The colored ring of muscle in the eye that surrounds and controls the size of the pupil.
Structure
The iris contains two smooth muscle layers: the circular sphincter pupillae that constricts the pupil and the radial dilator pupillae that opens it. Pigmented cells in its stroma determine eye color.
Function
In bright light the sphincter contracts to narrow the pupil and reduce glare. In dim light the dilator pulls the pupil open to gather more light.
Clinical note
Pupil size and reactivity are key parts of the neurological exam, since they reflect autonomic function and brainstem health.
Find more body parts by letter
Iris starts with I and ends with S. Browse other body parts along the same letter.
Body Parts that contain a letter from "Iris":