Evaporation rate of water from the precorneal tear film and cornea in the rabbit.

A technique for the study of evaporation from the precorneal and cornea! surfaces has been developed. Evaporation from the superficial lipid layer (SLL) has been measured to be 10.1 x 10~ Gm.cm.^secr When this lipid layer is removed, evaporation increases around fourfold. Evaporation from the dry epithelial surface occurs at a rate of 1.8 x JO" Gm.cmr^secr After the epithelium is removed, a twentyfold increase in evaporation rate occurs. The specific resistances (u) to evaporation of these layers have been calculated: epithelium, = 82.5 sec./ cm.; SLL, = 12.9 sec/cm.; aqueous tears, < 1 sec/cm.; stroma, < 0 sec/cm. Thus, the superficial lipid layer is effective in retarding evaporation of the precorneal tear film. Its effectiveness compares favorably to that of condensed monolayers of long-chain fatty alcohols on pure water, known to be efficient in retarding evaporation. The epithelium functions as a barrier to water flow and is highly effective in retarding evaporation in the absence of a tear film.