The tear film is a relatively instable, complex, trilaminar fluid structure that covers the cornea and conjunctiva externally. Given its importance and close anatomical relationship with the cornea and conjunctiva, it tends to be considered an additional corneal layer. It is formed of an external lipid layer, an intermediate aqueous or serous layer and an internal mucinous layer adherent to the cornea.
It is incorrect to consider the three layers as separate from each other; in fact, their anatomical, physiological and functional relationships make them a single structure. The health of the cornea and conjunctiva and, consequently, that of all the ocular structures depends not only on the continuous secretion, distribution and elimination of the tear film, but also on its correct composition. The volume, composition and structure of the tear film change in response to external stimuli.
The tear film protects the surface of the eye from stimuli from the external environment, such as intense light, excessive dryness, cold, heat, mechanical stimuli, trauma, chemical substances and infections of various types. It is the first interface between the air and the eye and has dioptric properties, acting as the most external refractive layer of the eye. It contributes to smoothing out irregularities of the cornea by filling the depressions created by the microvilli on the membrane of epithelial cells.
The tear film regulates the hydration of the cornea through changes in its own tonicity resulting from the physiological evaporation of the film and is the main source of oxygen, glucose and other essential elements for normal corneal metabolism. It acts as a lubricant between the eyelids, conjunctiva and cornea. It also has a cleansing action and consequently removes foreign bodies, cells, debris and bacteria from the surface of the cornea and conjunctiva. Finally, the tear film contributes to the healing of central corneal lesions by enabling cells from the limbus to reach the central area of the cornea.