The iris is the coloured, ring-shaped diaphragm at the front of the eye, with the pupil at its centre. It sits between the cornea and the lens and divides the space in front of the lens into anterior and posterior chambers. Two muscles in it, the sphincter and dilator pupillae, change pupil size.
| Location | Front of the lens, behind the cornea, attached peripherally to the ciliary body |
|---|---|
| Parts |
|
| Nerve Supply | Short ciliary nerves (parasympathetic to sphincter pupillae); Long ciliary nerves (sympathetic to dilator pupillae) |
| Blood Supply | Major arterial circle of the iris (from long posterior and anterior ciliary arteries) |
Controls how much light enters the eye by changing pupil size, and its pigment blocks light from passing anywhere except through the pupil.
Eye colour depends on the amount of melanin in the iris stroma. In angle-closure glaucoma the peripheral iris blocks drainage at the iridocorneal angle, and a laser iridotomy creates a bypass hole. Iris abnormalities such as heterochromia can point to Horner syndrome from early childhood, and Lisch nodules suggest neurofibromatosis type 1.
Inflammation of the iris (anterior uveitis) with eye pain, light sensitivity and a small pupil. It is treated with steroid and dilating drops.
Tearing of the iris root from the ciliary body after blunt trauma, producing a D-shaped pupil and sometimes double vision.
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