The tear film consists of three layers:
An oily layer
A watery layer
A layer of mucus
Each layer has its own purpose. The oily layer, produced by the meibomian glands, forms the outermost surface of the tear film. The main purpose of the oily layer is to help smooth the tear surface and reduce tear evaporation. The middle, watery layer, is produced by the lacrimal glands in the eyelids. This layer helps to cleanse the eye and wash away foreign particles or irritants. The inner layer consists of mucus produced by the conjunctiva. Mucus allows the watery layer to distribute evenly over the surface of the eye and helps the eye remain lubricated. Without mucus, tears would not stick to the eye. By constantly producing tears at a slow and steady rate, the eye stays moist and comfortable.
The eye uses two different methods to produce tears. It can make tears at a slow, steady rate to maintain normal eye lubrication. It can also produce a lot of tears in response to eye irritation, including dry eyes, or emotion. It may sound counterintuitive that dry eyes would cause excess tearing, but it's the eye's response to discomfort. If an insufficient amount of tears are being produced the eye becomes irritated. Eye irritation prompts the gland that makes tears (called the lacrimal gland) to release a large volume of tears, exceeding the rate of tear drainage. These excess tears then overflow from your eye.
Dry eye symptoms may include:
• Stinging or burning eyes
• Stringy mucus in or around the eyes
• Excessive eye irritation from smoke or wind
• Excess tearing
• Discomfort when wearing contact lenses
Your tear film is responsible for keeping the surface of the eye smooth and clear. Without this tear film, good vision would not be possible.
Sometimes people don't produce a sufficient amount of tears or the right quality of tears to keep their eyes healthy and comfortable. This condition is known as dry eye.