**3. Balance of tear film**

The dynamics of the precorneal tear film are balanced through drainage and evaporation. The drainage of the tear film is regulated by neural reflexes, as opposed to evaporation which depends on the blink rate, temperature, humidity, and air velocity [21].

The drainage of the tear film is maintained by the lacriminal portion of the orbicularis muscle with blinking and is termed the "lacriminal pump mechanism" [22, 23]. This mechanism is controlled mainly by the closure and opening of the eyelids by the orbicularis oculi muscle, in turn effecting the pressure on the lacriminal sac as seen below (e.g. **Figures 3** and **4**):
