Say it fast! Post mortem lividity! Has a certain ring to it, doesn't it? Nah, not really. But, it is the bread-and-butter to much of what I do. In kindergarten terms, (and I don't say this to be condescending, I just happen to THINK in kindergarten terms!) post mortem lividity means "where your blood settles when you die" (and for those of you who live in Inner-City Anywhere, the answer is not "on the sidewalk").
Back to our lesson . . .
When you die, your heart stops pumping blood, so blood stops moving around your body. Slowly, it begins to settle to the lowest parts. So if you die laying face-up in your bed, your blood will settle in your back, the back of your arms, the back of your legs, etc. If you hang yourself, the blood will settle in your feet (except for the blood that's trapped in your head by the rope you're hanging from). Then you can end up with an eggplant-colored face. (I know . . . say it together, class, "Ewwwwwwwwwwwwwww!")
Since this is kindergarten, there are only two things we need to know: lividity pattern and whether the pattern is "fixed."
When blood settles to the bottom, it stays there. After it's there for a long time, the pattern becomes "fixed," i.e., when you press your finger into the skin, the skin doesn't blanch much. Try this on your hand right now . . . push your finger deep into the skin. The skin will blanch (turn white) as the blood moves away from the pressure. If the lividity pattern is fixed or close to fixed, the blood won't want to move, so the skin won't blanch much. Whether or not the pattern is fixed helps us to determine how long you've been dead.
Another thing that's important about lividity patterns is "blanching pattern," because although the blood will settle to the lowest part of the body, it often doesn't settle at points of contact with hard objects. For instance, if you are lying on your back, there will often be light spots where there is very little blood, such as on your butt cheeks or shoulder blades.
A fixed lividity pattern is a wonderful way for a quick check on whether or not your body has been moved. For instance, let's say you died face-down in bed. The blood will settle where? Yes! In your tummy and the fronts of your legs and arms and your face! Again, we're talking about the possibility of an eggplant-face here. I know . . . get it out of your system. "Ewwwwwwwwwww!"
And let's say, you're lying there, (minding your own business) when someone finds you. They freak out. (People do that when they find dead people.) Then they call 911. Now you've been lying there, dead, for a while. You've got an eggplant-face, but the 911 dispatcher will often still tell the person to put you on a hard surface, (i.e., the floor) and turn you face-up to do CPR. (Important note: CPR doesn't work on people who have been dead so long they have an eggplant-face.)
Eventually, I end up there (it's my JOB) and I see that you have an eggplant-face, and it looks like you have a horrendous sunburn or bruising on the front of your body with white spots on your knees. I KNOW you have been turned over. Depending on the situation, this may, or may not be a problem. Sometimes people move a body for completely innocent reasons, they cut down folks who are hanging, or they try CPR on folks who are already long-dead. Other times, folks move a body to change or conceal a crime scene. Part of my job is to figure out IF a body had been moved, and if so, why.
And that . . . is your Forensic Lesson of the Day! (Clear as mud, right?) I wanted to provide photographs, but obviously I couldn't use pictures from my own files, and wonder of wonders, the internet wasn't filled with tasteful photos of post mortem lividity patterns (now that's an oxymoron if I've ever heard one! There is no such thing as a "tasteful lividity pattern" photograph!) Anyway . . . here is Professor Puppy to teach Kindergarten Crime Scene!