[[image:Tear system.svg|250px|thumb|The tear system. A) Tear gland / Lacrimal gland B) Superior lacrimal punctum C) Superior lacrimal canal D) Tear sac / Lacrimal sac E) Inferior lacrimal punctum F) Inferior lacrimal canal G) Nasolacrimal canal ]]

Tears are the liquid product of a process of lacrimation to clean and lubricate the eyes. The word lacrimation may also be used in a medical or literary sense to refer to crying. Strong emotions, such as sorrow or elation, may lead to crying. The process of yawning may also result in lacrimation. Although most land mammals have a lacrimation system to keep their eyes moist, humans are the only mammal generally accepted to cry emotional tears.

Physiology

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

1. The lipid layer contains oils secreted by the meibomian glands (or tarsal glands). The outer-most layer of the tear film coats the aqueous layer to provide a hydrophobic barrier that retards evaporation and prevents tears spilling onto the cheek.
2. The aqueous layer contains water and other substances such as proteins (e.g. tear lipocalin, lactoferrin, lysozyme and lacritin) secreted by the glands and the lacrimal gland. The aqueous layer serves to promote spreading of the tear film, control of infectious agents and osmotic regulation.
3. The mucous layer contains mucin secreted by the conjunctival goblet cells. The inner-most layer of the tear film, it coats the cornea to provide a hydrophilic layer that allows for even distribution of the tear film, as well as mucus covering of the cornea.

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.

Drainage of tear film

One lacrimal gland is located superiortemporally to each eye, behind the upper eyelid. 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 through 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

Types of tears

There are three very basic types of tears: