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Decomposition is the reduction of bodies and other formerly living organisms into simpler forms of matter and, most particularly, to the fate of the human body after death. The science which studies decomposition generally is called taphonomy.
The rate and the manner in which a human or animal body decomposes is strongly affected by a number of factors. In a roughly descending degree of importance, those factors include:
- Access by insects
- Burial, and depth of burial
- Access by carnivores or rodents
- Trauma, including wounds and crushing blows
- Humidity, or dryness
- Body size and weight
- Prior embalming
- The surface the body rests on
Decomposition begins at the moment of death. At this stage it is caused by two factors: autolysis, the breaking down of tissues by the body's own internal chemicals and enzymes; and putrifaction, the breakdown of tissues by bacteria. These processes release gases that are the chief source of the characteristic odour of dead bodies.
Insects and other animals are typically the next agent of decomposition, assuming the body is accessible to them. The most important insects that are typically involved in the process include the fleshflies (Sarcophagidae) and blowflies (Calliphoridae). The green-bottle fly seen in the summer is a blowfly.
Embalming affects the process, slowing it somewhat, but does not forestall it indefinitely. Embalmers typically pay the greatest attention to the parts of the body seen by mourners, such as the face and hands. The chemicals that are used in embalming will repel most insects, and slow the process of bacterial putrefaction, but will not preserve a corpse indefinitely. In sufficiently dry environments, an embalmed body may end up mummified. Bodies submerged in peat bogs may become naturally "embalmed", arresting decomposition and resulting in a preserved specimen known as a bog body.
The time for the reduction of an embalmed body to be reduced to a skeleton varies greatly. An unembalmed adult body buried six feet deep in ordinary soil without a coffin normally takes ten to twelve years to decompose fully to a skeleton, given a temperate climate. Immerse the body in water, and skeletonization occurs approximately four times faster; expose it to air, and it occurs eight times faster. The skeleton itself is not permanent; acids in soils can reduce it to unrecognisable components as well (this is one reason given for the lack of human remains found in the wreckage of the Titanic, even in parts of the ship considered inaccessible to scavengers). Bodies exposed to cool, damp soil may develop a waxy substance called adipocere, caused by the action of soil chemicals on the body's proteins and fats. The formation of adipocere slows decomposition by inhibiting the bacteria that cause putrefaction.
Various sciences study the decomposition of bodies. These sciences fall under the general rubric of forensics, because the usual motive for study of the decomposition of human bodies is to determine the time and cause of death, for legal purposes:
- Forensic pathology studies the clues to the cause of death found in the corpse as a medical phenomenon
- Forensic entomology studies the insects and other vermin found in corpses; the sequence in which they appear, the kinds of insects, and where they are found in their life cycle are clues that can shed light on the time of death, the length of a corpse's exposure, and whether the corpse was moved.
- Forensic anthropology is the branch of physical anthropology that studies skeletons and human remains, usually to seek clues as to the identity, race, and sex of their former owner.
The Body Farm, located at the University of Tennessee in Knoxville has a number of bodies laid out in various situations in a fenced in plot near the medical center. Scientists at the University study how the human body decays in various circumstances to gain a better understanding into decomposition.
Peter Greenaway's film A Zed and Two Noughts  (http://www.imdb.com/title/tt0090366/) has a sub plot which explores decomposition and is intercut with an escalating series of time-lapse sequences watching various plants and animals decomposing, culminating with the decomposition of the two main characters in the film.
- Kenneth V. Iserson, M.D., Death to Dust: What Happens to Dead Bodies; Galen Press, Tucson AZ (1994) ISBN 1883620074
- Jessica Snyder Sachs, Corpse: Nature, Forensics, and the Struggle to Pinpoint Time of Death, ISBN 0738207713de:Verwesung