Tears 143803 225982701 2008-07-16T09:17:56Z 91.3.77.212 {{otheruses3|Tear}} [[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 [[eye]]s. 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 tearing. Although most land mammals have a lacrimation system to keep their eyes moist, humans are the only mammal generally accepted to cry emotional tears. [http://ask.yahoo.com/20030313.html][http://www.aquaticape.org/tears.html] ==Physiology== In humans, the tear film coating the eye has three distinct layers, from the most outer surface: # The '''lipid layer''' contains [[lipid|oils]] secreted by the [[meibomian 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. # The '''aqueous layer''' contains [[water]] and other substances such as proteins (e.g. tear lipocalin, [[lactoferrin]], lysozyme<ref>[http://www.medrounds.org/ocular-pathology-study-guide/2005/10/tear-proteins.html Ocular Pathology Study Guide: Tear Proteins<!-- Bot generated title -->]</ref> 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. # 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== {{Refimprove|section|date=August 2007}} 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 lacrimalia|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. [http://www.academy.org.uk/tutorials/dilate3.jpg] ==Types of tears== There are three very basic types of tears: :1. '''Basal tears''': In healthy [[mammal]]ian 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]], [[immunoglobulin]]s, [[glucose]], [[urea]], [[sodium]], and [[potassium]]. Some of the substances in lacrimal fluid fight against [[bacterium|bacteria]]l [[infection]] as a part of the [[immune system]]. :2. '''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, [[tear gas]] or [[pepper spray]] in the eye's environment. These ''reflex tears'' attempt to wash out irritants that may have come into contact with the eye. [[Image:Crying-girl.jpg|thumb|A child producing tears due to emotional stress or pain|150px]] :3. '''Crying or weeping (psychic tears)''': The third category, generally referred to as ''crying'' or ''weeping'', is increased lacrimation due to strong [[emotional stress]], [[suffering]] or [[Pain|physical pain]]. This practice is not restricted to negative emotions; many people cry when extremely happy. In humans, ''emotional tears'' can be accompanied by reddening of the face and sobbing &mdash; 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 [[leucine]] [[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, specifically the hypothalamus, also has a degree of control over the autonomic system. The parasympathetic branch of the autonomic 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.<ref>Skorucak A. [http://www.scienceiq.com/ShowFact.cfm?ID=64 "The Science of Tears."] ScienceIQ.com. Accessed September 29, 2006.</ref> ==Diseases and disorders==<!-- This section is linked from [[Bell's palsy]] --> {{main|List of eye diseases and disorders}} Quality of vision is affected by the stability of the tear film.<ref>Szczesna DH, Jaronski J, Kasprzak HT, Stenevi U. "Interferometric measurements of dynamic changes of tear film." ''J Biomed Opt.'' 2006 May-Jun;11(3):34028. PMID 16822077.</ref> "Crocodile tears 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.<ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2261223&dopt=Abstract [Crocodile tears syndrome&#93; [Acta Otorrinolaringol Esp. 1990 May-Jun&#93; - PubMed Result<!-- Bot generated title -->]</ref> [[Keratoconjunctivitis sicca]], more commonly known as dry eye, is a very common disorder of the tear film. Paradoxically, sufferers can experience watering of the eyes which is in fact a response to irritation caused by the original tear film deficiency. "Leamy Eye" is a condition whereby there is excessive watering of one eye, seemingly for no apparent reason, in response to environmental stimuli. == Societal aspects == Most mammals will produce tears in response to extreme [[pain]] or other stimuli, but crying as an [[emotion]]al reaction is considered by many to be a uniquely [[human]] phenomenon, possibly due to humans' advanced [[self-awareness]]. Some studies suggest that [[elephant]]s and [[gorilla]]s may cry as well.<ref>Masson, Jeffrey Moussaieff, McCarthy, Susan, When Elephants Weep, Delta 1996 isbn: 978-0385314282 </ref> In nearly all cultures, crying is seen as a specific act associated with tears trickling down the cheeks and accompanied by characteristic sobbing [[sound]]s. Emotional triggers are most often [[anger]] and [[grief]], but crying can also be triggered by [[sadness]], [[happiness|joy]], [[fear]], [[laughter]] or [[humor]], [[frustration]], remorse or other strongly-experienced emotions. In many cultures, crying is associated with babies and [[children]]. The [[human brain]] is programmed to consider the crying of a child [[annoyance|irritating]], and thus make the person desire to aid the baby to stop the crying. {{Fact|date=May 2008}} 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]] to cry than [[man|men]], however this stereotype is slowly dying. {{Fact|date=March 2008}} Some modern [[psychotherapy|therapy]] movements such as [[Re-evaluation Counseling]] teach that crying is beneficial to health and mental well-being, encouraging it positively.<ref>[http://www.rc.org/theory/recovery.html Re-evaluation Counseling site: "The Recovery Process"]</ref> An insincere display of grief or dishonest remorse is called [[crocodile tears]], from the ancient [[anecdote]] that [[crocodile]]s would ''pretend'' to weep while luring or devouring their prey.<ref>[http://www.worldwidewords.org/qa/qa-cro1.htm World Wide Words: Crocodile tears<!-- Bot generated title -->]</ref> ==See also== * [[Epiphora (medical)|Epiphora]] * [[Eyelid]] ==References== <div class="references-small"> <references/> </div> ==External links== *[http://www.emedicine.com/ent/topic5.htm Nasolacrimal System Anatomy - eMedicine.com] [[Category:Human physiology]] [[Category:Eye]] [[Category:Body fluids]] [[gn:Tesay]] [[ay:Jacha]] [[cs:Slzy]] [[da:Tåre]] [[de:Träne]] [[es:Lágrima]] [[fr:Larme]] [[gl:Lágrima]] [[id:Air mata]] [[it:Lacrima]] [[he:דמעות]] [[jv:Luh]] [[la:Lacrimae]] [[lt:Ašara]] [[nl:Traan]] [[ja:涙]] [[pl:Łza]] [[pt:Lágrima]] [[qu:Wiqi]] [[sv:Tårar]] [[tr:Gözyaşı]] [[uk:Сльози]] [[yi:טרערן]] [[zh:淚]]