When this happens, not only does the chance of infection increase, the animal's vision becomes blurry or lost. Thus, terrestrial animals, had to evolve a way to protect the eyes from drying air and dust particles. Tears evolved in vertebrates to replace the saltwater bath that fish eyes constantly experience. The human lacrimal gland produces about 1 cubic cm of tears a day under normal conditions. These tears serve to wet and clean the cornea.
Tear fluids are rich in sugars and protein and act as a significant nutrition source for the cornea.
Tears also have anti-bacterial properties making them effective at fighting infections. Tears are salty to create an osmotic balance with the fluid inside the eye. The lacrimal system is a complicated series of ducts surrounding the eye. Tears secreted from above the upper eyelid mix first with mucus before covering the cornea. Interestingly, psychological or emotional crying is unique to humans and certain species of bears.