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4.6.1 How forensic pathologists determine the time of death

Time of death can be measured using the following factors;

  • Body temperature
  • Extent of rigor mortis
  • Level of decomposition
  • Forensic entomology

Body Temperature

A body cools following an S-shaped (sigmoid) curve. The initial plateau at 37˚C lasts 30 – 60 min, then the body cools quickly to ambient temperature.

After 24hrs a body has usually finished cooling and temperature is no longer useful.

Temperature is measured using a long thermometer with a wide range. Temperature is usually taken rectally or using an abdominal stab.

The rate of cooling depends on the situation the body is found in e.g.

Clothing – slows cooling

Found in water – speeds cooling

Found indoors – slows cooling

Air movements – speed cooling

Rigor mortis is the stiffening of joints and muscles. Small muscles stiffen first and unstiffen last.

Muscles stiffen because they run out of ATP, causing the actin and myosin muscle fibres to stick permanently to each other. Muscles unstiffen because the muscle fibres begin to break down.

On page 80 of your text book is a little more detail about the sequence of events that causes muscles to run out of ATP.

Level of Decomposition

Autolysis is the break down of body tissues using the body’s own enzymes from the digestive system and from Lysosomes.

 

After this, bacteria from the gut invade tissues and release more enzymes. This tends to happen in anaerobic conditions, which favours the growth of anaerobic bacteria.

Autolysis is increased by mild heat and slowed by intense heat. Humidity has a big involvement as well – dry conditions slow autolysis and, in some cases (e.g. mummies) stop it completely.

 

The presence of wounds, the clothing the person was wearing and the combination of gases released during decomposition also have an effect.

Forensic Entomology

The insects found in a dead body can help identify time of death in 3 ways;

 

  1. If the temperature of the body has remained relatively constant the age of the maggots growing in it can be determined by their starting length and the temperature of the part of the body they grew

e.g. a maggot 3mm long found growing at 28˚C will be roughly 3 days (8 hrs old)

2. Using the life-cycle of the maggot to identify age

3. If maggots are taken from the body, allowed to grow and the time taken to pupate is recorded; it is sometimes possible to work backwards from the pupation date and work out how old the maggots must have been when they were taken from the body. This works because maggots of different species usually take a fixed number of days to pupate.