The decomposition of a body has two stages: autolysis and putrefaction.
Autolysis is the self-digestion of tissues. After death, cell membranes break down, releasing digestive enzymes and causing self-digestion and the disintegration of organs. This chemical process first takes hold in glandular tissues and the brain.
The second stage – putrefaction – follows after rigor mortis.
In this stage there are changes in tissue color, gases are produced, and the liquefaction of tissues happens. The first external sign of putrefaction in a body is usually a greenish discoloration of the skin, which appears in 12 to 18 hours in summer and in one to two days in winter. This green appearance is more apparent on fair skin.
This green colour spreads over the entire abdomen and external genitals, and then patches appear successively on the chest, neck, face, arms and legs. The patches become dark-green, and later purple and dark-blue. The fingertips wrinkle and become leathery, and the nails become prominent. The greenish skin colour may become dark-green or almost black in 3 to 4 days. The viscera become softer and greasy to touch, before eventually breaking down into a soft disintegrating mass.
Gases collect in the intestines in 12 to 18 hours in summer, and 1 to 2 days in winter, producing sufficient hydrogen sulphide to burn with a blue flame. The abdomen becomes tense and distended. The eyeballs become soft, the cornea becomes white and flattened or compressed. Ultimately the eyeballs collapse. The gas-swelling is most pronounced in the face, genitalia and abdomen.
In males, gas is forced from the peritoneal space down the inguinal canals and up to the scrotum, resulting in massive scrotal swelling. The breasts and penis are greatly distended. The eyes bulge from their sockets, the tongue is forced out between swollen and discoloured lips. In 2 to 3 days the sphincters relax and urine and faeces may escape. The junction of the epidermis and dermis is weakened by the release of enzymes, which causes skin slippage and produces large, fragile sacs of clear or pink-red serous fluid.
Flies lay their maggots which start to appear in a couple of days, and they rapidly penetrate the skin. The maggots may raise the temperature to something near or even above that of normal body heat. After three days, the face is so discoloured and bloated that identification becomes very difficult. The hair becomes loose and is easily pulled out. The nails are also loose.
In 3 to 5 days or more, the sutures of the skull separate and liquid brain comes out. Teeth become loose and may fall out.
There is no standard time for a corpse to turn into a skeleton. That end process varies considerably and mainly depends on the temperature, and how many insects inhabit the corpse.
Picture source: Wellcome Institute via Wikimedia