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7 Filtered Facts About the Unappreciated Spleen

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The human body is an amazing thing. For each one of us, it’s the most intimate object we know. And yet most of us don’t know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy part by part. Think of it as a mini digital encyclopedia with a dose of wow.

1. IT'S SHAPED LIKE A SHOE.

The spleen is shoe-shaped, and it’s the largest organ in the lymphatic system at a whopping 5 inches, weighing as much as 6 ounces. The organ is situated between the fundus of the stomach and the diaphragm, and connected by one little ligament to your left kidney, too.

2. IT'S A BLOOD FILTER.

Your spleen is in charge of filtering blood as it moves through your body to control the production of red blood cells. It also defines the donut-like shape of red blood cells, which must squeeze through an opening in the spleen known as the interendothelial slit. This reshapes the cells before they pass back into the blood stream—or, if they are old, misshapen, or diseased, blocks them from returning.  

3. IT'S A POWERHOUSE INFECTION FIGHTER.

As recently as 2009, researchers discovered that the spleen plays host to a special cache of immune cells called monocytes. If the human body suffers a serious trauma, such as a heart attack, wound, or big infection, the spleen sends those monocytes into the blood stream to fight off infection.

4. ITS LOCATION IS BOTH IDEAL AND DANGEROUS.

From Anatomy of the Human Body by Henry Gray via Wikimedia Commons // Public Domain

 
The spleen is neatly tucked up under the left side of the rib cage, where it’s protected by those lateral bones. But this also means a broken rib could pierce the fragile organ and rupture it. If the organ is breached, usually during a traumatic injury like a car accident, it has to be removed immediately. Because the spleen has so many blood vessels, a person would otherwise bleed to death.

5. YOU CAN LIVE WITHOUT IT.

Unlike, say, your stomach or lungs, the spleen is not considered a “vital” organ, which simply means that it can be surgically removed and you’ll be mostly fine, though potentially prone to more infections.

6. THERE HAVE BEEN SPLEEN TRANSPLANTS.

A handful of these procedures have been done with some success on humans to boost infection-fighting power, but they are rare, and are usually transplanted in patients who have already lost their own spleens.

7. NO ONE LIKES A SPLEENFUL PERSON.

In medieval times, people believed your spleen was the source of “morose feelings and bad temper.” The connotation entered into common speech (via Latin, splen)—no one would want to be around you if you were spleenful, though spleenless was a prized way to be. Today, the word spleen is still in Webster’s Dictionary meaning “feelings of ill-will or anger, often suppressed.”

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What's Really Happening When We See 'Stars' After Rubbing Our Eyes?
Photo illustration by Mental Floss. Images: iStock.
Photo illustration by Mental Floss. Images: iStock.

It's likely happened to you before: You start rubbing your eyes and almost immediately begin seeing colors, specks, and swirls from behind your closed lids. So what's happening when you see these 2001-esque "stars"? Do they only occur upon rubbing? Does everyone experience them?

Before we can get to what causes the lights, we need to understand a bit about how the eyes work. Angie Wen, a cornea surgeon at New York Eye and Ear Infirmary of Mount Sinai, tells Mental Floss that the retina—the innermost layer of the eye—consists of millions of cells, or photoreceptors. These cells, she says, "are responsible for receiving information from the outside world and converting them to electrical impulses that are transmitted to the brain by the optic nerve. Then, the brain interprets them as images representing the world around us."

However, what we see doesn't just stop there. Sometimes "we see light that actually comes from inside our eyes or from electric stimulation of the brain rather than from the outside world," Wen says. "These bursts of seemingly random intense and colorful lights are called phosphenes, and appear due to electrical discharges from the cells inside our eyes that are a normal part of cellular function."

People have been writing and theorizing about phosphenes for thousands of years. Greek philosophers thought the bursts of light were the result of fire inside our heads: "The eye obviously has fire within it, for when the eye is struck fire flashes out," wrote Alcmaeon of Croton (6th–5th century BCE), a philosopher and early neuroscientist, of the swirls and specks someone sees after getting a blow to the head. A century later, Plato—who believed that a "visual current" [PDF] streamed out of the eye—wrote that "Such fire as has the property, not of burning, but of yielding a gentle light they [the Gods] contrived should become the proper body of each day."

