Atmospheric conditions modify to a large extent the duration of rigor mortis. Dry, cold air causes it to last for a long time, while warm, moist air shortens its duration. Also immersion in cold water brings on rigor mortis quickly and lengthens its duration.
Cadaveric Ecchymosis—Cadaveric Lividity or Hypostasis.
Within a few hours after death the skin of the body, which is of a pale, ashy-gray color, becomes covered by extensive patches of a bluish or purple color, which are most pronounced and are first seen on the back part of the trunk, head extremities, ears, face, and neck, and are due to the blood, before coagulating, settling in the most dependent parts of the body, producing a mottling of the surface with irregular livid patches. There is also a stagnation of blood in the capillary vessels, especially in those in the upper layer of the true skin or in the space between the cuticle and cutis. The discoloration continues to increase until the body is cold, when it is entirely arrested. Later on, just before putrefaction begins, the color deepens, and the change appears to proceed from an infiltration of blood pigment into the dependent parts of the body.
At the same time the discolorations are appearing on the surface of the body, internal hypostasis is also taking place, most marked in the dependent portions of the brain, lungs, intestines, kidneys, and spinal cord.
This condition in the brain may be mistaken for so-called congestive apoplexy; in the lungs, for pulmonary apoplexy or the first stage of lobar pneumonia; in the intestines and spinal meninges, for the beginning of inflammatory changes.
The position of these hypostases will afford the best correction for this possible error. The appearances presented by cadaveric ecchymoses have often been mistaken for the effects of violence applied during life. Innocent persons have been accused and tried for murder or manslaughter on charges afterward proved to be groundless. Therefore it is of the utmost importance that the medical jurist should be able to distinguish between ante-mortem and post-mortem ecchymoses.
The following are the points of difference:
1. Situation. Post-mortem ecchymoses are seen on that portion of the body which has been most dependent, generally the posterior aspect, and they involve principally the superficial layers of the true skin; ante-mortem ecchymoses may occur anywhere, and generally the deeper tissues are discolored.
2. In cadaveric lividity there is no elevation of the skin and the discoloration terminates abruptly.
3. After cutting into the tissues where an ecchymosis has been produced by violence, the blood without the vessels is free in the tissue; this is not so in cadaveric ecchymosis.