Overview of the Eye's Iris

The iris is the colored part of the eye that controls the amount of light that enters into the eye. It is the most visible part of the eye. The iris lies in front of the crystalline lens and separates the anterior chamber form the posterior chamber. The iris in part of the uveal tract which includes the ciliary body that also lies behind the iris.

The iris tissue makes up the pupil. The pupil is the hole in the iris in which light passes through to the back of the eye. The iris controls the pupil size. The pupil is actually located with its center a little below and slightly to the nasal side of the center of the cornea.

The size of the pupil is controlled by two muscles in the iris. The sphincter pupillae surrounds the border of the pupil and when it contracts, the pupil decreases in size. This is called miosis. The second muscle that controls pupil size is the dilator pupillae. This muscle contains fibers that are arranged in a radial pattern in the iris. When it contracts, the pupil dilates or increases in size. This is called mydriasis. 

The parasympathetic system controls the sphincter pupillae and the sympathetic system controls the dilator pupillae. There is a connection between these muscles in that that the dilator muscle must relax to allow the sphincter to constrict the pupil. Normal pupil size ranges from 2 to 4 mm in diameter in the light to 4 to 8 mm in the dark.

Iris color depends on the amount of melanin pigment in the iris. A person with brown eyes has the same color of melanin pigment that a person with a blue eye has. However, the blue-eyed person has much less pigment. The back of the iris is usually heavily pigmented to prevent light from shining through the iris.

The inheritance patterns of iris color is a heavily studied area. Eye color is controlled by three basic genes. Researchers understand two of those genes really well and one of them is still a bit of a mystery. These genes control the development of green, brown and blue eye color. Gray, hazel, and other combinations are more difficult to predict. In some families, eye color inheritance follows very predictable patterns whereas, in other families, it doesn't seem to make sense or follow any rules. In genetics, this is termed "polygenic." Polygenic means that there may be several complicated genes involved that interact to create eye color. Just stating that brown may be dominant to blue does make for easy explanations, but this model is too simplistic for all of the variations seen in real life.

Iris and pupil disorders include:

• Aniridia - Aniridia is a genetic defect in which the person is born with an iris.
• Coloboma - An iris coloboma is a large hole in the iris
• Synechiae - Synechia is adhesions that occur between the lens and the iris
• Corectopia - Corectopia is where the pupil is off-center
• Dyscoria - Dyscoria is a disorder where the pupil is distorted or irregular and does not dilate normally