Intro | Cornea | Optic Disk and Blind Spot | Fovea | Iris | Lens | Muscles (to move eye) | Muscles (to adjust lens) | Optic Nerve | Pupil | Retina

Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test

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The Iris is a colored ring of muscle that surrounds the pupil and allows adjustment of the size of the pupil opening, working much like the diaphragm in a camera. With contraction of the iris, the pupil opening dilates (becomes larger), letting in more light but creating a less sharper image. When the iris relaxes, the pupil opening constricts (becomes smaller), letting in less light but creating a sharper image. In conditions of dim lighting, the pupils will dilate to let in more light. Although the images are less clear, more of the environment will remain visible.