The sclera (from the Greek skleros, meaning hard), also known as the white of the eye, is the opaque, fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In humans the whole sclera is white, contrasting with the coloured iris, but in other mammals the visible part of the sclera matches the colour of the iris, so the white part does not normally show. In the development of the embryo, the sclera is derived from the neural crest. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, fatty deposits on the sclera can make it appear slightly yellow.