Physiology
In humans, the tear film coating the eye, known as the precorneal film, has three distinct layers, from the most outer surface:
| Name | Container(s) | Secretors | Functions |
|---|---|---|---|
| Lipid layer | oils | meibomian glands (or tarsal glands) | coats the aqueous layer; provides a hydrophobic barrier that envelopes tears and prevents their spilling onto the cheek. These glands are found among the tarsal plates. Thus, the tear fluid deposits between the eye proper and oil barriers of the lids. |
| Aqueous layer | water and other substances such as proteins (e.g., tear lipocalin, lactoferrin, lysozyme and lacritin) | lacrimal gland | promotes spreading of the tear film; promotes the control of infectious agents; promotes osmotic regulation |
| Mucous layer | mucin | conjunctival goblet cells | coats the cornea;provides a hydrophobic layer; allows for even distribution of the tear film; covers the cornea |
Having a thin tear film may prevent one's ability to wear contact lenses, as the amount of oxygen needed is higher than normal, and contact lenses stop oxygen from entering the eye. Eyes with thin tear film will dry out while wearing contact lenses. Special eye drops are available for contact lens wearers. Certain types of contact lenses are designed to let more oxygen through to the eye.
Read more about this topic: Tears
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—Samuel Butler (1835–1902)
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