Semen
Semen is an organic fluid (also known as seminal fluid) that usually contains spermatozoa. It is secreted by the gonads (sexual glands) and other sexual organs of male or hermaphroditic animals for fertilization of female ova. The process of discharge is called ejaculation.

# Physiological aspects

## Internal and external fertilization
Depending on the species, spermatozoa can fertilize ova externally or internally. In external fertilization, the spermatozoa fertilize the ova directly, outside of the female's sexual organs. Female fish, for example, spawn ova into their aquatic environment, where they are fertilized by the semen of the male fish.
During internal fertilization, however, fertilization occurs inside the female's sexual organs. Internal fertilization takes place after insemination of a female by a male through copulation. In low vertebrates (amphibians, reptiles, birds and monotreme mammals), copulation is achieved through the physical mating of the cloaca of the male and female. In marsupial and placental mammals, copulation occurs through the vagina.

## Composition of human semen
The components of semen come from two sources: sperm, and "seminal plasma". Seminal plasma, in turn, is produced by contributions from the seminal vesicle, prostate, and bulbourethral glands.
Seminal plasma of humans contains a complex range of organic and inorganic constituents.
The seminal plasma provides a nutritive and protective medium for the spermatozoa during their journey through the female reproductive tract. The normal environment of the vagina is a hostile one for sperm cells, as it is very acidic (from the native microflora producing lactic acid), viscous, and patrolled by immune cells. The components in the seminal plasma attempt to compensate for this hostile environment. Basic amines such as putrescine, spermine, spermidine and cadaverine are responsible for the smell and flavor of semen. These alkaline bases counteract the acidic environment of the vaginal canal, and protect DNA inside the sperm from acidic denaturation.
The components and contributions of semen are as follows:
A 1992 World Health Organization report described normal human semen as having a volume of 2 ml or greater, pH of 7.2 to 8.0, sperm concentration of 20x106 spermatozoa/ml or more, sperm count of 40x106 spermatozoa per ejaculate or more and motility of 50% or more with forward progression (categories a and b) of 25% or more with rapid progression (category a) within 60 minutes of ejaculation.

## Appearance and consistency of human semen
Most semen is white in colour, but grey or even yellowish semen can be normal as well.  Blood in the semen can cause a pink or reddish colour and may indicate a medical problem which should be evaluated by a doctor.
After ejaculation, semen first goes through a clotting process and then becomes more liquid.  It is postulated that the initial clotting helps keep the semen in the vaginal canal, but liquefaction frees the sperm to make their longer journey to the ova.  Immediately after ejaculation semen is typically a sticky, jelly-like liquid often forming globules. Within 5 to 40 minutes it will become more watery and liquid before finally drying. 

## Semen quality
Semen quality is a measure of the ability of semen to accomplish fertilisation. Thus, it is a measure of fertility in a man. It is the sperm in the semen that are of importance, and therefore semen quality involves both sperm quantity and sperm quality.

## Semen and transmission of disease
Semen can be the vehicle for many sexually transmitted diseases, including HIV, the virus that causes AIDS.
It is also hypothesized that components of semen, such as the spermatozoa as well as the seminal plasma, can cause immunosuppression in the body when introduced to the bloodstream or lymph. Evidence for this dates back to 1898, when Elie Metchnikoff injected a guinea pig with its own and foreign guinea pig sperm, finding that an antibody was produced in response; however the antibody was inactive, pointing to a suppression response by the immune system.
Further research, such as that by S. Mathur and J.M. Goust, demonstrated that non-preexisting antibodies were produced in humans in response to the sperm. These antibodies mistakenly recognized native T lymphocytes as foreign antigens, and consequently the T lymphocytes would fall under attack by the body's B lymphocytes. 
Other semen components shown to spur an immunosuppressive effect are seminal plasma and seminal lymphocytes.

## Blood in the semen (hematospermia)
The presence of blood in the semen may be undetectable (it only can be seen microscopically) or visible in the fluid. Its cause could be the result of inflammation, infection, blockage, or injury of the male reproductive tract or a problem within the urethra, testicles, epididymis and prostate.
Further semen analysis and other urogenital system tests might be needed to find out the cause of blood in the semen.

## Semen allergy
In rare cases people have been known to experience allergic reactions to seminal fluids, known as human seminal plasma hypersensitivity. Symptoms can be either localized or systemic, and may include vaginal itching, redness, swelling, or blisters within 30 minutes of contact. They may also include generalized itching, hives, and even difficulty breathing.
The best way to test for human seminal plasma sensitivity is to use a condom during intercourse. If symptoms dissipate with the use of a condom, it is possible that a sensitivity to semen is present. Mild cases of semen allergy can often be overcome by repeated exposure to seminal fluid. In more severe cases, it is important to seek the advice of a physician, particularly in the event that a couple is trying to conceive, in which case, artificial insemination may be indicated.

