Rheum (/ˈruːm/; from Greek: ῥεῦμα rheume a flowing, rheum), also known as gound, is thin mucus naturally discharged from the eyes, nose, or mouth during sleep (cf. mucopurulent discharge). Rheum dries and gathers as a crust in the corners of the eyes or the mouth, on the eyelids, or under the nose. It is formed by a combination of mucus (in the case of the eyes, consisting of mucin discharged from the cornea or the conjunctiva), nasal mucus, blood cells, skin cells, or dust. Rheum from the eyes is particularly common. Dried rheum is commonly called sleep, sleepy sand, eye goop or sleep dust. When the individual is awake, blinking of the eyelid causes rheum to be washed away with tears via the nasolacrimal duct.