Formation and residence of the tear film. (a) Sagittal view of the eye to show the tear distribution. (b) Schematic view of the lacrimal drainage system. Tears flow from the main and accessory lacrimal glands into the conjunctival sac, and thereafter into the tear meniscus. The negative hydrostatic pressure in the meniscus draws fluid from the tear film into the meniscus during the interblink period, flowing into the menisci via the upper and lower punctal openings, and thereafter into the canaliculi and then the lacrimal sac. From there the tears flow into the nasolacrimal duct where they are either absorbed or continue into the nasal passages (From Gaffney et al. [1]).