EY: v4

THE IRIS:  The iris is the black part of the eye.

THE IRIS:  The Iris is a muscle that acts like a kitchen faucet.

THE IRIS:  The iris changes the amount of light going to the eye by expanding or shrinking the hole–the pupil–which is in the center of it. You can see this movement of the iris by shining a flashlight into someone’s eye in a darkened room.  The iris will quickly close because it has sensors in it that detect that there is too much light. If you turn the flashlight off, the pupil will expand again to let more light in..

IRIS MUSCLE SLICE: Your iris is a muscle, not just any muscle, but a perfectly round muscle, kinda like a flattened cake.

IRIS MUSCLE SLICE: Your brain opens and closes the iris’s pupil–which is a black hole in the center of the eye–to allow in different amounts of light. You can shine a flashlight in someone’s eye in a darkened room and see how fast the brain adjusts the pupil.

IRIS MUSCLE SLICE: The iris is cleverly built so it opens and closes.  But how can it do this? All muscles only push.

IRIS MUSCLE SLICE: How does an iris open and close?  It can pull open.

IRIS MUSCLE SLICE: The colored iris of your eye is only 1 mm thick.  That’s thinner than 10 sheets of paper. Yet inside it there are 2 layers of thick muscles lying right on top of each other. 

IRIS MUSCLE SLICE: One layer of the iris is designed like the spokes of a bike wheel. They are attached at the opening of the iris and go to the outer edge of the circle of the iris. When these muscles contract they pull the iris open.

IRIS MUSCLE SLICE: How can the iris close? The second paper-thin muscle layer circles around the iris like ropes running around the opening of the pupil.   When these muscles pull and contract, they squeeze the opening shut.