Human penis

The human penis is a male body part found on the outside of the body. It is used for urination and for sexual intercourse. The main sexual function of the penis is to be inserted into a female's vagina and deliver semen which may cause pregnancy. The penis also provides pleasure during other types of sexual activity, including foreplay, oral sex, and solo sex (which doctors say is normal and healthy.)

There are many slang words for penis. They include dick, cock, and schlong. A euphemism for penis is male member. The general area where the penis is located is called the crotch. Sometimes people who do not want to use the word penis use the word crotch instead. For example, someone might say a rapper "grabbed his crotch" rather than "grabbed his penis."

Females have a clitoris, which is like a penis, although the outside part of the clitoris is much smaller than the penis. Most of the clitoris is inside the body. Part of the shaft of the clitoris is covered by a thin layer of skin called the clitoral hood, which is similar to the foreskin of the penis, discussed below. Both the penis and the clitoris are erectile organs, meaning they can become larger and harder when touched, rubbed, or stimulated in other ways.

Structure 

The human penis exits the abdomen above the scrotum and hangs freely outside the body. The main visible part is called the shaft.
The human penis has three parts.
Body of the penis: The main visible part of the penis. The urethra runs underside of the penis.
Root of the penis: Invisible part of the penis. It reaches near the anus and contains the bulb of the penis and the crus of the penis. The crus of the penis is attached to the pubic bone.
Skin of the penis: The shaft is covered by the skin. The skin is not attached tightly to the inner tissue. So it can move freely on the shaft.

The human penis is made up of three rod-shaped tissues. It has two corpora cavernosa (special pieces of muscle) next to each other and a corpus spongiosum (spongy muscle) between them.
The end of the corpus spongiosum forms the glans penis. The glans penis is covered by the foreskin in uncircumcised males. The rounded base of the glans is called the corona. The area on the bottom of the penis, where the foreskin is attached, is known as the frenulum of prepuce. 

The urethra, the tube where urine and semen travel through, runs down the corpus spongiosum (spongy tissue), and opens at the tip of the penis. The opening is called the urinary meatus. Sperm are made in the testes (ball-like organs) and stored in the epididymis (layer of tissue) around the testes. During ejaculation, sperm are pushed up the vas deferens. Fluids are added by the seminal vesicles, the prostate gland and the bulbourethral glands to make semen.

In sexual reproduction

Erection 

A penis can become erect ("hard") if a  male is sexually aroused (or sometimes during sleep, even though there is no sexual stimulation). In an erection, the penis fills with blood. The blood makes the penis become longer, thicker and harder. Veins taking blood away from the penis get smaller, so less blood is taken through. Arteries bringing blood to the penis get wider, bringing more blood to the penis. Erections sometimes occur in the male fetus (before birth), and occur naturally from birth onwards.  It is normal for the penis to be erect when a male wakes up. This is commonly called morning wood. It is also normal for male teenagers and men to have an erection several times per night when sleeping. If this does not happen, the person may have a bad medical problem or depression (a mental health problem.) Not being able to have an erection can also be caused by various common drugs, including antidepressants and drugs taken to control high blood pressure. During the day, an erection sometimes happens for no reason, or when undressing. This is most common in young teenagers and is normal.  People call this a spontaneous (sudden or surprise) erection. A slang word for an erection is a boner.

Ejaculation 

Ejaculation is when semen is pushed from the penis. It usually happens during an orgasm. Males can ejaculate during sexual intercourse or by masturbation. There are usually 40 million or more sperm in the semen ejaculated. Semen usually has a slight smell of chlorine and ammonia. It is not poisonous, but it is very bad if it gets in someone's eye. The enzymes semen contains to penetrate the egg in a female will also attack the eye. Be careful not to ejaculate into your eye or your partner's eye. Semen contains proteins and fructose (a kind of sugar.) Some people say it tastes sweeter if the man eats a lot of fruit. Scientists have not proven that. Smoking, or drinking coffee, tea or colas that contain caffeine can create a bitter flavor. Semen is slimy when wet. Eventually, it dries in the air and hardens like glue.

