A fluorescin-treated eye under ultraviolet light. This is the inner corner (medial canthus); tears appear yellow. Fluorescin is a harmless orange dye used in ophthalmology to detect damage to the cornea and conjunctiva. The conjunctiva (the delicate mucous membrane which covers the front of the eye and lines the inside of the eyelids) is protected by a fluid film. This fluid is produced by the lacrimal (tear) gland, which is situated in the upper outer margin of each eye socket. The tears drain diagonally across the front of the eye into the inferior and superior lacrimal canaliculi at the medial canthus. The canaliculi drain into a duct which opens into the nasal cavity.