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11 Amazing Facts About the Nipple

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.

Despite its relatively small size on the human body, the nipple gets a lot of attention. Biologically, the nipple serves two key functions: In women, nipples deliver milk out of the ducts within the breast to babies, and for women and men, they serve as erogenous zones. Check out our list of fascinating facts about this often-misunderstood body part.

1. THE NIPPLE HAS ITS OWN SWEAT GLANDS.

The nipple is the raised bump or protrusion on top of the breast that sits on the circular area known as the areola. The areola is often much larger in circumference than the actual nipple, as it holds small sweat glands called Montgomery glands (named for William Fetherstone Montgomery, an Irish obstetrician who first described them). The sole function of these glands is to secrete fluids during breastfeeding to lubricate the nipple and to produce a scent that attracts the baby to its mother's breast.

2. NIPPLES CAN POKE IN OR OUT.

Not all nipples point jauntily outward. Men's and women's nipples can be inverted, essentially pointing inward. In the worst-case scenario, "the skin adheres to itself and has to peel open to [turn outward] initially, and [that] can be painful," Constance Chen, a board-certified plastic surgeon and clinical assistant professor of plastic surgery at Weill Cornell Medical College in New York, tells Mental Floss. For most women, however, an inverted nipple causes neither pain nor prevents breastfeeding with proper technique or nipple shields.

3. SOME PEOPLE HAVE EXTRA NIPPLES.

While most people have two nipples, one atop each breast (yes, both women and men have breasts), some people have extra or "supernumerary" nipples outside of the typical location. One Indian man was even found to have seven.

Leigh Anne O'Connor, a certified lactation consultant in New York, says these extra nipples can appear on or below an area named the Tail of Spence (after Scottish surgeon James Spence), which extends from the breast up into the armpit.

"Some people have nipples in their armpits, or even tiny breasts, and these nipples may leak," O'Connor tells Mental Floss. But an extra nipple is just an extra nipple—no cause for alarm or shame.

4. HERE'S WHY MEN HAVE NIPPLES.

Since the main purpose of nipples is breastfeeding babies, and male breasts do not lactate, it begs the question: Why do men have nipples? Scientists Stephen Jay Gould and Richard C. Lewontin tried to get to the bottom of this conundrum in a seminal paper in 1993.

All human embryos start out essentially the same. If the embryo has XY chromosomes, a gene on the Y chromosome called SRY will activate within a couple weeks of conception and begin to differentiate the embryo into one with male genitals.

However, it turns out that breast tissue begins to develop before SRY kicks in, and since nipples in men essentially do no harm, Gould and Lewontin argue, there has simply never been a good enough reason—evolutionarily speaking—to do away with them. They linger because they're benign. Or as Andrew M. Simons, a professor of biology at Carleton University in Ottawa, Ontario wrote in Scientific American, "The presence of nipples in male mammals is a genetic architectural by-product of nipples in females. So, why do men have nipples? Because females do."

5. THE WORLD'S STRONGEST NIPPLES BELONG TO THE GREAT NIPPULINI.

Who needs pecs when you have nipples like those of The Great Nippulini, a.k.a. Sage Werbock, a performer who makes a living demonstrating the mighty power of his nipples? Each nipple can lift 70 pounds, and he holds a Guinness record for the heaviest vehicle pulled by nipples for 20 meters (66 feet)—988.5 kilograms (2179.27 pounds). He has also lifted a variety of dumbbells, anvils, and bowling balls.

6. THEY'RE VERY SENSITIVE TO STIMULATION.

It's no secret that many people take sexual pleasure from nipple stimulation. However, Michael Reitano, an expert in sexual health and wellness at Roman Health in New York, brings up a study published in 2011 in which researchers set out to map the neurology of sexual stimulation in women. Through MRI imaging, they determined that "when [the nipple is] stimulated, the sensations travel to the same part of the brain that is stimulated when the clitoris, vagina, or cervix is stimulated," Reitano says. The study, published in the Journal of Sexual Medicine, also confirmed that it was possible for some women to have an orgasm by nipple stimulation alone.

While the same brain mapping has not yet been done on men, "there is every reason to believe that it has some capacity to function as a source of sexual pleasure for men as well," Reitano tells Mental Floss.

7. NIPPLES ARE AS UNIQUE AS FINGERPRINTS.

Nipples come in many colors, including pale pink, reddish-beige, brown, and black. Your own two nipples can even vary from each other, as can the areolae. "They also come in many different shapes," O'Connor says. "Some are more flat, while others can be quite bulbous. A person can have two nipples that look very different from each other. Asymmetry is normal."