In reproductive sexual intercourse between a male and female, the erect penis is inserted into the vagina and moved in and out. The vagina places pressure on the penis, which can cause the male to have an orgasm and ejaculate into the vagina, causing insemination (meaning that semen enters the female's reproductive system.)    

Orgasm and ejaculation can happen during many types of sex acts including anal and oral sex, and when masturbating. A condom should be used during anal sex to avoid getting a urinary tract infection. Masturbation can include touching and rubbing the penis and other sensitive areas of the body, such as the scrotum, and other places in the genital area. This can be done naked or through clothes. Sometimes the penis is squeezed between the partner's thighs without being inserted into the body. This is called intercrural sex. Sometimes the penis is rubbed against the other person's body. The people can be naked or wearing clothes. When two males rub their penises together or against each other's body, it is called frottage. Ejaculation may also happen without touching during sleep (called a 'wet dream'). This is common, especially for pre-teen and teenage boys. It is uncommon in adults. While it can feel almost magical to ejaculate during a dream, boys don't like the mess it creates. Boys usually don't have wet dreams if they masturbate before sleeping every night.  A male must have started puberty before he can ejaculate. (Sometimes when a boy who has not started puberty masturbates, a small amount of urine comes out.) During orgasm, muscles push semen from the penis. Semen moves through the urethra and comes out of the hole at the tip of the penis.

Condom use 

A man can put a condom on his penis to prevent semen from entering the female's reproductive system. A condom is very thin, so having sex still feels good. It is normally made of latex. If used correctly, a condom usually stops the woman from getting pregnant and having a baby. The condom also lowers the chance of getting a sexually transmitted infection from the sexual partner or giving a sexually transmitted infection to the sexual partner. While condoms are most commonly used during vaginal sex, a condom can also be used during oral sex, anal sex, and masturbation.

Vaccination to reduce the spread of sexually transmitted infections 
Vaccinating young people (both girls and boys) before the age when they first have sex can reduce their risk of getting one infection called HPV. The United States CDC says the HPV vaccine should be given at age 11 or 12 years but can be started at age 9. HPV increases the risk of cancer, including cancer in the throat or neck if people have oral sex (licking, kissing or sucking on the penis.) HPV can be under the foreskin if the penis has a foreskin, and in the semen whether the penis is circumcised or uncircumcised.

Penis size in humans 

The length and thickness of the penis is different for different people. The size of a soft penis (flaccid or not erect) is much smaller than when it is erect. Some penises grow more when they get hard than other penises. In most cases, whether a penis is big or small, it can still be used for sex. It averages out to be around 3 to 6 inches long when not erect. The average size of an erect human penis is between 13 – 16 cm (5.1– 6 in). The average circumference of a penis is 12.3 cm (4.85 in) when fully erect. The penis grows bigger during puberty. At the start of puberty, the average length of the penis is . The penis reaches adult size about 5 years later. A study done in 1996 found the mean length of an adult man's penis is  when it is not erect. The average length of an erect penis is about .
The human penis is longer than in other primates except for the chimpanzee. The chimpanzee's penis is about the same length. The penis is bigger in circumference on humans than on other primates.

Circumcision 
The foreskin is a fold of skin that covers the end of the penis. The foreskin is a bit thicker than an eyelid. Cutting off the foreskin is called circumcision. The foreskin is connected to the head of the penis. During circumcision, most or all of the foreskin is removed from the penis. This simple change to the penis is very common in many countries of the world including the United States and Israel.  Circumcision is done for medical reasons because it lowers the chance of ever getting cancer of the penis to near zero. The foreskin removed during circumcision is a possible source of penile cancer. Childhood circumcision has a strong protective effect against penile cancer in later life. Penile cancer has been nearly eliminated in populations of males circumcised shortly after birth. Circumcision virtually eliminates the risk of invasive penile cancer. Being uncircumcised is a risk factor for penile cancer. Invasive penile cancer is cancer that spreads in the body. It is uncommon but often causes the man to die. Circumcision also reduces the risk of cancer of the cervix in female sexual partners.  Cervical cancer is very common. It causes over 300,000 deaths of women worldwide every year. Circumcision reduces, but does not eliminate, the risk of getting or spreading HIV/AIDS. Circumcision became more common in Africa after HIV/AIDS began. The use of condoms also reduces the risk of getting or spreading HIV/AIDS, but many people in Africa cannot afford condoms or choose not to use them. 

