After death the body cools (algor mortis), blood settles (livor mortis), muscles stiffen and relax (rigor mortis) and finally the body decomposes — each on a rough clock that helps estimate the time since death.
How it's asked: Order and timing of rigor (Nysten's rule), colour of hypostasis in specific poisonings, cadaveric spasm vs rigor, and the first sign of decomposition.
Why this is true
After death ATP is no longer regenerated. Without ATP, myosin heads cannot detach from actin, so muscles lock — rigor mortis. It passes off only when the proteins themselves break down during decomposition. Livor mortis is simple gravity: blood pools in dependent vessels while it is still fluid, and once haemoglobin leaks into the tissues the stain becomes 'fixed' and no longer shifts when the body is turned. The colour of that blood shows the pigment carried in it, which is why carboxyhaemoglobin looks cherry-red and methaemoglobin chocolate-brown.
Key points
Colour of hypostasis
| Colour | Cause |
|---|---|
| Cherry-red | Carbon monoxide (carboxyhaemoglobin) |
| Bright / brick-red | Cyanide; hypothermia |
| Chocolate-brown | Nitrites, chlorates, aniline (methaemoglobin) |
| Bluish-green | Hydrogen sulphide |
| Dark brown / yellow | Phosphorus |
Common traps
- Heat stiffening (pugilistic attitude in burns) and cold stiffening are not rigor mortis — they don't follow Nysten's rule.
- Hypostasis can mimic bruising; cutting the area shows blood confined to vessels in hypostasis, but extravasated blood in a bruise.
Mnemonic
Rigor '12–12–12': ~12 h to develop fully, stays ~12 h, passes off over ~12 h.
Clinical case
A man is found dead in a closed garage with the car engine running. Postmortem staining is a bright cherry-red colour.
High-yield
Rigor: myocardium first, then Nysten's order. Hypostasis fixed by ~6–8 h. CO cherry-red, nitrites chocolate-brown. Decomposition starts at the right iliac fossa. Cadaveric spasm = no primary flaccidity.
Quick check
Q1.Cadaveric spasm differs from rigor mortis because it:
Q2.Chocolate-brown postmortem staining suggests poisoning by: