Iris

The iris is the most anterior part of the vascular layer of the eyeball, it is a diaphragm with a central opening, the pupil, which regulates the amount of light entering the eye. The iris is a large circle that lies between the lens and the cornea, separating the anterior and posterior chambers of the eyeball.

The iris is continuous with the ciliary body around its periphery. It attaches to the corneal limbus at the iridocorneal angle.

It is coronal in shape, with its faces oriented anteriorly and posteriorly.

The anterior surface is covered with folds that are more numerous on the periphery, on the outer border of iris, than on the inner border of iris. The posterior surface of the iris, covered by the iridial part of retina, and part of the nonvisual retina. It is covered by pigmented epithelium overlying the myoepithelial cells forming the dilator pupillae.

Within the iris is the stroma, made up of collagen fibers and pigment cells that give the iris its color. It contains the many blood vessels of the iris, including the major circulus arteriosus of iris and the minor circulus arteriosus of iris in species where it exists. Within the pupillary margin, the circular fibers of the sphincter pupillae muscle pass through the iris stroma.