(Rise Earth) While many animals “cry” by making vocalizations as emotional expressions or sounds of pain, humans are the only animals that shed tears.
Your tears may be happy or sad, or they may be described clinically as lacrimation, particularly to describe the flow of tears that may occur when you cut an onion or get a few grains of sand in your eyes. No matter the cause of your tears, the scientific process that turns on the “waterworks” is the same. You have a lacrimal gland located between your eyeball and eyelid, which produces tears documented as emotion-related. When you blink, the fluid gets dispersed over your eye, then drains via your lacrimal punctum (and ultimately through your nose, which is why crying makes your nose run). If your tears are voluminous, however, they will overflow this drainage system and cascade down your cheeks. 
The three types of tears
Your body produces three different types of tears. There’s the basal variety, which are made as a form of lubrication and protection for your eyes. These are constantly secreted in tiny quantities (about one gram over a 24-hour period) and coat your eyes when you blink. 
You also produce reflex tears. These are another form of protection and are released in response to irritants, such as wind, dust, smoke or cut onions. The third form of tears — emotional or “psychic” tears as they’re sometimes called — are arguably the most talked about and the most mysterious.
Your psychic tears are produced in response to strong emotions — stress, happiness, sadness, physical pain and more. These emotions trigger tearing via an intricate connection with your autonomic nervous system. As explained in The Independent: 
” … [T]here is an area of your brain specifically to deal with your emotions, called the limbic system (specifically the part of it called the hypothalamus), which is hard-wired into your autonomic nervous system (that’s the part you don’t have any control over).
This system, via a neurotransmitter called acetylcholine, has a degree of control over the lacrimal ‘tear’ system; and it is this tiny molecule which then stimulates tear production.
So in short, your emotional reaction … triggers your nervous system, which in turn, orders your tear-producing system to activate.”
Crying involves many physical sensations
When you cry, it’s not only a matter of tears streaming down your face. It also typically involves an increase to your heart rate. You may begin sweating and, often, a lump will form in your throat, which is called the globus sensation. This isn’t a true lump, but rather is related to a strain in your throat muscles.
When you cry, the opening in your throat that allows air to pass from your larynx to your lungs (known as your glottis) becomes enlarged.
When you swallow, your glottis closes, but when you cry your body enlarges the glottis to let in more oxygen. It’s this fight between trying to open and close your glottis that leads to muscle strain and the feeling of a lump in your throat. 
Crying as a means of displaying submission or vulnerability
Some have theorized that shedding tears is a way to heighten the facial appearance of sadness, thereby providing a survival advantage of sorts. It may, for instance, help you to solicit support. As reported by the American Psychological Association (APA): 
“‘Tears add valence and nuance to the perception of faces,’ says the study’s lead author, Robert R. Provine, PhD, a professor of psychology and neuroscience at the University of Maryland, Baltimore County.
Tears become a sort of social lubricant, he says, helping to ensure the smooth functioning of a community by helping people communicate.”
Tearing up helps to build and strengthen personal relationships because it signals to others that your defenses are down, you’re less of a threat and it may evoke feelings of empathy in others. Researcher Oren Hasson, PhD explained: 
“My analysis suggests that by blurring vision, tears lower defenses and reliably function as signals of submission, a cry for help, and even in a mutual display of attachment and as a group display of cohesion.”
Do tears have medicinal properties?
Tears shed due to an emotional response contain a high concentration of adrenocorticotropic hormone (ACTH) — a chemical linked to stress.
One theory of why you cry when you’re sad is that it helps your body release some of these excess stress chemicals, thereby helping you feel more calm and relaxed.
Tears also contain nerve growth factor (NGF), which is a neuropeptide that plays a role in the development and survival of neurons, particularly sensory neurons involved in transmitting pain, temperature and touch.  According to Provine: 
“Several lines of evidence suggest that the NGF in tears has medicinal functions. The NGF concentration in tears, cornea, and lacrimal glands increases after corneal wounding, suggesting that NGF plays a part in healing.
More directly, the topical application of NGF promotes the healing of corneal ulcers and may increase tear production in dry eye … Although more of a scientific long shot, I suggest that tears bearing NGF have an anti-depressive effect that may modulate as well as signal mood.