Plato's take was still the dominant one through the Middle Ages. Eventually, Newton (1642–1727) theorized a concept that's more in line with what's believed today about these strange sparkly visions: The phenomenon is due to light that's produced and observed when pressure and motion is placed on the eyes.

Eleonora Lad, an associate professor of ophthalmology at Duke University Medical Center who has a background in neuroscience, explains exactly why eye rubbing generates these visions: "Most vision researchers believe that phosphenes result from the normal activity of the visual system after stimulation of one of its parts from some stimulus other than light," including putting external pressure on the eyes. (Interestingly, due to retinal damage, blind people can't see phosphenes caused by pressure, but they can see them when their visual cortex is electrically stimulated. In hopes of turning this phenomenon into improved vision for the blind, scientists have developed a cortical visual prosthesis, implanted in the visual cortex, that generates patterns of phosphenes. The device has been approved by the FDA for clinical trial.)

As Alcmaeon rightly pointed out, there are causes for the bursts of light beyond just rubbing your eyes: Getting hit in the eye can produce this phenomenon—as can a sneeze, a surprisingly powerful event that tends to clamp our eyes shut, Wen says.

Receiving an MRI or EEG may also trigger it. MRIs, for example, produce a changing magnetic field which can stimulate the visual cortex, making a person see these flashing lights. When it comes to an EEG, depending on the brain stimulation frequency band (Hz) used, some patients experience the phenomenon when closing their eyes, which is believed to come from retinal stimulation during the process.

And the activity doesn't only happen on Earth; astronauts in space have also been known to experience them. As reported in 2006 in the journal Vision Research, "over 80 percent of astronauts serving in today's NASA or ESA (European Space Agency) programs have perceived phosphenes at least in some missions and often over several orbits." They're mainly attributed to interactions between the eye and cosmic ray particles in space, outside the Earth's protective magnetic field.

No matter the cause, the bursts of light are perfectly normal—but that doesn't mean you should engage in excessive eye rubbing. Wen says ophthalmologists advise against rubbing your eyes or applying vigorous pressure; according to Lad, too much rubbing may be damaging to the cornea and lens or "result in a loss of fatty tissue around the eyes, causing the eyes to look deep-set."

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Why Your Knuckles Make That Satisfying Cracking Sound, According to Science
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Scientific curiosity is not always burdened by matters of great consequence. Over the years, considerable money and time has been applied to matters involving facial recognition between sheep, whether the flow of urine is impeded by someone watching you pee, and whether humans can capably swim in a pool full of syrup. (They can, almost as well as water.)

Now, researchers from Stanford University and Ecole Polytechnique in France have turned the roving eye of science to the phenomenon of knuckle-cracking. According to Gizmodo, a computer simulation was created to confirm an earlier theory that the audible noise that comes from the human hand after putting pressure on the knuckle was the result of gas bubbles popping inside the finger joint.

Conclusion: Probably true.

The study, published in Scientific Reports, demonstrated that microscopic bubbles inside the lubricating synovial fluid of the joint collapse when a knuckle-cracking session commences. To use an imperfect analogy, the cavitation bubbles are like the body’s Bubble Wrap. Popping them produces an audible—and for many, a very pleasing—sound.

To compile data, researchers took geometric representations of the joint's movements during a cracking session and turned them into mathematical equations. (Imaging has not been shown to be very productive in this field, as the crack takes only about 300 milliseconds and is not easily visualized.) The software models demonstrated that pressure shifts in the joint fluid increase pressure on the gas bubbles. Unlike packing material, however, the gas bubbles don't really perforate—they experience a partial collapse but remain suspended in the joint.

So does this solve the mystery surrounding cracked knuckles? Not entirely. Because it was a simulation, there's a possibility of mathematical error. Proponents of alternative theories—that it's not bubbles collapsing but bubbles being created that produce the noise—feel there's more work to be done. We can only hope a complete understanding will come in our lifetime. Fingers crossed. And cracking.

[h/t Gizmodo]

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