The tear system. A) Tear gland / Lacrimal gland B) Superior lacrimal punctum C) Superior lacrimal canal lacrimation leads to tears 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 crying to clean and lubricate the eyes. The word lacrimation (from L. Lacrima meaning Tear) (also spelled lachrymation) 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 and in response to other stimuli, humans are the only mammal generally accepted to cry emotional tears.^[1]

[edit] Physiology

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.

[edit] 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^[2], 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]

[edit] Types

There are three very basic types of tears:

A Frenchman weeps as German soldiers march into the French capital, Paris, on June 14, 1940.

[edit] Neural Aspects

The trigeminal (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 inhibits the reflex even under exposure to strong tear gases. The motor pathway is autonomic (involuntary), and generally 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." Lest the cornea be damaged in surgery or other failure of lacrimal function occur, 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.

[edit] Diseases and disorders

Quality of vision is affected by the stability of the tear film.^[5]

"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.^[6]

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.

Familial dysautonomia is a genetic condition which can be associated with a lack of overflow tears (alacria) during emotional crying.^[7]

[edit] Societal aspects

Most mammals will produce tears in response to extreme pain^{[citation needed]} or other stimuli, 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, gorillas, and even camels may cry as well.^[8]

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, happiness, fear, laughter or humor, frustration, remorse or other strongly-experienced 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.^[9][10][11]

Some modern therapy movements such as Re-evaluation Counseling teach that crying is beneficial to health and mental well-being, encouraging it positively.^[12]

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.^[13] Additionally, in medical terms, someone is said to have Crocodile tears syndrome as an uncommon consequence of recovery from Bell's palsy, where faulty regeneration of the facial nerve causes sufferers to shed tears while eating.

[edit] See also

• Epiphora
• Eyelid
• Dacryocystocele

[edit] References

[edit] External links

• Nasolacrimal System Anatomy - eMedicine.com
• PERSONAL HEALTH : "University of Minnesota researchers who are studying the chemical composition of tears have recently isolated two important chemicals, leucine- enkephalin and prolactin, from emotional tears. The first of these may be an endorphin, one of the body's natural pain-relieving substances. - Both chemicals are found only in tears that are shed in response to emotion. Tears shed because of exposure to a cut onion would contain no such substance." By JANE E. BRODY, published February 22, 1984