8. A SPECIFIC KIND OF BREAST CANCER TARGETS THE NIPPLE.

While most forms of breast cancer affect the whole breast, Paget disease of the breast is a rare cancer that targets the skin and ducts of the nipple. "Most patients get a rash on the nipples that looks like a severe case of eczema. It is a cancer of breast epithelial (skin) cells," Chen says. Paget disease of the breast represents between 0.5 and 5 percent of all breast cancers.

9. AFTER MASTECTOMY, NIPPLE SENSATION CAN OCCASIONALLY BE RESTORED.

In breast cancer cases where mastectomy—removal of the breast—is necessary, it is sometimes possible to spare the nipple, allowing for a more realistic post-treatment reconstruction, though sensation is often lost. However, Chen says that in certain cases, "it is possible to restore sensation to the nipples with nerve repairs and nerve grafts when a woman undergoes natural tissue breast reconstruction. Sensory restoration to the nipple after mastectomy is very cutting edge, but if you find the right surgeon, it is possible."

10. NIPPLE STIMULATION CAN HELP INDUCE LABOR IN PREGNANT WOMEN.

When a woman is ready to give birth but the baby isn't, one piece of often-shared advice is to stimulate the mother's nipples to induce labor. A 2005 analysis of six trial studies found a significant decrease in the number of women who hadn't gone into labor after 72 hours. Just under 63 percent of the women who received stimulation were not in labor versus 94 percent who hadn't received it.

The mechanism isn't entirely clear, but breast stimulation causes the uterus to contract. It may also help release the hormone oxytocin, which can start contractions. Once the baby's born, the baby's suckling also has benefits for the mother. "When a newborn suckles, the increased oxytocin causes the uterus to contract [and shrink to its original size over the subsequent weeks] following birth," Reitano explains.

11. BREAST MILK HAS MULTIPLE WAYS TO EXIT THE NIPPLE.

If you've ever pumped your own breast milk or seen it done, you may have noticed that the milk doesn't just come out in a single stream. In a typical nipple, "There are between four and 20 outlets for the milk to come out—it can look like one stream or [coming from] various holes," O'Connor says.

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11 Facts About Fingernails
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Whether there's dirt beneath them or polish atop them, your fingernails serve more than just decorative purposes: They help keep your fingertips safe and have a multitude of special functions that even your doctor might not be aware of. “The nails occupy a unique space within dermatology and medicine in general, particularly because they are such a niche area about which few people have expertise,” Evan Rieder, assistant professor in the Ronald O. Perelman Department of Dermatology at NYU Langone Health, tells Mental Floss.

1. FINGERNAILS HAVE FOUR MAIN PARTS.

Along with skin and hair, nails are part of the body's integumentary system, whose main function is to protect your body from damage and infection. Fingernails have four basic structures: the matrix, the nail plate, the nail bed, and the skin around the nail (including the cuticle).

Fingernail cells grow continuously from a little pocket at the root of the nail bed called the matrix. The pale, crescent-shaped lunula—derived from Latin for "little moon"—on the nail itself is the visible portion of the matrix. If the lunula is injured, the  nail won't grow normally (a scarred lunula can result in a split nail), and changes in the lunula's appearance can also be signs of a systemic disease.

Fingernail cells are made of a protein called keratin (same as your hair). As the keratin cells push out of the matrix, they become hard, flat and compact, eventually forming the hard surface of the nail known as the nail plate. Beneath that is the nail bed, which almost never sees the light of day except when there's an injury or disease.

Surrounding the matrix is the cuticle, the semi-circle of skin that has a tendency to peel away from the nail. The skin just underneath the distal end of the fingernail is called the hyponychium, and if you've ever trimmed your nails too short, you know this skin can be slightly more sensitive than the rest of the fingertip.

2. THEY GROW AT A RATE OF 0.1 MILLIMETERS A DAY ...

That's about 3 to 4 millimeters per month. But they don't always grow at the same speed: Fingernails grow more quickly during the day and in summer (this may be related to exposure to sunlight, which produces more nail-nourishing vitamin D). Nails on your bigger fingers also grow faster, and men's grow faster than women's. The pinky fingernail grows the slowest of all the fingernails. According to the American Academy of Dermatology, if you lose a fingernail due to injury, it can take up to six months to grow back (while a toenail could take as much as a year and a half).

3. ... BUT NOT AFTER YOU'RE DEAD.

You've probably heard that your fingernails keep growing after death. The truth is, they don't, according to the medical journal BMJ. What's actually happening is that the skin around the base of the fingernails retracts because the body is no longer pumping fluids into the tissues, and that creates a kind of optical illusion that makes the nails appear longer.

4. ITS ESTIMATED THAT 20 TO 30 PERCENT OF PEOPLE BITE THEIR NAILS.

Scientists say it's still unclear why, but they suspect nail-biters do it because they're bored, frustrated, concentrating, or because it just feels comforting (and anxiety doesn't seem to play a big role). Perfectionists who don't like to be idle are very likely to have the habit. Biters expose themselves to the dangerous crud that collects underneath the nail: The hyponychium attracts bacteria, including E. coli, and ingesting that through nail-biting can lead to gastrointestinal problems down the line. Biting can also damage teeth and jaws.

5. HUMAN FINGERNAILS ARE BASICALLY FLAT CLAWS.

Our primate ancestors had claws—which, like nails, are made of keratin. As human ancestors began using tools some 2.5 million years ago (or even earlier), evolutionary researchers believe that curved claws became a nuisance. To clutch and strike stone tools, our fingertips may have broadened, causing the claws to evolve into fingernails.

6. THE NAIL ACTUALLY MAKES YOUR FINGERTIP MORE SENSITIVE.

While the fingernail may be tough enough to protect tender flesh, it also has the paradoxical effect of increasing the sensitivity of the finger. It acts as a counterforce when the fingertip touches an object. "The finger is a particularly sensitive area because of very high density of nerve fibers," Rieder says.

7. FINGERNAILS CAN REVEAL LUNG, HEART, AND LIVER DISEASES.

"One of the most interesting facts about fingernails is that they are often a marker for disease within the body," Rieder says. Nail clubbing—an overcurvature of the nail plate and thickening of the skin around the nails—is a particularly significant sign of underlying illness, such as lung or heart disease, liver disease, or inflammatory bowel disease. Two-toned nails—whitish from the cuticle to the nail's midpoint and pink, brown, or reddish in the distal half—can be a sign of kidney and liver disease. Nails that are two-thirds whitish to one-third normal can also be a sign of liver disease. However, little white marks on your nails, known as milk spots (or punctate leukonychia) are just the remnants of any kind of trauma to the nail, from slamming it in a door to chewing on it too fervently.

8. YOU CAN GET A COMMON SKIN DISEASE ON YOUR NAILS.

Psoriasis is "typically thought of as a skin disease, but is actually a skin, joint, and nail disease, and when severe, a marker of cardiovascular risk," Rieder says. Psoriatic fingernails may have orange patches called oil spots, red lines known as splinter hemorrhages, lifting of the edges of the nails, and pits, "which look like a thumb tack was repeatedly and haphazardly pushed into the nails," he says.

Doctors often prescribe topical or injected corticosteroids to treat psoriatic nails, but using lasers is an emerging and potentially more cost-effective technique. Rieder relies on a pulsed dye laser, which uses an organic dye mixed with a solvent as the medium to treat nail psoriasis, "which can be both medically and aesthetically bothersome," he says. This laser is able to penetrate through the hard nail plate with minimal discomfort and "to treat targets of interest, in the case of psoriasis, blood vessels, and hyperactive skin," Rieder says.

9. ANCIENT CULTURES DISPLAYED SOCIAL STATUS WITH NAIL ART.

Painting and other forms of decorating nails have a history of offering social and aesthetic cues through variations in nail color, shape, and length, Rieder says. In fact, he adds, in some cultures ornate and well-decorated fingernails "serve as a proxy for social status."

Five thousand years ago in China, men and women of the Ming Dynasty aristocracy grew their nails long and covered them with golden nail guards or bright home-made polishes. The long nails allegedly announced to the world their social rank and their freedom from performing menial labor.

10. A FORMER BEAUTICIAN HELD THE WORLD RECORD FOR THE LONGEST NAILS.

Lee Redmond of Utah started growing her nails in 1979 and kept at it until she held the world record for "longest fingernails on a pair of hands ever (female)" in 2008. Her right thumbnail was 2 feet, 11 inches and the collective length of all her nails was 28 feet, 4 inches. She also applied nail hardener daily and painted them a reflective gold. Unfortunately, she broke her nails in a 2009 car accident and has no plans to regrow them.

11. THE FIRST NAIL CLIPPERS WERE PATENTED IN 1875.

Today, biters don't have to use their teeth to trim their nails. While the earliest tools for cutting nails were most likely sharp rocks, sand, and knives, the purpose-built nail clipper—though it might be more accurately called a circular nail file—was designed by a Boston, Massachusetts inventor named Valentine Fogerty and patented in 1875. The nail clippers we know today were the design of inventors Eugene Heim and Oelestin Matz, who were granted their patent for a clamp-style fingernail clipper in 1881.

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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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