What is the Pupil of the Eye? - Definition & Function

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  • 0:01 The Pupil of the Eye
  • 1:06 Controlling the Pupil
  • 1:48 The Mind's Eye
  • 2:16 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.

You may not think there is much to the pupil of the eye, but it's a window to your emotions and a key feature in managing the amount of light that gets into your eye. Learn about the pupil and all that it does in this lesson.

The Pupil of the Eye

The pupil of the eye is the black circle in the center of the iris. The iris is the colored ring in the eye with a color and structure unique to each person. From the outside of the eye, light passes through the clear lens, then through the pupil. When the light reaches the tissues at the back of the eye, it is absorbed, making the pupil look black.

Diagram of the eye
Eye diagram

The iris contracts and dilates involuntarily and changes the size of the pupil. The whole job of the iris and pupil is to control the amount of light that gets into the eye. It's called a pupillary reflex, and you have probably noticed that a person's pupils are smaller in bright light and bigger in low light.

This is the pupil controlling the amount of light that goes through the lens of the eyes to the retina in the back of the eye. There is some inconsistency in pupil size and shape though. In younger people, pupil size is usually larger, and about 20% of the population has two pupils that aren't the same size as each other. You probably also noticed that not all pupils are round. Some animals, like cats and snakes, have slit-shaped pupils.

A constricted pupil
Constricted pupil
A dilated pupil
Dilated pupil

Controlling the Pupil

So, what controls that constricting and dilating dance of the pupil? The iris has two sets of muscles:

  • Sphincter muscles constrict the pupil like a drawstring.
  • Dilator muscles that radiate out from the center of the iris help to dilate the pupil.

Behind the scenes, there is a long nerve that helps the eye react to changing conditions. This nerve takes a surprising route through the body, starting in the brain then traveling down the spinal cord, passing above the lungs, up through the neck, back through part of the brain and then, finally, to the pupil. Because of that strange route, problems with the brain, upper lungs, and some eye health issues may be discovered when there is an unexpected change in the pupils.

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