[The lacrimal film, structure and stability].

The precorneal tear film is important in preserving the integrity of the corneal epithelium. One of several quantitative and qualitative clinical evaluation of the tear film is the determination of break-up time (BUT). Although controversial, this test is an important index of tear film physiology. The test depends upon the viscosity, surface tension and geometry of the tear film. The system is therefore ill-defined and current practices of bio-microscopic examination do not take into account the real nature of the called "dry spot". In this paper, we describe an innovative method of microscopic examination of the wetting film formed on the eye. The method is based on physico-chemical studies of thin wetting film (thin liquid film formed on solid surface). A freshly enucleated eye is immerged in a physiological solution and a thin wetting film is formed by bringing an air bubble into contact with a corneal epithelium. The examination of the wetting film is done using a specially-constructed device which simultaneously enables the deliver of an air bubble and the ability to observe the formed bubble. The observed interference colours provide information about the thickness and homogeneity of the liquid film. One can also observe the nature of lipid droplets floating at the surface of the tear aqueous phase by this technique. The study of the stability and rupture of thin films formed on freshly enucleated normal rabbit or rat eyes shows that the corneal epithelium is perfectly wettable as long as the epithelial cells had not suffered any irreversible damage. "Dry spot" are in reality microscopically thin fluid films with a thickness of several hundred nanometers.