Iris (anatomy)
Ring-shaped eye structure controlling pupil size and color.
The iris is a thin, ring-shaped part of the eye found in most mammals and birds. Its main job is to control the pupil’s diameter—and therefore how much light hits the retina. Think of the pupil as the eye’s aperture and the iris as its diaphragm. The iris also determines eye color.
The word “iris” comes from the Greek word for “rainbow,” and from Iris, the rainbow goddess in the *Iliad*, because human irises can show so many colors.
The iris has two layers: a front, pigmented, fibrovascular layer called the stroma, and behind it, pigmented epithelial cells. The stroma connects to two muscles: the sphincter pupillae, which contracts the pupil in a circular motion, and the dilator pupillae, which pulls the iris outward radially to enlarge the pupil, creating folds. These two muscles oppose each other. When the pupil changes size, only its inner border moves; the outer border stays fixed. The constricting muscle sits on that inner border.
The back of the iris is covered by a two-cell-thick, heavily pigmented epithelial layer (the iris pigment epithelium), but the front surface has no epithelium—that anterior surface projects as the dilator muscles. This heavy pigmentation blocks light from passing through the iris to the retina, forcing light through the pupil. The outer edge, called the root, attaches to the sclera and the anterior ciliary body. Together, the iris and ciliary body form the anterior uvea. Just in front of the root is the trabecular meshwork, where aqueous humor drains out of the eye. Because of this, iris diseases often affect intraocular pressure and, indirectly, vision. The iris and anterior ciliary body also provide a secondary drainage route for aqueous humor.
The iris has two major regions: the pupillary zone (the inner region whose edge forms the pupil’s boundary) and the ciliary zone (the rest of the iris, extending to its origin at the ciliary body). The collarette is the thickest part of the iris, separating these two zones; it’s a remnant of the embryonic pupil’s coating and is where the sphincter and dilator muscles overlap. Radial ridges run from the periphery to the pupillary zone, supplying blood vessels. The root is the thinnest and most peripheral part.
Iris muscle cells are smooth muscle in mammals and amphibians, but striated muscle in reptiles (including birds). Many fish lack these muscles entirely, so their irises can’
- field
- Anatomy, Ophthalmology
- known_for
- Controlling pupil size and determining eye color
- structure
- Two layers: front pigmented fibrovascular stroma and posterior pigmented epithelial cells
- muscles
- Sphincter pupillae (constricts) and dilator pupillae (dilates)
- color_pigment
- Melanin (eumelanin and pheomelanin)
Lore & Background
The iris consists of two layers: the front pigmented fibrovascular layer known as a stroma and, behind the stroma, pigmented epithelial cells. The stroma is connected to a sphincter muscle (sphincter pupillae), which contracts the pupil in a circular motion, and a set of dilator muscles (dilator pupillae), which pull the iris radially to enlarge the pupil. The sphincter pupillae is the opposing muscle of the dilator pupillae. The outer border of the iris does not change size. The back surface is covered by a heavily pigmented epithelial layer that is two cells thick, but the front surface has no epithelium. The high pigment content blocks light from passing through the iris to the retina, restricting it to the pupil. The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body. The iris and ciliary body together are known as the anterior uvea. The iris is divided into two major regions: the pupillary zone and the ciliary zone, separated by the collarette, the thickest region of the iris. The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in reptiles (including birds). Many fish have neither, and their irises are unable to dilate and contract.
Reader's Guide
The iris is significant as the primary regulator of light entry into the eye, controlling pupil diameter from less than 2 mm to as large as 9 mm. Its pigmentation determines eye color, ranging from brown to blue, with melanin as the only substantial pigment. The iris also plays a role in aqueous humour drainage; diseases of the iris often affect intraocular pressure and vision. The iris's complex structure includes crypts, folds, and ridges that vary among individuals. Its development from neural crest and optic cup neuroectoderm underscores its embryological importance. The iris's ability to constrict and dilate in response to light, emotional state, and cognitive load demonstrates its integration with the nervous system. Very few humans can voluntarily control their iris muscles, though no clear purpose or advantage exists for this ability.
Did You Know?
- The word 'iris' is derived from the Greek word for 'rainbow' and the goddess Iris, due to the many colors the human iris can take.
- The iris consists of two layers: the front pigmented fibrovascular stroma and the posterior pigmented epithelial cells.
- The sphincter pupillae muscle contracts the pupil in a circular motion, while the dilator pupillae pulls the iris radially to enlarge the pupil.
- Many fish lack iris muscles, so their pupils always remain a fixed size.
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