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Portrait of Iris

Anatomy · Eye, ear & senses

Iris

The coloured diaphragm of the eye

Eye · middle (vascular) coat

A thin, circular, pigmented diaphragm that forms the front of the uveal tract, suspended between the cornea and the lens with the pupil at its centre. Two smooth muscles change the size of the pupil, and the amount of melanin in its front layers determines eye colour. It attaches at its outer edge, the root, to the ciliary body.

How to spot them

  • ▸The coloured disc of the eye, pierced by the round pupil
  • ▸Sits between the cornea and the lens, dividing the front of the eye into anterior and posterior chambers
  • ▸Two muscles: a circular sphincter that narrows the pupil and radial dilator fibres that widen it
  • ▸Eye colour depends on melanin in the front layers, not on any blue pigment

What to know

Lies in the aqueous humour, splitting the front of the eye into two small chambers joined only through its central opening

The iris — the anterior and posterior chambers communicate through the pupil

Sphincter fibres in a ring supplied by the oculomotor nerve, and radial fibres supplied by the sympathetic system

Sphincter pupillae (parasympathetic) and dilator pupillae (sympathetic) of the iris

Blue eyes contain no blue pigment; the colour comes from light scattering through a nearly pigment-free front layer

The iris, whose colour reflects melanin content, not pigment

The part of the eye named for the Greek goddess of the rainbow, because of its range of colours

The iris — the goddess Iris, messenger of the gods, personified the rainbow

A pattern of crypts, furrows and a zigzag collarette so individual that a Cambridge algorithm of the 1990s uses it to identify people

Iris recognition, developed by John Daugman

Two eyes of different colours in one person, or two colours in one eye

Heterochromia of the iris

Congenital absence, in whole or in part, of this structure, often linked to a mutation in the PAX6 gene

Aniridia

Key ideas

  • The iris controls light entry like a camera diaphragm, but it also changes the depth of field: a smaller opening gives a sharper image over a greater range of distances.
  • If the iris bows forward against the drainage angle, aqueous cannot escape, the pressure of the eye rises and angle-closure glaucoma follows.

Quick-fire facts

  • •About 12 mm in diameter and only about 0.5 mm thick, with its outer edge (the iris root) attached to the ciliary body
  • •The collarette is a zigzag ridge about 1.5 mm from the pupil margin, dividing the pupillary zone from the ciliary zone
  • •Its colour depends on melanin in the anterior border layer and the stroma; brown is the commonest colour worldwide
  • •Sphincter pupillae: oculomotor nerve (parasympathetic, via the ciliary ganglion). Dilator pupillae: sympathetic fibres from the superior cervical ganglion
  • •It, the ciliary body and the choroid together form the uvea, the eye's middle coat; inflammation is called iritis or anterior uveitis
  • •John Daugman of Cambridge patented the iris-recognition algorithm in 1994; it underlies India's Aadhaar identification programme
  • •Brushfield spots are pale flecks in the iris associated with Down syndrome; Lisch nodules are iris hamartomas seen in neurofibromatosis type 1
  • •Surgical removal of a segment of the iris (iridectomy) can relieve angle-closure glaucoma

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