[The preocular tear film: a clinical examination method].

The preocular tear film is formed of three superimposed layers: an internal mucus layer, the intermediate layer of water and the external lipid layer. These three layers are functionally necessary for the transparency of the cornea. If one of the constituent parts of the tears is lacking, epithelial repercussions are immediate: epithelial villosities disappear and the cells degenerate and desquamate. Qualitative and quantitative clinical tests can identify certain constituents of tears, establish diagnosis, and determine specific treatment.