Structure of tear film

Wolff was the first to describe the detailed structure of the fluid covering the cornea and called it precorneal film. He described this film to consist of three layers, which from posterior to anterior are mucus layer, aqueous layer and lipid or oily layer.

1. Mucus layer. It is the innermost and thinnest stratum of the tear film. It consists of mucin secreted by conjunctival goblet cells and glands of Manz. It converts the hydrophobic corneal surface into hydrophilic one.
2. Aqueous layer. The bulk of tear film is formed by this intermediate layer which consists of tears secreted by the main and  accessory lacrimal glands. The tears mainly comprise of water and
small quantities of solutes such as sodium chloride, sugar, urea and proteins. Therefore, it is alkaline and salty in taste. It also contains antibacterial substances like lysozyme, betalysin and lactoferrin.

3. Lipid or oily layer. This is the outermost layer of tear film formed at air-tear interface from the secretions of Meibomian, Zeis, and Moll glands. This layer prevents the overflow of tears, retards their evaporation and lubricates the eyelids as they slide over the surface of the globe.