The iris is the colored part of the eye that controls the amount of light that enters the eye. This is the most visible part of the eye. The iris is in front of the lens and separates the anterior chamber from the posterior chamber. The iris is part of the uveal tract, which includes the ciliary body, which is also behind the iris.
The tissue of the iris forms the pupil. The pupil is the opening in the iris through which light travels to the back of the eye. The iris controls the size of the pupil; the pupil is actually positioned so that its center is slightly lower and slightly closer to the nasal side of the center of the cornea.
The size of the pupil is controlled by two muscles of the iris. The sphincter pupils surround the edge of the pupil, and when it narrows, the pupil shrinks in size. This is called miosis. The second muscle that controls the size of the pupil is the dilating pupils. This muscle contains fibers located radially in the iris. When it narrows, the pupil dilates or enlarges. This is called mydriasis .
The parasympathetic system controls the sphincter pupils and the sympathetic system controls the dilated pupils. There is a connection between these muscles in that the dilator muscle must relax for the sphincter to contract the pupil. The normal size of the pupil ranges from 2 to 4 mm in diameter in the light to 4-8 mm in the dark .
The color of the iris depends on the amount of melanin pigment in the iris. A person with brown eyes has the same melanin pigment color as a person with blue eyes. However, the blue eye pigment is much less. The back of the iris is often heavily pigmented to prevent light from passing through the iris.
Iris color inheritance patterns are a well-studied area. Eye color is controlled by three main genes. Researchers know two of these genes very well, and one of them remains a mystery. These genes control the development of green, brown, and blue eyes. Gray, hazelnut, and other combinations are harder to predict. In some families, the inheritance of eye color follows very predictable patterns, while in other families it does not make sense and does not follow any rules. In genetics, this is called "polygenic." Polygenic means that several complex genes may be involved that interact to create eye color. The simple claim that brown can prevail over blue provides easy explanations, but this model is too simplistic for all the variations seen in real life.
Iris and pupil abnormalities
Disorders of the iris and pupil include:
- Aniridia : Aniridia is a genetic defect in which a person is born with an iris.
- Coloboma : The coloboma of the iris is a large hole in the iris.
- Synechiae : Synechiae are adhesions that arise between the lens and the iris.
- Corectopia : Corectopia is when the pupil is off-center
- Discoria : discoria is a condition in which the pupil is distorted or irregular and does not dilate normally.