Physiological Pharmaceutics

254 Physiological PharmaceuticsFigure 11.4 The structure of the tear film according to Wolff,The average tear volume in a human is 7 µl, 1 µl of which is contained in theprecorneal tear film, and 3 µl in each of the tear margins. Prior to blinking, the tear volumecan increase to about 30 µl. The maximum amount of fluid that can be held in theconjunctival sac is only about 10 µl As the tear layer is so thin, evaporation and lipidcontamination of the mucin component of the tear fluids quickly destroys its continuity.This results in dry spots that appear usually within 15 to 30 seconds after a blink, atscattered locations on the corneal surface. The blinking action of the eyelids, which usuallyoccurs before the actual formation of dry spots, is required to re-form the tear film layer.The blink interval should therefore, be shorter than the tear break-up time.The precorneal tear film was first thought to be a three layered structure consistingof a superficial oily layer, a middle aqueous layer, and an adsorbed mucus layer (Figure11.4), secreted by several glands (Figure 11.5) 1 . Recent research suggests that the mucusfraction extends through the tear film 2Figure 11.5 The glands secreting the components of the precorneal tear film