The iris is a part of the eye structure, referring to the colored ring around the black eyeball(pupil).

The color of the iris varies from person to person and is determined by the amount and distribution of melanin pigment.
When there is a lot of melanin, the pupil color becomes darker, resulting in black or brown eyes. On the other hand, if there is less melanin, the pupil color becomes lighter, resulting in blue or gray eyes.
Moreover, melanin absorbs harmful ultraviolet rays, helping to protect the eyes.

The main role of the iris is “adjusting the size of the pupil.”

• ・In bright environments, the iris contracts to make the pupil smaller, reducing the amount of incoming light.
• ・In dark environments, the iris expands to make the pupil larger, allowing more light to enter.

Through this function, the iris controls the amount of light reaching the retina, maintaining comfortable vision.

The iris is mainly composed of the following two muscles:

• ・Sphincter pupil muscle
  → Circular muscle that makes the pupil smaller, working in bright conditions.
• ・Dilator pupil muscle
  → Radial muscle that makes the pupil larger, working in dark conditions.

The patterns of the iris, like fingerprints, are unique to each person and are sometimes used in personal authe　ntication systems.

Thus, the iris not only controls the amount of light entering the eye but also determines eye color, playing a vital role in identity verification and protection against ultraviolet rays.