Non-emotional, healing tears may have originally signaled trauma to the eyes, eliciting caregiving by tribe members or inhibiting physical aggression by adversaries.
This primal signal may have later evolved through ritualization to become a sign of emotional as well as physical distress.
In this evolutionary scenario, the visual and possibly chemical signals of emotional tears may be secondary consequences of lacrimal secretions that originally evolved in the service of ocular maintenance and healing.”
Four interesting facts about crying
Why humans shed tears is still a question shrouded in mystery, but there are some intriguing facts worth learning. Mental Floss recently shared several of them: 
1. Crying (eventually) makes you feel better
Research published in Motivation and Emotion found crying may lead to both worsened and heightened mood depending on when your mood is measured. 
The research involved 60 people who watched an emotional movie and had their moods assessed immediately after as well as 20 and 90 minutes later. Those who cried during the film had significantly increased negative moods right after while non-criers’ moods remained unchanged.
By the next measurement, the criers’ moods had returned to baseline but, interestingly, by the final measurement their moods had not only recovered but also were enhanced compared to their pre-film measurements. So while crying might initially make you feel worse, it may ultimately boost your mood. The researchers explained: 
“After the initial deterioration of mood following crying that was observed in laboratory studies, it apparently takes some time for the mood, not just to recover, but also to become even less negative than before the emotional event, which corresponds to the results of retrospective studies.”
2. You can stop onions from making you cry
Onions release a gas called lachrymatory factor (LF), which causes tearing. Japanese researchers developed an onion that lacked the enzyme necessary to produce LF, and therefore wouldn’t cause tearing, but it also altered the beneficial sulfur-containing compounds in the onion.
So while the fact that onions make you tear up is a bit inconvenient, it’s also a reminder of the many potent health compounds they contain. That being said, the World’s Healthiest Foods shared a few tips to cutting onions that should help lessen eye irritation and tearing. If this is an issue for you, don’t give up on onions. Try these tips instead: 
“Use a very sharp knife and always cut the onions while standing; that way your eyes will be as far away as possible. Consider cutting onions by an open window. If cutting onions really makes you cry, consider wearing glasses or goggles.
Chill the onions for an hour or so before cutting; this practice can slow down the onion’s metabolism and thereby lessen the rate of LF gas production.
Cutting onions under cold, running water is a method that is often used to cut back on eye irritation, but it’s a method we view as a second-best choice since some of the nutrients found in onion can be lost into the flow of water.”
3. Crying might help you win a negotiation
Expressing sadness, including crying, during a negotiation might help you to get your way. This was true, however, only when certain conditions were met, namely when recipients: 
- Perceived the expresser as low power
- Anticipated a future interaction
- Construed the relationship as collaborative in nature
- Believed that it was inappropriate to blame others
4. Crying after sex is normal
Research suggests nearly half (46 percent) of women have cried after sex at some point in their lifetime  (and separate research suggests many men have cried after sex as well). Known as postcoital dysphoria, or PCD, this may be due to fluctuations in hormones that occur during and after sex. It may also be due to the intimate nature of sex, which allows people to express emotions they’ve been keeping bottled up.
What do tears look like under a microscope?
In a project called “Topography of Tears,” photographer Rose-Lynn Fisher used a microscope to examine what dried human tears look like close up. Over the course of several years, she examined more than 100 tears from herself, volunteers and even a newborn baby, under a microscope.
What resulted was a beautiful collection of strikingly different images, many resembling large-scale landscapes. Fisher described them as “aerial views of emotion terrain.”  She told Smithsonian magazine: 
” … Tears are the medium of our most primal language in moments as unrelenting as death, as basic as hunger and as complex as a rite of passage … It’s as though each one of our tears carries a microcosm of the collective human experience, like one drop of an ocean.”
As the saying goes, a picture is worth 1,000 words, so to see the photos for yourself, see Rose-Lynn Fisher’s website.  What is perhaps most intriguing is the different forms tears take depending on the emotions behind the. Tears of “laughing till I’m crying,” tears of grief, tears of change, onion tears and others all appear remarkably different.
Source: Rise Earth
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