The iris has received its name from its various colors in different individuals. It is a thin, circular, contractile disk, suspended in the aqueous humor between the cornea and lens, and perforated a little to the nasal side of its center by a circular aperture, the pupil.
By its periphery it is continuous with the ciliary body, and is also connected with the posterior elastic lamina of the cornea by means of the pectinate ligament; its surfaces are flattened, and look forward and backward, the anterior toward the cornea, the posterior toward the ciliary processes and lens.
The iris divides the space between the lens and the cornea into an anterior and a posterior chamber:
In the adult the two chambers communicate through the pupil, but in the fetus up to the seventh month they are separated by the membrana pupillaris.
Structure.—The iris is composed of the following structures:
1. In front is a layer of flattened endothelial cells placed on a delicate hyaline basement membrane. This layer is continuous with the endothelium covering the posterior elastic lamina of the cornea, and in individuals with dark-colored irides the cells contain pigment granules.
2. The stroma (stroma iridis) of the iris consists of fibers and cells. The former are made up of delicate bundles of fibrous tissue; a few fibers at the circumference of the iris have a circular direction; but the majority radiate toward the pupil, forming by their interlacement, delicate meshes, in which the vessels and nerves are contained. Interspersed between the bundles of connective tissue are numerous branched cells with fine processes. In dark eyes many of them contain pigment granules, but in blue eyes and the eyes of albinos they are unpigmented.
3. The muscular fibers are involuntary, and consist of circular and radiating fibers. The circular fibers form the Sphincter pupillæ; they are arranged in a narrow band about 1 mm. in width which surrounds the margin of the pupil toward the posterior surface of the iris; those near the free margin are closely aggregated; those near the periphery of the band are somewhat separated and form incomplete circles. The radiating fibers form the Dilatator pupillæ; they converge from the circumference toward the center, and blend with the circular fibers near the margin of the pupil.
4. The posterior surface of the iris is of a deep purple tint, being covered by two layers of pigmented columnar epithelium, continuous at the periphery of the iris with the pars ciliaris retinæ. This pigmented epithelium is named the pars iridica retinæ, or, from the resemblance of its color to that of a ripe grape, the uvea.
Vessels and Nerves.—The arteries of the iris are derived from the long and anterior ciliary arteries, and from the vessels of the ciliary processes. Each of the two long ciliary arteries, having reached the attached margin of the iris, divides into an upper and lower branch; these anastomose with corresponding branches from the opposite side and thus encircle the iris; into this vascular circle (circulus arteriosus major) the anterior ciliary arteries pour their blood, and from it vessels converge to the free margin of the iris, and there communicate and form a second circle (circulus arteriosus minor).
The nerves of the choroid and iris are the long and short ciliary; the former being branches of the nasociliary nerve, the latter of the ciliary ganglion. They pierce the sclera around the entrance of the optic nerve, run forward in the perichoroidal space, and supply the bloodvessels of the choroid. After reaching the iris they form a plexus around its attached margin; from this are derived non-medullated fibers which end in the Sphincter and Dilatator pupillæ their exact mode of termination has not been ascertained. Other fibers from the plexus end in a net-work on the anterior surface of the iris. The fibers derived through the motor root of the ciliary ganglion from the oculomotor nerve, supply the Sphincter, while those derived from the sympathetic supply the Dilatator.