Tears

Tearing (/ˈtiərɪŋ/), lacrimation, or lachrymation (from Latin lacrima, meaning "tear") is the secretion of tears, which often serves to clean and lubricate the eyes in response to an irritation of the eyes. Tears formed through crying are associated with strong internal emotions, such as sorrow, elation, love, awe and pleasure. Laughing or yawning may also lead to the production of tears.

In humans, the tear film coating the eye, known as the precorneal film, has three distinct layers, from the most outer surface:

Having a thin tear film may prevent one's ability to wear contact lenses, as the amount of oxygen needed is higher than normal, and contact lenses stop oxygen from entering the eye. Eyes with thin tear film will dry out while wearing contact lenses. Special eye drops are available for contact lens wearers. Certain types of contact lenses are designed to let more oxygen through to the eye.

The lacrimal glands secrete lacrimal fluid, which flows through the main excretory ducts into the space between the eyeball and lids. When the eyes blink, the lacrimal fluid is spread across the surface of the eye. Lacrimal fluid gathers in the lacrimal lake, and is drawn into the puncta by capillary action, then flows through the lacrimal canaliculi at the inner corner of the eyelids entering the lacrimal sac, then on to the nasolacrimal duct, and finally into the nasal cavity. An excess of tears, as with strong emotion, can thus cause the nose to run. [1]

...
Wikipedia