Henry Gray (18251861). Anatomy of the Human Body. 1918.
behind the peripheral part of the iris, and in front of the suspensory ligament of the lens and the ciliary processes. 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.
The color of the iris is produced by the reflection of light from dark pigment cells underlying a translucent tissue, and is therefore determined by the amount of the pigment and its distribution throughout the texture of the iris. The number and the situation of the pigment cells differ in different irides. In the albino pigment is absent; in the various shades of blue eyes the pigment cells are confined to the posterior surface of the iris, whereas in gray, brown, and black eyes pigment is found also in the cells of the stroma and in those of the endothelium on the front of the iris.
The iris may be absent, either in part or altogether as a congenital condition, and in some instances the pupillary membrane may remain persistent, though it is rarely complete. Again, the iris may be the seat of a malformation, termed coloboma, which consists in a deficiency or