Which layer is NOT part of the three basic layers of tears?

The three basic layers of tears are designed to provide protection, lubrication, and nourishment to the eyes. These layers consist of oil, water, and mucus. The oil layer helps to slow the evaporation of the water layer, the water layer provides hydration and nutrients, and the mucus layer keeps the tears evenly distributed across the surface of the eye.

Vapor, on the other hand, is not considered one of the basic layers of tears. It does not serve a function in the physiological composition of tears and is not involved in maintaining eye health or comfort. Therefore, understanding the distinct roles of each of the actual tear layers clarifies why vapor does not belong in the classification of basic tear components.