Iris: Definition & Function

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  • 0:00 Snowflakes And Uniqueness
  • 0:35 Iris Of The Human Eye
  • 2:07 How Does The Iris Work?
  • 2:55 Uniqueness Of The Iris
  • 3:33 Lesson Summary
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Lesson Transcript
Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

The iris of the human eye is unique in each individual. What makes it unique? What is it made of? What does it do? Learn the answer to those questions here.

Snowflakes and Uniqueness

Have you ever heard the expression that someone is a special snowflake? This expression stems from the very nature of snowflakes - that each one is entirely unique. But snowflakes aren't the only individually unique things in the world. We might just as easily refer to some people as special irises. That's because the iris in the human eye is just as unique as snowflakes. In the entire human population, not a human iris will be similar from one person to the next.

Iris of the Human Eye

The iris of the eye is the colored ring just behind the clear, outer layer of the eye's cornea. Behind the iris is the crystalline lens, which helps to refract light. In the center of the iris is the pupil, the black hole that changes size. The iris is made of connective tissue, smooth muscle fibers, and pigments that give the iris its color.

The pigments in the iris are made of melanin (the same pigment that gives skin its color) and lipochrome. The amount of pigment in the eye creates eye color. Before you were born, genetics determined your eye color and the unique structure of your iris was laid out. On one end of the spectrum are eyes with no pigment, which are the pink eyes of a person with albinism. A little pigment makes the eye blue. Then in increasing amounts of pigment are green, hazel, and finally, brown eyes, which have the most pigment.

Occasionally, someone has an eye that is one color and another eye that is a completely different color. And just to set the record straight, once you are an adult, your eyes don't change color. They can look different depending on what you are wearing or the light that surrounds you, but they aren't changing color.

Because light eyes have less pigment, they need a little extra protection from the ultraviolet rays of the sun. A lot of sun exposure has been connected to a higher chance of cataracts and macular degeneration. Now there's a good reason to wear cool shades, especially if you are light-eyed!

How Does the Iris Work?

Besides giving you a unique look, the iris does serve a functional purpose. The muscles of the iris contract and dilate the pupil of the eye to let in more or less light. The iris dilator muscle pulls away from the center of the iris to dilate (open) the pupil and the iris sphincter muscle pulls towards the center of the iris to constrict (close) the pupil.

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