Although usually thought of as primarily composed of water, the structure of the tears and their relationship to the underlying ocular surface is extremely complex. The eye’s surface is brilliantly adapted to help stabilize the tears and hold them in place. Imagine water splashed on a window and how quickly it would drip downward pulled by gravity. The tears stay in place even over the convex surface of the cornea because of these special adaptations.
Tears serve several key functions. The most important of these is vision. The tears coat and effectively smooth the surface of the cornea – the clear tissue that focuses light into the internal structures of the eye. A cohesive tear layer assures crisp and stable vision regardless of the environmental fluctuations. The eye functions normally regardless or how high or low the humidity or how hot or cold the temperature. Such flexibility is not only essential for normal function but critical for survival when sharp stable vision can make the difference between life and death.
The tears are intrinsically protective and keep the ocular surface from drying. Traditionally the tears have been described as having three discrete layers; however, most experts now recognize how overly simplistic that view was. Mucins, a combination of proteins and carbohydrates along with the watery component of the tears, provide a dynamic viscoelastic structure that is transformed with each blink to protect underlying structure. These mucins also help bond the tears to the underlying cornea.
The outermost structure of the tears is made up of lipids – oils that are injected into the tears with each blink. These lipids are complex and are produced by multiple meibomian glands, which run radially in both upper and lower lids. The glands are compressed with each blink and express minute amounts of oils. These oils serve a variety of functions including increasing lubrication and decreasing evaporation. They also provide the structure for the tear film and stabilize it, much as a roof provides structure for a house.
Normal healthy tears are a symphony of substance, form and motion. Understand that sight is so critically important to our survival that control of tear function is delegated to the brain. The tears should not be taken for granted as their absence or abnormality can have dire consequences.