Dark black segment contained by middle of eye??

Why does it change sizes. I have blue eyes, and i notice the black part in my mid-eye be very very small. Sometimes it's big. Why does it modify?

Answers:    the pupil dilates and gets bigger to let more pale in when its dark so you can see. Also when you want to see more of something. It get smaller when you want to less light within when its too bright
You are referring to the pupil.The pupil is a hole, the Iris, (the colored area around the pupil) opens and closes to compensate for brighter or dimmer muted. yup the muscles in your eye contract to make it smaller, so that it doesn't suffer despoil from the light. When it's pitch black around you, they get greatly large.

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