Tears



Tears are the secretions of the glands that clean and lubricate the eyes. Lacrimation (from L. Lacrima, meaning tear) (also spelled lachrymation) may also be used in a medical or literary sense to refer to the production or shedding of tears.

Strong emotions, such as sorrow or elation, along with irritation of eye, may lead to an increased production of tears, or crying. The process of yawning may also result in increased lacrimation.

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.

Drainage of tear film
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. 

Types
There are three very basic types of tears:

Neural Aspects
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 lacrimal gland malfunctions but does not cause any severe drying of the cornea, and the main lacrimal gland may 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.

Diseases and disorders
Quality of vision is affected by the stability of the tear film.

"Crocodile tears 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 (tear ducts), 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.

Familial dysautonomia is a genetic condition that can be associated with a lack of overflow tears (alacrima) during emotional crying.

Societal aspects
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 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
In a research conducted by the Weizmann Institute of Science in Rehovot, Israel, emotional tears from women have been found to reduce sexual arousal in men. 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 the drop in testosterone provoked by the chemicals in the tears meant to reduce aggression. In the animal world, it has been found that some blind mole rats would rub tears all over their bodies as a strategy to keep aggressive mole rats away.