The decomposition of a dead body depends on many factors, any of which can affect the time necessary to break it down. If a body is buried in a coffin deep in the ground, for example, it could take as long as 50 years for all of the tissue to disappear. But if it is exposed to the elements, it will decay very quickly. The most important factor in decomposition time is how much exposure the body has had to bacteria. Bacteria need oxygen to survive and are generally found in heavy concentrations in water. Therefore, exposure to air or water will speed up the process of decomposition dramatically. Animals and insects will feed on the tissue if a body is exposed, also quickening the process. Regardless of how long it takes for tissue to be completely broken down, it may take the underlying bones hundreds of years to fully decay.
Much faster than decomposition is, of course, death itself, when the body goes through a host of grisly changes in a relatively fast time frame. When a person's heart stops beating at death, the body's cells are deprived of oxygen and die. Blood then begins draining from the circulatory system and pools in the low-lying areas of the body. Stiffening of the muscles -- rigor mortis -- sets in after about three hours, and by the time 24 hours have passed, the body will have lost all of its internal heat. The muscles will begin to lose rigor mortis after about 36 hours, and the stiffness will be completely gone after 72 hours. Meanwhile, as cells die, bacteria living in the body begin to break them down. The decomposing tissue will take on a horrific look and smell and emit a green liquid. The tissue will also release hydrogen sulfide and methane, as well as other gases. Animals and insects will consume parts of the body if it's exposed to them. No one ever said death was pretty, and the body makes no bones about reminding us of that fact.
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