Rheum (/ˈruːm/; from Greek: ῥεῦμα, rheuma, a flowing, rheum), also known as Gound is thin mucus naturally discharged as a watery substance from the eyes, nose ormouth during sleep (cf. mucopurulent discharge). Rheum dries and gathers as a crust in the corners of the eyes or 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 conjunctiva), nasal mucus, blood cells, skin cells, or dust. Rheum from the eyes is particularly common. Dried rheum is in common usage called sleep, e.g., to have sleep in one's eyes.