Fig. 2. Schematic representation of the three layers of the tear film. The tear film covers the cells of the ocular surface (cornea and conjunctiva). The upper lipid layer is secreted by the meibomian glands, and the middle aqueous layer is secreted by the main and accessory lacrimal glands and the conjunctival epithelium. The inner mucous layer is secreted by conjunctival goblet cells and stratified squamous cells of the conjunctiva and cornea, and the glycocalyx of the cornea and conjunctiva. (Reprinted from Hodges RR, Dartt DA: Regulatory pathways in lacrimal gland epithelium. Int Rev Cytol 231:129–196, 2003.)