Foreskin

Male mammals have a penis, and a glans penis. Usually, some skin covers the glans penis. This skin is called foreskin. Most male mammals either have a foreskin that covers the glans penis or a sheath in which the whole penis can retract.

Circumcision is the removal or shortening of this skin. It is often done for religious reasons. Sometimes it is done for medical reasons.

Female mammals have a clitoris. It also has a piece of skin protecting it, called clitoral hood.

Human foreskin
The outside of the foreskin is like normal skin but the inside of the foreskin is a membrane like the inside of an eyelid or  mouth. The foreskin is attached to the penis by the frenulum, but it can move. It is very stretchy. On the penis, the most sensitive areas to fine-touch are located on the foreskin. The foreskin can be pierced (have a hole for jewelry put through it) or slit (cut) for fashion reasons. If the foreskin is not cared for (by washing the inside) daily it produces smegma. Smegma is a mixture of epithelial (skin) cells, skin secretions and fluid, which accumulate under the foreskin of the penis. Smegma has strong bad-smelling odor and a bad taste, caused by lactic acid bacteria. The foreskin must be rolled back and washed to prevent smegma.

Retractability

In older boys and adult males, the foreskin can be pulled back. In baby boys, the foreskin is usually attached to the glans of the penis and should not be forcibly pushed or pulled back for cleaning. In the United States, medical advice is usually that parents should gently pull back their son's foreskin part way back for cleaning after the baby is one year old. Soap should not be used -- just warm water. Pulling a baby's foreskin back too early (especially before six months of age) can damage it and cause scar tissue to form. When a parent is able to pull the foreskin back, it is important to do so gently and only pull it back as far as it will go. The parent should not make the baby cry or bleed. When the foreskin can be pulled back, the area under the foreskin needs to be cleaned every day. In Europe, where uncircumcised penises are much more common, parents are often told they do not need to pull back a boy's foreskin for washing. 

American doctors may be more worried about possible kidney damage, especially scarring inside the kidneys. That can happen because of a bacterial infection that starts in the foreskin and urethra. The urethra is the tube urine (pee) comes out of. The infection often spreads to the bladder. While not as common, the infection can spread further to the kidneys. A serious kidney infection (called pyelonephritis) can cause scarring of the kidney. The boy may have very serious kidney problems and may even die because of a childhood urinary tract infection. Acute (very serious) pyelonephritis in the first years of life often leads to significant renal (kidney) damage that may progress to end-stage renal disease during adolescence. End-stage renal disease means the kidneys  no longer function well enough to meet the body's needs. If the person does not receive frequent kidney dialysis or a kidney transplant, the person will die. 

European doctors may think it is more important that parents not touch their young boy's penis too much. The European attitude is that a boy should discover how to pull back his foreskin on his own. 

In any case, sometime between infancy and the onset of puberty, the foreskin separates from the glans of the penis and can slide back and forth over the glans of the penis. Often this happens sometime after age five. This happens because of normal erections during childhood. Erections stretch the foreskin. There is no "right" age for this to happen. Each child is different. 99% of foreskins are retractable by puberty. Once the foreskin becomes retractable, usually it will automatically retract with an erection. Alternatively, a male (or his sexual partner) may slide the foreskin back and forth over the glans of the penis to cause an erection. 

For a few boys, the foreskin does not separate on its own from the glans of the penis. In the few cases where a boy can't pull his foreskin all the way back by puberty, the boy should ask a doctor or nurse. Usually a doctor will correct the problem or give advice about how to gradually stretch the foreskin. The doctor may give a prescription for a steroid cream which will help to stretch the skin. Circumcision is another common option, especially in the United States.

Sensitivity in humans
The foreskin contains Meissner’s corpuscles, which are nerve endings involved in fine-touch sensitivity. They are most numerous in the “ridged band”, the junction of the inner and outer foreskin layers, and least numerous in the smooth inner layer of foreskin. Compared to other hairless skin areas on the body, the Meissner's index was highest in the finger tip (0.96) and lowest in the foreskin (0.28). The foreskin is the least sensitive hairless tissue of the body. A study also found that “the number of these nerve endings decreases significantly after the teenage to young adult years when sexual activity begins. This makes it very difficult to propose any sexual function for Meissner’s corpuscles. A more feasible hypothesis is to regard them as a juvenile phenomenon, perhaps serving to protect the penis until the onset of puberty reveals its sexual function.”

Studies in sexual sensation concluded that the glans, not the foreskin, is involved in sexual sensation, particularly the corona and frenular (the area under the frenulum) areas. Thus, speculation and outdated opinion pieces claiming special properties of the foreskin, such as in penile function and masturbation, should be viewed with skepticism.

Perhaps sensitivity of the foreskin to fine touch might have served as an “early warning system” in our naked upright forebears from the intrusion of biting insects and parasites while protecting the glans.

Use 

The foreskin can keep the glans penis comfortable, moist, and protect it.

In modern times, there is controversy regarding whether the foreskin is a vital or vestigial structure. During the physical act of sex, the foreskin reduces friction, which can reduce the need for additional sources of lubrication. "Some medical researchers, however, claim circumcised men enjoy sex just fine and that, in view of recent research on HIV transmission, the foreskin causes more trouble than it’s worth." The area of the outer foreskin measures between 7–100 cm2, and the inner foreskin measures between 18 and 68 cm2, which is a wide range. Regarding vestigial structures, Charles Darwin wrote, “An organ, when rendered useless, may well be variable, for its variations cannot be checked by natural selection.” In the March 2017 publication of the Global Health Journal: Science and Practice, Morris and Krieger wrote, "The variability in foreskin size is consistent with the foreskin being a vestigial structure." It has been found that larger foreskins place uncircumcised men at an increased risk for HIV infection most likely due to the larger surface area of inner foreskin and the high concentration of Langerhans cells.

Moses and Bailey (1998), say that "it has not been demonstrated that [the foreskin] is associated with increased male sexual pleasure."

Circumcision of the foreskin 

Circumcision is when some or all of the foreskin is cut off. The removal of the foreskin can protect against certain medical conditions and infections. It is done to satisfy medical, religious, hygienic, ritual, and aesthetic views. Circumcision is common in many countries such as the United States, South Korea, Israel and Muslim countries.

Related pages
 Anatomy
 Circumcision
 Genitals
 Masturbation
 Human penis
 Smegma

References

Other websites
 Foreskin.org - Many pictures of human foreskin 

Anatomy of the male reproductive system