The first effect of death from any cause is general relaxation of the entire muscular system. The lower jaw drops, the eyelids lose their tension, the limbs are flabby and soft, and the joints become flexible.
In from five to six hours after death, and generally while the body is in the act of cooling, the muscles of the limbs are observed to become hard and contracted, the joints stiff, and the body unyielding. Muscles which are contracted in the death-agony do not necessarily become relaxed at any time.
The muscular tissues in the dead body can be considered as passing through three stages: (1) flaccid but contractile, (2) rigid and incapable of contraction, (3) relaxed and incapable of further contractility.
Rigor Mortis.
This is sometimes called cadaveric rigidity and occurs generally within six hours after death and disappears within sixteen to twenty-four hours. Many theories have been advanced to account for it, but the most probable one is that the rigidity is due to the coagulation of the myosin in the muscles by the weak acids which are no longer removed from the system; the muscles always give an acid reaction and are opaque instead of transparent; after putrefaction has set in ammonia is developed, the myosin dissolved, and so flaccidity results.
Rigor mortis occurs first in the muscles of the eyelid, next the muscles of the lower jaw and neck are affected, then the chest and upper extremities; afterward it gradually progresses from above downward, affecting the muscles of the abdomen and lower limbs. The rigidity disappears in the same sequence. The period after death when rigor mortis manifests itself, together with its duration, is chiefly dependent upon the previous degree of muscular exhaustion. Brown-Séquard has demonstrated that the greater the degree of muscular irritability at the time of death, the later the cadaveric rigidity sets in and the longer it lasts. He has also shown that the later putrefaction sets in, the more slowly it progresses.
The more robust the individual and the shorter the disease, the more marked and persistent is this muscular rigidity. It has been noticed that the bodies of soldiers killed in the beginning of an engagement become rigid slowly, and those killed late quickly. This explains the reason why bodies are sometimes found on the battle-field in a kneeling or sitting posture with weapons in hand.
If the rigidity of rigor mortis after it is once complete is overcome, as in bending an arm, it never returns; but if incomplete it may return. This will serve at times to distinguish real death from catalepsy and its allied conditions. While the average duration of rigor mortis has been given as sixteen to twenty-four hours, it must be remembered that in some cases it has been known to last only a few hours, as in death by lightning or by electrocution. In other cases it has persisted for seven and fourteen days.
This long continuance of rigor mortis has been noted in death from strychnine and other spinal poisons, in suffocation, and in poisoning by veratrum viride.