Vestigial Structures

This is my sister's eye.  A vestigial structure is a structure in an organism that has lost all or most of its original function in the course of evolution.  The plica luminaris is the flap of skin that covers the outside corner of the human eyeball.  It no longer has a purpose but it is still there from our ancestors.  It is thought to have been part of a nictitating membrane, which is now unnecessary to the human eye.