Post mortem interval (PMI) is the time that has elapsed since a person has died. If the time in question is not known, a number of medical/scientific techniques are used to determine it.

Many types of changes to a body occur after death. Some of those that can be used to determine the post mortem interval are:

• Algor mortis - body cooling;
• Rigor mortis - stiffening of limbs;
• Forensic entomology - insect activity on the corpse;
• Vitreous humour changes - eye chemistry;
• State of decomposition - autolysis (process of self digestion) and putrefaction (process caused by bacteria found within the body)

A person who judges the time of death by the means of decomposition is privy to a simple five stage process:

• Stage 1: Initial Decay - This is basically where the body will stop producing antigens and enzymes that are used to fight off bacteria located mainly in the lower intestine.
• Stage 2: Putrefaction - Because the body no longer has a defense system in place the bacteria grow and multiply by feeding off the body. They will begin to bring forth certain gases, which in turn will give the dead body a sort of bloated look and will cause a rather unpleasant odor.
• Stage 3: Black Putrefaction - This stage will bring further discoloration to the body (whether it be black, blue, purple, green etc). It will also have an even more horrendous odor as the gases caused by the bacteria begin to escape out of the body.
• Stage 4: Butyric Fermentation - Where the internal organs begin to liquefy and the body will begin to desiccate-or dry out.
• Stage 5: Dry Rot - This is the slowest process out of the five stages. Basically, as the name indicates, the body will slowly begin to dry out and ultimately skeletonize.

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