A very thin layer of tears covers the cornea known as precorneal tear film. It has three layers :
1. Mucin layer:
It is secreted by mucin secretory glands of conjunctiva namely Goblet cells, Crypts of Henle and Glands of manz.
It converts hydrophobic surface of cornea into hydrophilic surface thus stabilizing the aqueous layer of tear film on cornea.
2. Aqueous layer:
It constitutes main bulk of tear film. It is secreted by accessory lacrimal glands namely Glands of Krause and Glands of Wolfring. It forms the Basic tear secretion.
The main lacrimal gland secrets tears during hours of emotional crisis known as Reflex tear secretion. This layer keeps the cornea and conjunctiva moist, flushes away the metabolic waste products from conjunctiva.
This layer contains sodium chloride, sugar, and urea. It is alkaline in nature and saltish to taste. It also contains antibacterial substances like Lysozyme, Betalysin and Lectoferrin.
3. Lipid layer:
It is secreted by Meibomian glands. It spreads over the aqueous layer and prevents its evaporation. It also lubricates the eyelids to facilitate blinking.
Functions of Tear Film
1. It supplies oxygen to cornea.
2. It keeps the cornea and conjunctiva moist.
3. It contains antibacterial substances.
4. It flushes away the metabolic waste products from conjunctiva and cornea.
5. It facilitates blinking. Normal blink rate is 12 per minute.
Tear production is decreased in old age, during hours of fatigue. Use of certain drugs like Timolol, Pilocarpine, Oral contraceptives, Antihistaminics decreases tear production.
Thus such medicines should not be used by a contact lens wearer. Pregnancy also decreases tear production.