"Tears have three separate components. The innermost tear layer coats the surface of the cornea and is called the mucous layer, or mucin. The middle tear layer is an aqueous [coating] produced by the lachyrmal glands and supplies salt, proteins and other compounds to the cornea. The outer tear layer is composed of oil from the meibomian sebaceous glands in the eyelids. This helps to prevent evaporation of the watery tears from the surface of the eyes".
Joh Uys of Bellville, South Africa quoted in Does Anything Eat Wasps? (page 23)