After the death of a person whole spectrum of different changes begin to occur on the body. Some of these changes start to appear soon after the death and some start to appear later. Therefore the post mortem changes are used to determine how long the person may have been dead. Depending on the time interval these post mortem changes start after the death they can be classified in the following way.

1. Early post mortem changes
2. Late post mortem changes 

Early post mortem changes

The following types of post mortem changes can be classified as early post mortem changes.

1. Primary Flaccidity

complete loss of the tone of muscles which develops soon after the death and disappears in 2 hours when the rigor mortis starts.

• Rigor Mortis

It is the post mortem stiffening of the body which starts around 2 hours after the death. It is usually detected first around the eyes and the jaw muscles followed by the development of stiffening on large joints and complete stiffening of the body in about 10 hours after the death. Rigor mortis gradually disappears in next 10 hours in the same order due to the onset of decomposition.

• Hypostasis

When the heart beat stops after the death, blood circulation in the blood vessels also stop. The blood in the blood vessels stagnates in the lowest levels within the blood vessels. For example if the body lies on face up position the hypostasis will be on the back aspect of the body. It starts appearing around 2 hours after the death as red or brown patches on the skin and completes fixation 8- 10 hours after the death which means helpful in determining the time since death. The hypostasis appears as red or brown area on a fair skinned dead body. But it may not be visible in dark skinned bodies. The hypostasis may be helpful in determining whether the position of the body has been changed after death. The colour of the hypostasis may be helpful to determine the cause of death; Eg. it takes pink-red colour in carbon monoxide intoxication

• Cooling of the body after the death

The temperature of a human body during life is around 37⁰ C. After the death all the activities of producing body heat ceases and the body cools down very approximately until the temperature approaches that of the environment.  The body temperature usually remains constant for first two hours of death and then rapidly decreases. The loss of body temperature depends on many internal and external factors such as weight of the body, clothing, environmental factors, humidity, enclosement  etc. The rate of cooling of the body during the first few hours (around 6 hours) after death is useful in determining the post-mortem interval.

Late post mortem changes

The dead bodies undergo a variety of changes which eventually return the tissue components to the food chain and rid the surface of earth of corpses. These changes are brought about by chemical processes, bacterial and fungal attack and post mortem animal attacks.

1. Decomposition

The decomposition incorporates two main mechanisms; the soft tissue of the body undergoes the process of liquefaction due to intracellular enzymatic activity known as autolysis and action of endogenous and extraneous bacterial action on tissues after the death termed as putrefaction. The process of putrefaction is initially visible as greenish discolouration on the right side of the lower part of the abdominal wall 18 to 23 hours after the death. Then the body starts swelling due to gas formation in the tissues following bacterial action which is evident 36 to 48 hours after the death. The body gets discoloured becoming blue then black. The tongue protrudes out and eyes bulged out. Outer parts of the skin peels off. All the soft tissues; skin, muscles and internal organs become soft and liquefies with the time leaving the skeleton.

• Mummification   

A body may mummify if left in a warm, dry place especially with a flow of air. The mummified tissue is dry, lathery and brown in colour, the tissues are dehydrated and the skin is stiffened. The mummified body may be in an unchanged state for a longer period and the identification features may be well preserved. 

• Adepocere Formation

The body which is left in wet conditions such as submerged body may cover with a greyish greasy compound which formed due to the change of the body fat. Usually it takes several weeks to several months to form adepocere formation.

• Skeletalization

Due to the decomposition the soft tissues on the body disappears and the body becomes a skeleton. Most of the soft tissues will disappear after 1 or 2 years. The bones get disarticulated, dry several years after the death. 

Different factors influence the time of appearance and distribution of the post mortem changes. However from the observation of the different post mortem changes the doctor can determine a range of time interval since the death. But one has to bear in mind that the estimation of time since death cannot be determined from observation of one post mortem change. The history of last seen alive, scene visit findings and observation of all available post mortem changes should be considered when determining the time since death. In fact body may undergo natural processing due to complex chemical nature of the media, especially when disposed in marshy lands, and remain unchanged for indefinite period.