Rheum (/ˈruːm/; from Greek: ῥεῦμα rheuma 'a flowing, rheum') is a thin mucus naturally discharged from the eyes, nose, or mouth, often 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.