There are a variety of tests and definitions of death, and these have evolved over time. The goal, of course, is to make sure that nobody is buried alive.

The tests begin with basic signs of death that result from the cessation of functioning of the main organs. The temperature drops; the blood stops circulating and forms blue patches; muscles contract; the body get stiff, though it later becomes limp again. Previously, tests consisted of placing a feather or mirror at the lips to check for breath and a finger or ear used to check for pulse. Now, people are checked for response to pain, light, inability to breathe without a machine, lack of muscle movement, lack of reflexes, no involuntary actions such as blinking or swallowing occur, and lack of brain waves as detected by machine.

Algor Mortis (body temperature)

After death, body temperature declines progressively until it reaches the temperature of its surroundings. This process generally takes about 8 to 12 hours on the skin, but the centre of the body takes about three times as long to cool. This fact can be used to estimate time of death. However, many factors may influence the rate of heat loss and this is only an estimate. After the onset of putrefaction (about two days after death) the body temperature will increase again, due to the metabolic activity of the bacteria and other decomposing organisms.

Rigor mortis

Rigor mortis is a well known phenomenon, and is due to a complex chemical reaction in the body. In the living body muscles can function both aerobic and anaerobic. In the dead body muscle cells can only function anaerobically. When muscle cells work anaerobically the end product is lactic acid. In the living body, lactic acid can be converted back, by means of excessive oxygen uptake after an anaerobic exercise. In the dead body this can not happen, and the breakdown of glycogen in the muscles leads irreversebly to high levels of lactic acid in the muscles. This leads to a complex reaction where actin and myosin fuses to form a gel. This gel is responsible for the stiffness felt in the body. This stiffness will not be over before decomposition begins.
As rigor mortis is due to a chemical reaction, the reaction time is due to temperature and the initial concentrations of lactic acid. High metabolic activity in the time just before death, for example when running, leads to higher levels of lactic acid, and shorter time for the rigor mortis to develop. Higher environmental temperature also leads to a shorter reaction time.
In temperate regions the following rules of thumb can be used in estimating death, but must be used with caution:
 

Temperature of body Stiffness of body Time since death Warm  Not stiff Not dead - more than three hours Warm  Stiff Dead between 3 to 8 hours Cold  Stiff Dead between 8 to 36 hours Cold Not stiff Dead in more than 36 hours

Rigor mortis should never be the only basis for estimating time of death.

Livor Mortis (discoloration)

Livor mortis is the settling of blood. It is also called Hypostasis. When the heart stops, the blood stops circulating and gravity makes it settle. This makes the areas where the blood has settled turn dark blue or purple. This starts happening immediately and is visible with a couple of hours. At this point skin is bluish and blotchy. After five or six hours the blotches have joined up but the skin still goes white when pressed. After ten to twelve hours the blue colour remains even when pressed.
The lividity doesn't show where the body is in contact with something. This a body lying on its back will show lividity in the small of its back, its neck etc but not is parts of the body directly touching the ground. This a very useful when determining if a body has been moved after death. With some poisons the discoloration is different. Carbon Monoxide, for example, turns the skin cherry pink.

Decomposition

The process is faster in damp places or when the body is exposed to air. Left to nature, scavengers will quickly take over a body, from vultures to flies. The further decomposition of the carrion is carried out by bacteria, enzymes and fungi, which allow for recycling of the materials. Escherishia coli and others internal organisms in the intestine will start multiplying.

First the intestine and the blood will be attacked, and when gas formation and other things leads to rupture of the intestine other organs will be attacked. Decay is about eight times faster in the air than underground. Heat speeds up decay, but a dry climate can lead to mummification. People with a lot of fat will decay faster. People who died of bacterial disease will also decay faster. However, some poisons preserve the body.

The decomposition of a body can be divided into several stages, even if the duration of each stage will vary a lot:

• 2-3 days: green staining begins on the right side of the abdomen. Body begins to swell.
• 3-4 days: staining spreads. Veins go "marbled" - a browny black discoloration
• 5-6 days: abdomen swells with gas. Skin blisters
• 2 weeks: abdomen very tight and swollen.
• 3 weeks: tissue softens. Organs and cavities bursting. Nails fall off
• 4 weeks: soft tissues begin to liquefy. Face becoming unrecognisable
• 4-6 months: formation of adipocere, if in damp place. This is when the fat goes all hard and waxy.
• A body without a coffin will be decayed within 12 years.
   

Stage Description Initial Decay Carcase appears fresh externally but is decomposing internally due to the activities of bacteria, protozoa and nematodes present in the animal before death Putrefaction Carcase swollen by gas produces internally, accompanied by odour of decaying flesh Black putrefaction Flesh of creamy consistence with exposed parts black. Body collapses as gases escapes. Odour of decay very strong Butyric fermentation Carcase drying out. Some flesh remains at first, and cheesy odour develops. Ventral surface mouldy from fermentation Dry decay Carcase almost dry; slow rate of decay