Iris

The iris is the colored muscular tissue surrounding the pupil. Its muscles change pupil diameter to regulate how much light enters the eye.

Pupillary light reflex: A reflex that changes pupil size in response to changes in light. This reflex activates iris muscles to constrict the pupil in bright conditions.
Pupil: The variable opening at the center of the iris through which light enters the eye. The iris changes this opening's diameter rather than acting as a transparent lens.
Iris recognition: A biometric identification method that matches patterns in the iris. Its distinctive tissue patterns can identify a person independently of pupil size.
Lens (anatomy): A transparent structure behind the iris that changes shape to focus light on the retina. Unlike the iris, the lens focuses light by changing curvature rather than adjusting an opening.
Sphincter pupillae muscle: A circular smooth muscle in the iris that constricts the pupil. Its contraction narrows the pupil, especially in bright light and during near focus.
Cornea: The transparent front surface of the eye that provides much of its focusing power. Light reaches the pupil after passing through the cornea.
Aniridia: A rare disorder characterized by complete or partial absence of the iris. It shows how loss of iris tissue affects light control and vision.
Choroid: A vascular, pigmented layer between the retina and the outer coat of the eye. It is also pigmented, but lies behind the iris and nourishes the retina.
Dilator pupillae muscle: A radial smooth muscle in the iris that enlarges the pupil. Its contraction pulls the pupil wider in dim light.
Ciliary body: A ring of tissue behind the iris that produces aqueous humor and helps control lens shape. It joins the iris at its root and shares the eye's anterior anatomy.

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