The iris is the “colored” muscle inside the eye that controls pupil size. Some people have brown irises and others have blue. The color of the eye is determined by the amount of pigment in the iris, with dark brown eyes having more pigment than lighter eyes. The iris has rings of muscle fiber that contract and changes the pupil shape in response to light entering the eye. When inflamed, a condition called iritis, the iris can cause pain and sensitivity to light (photophobia). The iris is thin, like a drumhead, and very mobile. If the iris bows forward, the trabecular meshwork (the drain of the eye) may become blocked, leading to angle closure and acute glaucoma. Floppy iris syndrome (usually caused by urinary medications like Flomax) can cause difficulties during cataract surgery. Pupil abnormalities can occur from nerve blockage to the iris muscle from many sources including Horner’s syndrome, Adie’s pupil, third nerve palsy, or pharmacologic dilation.