12 Facts About the Penis

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Of all the body parts, none may elicit more questions—and myths—than the penis. One of the male sexual organ's main roles is to make procreation happen, but the penis also has cultural significance. Here are 12 facts to clear up the confusion.

1. THE PENIS HAS TWO PRIMARY FUNCTIONS.

The penis has two main biological roles, according to Michael Reitano, a physician-in-residence and an expert in sexual health and wellness for Roman Health. One is the elimination of waste in the form of urine; the second is the means for transferring semen, which carries sperm from the testes out of the body to somewhere else, such as the vagina for procreation. Another of its functions is, of course, sexual pleasure.

2. IT DEVELOPS FROM A CLITORIS-LIKE ORGAN.

All mammalian embryos start life as female in presentation, before the chromosome process is activated, with an external, undifferentiated clitoris-like structure. Eventually, embryos with XX chromosomes will develop a clitoris, labia, and vagina, while those with XY chromosomes will grow a penis and testes.

3. THE PENIS IS COMPOSED OF THREE TUBES.

The penis may look like one long tube, but there are actually three columns of tissue that run along the inside of the penis. Two are corpus cavernosum columns, which extend from the base to the end of the penis and fill with blood to allow for erection. The other is the corpus spongiosum; it surrounds the urethra, the tube through which urine and semen passes. The corpus spongiosum also fills with blood during erection, but remains pliable to keep the urethra open.

4. HUMANS MAY HAVE THE LARGEST PENIS OF ALL PRIMATES ...

When girth is considered, the human penis is quite a bit larger than those of its primate cousins. "A gorilla, for example, has a penis just 2 inches long. Human males walk upright, and it is thought that larger genitalia might have acted as a sexual attractant in competitive situations," Reitano says. But according to the Center for Academic Research and Training in Anthropogeny, chimpanzees and bonobos' penises are more slender but comparable to the length of an average human penis (which is 5.16 inches long and 4.59 inches around when erect, 3.61 inches and 3.66 inches around when not). Scientists theorize that our unique proportions are a result of natural selection through female mate choice.

5. ... AND THOSE PENISES MAY ONCE HAVE BEEN BARBED.

The human penis is most definitely among the smoothest in the animal kingdom. "In the distant past, it probably had sharp barbs, like some of our primate cousins, to make sex with another partner soon after coitus unlikely," Reitano says. Chimp penises still have small barbs that "hold the female in place, and when the penis is removed, it irritates the female's vagina so she avoids other chimps who might want to mate with her," he explains. (Let's not get started on the exploding corkscrew penis of the Muscovy duck, which looks like it makes for some irritating coitus.)

6. IN THE 15TH CENTURY, MEN FEARED WITCHES WHO COULD STEAL THEIR PENISES.

One of many bizarre beliefs put forth in the Malleus Maleficarum, a 15th-century German witch-hunting manual by Heinrich Kramer and Jacob Sprenger, was the opinion that witches could steal men's penises. Kramer wrote that witches "can take away the male organ." He didn't mean Lorena Bobbitt-style, clarifying, "not indeed by despoiling the human body of it, but by concealing it with some glamour."

In a 2002 paper published in the Journal of Folklore Research, Moira Smith pointed to male sexual insecurity as a driver of the witch hunts. "Many of the crimes (maleficia) attributed to witches concerned sexuality: copulation with incubus devils, procuring abortions, causing sterility and stillbirth, and impeding sexual relations between husbands and wives," she wrote.

7. ERECTIONS ARE MORE COMPLICATED THAN THEY SEEM.

Achieving an erection is one of the most complex functions to happen in a man, Reitano says: "For starters, hormones must be released on demand, arteries need to carry six times more blood to the penis with perfect efficiency, the nervous system must transmit its signals without a hitch, and the mind must be working in perfect harmony with the body."

The ability to get and sustain an erection, he says, depends upon "a body that is perfectly tuned physically, psychologically, and emotionally." The inability to achieve an erection, a.k.a. erectile dysfunction, is usually the first sign of poor health, according to Reitano.

8. UNLIKE OTHER MAMMALS, HUMANS LACK A PENIS BONE …

Many mammals, including gorillas and chimpanzees, have a penis bone or "baculum," says Arash Akhavein, a urologist at Cedars-Sinai in Los Angeles. In some animals, like whales, the baculum hangs back inside the abdomen until mating time, and then it slides into the penis to help maintain an erection. Instead of relying on this kind of structure, the human penis requires blood flow and engorgement for erection.

9. … AND YET IT'S POSSIBLE TO "FRACTURE" A PENIS.

Unfortunately, the penis can be fractured during sex, Reitano says. While penile fracture is relatively uncommon, it can happen when the thick sheath called the tunica albuginea, which gives an erect penis its rigidity, is injured by blunt force. "When it fractures, there is usually a clear popping sound, extreme pain, and the rapid loss of erection," Reitano says.

