Whether it’s a sad movie, news of a recent death or chopping onions, certain events can move you to tears. There are many different purposes to your tears, which is why your eyes produce anywhere from 15 to 30 gallons a year, according to the American Academy of Ophthalmology.
Although other animals shed tears, scientists have long said humans are the only animals capable of shedding emotional tears. But a small new study from Japan found dogs’ eyes welled up with tears of joy when being reunited with their owners. (See box at the end of the article for details.)
Why do we cry?
Just as there are many different reasons why your eyes well and sometimes spill over with tears, there are different types of tears, each made of different substances. We cry to protect our eyes, to wash out irritants and because, well, we are moved to tears. “There are three types of tears: basal tears, emotional tears and reflex tears,” explains David Silverstone, M.D., a professor of ophthalmology at the Yale School of Medicine.
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The three forms of tears have some things in common. They all share certain key ingredients and are all made up of three layers, known as the tear film:
- An inner mucus layer to keep the tear stuck to the eye
- A watery middle layer to keep the eye wet, fight off bacteria and the cornea, or the clear outer layer of your eye
- An oily outer layer to prevent the tears from drying out
How are tears made?
The water in your tears is produced by the lacrimal glands above your eyes, Silverstone says. This fluid is made up of salt and water, which helps keep the surface of your eyes lubricated and healthy. The mucus and oil come from your meibomian glands, oil glands along the edge of your eyelids, where your eyelashes are. “In order for your tears to work properly, you need both components,” he adds. As you blink, these substances come together and spread across the surface of your cornea. They then drain into your tear ducts, the small holes in the inner corners of your eyelids, and down through your nose, where they either evaporate or are reabsorbed.
How are the three types of tears different?
There are also some key differences among the three types of tears. Here’s a closer look.
1. Basal tears
These are the tears that your eyes are bathed in all day. When you blink, you spread them evenly over your eye’s surface, to improve your vision and focus. They wash away dirt and germs, to keep your eyes healthy and free of infection.
Famous Quotes on Tears
- “If you have tears, prepare to shed them now.” —Mark Antony, Julius Caesar
- “But mermaids have no tears and therefore she suffers so much more.” —Hans Christian Andersen, The Little Mermaid
- “Heaven knows we need never be ashamed of our tears.” —Charles Dickens, Great Expectations
- “I often cry when I am happy, and smile when I am sad.” —Anne Brontë, The Tenant of Wildfell Hall
- “For every laugh, there should be a tear.” —Walt Disney in The New York Times
2. Reflex tears
These are the tears that your eyes produce when you chop an onion. They wash away harmful irritants like smoke or particles. They come mainly from your lacrimal gland, so they are mostly water. Your eyes make more of them than basal tears, and they contain substances, such as antibodies, to help fight germs.
3. Emotional tears
These are the tears that pour out of you when you go through a crying jag. They contain additional proteins and hormones you won’t find in other tears, such as prolactin, potassium, manganese and stress hormones. One theory as to why we cry tears of emotion is that you release stress through these tears, to help calm your body down. Unlike basal tears, your body doesn’t make them automatically. For emotional tears to kick in, your limbic system — the part of your brain that regulates emotions — sends a signal to your brain’s message system to activate your lacrimal glands to produce tears. The result? A full-on cry-fest. When you make a lot of them, they overwhelm your tear ducts. As a result, they spill out of your eyes down your cheeks and even sometimes drip down into your nose.
Why do we cry emotional tears? It’s thought that emotional tears themselves have an evolutionary purpose: They’re a way to get empathy. Research has found that people who get social support when they shed tears report that they feel better than those who hold back their waterworks, or who don't have any support.
What is dry eye?
As you get older, your lacrimal gland’s production of water starts to slow down, says Silverstone. This can make you more susceptible to develop a condition known as dry eye, which may make eyes feel painful and impact vision. You’re also more susceptible to blepharitis, a condition that causes eyelid irritation and swelling. “Your meibomian glands can become inflamed over time, so that its secretions get stuck,” explains Silverstone. “I tell my patients that they need to take care of these glands by applying a warm compress to their eyes for 15-20 seconds a day. It’s like their eyes’ version of flossing.” Adding in artificial tears and possibly an omega 3 fatty acid supplement may also help to treat dry eye, he adds. Your eye doctor can help you find solutions.
Scientist Takefumi Kikusui and other researchers at the Center for Human and Animal Symbiosis Science at Azabu University in Japan compared the amount of tears in dogs eyes after reunions with owners to greetings with other people the animals knew. They found that only reunions with owners increased the amount of doggy tears. They also experimented by administering the “feel-good” hormone oxytocin to dogs’ eyes and found that the dogs who had received oxytocin produced more tears than those who got a control substance. Finally, they showed people photos of dogs with and without tears in their eyes and found that humans had more positive thoughts about the teary-eyed pups, which “can stimulate caring emotions in humans.” In other words, we might find giving into those big, wet puppy eyes with a treat — or food, or a walk, or water — irresistible, according to the new study they published in Current Biology. Some tear scientists, however, remain skeptical that any animal other than humans can shed emotional tears; they say more research needs to be done before we really know if our pups are crying for us.
Hallie Levine is a contributing writer and an award-winning medical and health reporter. Her work has appeared in The New York Times, Consumer Reports, Real Simple, Health and Time, among other publications.