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Tears

Lacrimation is the body's process of producing tears, which are a liquid to clean and lubricate the eyes. The word lacrimation may also be used in a medical or literary sense to refer to crying.

Physiology
In humans, the tear film coating the eye has three distinct layers:

1. 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 hydrophobic layer that allows for even distribution of the aqueous layer.

2. The aqueous layer contains water and other substances secreted by the glands of Kraus and Wolfring and the lacrimal gland.

3. The lipid layer contains oils secreted by the Meibomian glands. The outer-most layer of the tear film, it coats the aqueous layer to provide a hydrophobic barrier that retards evaporation.

One lacrimal gland is located above and beside each eye, behind the upper eyelid. The lacrimal glands secrete lacrimal fluid which flows through lacrimal ducts into the space between the eyeball and lids. When the eyes blink the lacrimal fluid is spread across the surface of the eye. The lacrimal fluid flows through the lacrimal caniliculi at the inner corner of the eyelids, to the lacrimal lake, throught the nasolacrimal duct, and finally into the nasal cavity. An excess of tears, as with strong emotion, can thus cause the nose to run.

Types of tears
There are three basic types of tears. In healthy mammalian eyes, the cornea is continually kept wet and nourished by basal tears. They lubricate the eye and help to keep it clear of dust. Tear fluid contains water, mucin, lipids, lysozyme, immunoglobulins, glucose, urea, sodium, and potassium. Some of the substances in lacrimal fluid fight against bacterial infection as a part of the immune system.

The second type of tears results from irritation to the eye by foreign particles, or substances such as onion vapors, tear gas, or pepper spray. These reflex tears attempt to wash out irritants that may have gotten into the eye.

The third category, also referred to as crying or weeping, is increased lacrimation due to strong emotional stress or pain. In humans, emotional tears can be accompanied by reddening of the face and sobbing — cough-like, convulsive breathing, sometimes involving spasms of the whole upper body.

Tears brought about by emotions have a different chemical make up than those for lubrication. It has been suggested from their stress hormone content that tears may be a method of expelling excess hormones from the body.

Meniscometry is used to evaluate tear volume over the ocular surface.

Crocodile tear syndrome is an uncommon consequence of recovery from Bell's palsy where faulty regeneration of the facial nerve causes sufferers to shed tears while eating.

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Societal aspects
Most mammals will produce tears in response to extreme pain or other stimulus, but crying as an emotional reaction is considered by many to be a uniquely human phenomenon, possibly due to humans' advanced self-awareness. Some studies suggest that elephants and gorillas may cry as well.

In nearly all cultures, crying is seen as a specific act associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most often anger and grief, but crying can also be triggered by sadness, joy, fear, humor, et cetera.

In many cultures, crying is associated with babies and children. Some cultures consider crying to be undignified and infantile, casting aspersion on those who cry publicly. In most human cultures, it is more societally acceptable for women to cry than men.

An insincere display of grief or dishonest remorse is called crocodile tears, from the mistaken notion that crocodiles would weep over their prey or that crocodiles don't cry, therefore that the tears are "false" (though in an obviously different way).

Many religions describe gods or prophets as crying. The shortest verse in the Christian Bible (in English) is simply: "Jesus wept". Jade is sometimes known as tears of the Buddha. The Qur'an describes Muhammad crying, and proclaiming: "This is an expression of the tenderness and compassion, which the Lord hath put into the hearts of His servants."