Anatomy of Uvea by Parthopratim Dutta Majumder
Uvea is the middle vascular coat of eye ball. From anterior to posterior, the uvea or uveal tract can be divided into three parts- Iris, Ciliary body and choroid. The name “uvea” has originated from Latin word grape. Why a grape? If the stem is removed from a grape, the hole looks like the pupil and the grape the eyeball.
Iris is the anterior most part of the uveal tract. It is a thin and circular structure which forms a diaphragm like structure in front of the crystalline lens. The word “iris” has originated from a Greek word. In Greek mythology the iris is the name of Greek goddess of rainbow.
The diaphragm formed by iris contains a central aperture known as pupil. The location of the pupil is not exactly central, its little nasal to the center. The pupil determines the amount of light entering the eye. The normal size of pupillary aperture is 3-4 mm. Details on pupil will be discussed in chapter "Pupil".
Iris is attached to the middle of anterior surface of ciliary body. The iris divides the space in front of the lens into anterior chamber and posterior chamber.
Topography of IRIS: Average diameter of the iris is 10 to 11 mm. It is thickest at collarette, which is located approximately 1.5 mm from the pupillary margin and thinnest at iris root, the part of iris which joins with the ciliary body. The thickness of iris root is approximately 0.5 mm.
During blunt trauma, damage to iris occurs most commonly at the iris root, where the iris rips away from the ciliary body
Anterior surface of the iris is divided into a pupillary zone and a ciliary zone by a circular ridge, located 1.5 mm away from pupillary margin, called collarette (also known as iris frill).
Collarette is the site of foetal pupillary membrane attachment.
Pupillary Zone: Pupillary zone extends from pupillary margin to collarette. Pupillary zone is relatively flat. Pupillary margin is marked by a dark border, known as pupillary ruff. Pupillary ruff is the anterior termination of the pigmented layer, which lines the posterior surface of iris.
Ciliary Zone: Ciliary zone of iris extends from collarette to iris root. There are some depressions or pit arranged in rows present in this area known as crypts. Crypts are found in two locations. Those present near collarette are relatively larger and known as Fuchs’s crypt and few are seen in periphery of the iris.
In laser iridotomy, the opening is created in areas of iris crypts, as it requires less amount of energy in these thinnest areas of iris thickness.
Posterior surface of Iris: Posterior surface of the iris is much more uniform.Posterior surface of the iris is darker than the anterior surface and shows numerous radial contraction folds. However circular folds are also seen.