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The iris (plural: irises or irides) is a pigmented muscular structure which modifies the amount of light entering the eye, by controlling the size of the pupil, its central aperture.
- location: between the anterior and posterior chambers of the globe
- function: controls the amount of light entering the eye
- blood supply and drainage: anterior ciliary vessels
The iris is a ring-shaped structure, secured to the inner aspect of the ciliary body, which encircles it. The iris has a central aperture, the pupil, the variable diameter of which modifies the quantity of light reaching into the eye.
Most of the iris comprises vascularized stroma with scattered melanocytes. The coloration of the iris varies with its pigment content. At birth the iris is blue, due to absence of pigment. As the infant grows, pigment cells may proliferate, resulting in darker iridal colors.
The iris has radially-arranged dilator pupillae muscles, and concentrically-arranged sphincter pupillae muscles. These muscles dilate and constrict the pupil, in response to sympathetic and parasympathetic nerve impulses, from the long ciliary nerve and oculomotor nerve (III), respectively.
- anterior: anterior chamber and cornea
- posterior: posterior chamber and lens
- in vivo, the margin of the pupil is in contact with the lens
- lateral: ciliary body
- medial: pupil
- anterior ciliary veins
- there is no lymphatic drainage of the globe or its parts
Most of the iris comprises markedly vascularized loosely-compacted stroma, with melanocytes dispersed throughout the tissue. The anterior aspect of the iris is uneven, formed by an incomplete layer of fibroblasts and melanocytes. Conversely its posterior aspect is fairly even, formed by an epithelial layer, which originates embryologically in continuity with the double layer of cuboidal epithelium lining the ciliary body.
The superficial layer of this iridal epithelium is so heavily pigmented (cf. non-pigmented in the ciliary body) that the cells can no longer be distinguished from one another. The deeper layer consists of radially-oriented myoepithelial cells which are only lightly colored and constitute the dilator pupillae muscle.
The constrictor pupillae muscle is formed by intrastromal muscle fibers located near the inner margin of the iris, which are laid down concentrically.
The coloration of the iris relates to the quantity of pigment in the anterior stroma, which shows individual variation across the population. The melanocytes of the posterior epithelium exhibit little variation in pigmentation between individuals. People lacking stromal pigment have blue eyes, conversely those whose stroma is hyperpigmented have brown eyes.
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- 3. John W. Heath (Ph. D.). Wheater's Functional Histology. (1993)