And yes, researchers have studied the sexual positions associated with the problem in heterosexual couples. A 2014 study in Advances in Urology found that the woman being atop the man was the primary position associated with penile fracture; the man being behind the woman was the second most commonly linked. A 2017 study in the International Journal of Impotence Research found that man-behind-woman was most often associated with fracturing a penis, followed by man-atop-woman.

10. MASTURBATION FEARS MAY HAVE DRIVEN MASS CIRCUMCISION.

In an uncircumcised penis, the glans (head) of the penis is covered by skin known as the foreskin, which can be pulled back from the glans. Circumcision, the removal the foreskin, is an ancient surgical practice performed for religious reasons and to prevent issues stemming from poor hygiene.

One thing circumcision doesn't prevent is masturbation. But in English-speaking countries in the 19th century, a general attitude against sexual profligacy, fueled by religious ideology, led to fears about the effects of masturbation and a spike in circumcision. Religious leaders and physicians warned people to avoid "self-abuse" for fear it could cause physical and mental disorders such as tuberculosis, memory loss, and epilepsy. As Tom Hickman writes in his book God's Doodle: The Life and Times of the Penis, "What made circumcision common among the proliferating 19th-century middle classes on both sides of the Atlantic was the hysteria about masturbation; removing the foreskin helped its prevention, doctors declared, and also cured bed-wetting and other conditions."

11. ADULT MEN GET CIRCUMCISED, TOO.

It's most common for boys to be circumcised as newborns, and in some cultures as adolescents, but there are a number of reasons why adult men may choose to have a later-in-life circumcision, Akhavein says. These include tears in the skin where the tip of the penis attaches to the foreskin; when the foreskin is too tight to be retracted easily, a condition called phimosis; and a buildup of smegma, a whitish waxy substance made up of dead skin cells and oils. Men who had an incomplete circumcision (too little skin removed) in childhood would also be good candidates.

12. ICELAND HAS A PENIS MUSEUM.

In 1974, an Icelandic history teacher named Sigurður Hjartarson received a cattle whip made out of a bull penis, which apparently gave him the idea to collect other penises, because hey, why not? The result is the Icelandic Phallological Museum, which holds a collection of more than 238 penises and penile parts from nearly all the fauna in Iceland. Yes, including Homo sapiens.

The largest penis on hand, from a sperm whale, stands at 6 feet tall and weighs nearly 150 pounds. "You’ll learn that as with everything in nature, the diversity in this department is as great as in any other; even within the same species the difference in size and shape is often quite remarkable," Hjartarson told Mental Floss in 2015.

25 Amazing Facts About the Human Body

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The human body is an amazing piece of machinery—with a few weird quirks.

  1. It’s possible to brush your teeth too aggressively. Doing so can wear down enamel and make teeth sensitive to hot and cold foods.

  2. Goose bumps evolved to make our ancestors’ hair stand up, making them appear more threatening to predators.

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  1. Wisdom teeth serve no purpose. They’re left over from hundreds of thousands of years ago. As early humans’ brains grew bigger, it reduced space in the mouth, crowding out this third set of molars.

  2. Scientists aren't exactly sure why we yawn, but it may help regulate body temperature.

  3. Your fingernails don’t actually grow after you’re dead.

  4. If they were laid end to end, all of the blood vessels in the human body would encircle the Earth four times.

  5. Humans are the only animals with chins.

    An older woman's chin
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    1. As you breathe, most of the air is going in and out of one nostril. Every few hours, the workload shifts to the other nostril.

    2. Blood makes up about 8 percent of your total body weight.

    3. The human nose can detect about 1 trillion smells.

    4. You have two kidneys, but only one is necessary to live.

    5. Belly buttons grow special hairs to catch lint.

      A woman putting her hands in a heart shape around her belly button
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      1. The satisfying sound of cracking your knuckles comes from gas bubbles bursting in your joints.

      2. Skin is the body’s largest organ and can comprise 15 percent of a person’s total weight.

      3. Thumbs have their own pulse.

      4. Your tongue is made up of eight interwoven muscles, similar in structure to an elephant’s trunk or an octopus’s tentacle.

      5. On a genetic level, all human beings are more than 99 percent identical.

        Identical twin baby boys in striped shirts
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        1. The foot is one of the most ticklish parts of the body.

        2. Extraocular muscles in the eye are the body’s fastest muscles. They allow both of your eyes to flick in the same direction in a single 50-millisecond movement.

        3. A surgical procedure called a selective amygdalohippocampectomy removes half of the brain’s amygdala—and with it, the patient’s sense of fear.

        4. The pineal gland, which secretes the hormone melatonin, got its name from its shape, which resembles a pine nut.

        5. Hair grows fast—about 6 inches per year. The only thing in the body that grows faster is bone marrow.

          An African-American woman drying her hair with a towel and laughing
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          1. No one really knows what fingerprints are for, but they might help wick water away from our hands, prevent blisters, or improve touch.

