website=[[Quackwatch]]|author1=[[Stephen Barrett]]}}</ref>
==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. </gallery>
==See also== {{Portal|Anatomy}} *[[Blood–ocular barrier]] *[[Coloboma]] *[[Gonioscopy]] *[[Human eye]] *[[Iris recognition]]
==References== {{Reflist}}
==External links== {{Commons category|Iris (eye)}} *[https://web.archive.org/web/20090318053728/http://www.art-dept.com/artists/rankin/portfolio/specialprojects/eyescapes/portfolio.html?source=20i Detailed photographs of human irides] *{{BUHistology|08010loa}} *{{UMichAtlas|eye_1}} – "Sagittal Section Through the Eyeball"
{{Eye anatomy}} {{Authority control}}
[[Category:Iris (anatomy)| ]] [[Category:Eye color]]