Human eye. Close-up of a human eye with a dilated pupil (black, centre). This aperture allows light to enter the eye. A ring of coloured muscles, the iris (blue), controls the size of the pupil. The eye's sclera (white area) is the tough outer wall that keeps the eyeball spherical. The sclera is replaced at the front of the eye by a circular transparent area, the cornea, which covers the iris and pupil. The visible surface of the eye is covered by the conjunctiva, a transparent membrane. The eyelashes are modified hairs that trap airborne debris. Eyelids protect the eyes and help clean them by blinking. Bony ridges round the eye protect it, and eyebrows trap sweat.