Rheum is a yellowish discharge, usually developing in the corner of the eye during sleep. 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, snot, blood cells, skin cells, or dust. Enhanced SEM. Image width: 2.35 mm. Magnification: 42.5x if the image is printed 10 cm wide.