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Eye Anatomy: Iris and Pupil

The eye’s iris and pupil are the most visible or obvious eye structures that people see immediately when they look at you. The iris is the colored part around the black pupil. It is a flat structure and each iris is unique in its color, patterns and texture. Your two irises can identify you as definitely as your fingerprints can.

Iris Color

Eye color is genetic, with blue (low pigment amount) being recessive. It is not fully understood how many genes contribute to eye color. There are three inherent colors in the iris that determine the particular color your eyes appear to have: blue, brown, and yellow. The pigment involved is melanin, which can be of differing shades from yellow-brown to black. An iris with no pigment is naturally a pale blue, as can be seen in an albino person’s eyes.

Brown is the most common eye color and green is the least common. There are also unusual genetic mutations that can create shades of red or violet. The shades of light brown, hazel, green, and gray are created by different degrees of melanin, of different shades, in varying distributions within the iris.

Aside from whatever pigment is present, eye color is also affected by the presence of blood vessels and the nature of fibrous tissue in the iris. Ambient lighting can change one’s subjective impression of a person’s eye color, as can the color of upper body clothing and jewelry and use of eye makeup.

Melanin also provides skin and hair color although its structure is slightly different in the iris.

Pupil Size

The iris has an opening in the center called the pupil. It looks black. In the iris are two tiny muscles:

  • A sphincter muscle around the edge of the pupil – it acts to constrict the pupil in bright light, restricting how much light can enter the eye and travel through the lens to the retina. Constricting the pupil makes the iris larger.
  • A dilator muscle structured radially like spokes on a wheel – it widens (dilates) the pupil in dim light, to maximize how much light can enter the eye, and this reduces the size of the iris.

Pupil diameter can range between two and eight millimeters. This means that it can change how much light is entering the eye by a factor of 30.

When your eye doctor examines your eyes, he may first administer eyedrops to dilate the pupils. This gives him or her a wider opening through which to view the eye’s internal structures.

Pupil size is a factor in LASIK candidacy. Laser vision correction is done on the cornea, which is directly over the iris and pupil. If a person’s pupils dilate in dim light to an extra-large size, there may not be enough corneal area for adequate vision improvement. The pupil might dilate to admit light which has passed through the untreated corneal area, and this would give distorted vision.

For more about eye anatomy, please refer to Eye Anatomy: Overview.

Please visit our Eye Doctor Directory if you would like to speak with a qualified ophthalmologist in your area.

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