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Iris

The iris is a membrane in the eye, responsible for controlling the amount of light reaching the retina. The iris consists of pigmented fibrovascular tissue known as a stroma. It is the most forward portion of the eye and the only one seen on superficial inspection. The stroma connects a sphincter muscle (sphincter pupillae), which contracts the pupil, and a set of dilator muscles (dilator pupillae) which open it. The back surface is covered by a heavily pigmented epithelial layer two cells thick (the iris pigment epithelium), but the front surface has no epithelium. The high pigment content blocks light from passing through the iris and restricts 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 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. Depending on the amount of light, the iris makes the pupil larger or smaller. The plural form of iris is irides.

The iris is divided into two major regions:

1 - The capilary 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 region of the iris separating the pupillary portion from the ciliary portion. It is typically defined as the region where the sphincter muscle and dilator muscle overlap.

The iris is usually strongly pigmented, with colors ranging from brown to green, blue, grey, and hazel. Occasionally its color is due to lack of pigmentation, as in the pinkish-white of oculo-cutaneous albinism, or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, there is only one pigment that contributes substantially to normal human iris color, the dark pigment called melanin. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair.

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