Iris

At a glance

English

Iris

Latin (TA2)

Iris

System

Eye

Region

Head

Side

Paired, left and right

About the iris

The iris is the most forward part of the uvea, the vascular middle coat of the eye. It is a thin, circular sheet suspended in the aqueous humour, with the anterior chamber in front and the posterior chamber and lens behind. Its outer edge, the root, is its thinnest point and is fixed to the front of the ciliary body just behind the corneoscleral junction. Its free inner edge outlines the pupil and rests lightly on the front of the lens. Together with the ciliary body it forms the anterior uvea.

Two main layers make it up. In front lies a loose, pigmented stroma of connective tissue, blood vessels and melanocytes, with no covering epithelium, so aqueous humour can enter it through pits called crypts. Behind lies a double layer of densely pigmented epithelium that stops light passing anywhere but through the pupil. A thickened ring, the collarette, divides the front surface into an inner pupillary zone and a broader outer ciliary zone. Eye colour depends mostly on how much melanin the stroma holds and how it scatters light.

Two smooth muscles change the pupil. The sphincter pupillae, a ring near the pupillary margin, narrows it under parasympathetic control from the oculomotor nerve, and the dilator pupillae, a thin radial sheet derived from the back epithelium, widens it under sympathetic control. Only the inner, pupillary circumference changes in size; the root stays fixed. The pupil thus works like the aperture of a camera, shrinking in bright light and for near work, and enlarging in the dark.

The iris matters in disease of the whole front of the eye. Its root lies just behind the trabecular meshwork, so a forward-bowed or thickened iris can block drainage and cause angle-closure glaucoma, which may be relieved by making a small hole in it (iridotomy). Inflammation, iritis, makes the eye red and painful and the pupil small. Unequal pupils or loss of the light reflex point to nerve lesions such as oculomotor palsy or Horner syndrome.

Function and supply

Function

Regulates the size of the pupil, and so how much light gets to the retina and the depth of focus, while its pigment blocks stray light.

Nerve supply

Parasympathetic fibres from the oculomotor nerve via the ciliary ganglion and short ciliary nerves to the sphincter; sympathetic fibres from the superior cervical ganglion to the dilator; sensory fibres from the nasociliary nerve.

Blood supply

The major and minor arterial circles of the iris, formed by the long posterior and anterior ciliary arteries from the ophthalmic artery; veins drain to the vorticose veins.