In the late 1700s, the French pediatrician and chemist Pierre Nysten revisited the postmortem muscle lock that the Greeks had dubbed rigor mortis. Like others before him, Nysten noted that death initially released all hold on the muscles. The body is at first utterly flaccid, before gradually stiffening in the hours that follow death.

He was the first, however, to build a clock out of the joint-by-joint progress of the subsequent rigidity. The result, in 1811, was the first scientific description of rigor mortis and "Nysten's law," which states: "The progress of cadaveric rigidity is descending." That is to say, it begins with the muscles of the face, then progresses to the neck, trunk, arms, and finally the lower limbs.

Nysten concluded that the pattern reflected the increasing distance between different muscles and the brain, although he puzzled over the fact that decapitation didn't seem to affect the process! Eeek! It is now known that the general progression of joint paralysis is actually from smaller to larger muscle groups. But that understanding -- as well as an appreciation of rigor's many vagaries -- would only come with the twentieth-century discovery of the biochemical nature of muscle movement.

What we know now is that the membranes of muscle cells become more permeable to calcium ions. Living muscle cells expend energy to transport calcium ions to the outside of the cells. The calcium ions that flow into the muscle cells promote the cross-bridge attachment between actin and myosin, two types of fibers that work together in muscle contraction. The muscle fibers ratchet shorter and shorter until they are fully contracted or as long as the neurotransmitter acetylcholine and the energy molecule adenosine triphosphate (ATP) are present. However, muscles need ATP in order to release from a contracted state -- it is used to pump the calcium out of the cells so the fibers can unlatch from each other. ATP reserves are quickly exhausted from the muscle contraction and other cellular processes. This means that the actin and myosin fibers will remain linked until the muscles themselves start to decompose.

The simplicity of Nysten's law gave early forensic pathologists a deceptively precise chart of time since death. The generally accepted timetable began with the first signs of jaw stiffness an hour after death and wrapped up with the lock of hips and knees ten hours later. The twelfth hour brought "full rigor," a fascinating state in which the body appears fully petrified. Rest the head on one chair and the feet on another, and the corpse will remain suspended like some bewitched volunteer in a vaudeville magician show. At typical room temperatures this rock-solid state lasts for twenty-four to thirty-six hours, before advanced decomposition begins to loosen the muscle groups in the same order that they seized.

About the same time that Nysten was refining his rigor chart, English physician John Davey became the first to thrust Gabriel Fahrenheit's mercury thermometer (invented in 1710) into a human body at autopsy. With this instrument, pathologists had been given the means to add hatch marks to the temperature standards of death known to the ancient Egyptians and Chinese.

Estimating time of death by temperature is called algor mortis. Unfortunately, Davey began his experiments, not in his temperate homeland, but in the sweltering heat of Malta, the Mediterranean stronghold captured by the British in 1800. Consequently, the corpses of Davey's British soldiers actually rose as high as 108 to 113 degrees Fahrenheit in the hours after death. Nonetheless, Davey saw the implications -- that the gradual equilibration of a body's temperature with that of its surroundings could be used to estimate time elapsed since death. Years later, he would become the first person to mention the forensic potential of cadaver temperature in his 1839 textbook, Researches, Physiological and Anatomical.

Davey's widely read book prompted various pathologists to attempt their own measurements. Unfortunately, the pathologists Davey so inspired failed to heed his wisdom in placing his thermometer inside the body. Temperature readings on the skin, typically in the armpit, resulted in what would become the near unshakeable dogma that body temperature dropped at a precise and steady rate of 1.6 degrees an hour (soon rounded to 1.5 for ease of calculation). Greatly popularizing this belief, in 1887 Frederick Womack published his to-the-minute, time-of-death calculations on 118 cadavers. Womack performed his temperature calculations in the mortuary and Anatomy Theatre of London's St. Bartholomew's Hospital, allegedly without being told the actual time of death recorded on each patient's death certificate.

In the preamble to his first case, he apologizes for his "gross" error of estimating the patient's time of death at 4:54 p.m., when in fact the attending physician had recorded it as 5:05 p.m. That's Victorian science for you! Womack's reported accuracy was no doubt impossible, given his, or any other known method, of estimating postmortem interval. Nevertheless, his reports solidified nineteenth-century beliefs in the pinpoint accuracy of temperature in determining the time of death.

Together, the triple stopwatches of rigor, livor, and algor gave nineteenth-century pathologists the confidence to estimate time of death over the first twenty-four to forty-eight hours, up to the point where lividity became fixed, bodies reached room temperature, and rigor melted away. To expand their timetables into the long term, they turned to the scientific records of the previous century.