Normally, you can see the white membrane next to the black eye, called the sclera. In front of the sclera is a transparent layer called the bulbar conjunctiva, which is the thinnest part of the conjunctiva and can be pushed. In addition, when the eyelid is turned over, you can see a layer of conjunctiva on the surface of the blood vessels, called the lid conjunctiva. The lid conjunctiva and the bulbar conjunctiva extend at the back of the eye and will have a fold back called the conjunctiva of the fornix. These three together form a sac-like gap in the conjunctival sac, which is the base of the eye slightly behind the eyeball, and the opening is in the direction of the lid fissure, like a sac, so the structure formed is called the conjunctival sac. Many foreign objects, such as eyelashes and contact lenses, are actually present in the conjunctival sac, which acts as a barrier so that these things cannot enter the eye directly.