The root of the iris is the thinnest and most peripheral. Radial ridges extend from the periphery to the pupillary zone, to supply the iris with blood vessels.
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It is typically defined as the region where the sphincter muscle and dilator muscle overlap. The collarette is a vestige of the coating of the embryonic pupil. The collarette is the thickest region of the iris, separating the pupillary portion from the ciliary portion.
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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, pulling it in folds. The iris consists of two layers: the front pigmented fibrovascular layer known as a stroma and, beneath the stroma, pigmented epithelial cells. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm. Eye color is defined by that of the iris. In humans and most mammals and birds, the iris (plural: irides or irises) is a thin, annular structure in the eye, responsible for controlling the diameter and size of the pupil, thus the amount of light reaching the retina.