The kinetics of tear fluid under the lower lid.

A very small drop of fluorescein solution was placed at the bottom of the lower fornix in human volunteers and its appearance in the tear film was measured. This was quite variable, but its appearance time averaged about 4 min and the peak of fluorescence 8 min. The time was shortened by blinking. The appearance time was too fast to be accounted for by simple diffusion in an unmixed tear fluid. Under normal circumstances, the release of the dye from the fornix is faster than its loss from the conjunctival sac, so that it does not seem to be a controlling factor in this loss.