 Hello, today we're going to talk a little bit about the post-mortem interval. And what that is, the tool that forensic scientists can use to estimate how long an individual has been deceased, how long a body has been left behind at a crime scene or in the elements. So let's take a look and let's go through some of the methods that we commonly use to estimate this. So there are three common methods that we use in estimating the post-mortem interval, or PMI for short. We look at stages of decomposition, so that's what happens to the body during the stages of death, or after death has occurred. And then we also look at insect behavior and life cycles. And this is forensic entomology, the study of insects. And we also observe the environmental factors that might have contributed to the state that we see the body in. So how long it's been in a certain location and then what has actually happened to it while it's been in that location based on the weather or other environmental changes. The stages of decomposition can be divided into four overall broad stages. We have the fresh state, the bloat state, and the decay and then dry state. And it's pretty interesting because a body can actually be in several different stages of decomposition at one time, depending on if it's laying maybe partially in a wet environment or partially in a shady area. These are going to contribute a little bit to the stages that we're observing. So first you start out, oftentimes in forensic science we use pigs as a proxy for humans because they have a similar body composition to humans and similar texture of the skin and organs and things like that. So we use pigs commonly as proxies in experiments. So here we have pigs, they start out living or in the anti-mortem state before death. And then our first stage, we have a fresh body. And during this stage we see at the time of death the heart is going to stop. We have the skin becoming tight and gray in appearance. The cells start to die, so your whole body doesn't die all at once. The cells die sort of one by one. And it starts out with your brain cells dying in about three to seven minutes. And then after that skin will die a little bit, it takes a little bit longer for skin to die, so up to about 24 hours or so. And at this stage you're gonna see the muscles which start to relax. And you'll see the bladder and the bowels will empty from the body. Then we observe what's called algoramortis. This is commonly referred to as the cooling of the body. And this is based on the idea that once an individual is deceased, their body no longer generates warmth and no longer has to maintain a certain temperature in order to survive. So we can use this drop in temperature to estimate how long an individual has been deceased or the time since death. So our normal body temperature is about 98.6 degrees Fahrenheit. And some people run a little bit hot, and some people run a little bit cold. But on average, our body is going to cool at a rate of about 0.32 degrees Celsius per hour. So if we have that information, if we can take the temperature from a corpse, then we can insert that into a formula and we can determine how long the individual has been deceased based on the cooling of the body. Then we have liver mortis, and this is commonly called the discoloration of death. And so the body starts to turn different colors as the blood in the body starts to settle. So it's no longer moving and pumping through the vein, so it starts to settle in different locations. So if someone is laying on their back when they die, you'll see the blood start to pull in their back. If they're in a more upright position or sitting, you might see the blood start to pull in their legs first. And so we'll see that discoloration in those regions that are closer to the floor or closer to the lower area of gravity there. And we'll see liver mortis setting in in about 30 minutes to three hours after death. And during this time, we see the corpse starting to get purple and waxy in the skin. We see lips, finger and toenails start to become pale or turn white. The hands and feet will turn blue. And this is when the eyes will sort of start to sink back into the skull a little bit. Then rigor mortis. This is commonly referred to as the stiffness of death. And what happens after death is immediately afterward, the body becomes limp due to the just overall relaxation of the muscles. And these muscles are going to start to stiffen due to some chemical changes in the muscle tissue, the lactic acid and things like that. And what happens with rigor mortis is it actually sets into different muscle groups at a different rate. So you'll see it happening in the smaller muscle groups like those in your hands and feet, and then moving into the larger muscle groups like those in your hips and your torso region. Rigor mortis happens in a predictable pattern. And here are three rules that you need to know in order to determine the states of rigor. So within about 10 hours of death, that's when rigor mortis starts to set in. That's when you start to see stiffness developing in the small muscle groups. Then the whole body will be stiff within about 12 to 18 hours. And then that stiffness starts to disappear after 24 hours. So that means a body that's been dead longer than 24 hours is no longer going to be stiff. And so you can't use rigor mortis anymore to help to determine with the post-mortem interval, help to determine how long that individual's been deceased. So let's take a look in our pig, in our pig sample and see what some of these stages look like in our proxy. So here we have our pig in the first state of decay in the fresh state. And at this point, you can see that the body overall appears normal, just like it would be if the pig were alive. It even looks like it's sleeping. But what we're actually seeing is on the inside the cellular death, the death at the cellular level. So at this point, the cells of the brain and the skin cells are decomposing and dying, and we're seeing that the cells are no longer moving and shaking and doing what they usually do. At this point, we don't sense the odor of decomposition that's common with other stages of decay. But there are other indicators that this individual is deceased in this first stage of decomposition, like we have insect activity already starting. So here we have a fly, and this is drawn to the ear region of the pig. And the fly is already landing on the deceased individual and starting to find a place to lay eggs. And then here you can see we've got some eggs that have been laying on this individual, on this corpse. And so insects are usually gonna try and find some moist areas to lay their eggs. And that's exactly what we're seeing here, again, in the first stage of decomposition. So why don't you take a minute and think about what you might expect to see in some of the later stages of decomposition.