CorpusHuman Anatomy
Special SensesOrgan part

Iris

Iris

Also known as: sphincter pupillae, dilator pupillae, iris stroma

The pigmented, contractile diaphragm of the uvea in front of the lens; its two smooth muscles alter the diameter of the central aperture, the pupil, under autonomic control.

Show on body map Eye — Horizontal Section

Overview

The iris arises from the anterior ciliary body (iris root — its thinnest part, prone to tearing in trauma) and rests on the anterior lens surface. It separates the anterior from the posterior chamber, which communicate through the pupil. Its anterior surface shows a collarette dividing a pupillary zone from a ciliary zone, with radial ridges and crypts.

Layers (anterior → posterior): the anterior border layer (fibroblasts and melanocytes; no epithelium), the vascular stroma (loose connective tissue with melanocytes, vessels and the sphincter pupillae), the dilator pupillae (radially oriented myoepithelial processes of the anterior pigmented epithelium) and the posterior pigmented epithelium (two layers, heavily pigmented, continuous with the ciliary epithelium). Both muscles are unusual in being derived from neuroectoderm (the optic cup).

Eye colour depends mainly on the amount of melanin in the anterior border layer and stroma — the posterior epithelium is densely pigmented in everyone (except in albinism). Blue irides have little stromal melanin; short-wavelength light scattered by the stroma (Tyndall/Rayleigh-like scattering) against the dark posterior epithelium produces the blue colour. Iris colour can still darken in the first year of life.

Location

Coronal diaphragm between cornea and lens, attached peripherally to the ciliary body; its free margin bounds the pupil.

Parts & subdivisions

  1. Sphincter pupillae
    Circular ring of smooth muscle (≈0.75–1 mm wide) at the pupillary margin. Parasympathetic: Edinger–Westphal nucleus → CN III → ciliary ganglion → short ciliary nerves (M3 receptors). Contraction → miosis.
  2. Dilator pupillae
    Radial myoepithelial fibres. Sympathetic: hypothalamus → ciliospinal centre (C8–T2) → superior cervical ganglion → internal carotid plexus → long ciliary nerves (α₁ receptors). Contraction → mydriasis.
  3. Stroma and anterior border layer
    Loose vascular connective tissue with melanocytes and clump cells; determines eye colour.
  4. Posterior pigmented epithelium
    Two heavily pigmented layers preventing light passing through the iris; its edge forms the pupillary ruff.
  5. Major and minor arterial circles
    Major circle at the iris root (long posterior + anterior ciliary arteries); minor circle near the collarette.
  6. Pupil
    The central aperture, 2–8 mm in diameter.

Anatomy

Innervation

Sphincter pupillae — parasympathetic (CN III via ciliary ganglion and short ciliary nerves). Dilator pupillae — sympathetic (superior cervical ganglion via long ciliary nerves). Sensory — long ciliary nerves (V1).

Arterial supply

Major arterial circle of the iris (long posterior and anterior ciliary arteries), radial vessels to the minor circle.

Histology

No epithelium on the anterior surface; stroma of collagen, melanocytes and vessels; sphincter is true smooth muscle, dilator is myoepithelial; posterior surface covered by a double-layered pigmented epithelium.

Development

Stroma from neural crest; both pigmented epithelial layers and both pupillary muscles from the anterior rim of the optic cup (neuroectoderm).

Function

  • Regulate light entering the eye (≈16-fold range of pupil area)
  • Increase depth of focus and reduce optical aberrations when constricted
  • Absorb stray light (posterior pigment epithelium)

Clinical significance

Horner syndrome

Interruption of the oculosympathetic pathway (central — brainstem stroke; preganglionic — Pancoast tumour at the lung apex; postganglionic — carotid dissection, cavernous sinus lesions) causes ptosis, miosis and anhidrosis with enophthalmos-like appearance and dilation lag. The anisocoria is greater in dim light.

Iris colour changes

Heterochromia may be congenital (congenital Horner syndrome — lighter iris on the affected side; Waardenburg syndrome) or acquired (siderosis, iris melanoma, prostaglandin-analogue eye drops, which darken the iris). Lisch nodules (pigmented iris hamartomas) are a diagnostic criterion for neurofibromatosis type 1.

Aniridia

Absence of the iris due to PAX6 mutation; associated with foveal hypoplasia, glaucoma and, in the WAGR deletion syndrome, Wilms tumour.

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.