          2. The heart beats more than 3 billion times in the average human lifespan.

          3. Blushing is caused by a rush of adrenaline.

12 Intriguing Facts About the Intestines

When we talk about the belly, gut, or bowels, what we're really talking about are the intestines—long, hollow, coiled tubes that comprise a major part of the digestive tract, running from the stomach to the anus. The intestines begin with the small intestine, divided into three parts whimsically named the duodenum, jejunum, and ileum, which absorb most of the nutrients from what we eat and drink. Food then moves into the large intestine, or colon, which absorbs water from the digested food and expels it into the rectum. That's when sensitive nerves in your rectum create the sensation of needing to poop.

These organs can be the source of intestinal pain, such as in irritable bowel syndrome, but they can also support microbes that are beneficial to your overall health. Here are some more facts about your intestines.

1. The intestines were named by medieval anatomists.

Medieval anatomists had a pretty good understanding of the physiology of the gut, and are the ones who gave the intestinal sections their names, which are still used today in modern anatomy. When they weren't moralizing about the organs, they got metaphorical about them. In 1535, the Spanish doctor Andrés Laguna noted that because the intestines "carry the chyle and all the excrement through the entire region of the stomach as if through the Ocean Sea," they could be likened to "those tall ships which as soon as they have crossed the ocean come to Rouen with their cargoes on their way to Paris but transfer their cargoes at Rouen into small boats for the last stage of the journey up the Seine."

2. Leonardo da Vinci believed the intestines helped you breathe.

Leonardo mistakenly believed the digestive system aided respiratory function. In 1490, he wrote in his unpublished notebooks, "The compressed intestines with the condensed air which is generated in them, thrust the diaphragm upwards; the diaphragm compresses the lungs and expresses the air." While that isn't anatomically accurate, it is true that the opening of the lungs is helped by the relaxation of stomach muscles, which does draw down the diaphragm.

3. Your intestines could cover two tennis courts ...

Your intestines take up a whole lot of square footage inside you. "The surface area of the intestines, if laid out flat, would cover two tennis courts," Colby Zaph, a professor of immunology in the department of biochemistry and molecular biology at Melbourne's Monash University, tells Mental Floss. The small intestine alone is about 20 feet long, and the large intestine about 5 feet long.

4. ... and they're pretty athletic.

The process of moving food through your intestines requires a wave-like pattern of muscular action, known as peristalsis, which you can see in action during surgery in this YouTube video.

5. Your intestines can fold like a telescope—but that's not something you want to happen.

Intussusception is the name of a condition where a part of your intestine folds in on itself, usually between the lower part of the small intestine and the beginning of the large intestine. It often presents as severe intestinal pain and requires immediate medical attention. It's very rare, and in children may be related to a viral infection. In adults, it's more commonly a symptom of an abnormal growth or polyp.

6. Intestines are very discriminating.

"The intestines have to discriminate between good things—food, water, vitamins, good bacteria—and bad things, such as infectious organisms like viruses, parasites and bad bacteria," Zaph says. Researchers don't entirely know how the intestines do this. Zaph says that while your intestines are designed to keep dangerous bacteria contained, infectious microbes can sometimes penetrate your immune system through your intestines.

7. The small intestine is covered in "fingers" ...

The lining of the small intestine is blanketed in tiny finger-like protrusions known as villi. These villi are then covered in even tinier protrusions called microvilli, which help capture food particles to absorb nutrients, and move food on to the large intestine.

8. ... And you can't live without it.

Your small intestine "is the sole point of food and water absorption," Zaph says. Without it, "you'd have to be fed through the blood."

9. The intestines house your microbiome. 

The microbiome is made up of all kinds of microorganisms, including bacteria, viruses, fungi, and protozoans, "and probably used to include worm parasites too," says Zaph. So in a way, he adds, "we are constantly infected with something, but it [can be] helpful, not harmful."

10. Intestines are sensitive to change.

Zaph says that many factors change the composition of the microbiome, including antibiotics, foods we eat, stress, and infections. But in general, most people's microbiomes return to a stable state after these events. "The microbiome composition is different between people and affected by diseases. But we still don't know whether the different microbiomes cause disease, or are a result in the development of disease," he says.

11. Transferring bacteria from one gut to another can transfer disease—or maybe cure it.

"Studies in mice show that transplanting microbes from obese mice can transfer obesity to thin mice," Zaph says. But transplanting microbes from healthy people into sick people can be a powerful treatment for some intestinal infections, like that of the bacteria Clostridium difficile, he adds. Research is pouring out on how the microbiome affects various diseases, including multiple sclerosis, Parkinson's, and even autism.

12. The microbes in your intestines might influence how you respond to medical treatments.

Some people don't respond to cancer drugs as effectively as others, Zaph says. "One reason is that different microbiomes can metabolize the drugs differently." This has huge ramifications for chemotherapy and new cancer treatments called checkpoint inhibitors. As scientists learn more about how different bacteria metabolize drugs, they could possibly improve how effective existing cancer treatments are.

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