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Tearing (//), 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:
|Lipid layer||Oils||Meibomian glands (or tarsal glands)||Coats the aqueous layer, provides a hydrophobic barrier that envelops tears and prevents their spilling onto the cheek. These glands are found among the tarsal plates. Thus, the tear fluid deposits between the eye proper and oil barriers of the lids.|
|Aqueous layer||Electrolytes, 60 metabolites—Amino Acids (1-Methylhistidine/3-Methylhistidine, Arginine, Asymmetric, Asymmetric dimethylarginine/Symmetric dimethylarginine, Citrulline, Creatine, Glutamine, Homoarginine, Hydroxyproline, Phenylalamine, Proline, Pyroglutamic acid, Serine, Taurine, Theonine, Tryptophan, Tyrosine, Urocanic acid, Valme), Amino Alcohols (Panthenoll); Amino Ketones (Allantoin, Creatine), Aromatic Acids (Cinnamic acid, o-Coumaric acid/m-Coumaric acid/p-Coumaric acid), Carbohydrates (N-Acetylneuraminic acid), Carnitines (Acetylcarnitine, Carnitine, hexanoylcarnitine, Palmitoylcarnitine); Cyclic Amines (Niacinamide); Dicarboxylic Acids (Fumaric acid/ Maleic acid), Nucleosides (1-Methyladenosine, Adenoisine, Cytidine, Guanosine, Inosine, S-Adenosyl-homocysteine, S-Adenosyl-methionine, Uridine, and Xamthosine), Nucleotides (ADP, AMO, CMP, Cytidine diphosphate choline, GMP, IMP, UDP, UMP, UDP-N-acetylgalactosamine/UDP-N-acetylglucosamine), Peptides (Oxidized glutathione), Phospholipids (1-Palmitoyl-lysophosphatidylcholime), Purines and derivatives (Hypoxanthine, Theobromine, Uric acid, Xanthine), Purines and derivatives (4-Pyridoxic acid), Quaternary Amines (Acetylcholine, Glycerolphosphocholine, Phosphocholine), and Tricarboxylic Acids (Citric acid)--and other substances such as proteins (e.g., antibodies, lipocalin, lactoferrin, lysozyme, and lacritin)||Lacrimal gland||Promotes spreading of the tear film, the control of infectious agents and osmotic regulation.|
|Mucous layer||Mucins||Conjunctival goblet cells||Coats the cornea, provides a hydrophilic layer and allows for even distribution of the tear film.|
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.
Drainage of tear film
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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. 
There are three very basic types of tears:
|Basal 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, lactoferrin, lipocalin, lacritin, immunoglobulins, glucose, urea, sodium, and potassium. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial infection as a part of the immune system. Lysozyme does this by dissolving a layer in the outer coating, called peptidoglycan, of certain bacteria. It is a typical body fluid with a salt content similar to blood plasma. Usually, in a 24-hour period, 0.75 to 1.1 grams (0.03–0.04 ounce avoirdupois) of tears is secreted; this rate slows with age. In addition, the basal tears are composed of antioxidants such as Ascorbate, Urate, Cysteine, Glutathione, and Tyrosine. Ascorbate and Urate constitute half of the tears.|
|Reflex tears||The second type of tears results from irritation of the eye by foreign particles, or from the presence of irritant substances such as onion vapors, perfumes and other fragrances, tear gas, or pepper spray in the eye's environment, including the cornea, conjunctiva, or nasal mucosa, which trigger TRP channels in the ophthalmic nerve. It can also occur with bright light and hot or peppery stimuli to the tongue and mouth. It is also linked with vomiting, coughing and yawning. These reflex tears attempt to wash out irritants that may have come into contact with the eye.|
|Crying or weeping (psychic tears)||The third category, in general, referred to as crying or weeping, is increased tearing due to strong emotional stress, pleasure, anger, suffering, mourning, or physical pain. This practice is not restricted to negative emotions; many people cry when extremely happy such as during times of intense humour and laughter. 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; emotional tears contain more of the protein-based hormones prolactin, adrenocorticotropic hormone, and Leu-enkephalin (a natural painkiller) than basal or reflex tears. The limbic system is involved in production of basic emotional drives, such as anger, fear, etc. The limbic system, to be specific, the hypothalamus, also has a degree of control over the autonomic system. The parasympathetic branch of the autonomic nervous system controls the lacrimal glands via the neurotransmitter acetylcholine through both the nicotinic and muscarinic receptors. When these receptors are activated, the lacrimal gland is stimulated to produce tears.|
The trigeminal V1 (fifth cranial) nerve bears the sensory pathway of the tear reflexes. When the trigeminal nerve is cut, tears from reflexes will stop, but not emotional tears. Likewise, application of cocaine to the surface of the eye, due to its paralyzing effect on the sensory nerve endings, inhibits the reflex even under exposure to strong tear gases. The motor pathway is autonomic (involuntary), and, in general, uses the pathway of the facial (seventh) nerve in the parasympathetic division. In parasympathetic imitators (such as acetylcholine), more tears are produced, and an anticholinergic drug like atropine inhibits tear production. A newborn infant has insufficient development of nervous control, so s/he "cries without weeping." If the lacrimal gland malfunctions or is damaged (e.g. by surgery), but does not cause any severe drying of the cornea, it is not a serious matter, for the accessory glands are enough for general secretion. In reflex situations, copious tears are produced mainly in emergencies.
