PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Human eye. The three visible parts of the eye are the sclera (white), the iris (blue/green) and the pupil (black). The iris muscles contract and relax to control the diameter of the pupil. Light passes through the pupil and is focused on the back of the eye by a lens. This produces nerve impulses that are transmitted to the brain and processed to create the sensation of vision. Eyelashes (black) are specialised hairs that trap dust and other particles. The eyelids are used in blinking to clean the surface of the eye. The eyelids can also be closed, instinctively to protect against bright light, or voluntarily when resting and sleeping.
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