The sclera is the tough white outer layer of the eyeball.
Made largely of dense collagen fibers, the sclera forms the protective outer coat around most of the eye. It helps the eyeball keep its shape and provides attachment points for the extraocular muscles that move the eye. The optic nerve passes through an opening at the back, while the sclera continues forward and meets the transparent cornea at the corneal limbus.
The sclera is white because its collagen fibers are arranged irregularly, scattering light and making the tissue opaque. It is not the same as the conjunctiva, a thin, transparent membrane that covers the visible front of the sclera and lines the inside of the eyelids. The iris is the colored ring that controls the pupil, and the cornea is the clear front surface through which light enters.
The sclera can look slightly different with age or illness. In children it may appear faintly bluish because it is thinner, while fatty deposits can give it a yellowish cast in older adults. Its familiar name, “the white of the eye,” is therefore a useful clue to the answer.