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.

10 Smart Facts About Your Gut

Colorized scanning electron micrograph of E. coli, a common gut bacteria
Colorized scanning electron micrograph of E. coli, a common gut bacteria
National Institute of Allergy and Infectious Diseases, Flickr // CC BY 2.0

Gut feelings get all the press, but your gut may be more of a thinker than you know. Some scientists now consider it a second brain. While it won’t necessarily help you study for an exam or get a promotion, your gut can influence the chemistry of your mood, emotions, immune system, and long-term health. Research even suggests the gut can “learn” new tricks through conditioning. These powerful connections are part an emerging field of science called neurogastroenterology designed to study the gut-brain link. Here are 10 facts you may not know about your gut.

1. THE GUT DOESN'T NEED THE BRAIN'S INPUT. 

You might think of your gut as a rebel against authority. It doesn’t wait for your brain’s impulses to do the important work of digestion, because it doesn’t need to—it acts as its own “brain.” No other organ, not even the all-powerful heart, can pull that off.

2. THERE ARE MORE THAN 100 MILLION BRAIN CELLS IN YOUR GUT.

Your gut’s power to think for itself is no surprise; there are millions of neurons in its lengthy coils (9 meters of intestines, from esophagus to anus). That’s more neurons than are found in the spinal cord or peripheral nervous system.

3. YOUR GUT HAS ITS OWN NERVOUS SYSTEM.

The enteric nervous system—the controlling mechanism of digestion and elimination—is the overlord of your gut, and functions all on its own. Some scientists see it as part of the central nervous system, while others consider it its own entity. It likely evolved to give the gut the go-ahead when the “got to go” impulse strikes, without requiring the brain’s sign-off, particularly when you consider the helplessness of an infant with its brand-new brain.

4. THERE'S AN INFORMATION HIGHWAY FROM YOUR GUT TO YOUR BRAIN.

There’s one big visceral nerve embedded in your gut—the vagus nerve. Research has revealed that up to 90 percent of its fibers carry information from the gut to the brain, rather than the other way around. In other words, the brain interprets gut signals as emotions. So you really should trust your gut.

5. MOST OF YOUR SEROTONIN IS IN YOUR GUT.

Some 95 percent of your body’s serotonin, that marvelous mood molecule that antidepressant drugs like Prozac keep in your body, can be found in the gut. So, it’s no wonder that diet, medications, and antibiotics can wreak havoc on one’s mood.

6. A HEALTHY GUT MAY PROTECT YOUR BONES.

In a study of the serotonin-gut relationship, scientists discovered an unexpected link between the gut and the bones. Inhibiting the gut’s release of serotonin counteracted the bone-density reduction of osteoporosis in mice. This research is going into studies on new osteoporosis-fighting drugs.

7. RESEARCH SHOWS LINKS BETWEEN AUTISM AND HAVING FEWER STRAINS OF GUT BACTERIA. 

In as many as nine out of 10 cases, autistic people have common gut imbalances such as leaky gut syndrome, irritable bowel syndrome, and fewer strains of “good” bacteria. Research on mice is looking at possible treatments of some of the behavioral disorders of autism by balancing microbes in the guts, though many warn that such treatments can’t produce a “cure” for autism.

8. FOOD REALLY DOES AFFECT YOUR MOOD. 

Different foods, when introduced to the gut via feeding tubes, have been shown to change a person's moods without the person’s awareness of what they were "eating." Fat, for instance, increased feelings of happiness and pleasure (no surprise there) because appeared to trigger the release of dopamine—the brain’s natural opiate. Carbohydrate consumption stimulated the release of serotonin, the “feel good” neurotransmitter.

9. YOUR GUT IS YOUR BEST FRIEND IN COLD AND FLU SEASON.

Not only does your gut hold brain cells, it also houses the bulk of your immune cells—70 percent—in the form of gut associated lymphoid tissue, or GALT, which plays a huge part in killing and expelling pathogens. GALT and your gut microbiome—the trillions of bacteria that live, like an immense microbial universe, in your gut—work hard to help you get over what ails you. That’s all the more reason to be careful with the use of antibiotics, which wipe out the good bacteria along with the bad.

10. YOUR GUT CAN BECOME ADDICTED TO OPIATES.

Inside your gut are opiate receptors, which are also found in the brain. The gut is just as susceptible to addiction as the brain and may contribute to the intense difficulty some addicts have trying to kick the habit.

What Causes Hiccups?

iStock/damircudic
iStock/damircudic

The cause of hiccups depends on whom you ask. The ancient Greek physician Galen thought hiccups were violent emotions erupting from the body, while others thought they were a sign of liver inflammation. Today, evidence points to spasms in the diaphragm, the large muscle between the chest and abdomen that aids airflow during breathing. This involuntary contraction can be brought on by a number of things that might irritate the nerves that control the movement of the muscle. A full stomach, heavy boozing, rapid shifts in temperature either inside or outside of the stomach, and certain emotions like shock or excitement are all common culprits.

No matter the cause, the result is the same: The diaphragm spasms and causes us to take a quick breath. The sudden rush of air causes the epiglottis (the flap that protects the space between the vocal cords) to shut and interrupt the breath, which makes the familiar "hic" sound.

WHAT CURES THEM?

The best cure for hiccups also depends on the person you ask. Almost all cures are based on one of two principles: One type works its magic by overwhelming the vagus nerve with another sensation. The vagus nerve is a cranial nerve that innervates the stomach and conveys sensory information about the body's organs to the brain. When distracted by overwhelming information of another sort, it basically tells the brain that something more important has come up and the hiccuping should probably be stopped (vagus nerve stimulation is also used to control seizures in epileptics and treat drug-resistant cases of clinical depression). The other method for curing hiccups is to interfere with the breathing, increasing the amount of carbon dioxide in the blood, and causing the body to focus on getting rid of the of the CO2 and not making hiccups.

Swallowing a spoonful of sugar is probably the most commonly prescribed hiccup cure and falls into the first category. A teaspoon of sugar is usually enough to stimulate the vagus nerve and make the body forget all about the hiccups. Even ardent supporters of the sugar cure disagree if the sugar should be taken dry or washed down with water, though.

If this home remedy doesn't work, and your hiccups are both severe and persistent, you may need to bring out the big guns. For chronic cases like this, doctors sometimes use a cocktail of Reglan (a gastrointestinal stimulant) and Thorazine (an anti-psychotic with sedative properties) to quiet things down. In some cases that resist these drugs, Kemstro, an anti-spasmodic, is also used. Other doctors have used vagus nerve stimulators implanted in the upper chest of patients. The pacemaker-like devices send rhythmic bursts of electricity through the vagus nerve to the brain to keep the hiccup cycle in check.

Many people prefer home remedies to battle their hiccups, which may include holding your breath, gargling ice water, or breathing into a paper bag. While the same people will swear by the treatment they've been using all these years, there's no firm scientific consensus that any of them actually work. But if it helps you, isn't that all that matters?

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

An earlier version of this article appeared in 2012.

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