The structure of the eyelid is completed by a thin membrane (the conjunctiva), which covers the posterior surface of each eyelid (see Fig. 8.77). This membrane covers the full extent of the posterior surface of each eyelid before reflecting onto the outer surface (sclera) of the eyeball. It attaches to the eyeball at the junction between the sclera and the cornea. With this membrane in place, a conjunctival sac is formed when the eyelids are closed, and the upper and lower extensions of this sac are the superior and inferior conjunctival fornices (Fig. 8.77).