- The iris is the colored, circular structure of the eye that surrounds the pupil and controls the amount of light entering the eye by adjusting the size of the pupil.
- It functions much like the aperture of a camera, expanding or contracting to regulate light exposure to the retina, where visual information is processed.
- While most commonly known for giving eyes their distinctive color—ranging from brown, blue, green, to hazel—the iris is far more than a cosmetic feature; it is a vital component of the eye’s optical and protective systems.
- Anatomically, the iris is a thin, flat, and muscular structure located between the cornea and the lens, forming the anterior boundary of the eye’s uveal tract, which also includes the ciliary body and the choroid. It consists of two main layers: the stroma, which contains blood vessels, pigment cells (melanocytes), and connective tissue; and the pigmented epithelium, a darker, posterior layer that prevents excess light from entering the eye. Embedded within the stroma are two groups of smooth muscles: the sphincter pupillae, which constricts the pupil in bright light (pupillary miosis), and the dilator pupillae, which enlarges the pupil in dim light (pupillary mydriasis). These muscles are controlled by the autonomic nervous system, allowing the iris to respond reflexively to changes in lighting and emotional stimuli.
- The color of the iris is determined by the density and distribution of melanin in the stroma. Brown eyes contain a high concentration of melanin, while blue and green eyes have less. Although eye color is largely genetic, it may change in early childhood and is also influenced by the scattering of light (Rayleigh scattering) in the iris tissues. In certain medical conditions, such as albinism, the lack of melanin in the iris results in a translucent appearance and increased light sensitivity (photophobia).
- The iris also plays a role in protecting the retina from overexposure to bright light, and in enhancing visual acuity by improving the eye’s depth of field. Disorders of the iris can impair these functions. For example, aniridia is a rare congenital condition in which the iris is partially or completely absent, leading to vision problems. Iritis or anterior uveitis, the inflammation of the iris, can cause pain, light sensitivity, redness, and blurred vision. Trauma to the iris or diseases such as glaucoma and cataracts may also affect its shape and function. Additionally, heterochromia iridum, a condition in which the two irises are different colors (or one iris has multiple colors), may be congenital or caused by injury or disease.
- In ophthalmology, examination of the iris can provide diagnostic clues to systemic and ocular conditions. Techniques such as slit-lamp examination and pupillary light reflex tests help assess the integrity of the iris muscles and their neurological connections. In some cases, iris recognition technology is used in biometric security systems due to the iris’s unique and stable pattern in each individual.
