The sclera is commonly known as "the white of the eye."  It is the tough, opaque tissue that serves as the eye's protective outer coat.  Six tiny muscles connect to it around the eye and control the eye's movements.  The optic nerve is attached to the sclera at the very back of the eye.

In children, the sclera is thinner and more translucent, allowing the underlying tissue to show through and giving it a bluish cast.  As we age, the sclera tends to become more yellow.