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History - Science
As forensic pathologist Dr. Judy Melinek notes, "Many people die with their eyes open, and when the whites of the eyes start to dry out, they turn blue or gray. This is called 'tache noir' and is frequently part of forensic board examinations." The phenomenon arises hours after somebody dies with their eyes open, as dry air dehydrates the irises and augments any subsequent discoloration.
Over the course of the next few hours and days, the cornea – that transparent outer layer of the eye — becomes contaminated with dry air and debris, and the eyes start to appear more gray and opaque. The eyeballs will eventually start to recess back into the skull once all reflexes and blood flow to them have been cut off.
Studying the level of opaqueness that has consumed a dead person's cornea, scientists can more or less estimate how long it has been since that person died — though a more accurate method involves measuring increased postmortem potassium levels in the eyes. The eyes also provide glimpses of the truth that other parts of the deceased anatomy do not.
Petechial hemorrhages, for example, are tiny capillary ruptures that appear in both the conjunctivae and eyelids after a person has been strangled. Dr. Randy Hanzlick says, "They're caused by an increase in the hydrostatic pressure in the capillaries. You can see them in many forms of asphyxia, like strangulation or chest compression, but you can also see them if there's a [build-up of] pressure from heart failure or a sudden cardiac death."