# Cultural aspects

## Semen and martial arts
Chi Kung and Chinese medicine places huge emphasis on a form of energy called (pinyin: jing1, also a morpheme denoting "essence" or "spirit")  - which one attempts to develop and accumulate, jing is sexual energy and is considered to dissipate with ejaculation so masturbation is considered "Energy Suicide" amongst those who practice this art. According to Chi Kung theory, energy from many pathways/meridians becomes diverted and transfers itself to the sexual organs during sexual excitement, the ensuing orgasm and ejaculation will then finally expel the energy from the system completely. The Chinese proverb ,(pinyin: yi4 di1 jing1, shi2 di1 xue3, literally: a drop of semen is equal to ten drops of blood) illustrates this point.
The scientific term for semen in Chinese is (pinyin: jing1 ye4, literally: fluid of essence/jing) and the term for sperm is (pinyin: jing1 zi3, literally: basic element of essence/jing), two modern terms with classical reference.

## Cultural views
In some cultures, semen is attributed with special properties of masculinity. For instance, among the Etoro people of Papua New Guinea, it is believed that young boys must fellate their elders and ingest their sperm to achieve proper sexual maturation. This act may also be attributed to the culturally active homosexuality throughout these and other tribes.

### Aristotle
Aristotle wrote on the importance of semen as follows: 
"For Aristotle, semen is the residue derived from nourishment, that is of blood, that has been highly concocted to the optimum temperature and substance. This can only be emitted by the male as only the male, by nature of his very being, has the requisite heat to concoct blood into semen."
"Sperms are the excretion of our food, or to put it more clearly, as the most perfect component of our food"
If men start to engage in sexual activity at too early an age... this will affect the growth of their bodies. Nourishment that would otherwise make the body grow is diverted to the production of semen. ... Aristotle is saying that at this stage the body is still growing; it is best for sexual activity to begin when its growth is 'no longer abundant', for when the body is more or less at full height, the transformation of nourishment into semen does not drain the body of needed material.

## Sacred semen
In some pre-industrial societies, semen and other body fluids were revered because they were believed to be magical. Blood is an example of such a fluid, but semen was also widely believed to be of supernatural origin and effect and was, as a result, considered holy or sacred.
Semen is currently and has long been revered by Buddhist, and Daoist traditions as a very important constituent of human physiology.
Dew was once thought to be a sort of rain that fertilized the earth and, in time, became a metaphor for semen. The Bible employs the term dew in this sense in such verses as Song of Solomon 5:2 and Psalm 110:3, declaring, in the latter verse, for example, that the people should follow only a king who was virile enough to be full of the dew of youth.
It was widely believed, in ancient times, that gemstones were drops of divine semen which had coagulated after having fertilized the earth. There is an ancient Chinese belief that jade, in particular, was the dried semen of the celestial dragon.
Based upon the resemblance of dandelion juice to human semen, it was believed that the flower magically promoted the flow of sperm.
The orchids twin bulbs were thought to resemble the testicles, and there was an ancient Roman belief that the flower sprang from the spilled semen of copulating satyrs.
Barbara G. Walker recounts these examples of sacred semen in The Womans Dictionary of Symbols and Sacred Objects, the thesis of which is that myth and folklore show a pre-patriarchic rule by women that was later supplanted by masculine culture.
In primitive mythology semen is considered to be the returning or refunding of the milk of the mother in an alimentary metaphor. The wife feeds her husband who returns to her his semen, the milk of human kindness, as it were.

## Semen in popular culture
Depiction of semen in art and popular culture has, for a long time, been considered a taboo subject.
The Japanese artist Takashi Murakami is famous for a manga style piece entitled My Lonesome Cowboy, which features a naked cowboy superhero wielding his own semen as a lasso.
Andres Serrano, whose photos depict bodily fluids such as "Blood and Semen II" (Semen y Sangre II) (1990), became a controversial figure for featuring semen in his work. He was criticized by some for producing offensive art, while others defended him in the name of artistic freedom. His photos were featured on the cover art of two Metallica albums, Load and ReLoad, which feature images made by shining light through a piece of clear plastic on which semen, blood and urine have been splattered and swirled around.
Only recently has semen been depicted (albeit controversially) in movies such as Kika (1993),  There's Something About Mary (1998) ("a hard-core staple making its debut in a mainstream Hollywood comedy"), Happiness (1998), American Pie (1999), Scary Movie (2000), Scary Movie 2 (2001), and National Lampoon's Van Wilder (2002). Jackass Number Two (2006) features a scene where Chris Pontius drinks horse semen. It has also appeared in the anime movie End of Evangelion, which is not otherwise an adult-oriented film.

## Euphemisms
A huge variety of euphemisms and dysphemisms have been invented to describe semen. For a complete list of terms, see Sexual slang.