Diseases and disorders
Quality of vision is affected by the stability of the tear film.
"Crocodile tears syndrome", also known as Bogorad's syndrome, is an uncommon consequence of nerve regeneration subsequent to Bell's palsy or other damage to the facial nerve in which efferent fibers from the superior salivary nucleus become improperly connected to nerve axons projecting to the lacrimal glands, causing one to shed tears (lacrimate) during salivation while smelling foods or eating. It is presumed that one would also salivate while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but this would be less noticeable.
Keratoconjunctivitis sicca, known as dry eye, is a very common disorder of the tear film. However, sufferers can experience watering of the eyes, which is in fact a response to irritation caused by the original tear film deficiency. Lack of Meibomian gland secretion can mean the tears are not enveloped in a hydrophobic film coat, leading to tears spilling onto the face.
Obstruction of the punctum, nasolacrimal canal, or nasolacrimal duct can cause even normal levels of basal tear to overflow onto the face (epiphora), giving the appearance of constant psychic tearing. This can have significant social consequences.
In nearly all cultures, crying is associated with tears trickling down the cheeks and accompanied by characteristic sobbing sounds. Emotional triggers are most often sadness and grief but crying can also be triggered by anger, happiness, fear, laughter or humor, frustration, remorse, or other strong, intense emotions. In many cultures, crying is associated with babies and children. Some cultures consider crying to be undignified and infantile, casting aspersions on those who cry publicly, except if it is due to the death of a close friend or relative. In most cultures, it is more socially acceptable for women and children to cry than men. In some Latin regions, crying among men is acceptable.
Some modern therapy movements such as Re-evaluation Counseling teach that crying is beneficial to health and mental well-being, encouraging it positively. An insincere display of grief or dishonest remorse is sometimes called crocodile tears in reference to an Ancient Greek anecdote that crocodiles would pretend to weep while luring or devouring their prey. In addition, in medical terms, someone is said to have crocodile tears syndrome as an uncommon consequence of recovery from Bell's palsy, in which faulty regeneration of the facial nerve causes sufferers to shed tears while eating.
Chemicals in tears
On a study conducted by the Weizmann Institute of Science in Rehovot, Israel, emotional tears from women have been found to reduce sexual arousal in men. Also, emotional tears are made up of a different chemical component than those evoked by eye irritants and can relay chemical messages to others. The change in sex drive could be attributed to a drop in testosterone provoked by the tear chemicals, reducing aggression. In the animal world, it has been found that some blind mole rats rub tears all over their bodies as a strategy to keep aggressive mole rats away.
Tear composition varies from tear types. Mainly, tears are composed of water, salts, antibodies and lysozymes (antibacterial enzymes). According to a discovery by Dr. William H. Frey II, a bio-chemist from St. Paul Ramsey medical center in Minnesota, the composition of tears caused by emotion differs from that of tears as a reaction to irritations, such as onion fumes, dust or allergy. Emotional tears are composed of more protein-based hormones, such as prolactin, adrenocorticotropic, and leucine enkephalin (a natural pain killer), which is suggested to be the mechanism behind the experience of crying from emotion making an individual feel better.
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