part of part of has quality has quality composed primarily of protects liquid configuration secretion of exocrine gland anterior segment of eyeball secretion of lacrimal gland bodily fluid ocular surface region tear film maintenance of the avascular transparent corneal epithelium The tear film is structurally complex with three distinct layers: a surface lipid layer, a middle aqueous layer, and an inner mucus layer[http://physiologyonline.physiology.org/content/13/2/97] UBERON:0022287 precorneal film the thin acellular fluid layer that adheres to the most superficial layer of the corneal epithelium and lubricates and protects the ocular surface; the film is generally composed of three layers: (i) an inner mucous layer, secreted largely by the conjunctival goblet cells with contributions from the lacrimal glands, which coats the cornea, provides a hydrophobic layer, and allows for even distribution of the tear film; (ii) a middle aqueous layer, secreted by the lacrimal glands, which keeps the corneal surface moist, and provides a mechanism for oxygenation and nutrient exchange with the avascular anterior corneal tissue; and (iii) an outer lipid layer, secreted principally by the Meibomian (tarsal) glands although lipid tear elements also are contributed by the Harderian glands; tear film lipids form a monolayer on the tear surface, and function by enhancing the surface tension which supports the integrity of the tear film against collapse due to gravity and also slows down the loss of the tear film by evaporation; in addition , the lipid layer provides a glassy, smooth interface between the air and the transparent cornea An aqueous substance that covers the anterior surface of the eyeball, keeping the cornea wet.