The eyelids (palpebrae) are the most external structure of the eye. They protect the eyeball from harmful contact and ensure the tear film is distributed over the anterior surface of the eye, keeping it moist. Eyelids are remarkable in that they are extremely flexible. They are among the most mobile structures in the human body.
The upper lid can be subdivided into two zones. The tarsal zone (tarsus or tarsal plate = dense fibrous tissue) stretches from the lid margin to the upper lid crease. The orbital zone starts at the upper lid crease and goes up to the eyebrows.
The eyebrows form a kind of boundary between the forehead and upper eyelid and protect the eye by preventing sweat and dirt from entering the anterior part of the eye.