iris /ī′ris/ [Gk, rainbow] , an annular, colored membrane shaped like a disc, suspended in aqueous humor between the cornea and the crystalline lens of the eye and enclosing a circular pupil. Smooth muscle fibers of the iris contract and relax to allow more or less light to enter the eye through the pupil. The periphery of the iris is continuous with the ciliary body and is connected to the cornea by the pectinate ligament. The iris divides the space between the lens and the cornea into an anterior and a posterior chamber. The involuntary muscle of the iris is composed of circular fibers and radiating fibers. Dark pigment cells under the translucent tissue of the iris are variously arranged in different people to produce different colored irises. The pigment is absent in albinos. In blue eyes the pigment cells are confined to the posterior surface of the iris, but in gray eyes, brown eyes, and black eyes the pigment cells appear in the anterior layer of epithelium and in the stroma. See also dilator pupillae, sphincter pupillae. −iridic, adj.