Circumcision is usually performed on infant males for medical reasons, or for religious reasons, especially for Jews and Muslims, or cultural reasons in the United States, the Philippines, and South Korea. "Cultural reasons" means reasons such as beliefs that the penis stays cleaner or looks more beautiful when circumcised. "Cultural reasons" also includes tradition, meaning parents wanting their son's penis to look like the father's penis and like other boys' penises. In the United States, one of the reasons for it being a tradition is that it was once a requirement in the navy and popular in other parts of the military. The World Health Organization estimates that the overall male circumcision rate in the United States is somewhere between 76 and 92 percent. Circumcision was also very common in Canada, although that is no longer the case, due in part to changes in government rules about paying for circumcision. Through the first half of the twentieth century, up to about the late 1960s and even the early 1970s, it was common for men in the United States and Canada to swim naked (without bathing suits) at high school, college, and YMCA pools. It was also common that men showered together in schools and in the military. Since it was common for boys and young men to be naked, many parents wanted their sons to look like other American males. Parents did not want their sons to look different from the majority. 

Some males have the foreskin cut off when they are adults because they have problems with their foreskin. Some males have the foreskin cut off because they want to change how their penis looks. Few boys are born without a foreskin or with a very short foreskin, called aposthia, naturally circumcised. 

In common speech, you can say that someone is cut or uncut, meaning circumcised or not circumcised. For example, one teenage boy might ask another teenage boy, "Are you cut?" While this is slang English, it is not considered profanity. 

In some religions, babies and young boys have their foreskins cut off. This is expected in Islam and Judaism. It is not required according to Christianity. Among Christians in the United States, it is more common among Protestants than among Catholics. In the United States, circumcision is less common among boys and men whose families came from Mexico, who are usually Catholic. In Judaism, infant males are required to have their foreskin removed as a sign of the covenant (ancient promise) made with God.

When a foreskin is removed from a baby, doctors can use the skin for important medical purposes. Doctors can replace skin on someone with burns or with foot sores caused by diabetes. Foreskins that have been removed can be used for medical research.

If a male is uncut (uncircumcised) he should wash under the foreskin every day. However, young boys should not pull or push their foreskin back when washing. Not washing can cause a bad smell. People should not use soap when washing their penis. Getting soap inside the penis (in the urethra) can hurt. It is like getting soap in the eyes.

Skin color
Penile skin is often slightly darker than skin in surrounding areas. This is due to a higher concentration of melanocytes in genital skin tissue. The tip (glans) of the penis is usually pink or red color even if the penis shaft skin is dark. If the man is not circumcised, the tip is usually covered by the foreskin. The foreskin is about the same color as the skin on the shaft of the penis.

If a man's penis skin changes color and he doesn't know why, he should go to a doctor. He could have an infection, diabetes or cancer. A sunburn on the penis can happen quickly.  If a man is outside without clothes, he will get a sun tan. That is normal. Too much sun causes sunburn. Doctors say to use sunscreen (sun cream) before and after nude swimming. A sunburn can happen in a few minutes on a very sunny day.

Culture
 
Since the oldest cultures the penis has assumed a strong importance as a symbol of fertility used also in religious ceremonies to celebrate and (people hoped) to improve the reproductive capacity of men and beasts. For example, in Roman paganism, the god Priapus is characterized by a huge penis. In some places in Japan and South Korea, there are special days and public events to celebrate the penis. When an erect penis is shown in art or in cultural events, it is often called a phallus.

Piercing 
Some men put jewelry on their penis. This is a type of body piercing called genital piercing. One main reason is that it can make sex more pleasurable. Jewelry is sometimes at the tip (through the glans and urethra), sometimes through the foreskin, corona, or frenulum, or sometimes under the shaft of the penis.

Related pages 
 Male reproductive system
 Phallus
 Scrotum
 Testicles
 Vagina
 Fellatio

References

Other websites 

 Kinsey Institute on the penis  

Anatomy of the male reproductive system