The tear film is a thin layer of fluid which covers the cornea at the front of the eye. This layer provides a nice, smooth surface for incoming light and helps keep the eye’s surface comfortable. Additionally, the tear film helps prevent the surface of the eye from drying out, and from infections and dirt.
The lacrimal gland, glands in the eye’s mucous membrane (conjunctiva), and the eyelids continuously produce new tear fluid. A thin layer of fresh tear fluid is uniformly distributed over the eye each time you blink. Part of the tear film evaporates on the surface of the eye, and the rest is drained to the nose via the tear ducts. The tear duct produces more tears (reflex tears) when you are emotional or if your eyes are irritated. The eyes overflow with emotion.
The tear film consists of 3 parts:
- A lipid layer (an oily layer), which makes up the outer layer of the tear film. This layer prevents the evaporation of the tear film and is produced by tiny glands in the eyelids, known as the Meibomian glands.
- The watery middle layer, which flushes dirt and bacteria off the surface of the eye, and which provides the cornea with nutrients. This layer is produced by glands (Krause’s and Wolfring’s glands) in the conjunctiva and the tear duct.
- A slimy layer (mucin layer) adjacent to the surface of the eye ensures optimal bonding to the tear film and its uniform distribution across the surface of the eye. This layer is formed by glands (goblet cells) in the conjunctiva.
A properly functioning tear film is based on a complex balance between the rate and frequency of blinking, tear production, eye surface and the evaporation and drainage of tears. The disruption of this balance will result in a defective tear film and symptoms related to dry eyes.
