Fig. 1 Ocular surface defense. Ocular surface health is maintained by a stable tear film, which requires both compositional and hydrodynamic factors. The former includes ocular surface epithelium producing mucins, meibomian glands producing meibum, and lacrimal glands producing aqueous tears. The latter includes eyelids, which blink at desirable frequencies to allow tears to spread and clear, and close completely during the close eye state to prevent evaporation.