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In anatomy, the iris (correct classical plural, irides) is the most visible part of the eye of vertebrates, including humans. Some cephalopods also have an iris in their eyes, which evolved separately from the vertebrate iris. The following covers only the iris of vertebrates.
It is an annulus (a flattened ring) of pigmented fibrovascular tissue, the stroma, which also contains bundles of smooth muscle to contract the pupil (the sphincter muscle) and other muscle fibres to dilate the pupil. The back surface is covered by a two-cell thick epithelial layer, the iris pigment epithelium, but the front surface has no epithelium. 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 through which the aqueous humour constantly drains out of the eye, with the result that diseases of the iris often have important effects on intra-ocular pressure, and indirectly on vision.
The iris is usually strongly pigmented, with colours ranging from brown to green, blue, gray, and hazelThis article is about the tree; for other meanings of hazel see Hazel (disambiguation). The hazels are a genus of about ten species of deciduous trees and large shrubs native to the temperate northern hemisphere. The botanical name is Corylus and is place. Occasionally its colour is due to lack of pigmentation, as in the pinkish-white of oculo-cutaneous albinismAlbinism is a genetic condition resulting in a lack of pigmentation in the eyes, skin and hair. It is an inherited condition arising from the combination of recessive genes passed from both parents of an individual. A variety of problems with photosensiti, or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colours, there is only one pigment that contributes substantially to normal human iris colour, the dark pigment called melaninMelanin is a pigment that affects skin, eye, and hair colour in humans and other mammals. It is produced by melanocytes, which are found in the bottom layer of the skin's epidermis. Some individual animals and humans have no or very little melanin in thei. Structurally, this huge molecule is only slightly different from its equivalent found in skinThis article is about skin in the biological sense. For other uses of skin see skin (computing) and skinhead. Regarding exposure of the skin to be seen by others, or prevention of this, see nudity, nudism, modesty, clothing. Fruits such as oranges also ha and hairHair is also a musical: see Hair (musical) and Hair (movie Hair is the filamentous outgrowth of the epidermis found in mammals. Hair is a characteristic of all mammals, though in some species hair is absent at certain stages of life. Hairs" are also found.
Despite periodical reports that specific eye colour genes had been "discovered" (but not so far independently confirmed), iris colour remains a highly complex phenomenon due to the combined effects of texture, pigmentation, fibrous tissue and blood vessels within the iris stroma, which together make up an individual's epigenetic constitution. A person's "eye colour" is actually the colour of one's iris, since the cornea is transparent and the white sclera lies entirely outside the area of interest. It is a common misconception that the iris colour is all due to its melanin pigment, but this varies only from brown to black.
Melanin is yellowish-brown to dark brown in the stromal pigment cells, and black in the iris pigment epitheliumAnatomy The iris pigment epithelium (IPE) is a two-cell thick layer of cuboidal cells lying behind the iris. Both layers are jet black due to the numerous large melanosomes which pack the cytoplasm of each cell. Towards the central axis, the IPE terminate, which lies in a thin but very opaque layer across the back of the iris. Most human irides also show a condensation of the brownish stromal melanin in the thin anterior border layer , which by its position has an undue influence on the overall colour. The degree of dispersion of the melanin, which is in subcellular bundles called melanosomes, has some influence on the observed colour, but melanosomes in the iris of man and other vertebrates are not mobile, and we cannot reversibly change the degree of pigment dispersion (so that we cannot flash coloured messages with our irises, the way that squid can flash changes in skin colour!). Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris colour (see heterochromia, below). Colours 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 colour, especially in aged or diseased green eyes (but not in healthy green human eyes).
The optical mechanisms by which the non-pigmented stromal components influence eye colour are complex, and many erroneous statements exist in the literature. Simple selective absorption and reflection by biological molecules (hæmoglobin in the blood vessels, collagen in the vessel walls and stroma) is the most important element. Rayleigh scattering (which happens in the sky), Tyndall scattering, and diffraction also occur. Raman scattering (named after Chandrasekhar Venkata Raman ), and constructive interference , as in the feathers of birds, do not contribute to the colour of the human eye, but interference phenomena are important in the brilliantly coloured iris pigment cells (iridophores) in many animals. Interference effects can occur at both molecular and light microscopic scales, and are often associated (in the melanin-bearing cells) with quasi-crystalline formations which enhance the optical effects. Interference is recognised by characteristic dependence of colour on the angle of view, as seen in eye spots of some butterfly wings, although the chemical components remain the same.