 It wasn't that long ago. It probably feels like a million years to you, but it wasn't that long ago that we were in pretty deep with cavities and the mystery of body cavities. You probably look back at that time and go, holy crud, I wish we could be studying body cavities right now because we've definitely taken lots of steps up in our survey of anatomy. So hopefully thinking about the pericardial cavity would definitely review. If you remember, the pericardial cavity was one of our three cavities that had that wild cirrus membrane inner layer, outer layer, cirrus fluid producing double membrane thing, just like the pleural cavity with the visceral and parietal peritoneum, the pleura, and the peritoneal cavity with the visceral and parietal peritoneum. So totally cool. We're just seeing the same thing again, and I want to remind you where the pericardial cavity is because the pericardial cavity, if you remember back when these things were inconceivable to you, the pericardial cavity is inside the mediastinum. The mediastinum is inside the thoracic cavity. So here is my picture of the entire thoracic cavity. This image is actually from the open stacks textbook, anatomy and physiology textbook, go open stacks, go. And the thoracic cavity is illustrated in purple here. The thoracic cavity contains the pleural cavities, which are these, I guess, the thoracic cavity is the whole thing. The thoracic cavity are the little purple ones on the edges. The mediastinum is shown in yellow. What I would love you to remember is that the mediastinum comes all the way down and contains this blue pericardial cavity. Now, that's where you are. What's inside there? The heart. And if you had to guess about, like if you just had to stop and go, okay, what are all my layers that I'm going to be seeing in the pericardial cavity? Do you agree that we're probably going to have a visceral layer of some sort of serous pericardium and a parietal layer of that? And if you remember looking at our cadavers, there's actually a sac, like there's a little structure that surrounds the heart and you can like open it up and go, peek-a-boo, bye-bye, peek-a-boo. And the heart is just sitting in there, chilling. All of those things have layers that we're going to name. So don't forget where we are. We're in the mediastinum in the pericardial cavity. Now, I'm going to draw you a picture. This picture, I'm going to start out and I'm going to name, I'm going to start out with the heart itself. And again, totally diagrammatic, which I'm sure is shocking to you. But this is my picture of heart. It's intentionally hollow, not drawn in any of the anatomy this time because I don't want you to get distracted by the layers that we're talking about. So it's hollow just like a heart actually is, which means, do you agree that there's probably some sort of, I don't know, would you agree some sort of layer lining inside of the heart itself? In lab, I can do this, I really can label that. In lab, we're actually going to have some fresh hearts to mess with and if you cut the heart open, the fresh heart is lined with this really smooth structure and it's called endocardium. That's actually super amazing that it's called endocardium. It's going to be all over the place here. So my little blue, I should leave it blue. My little blue thing here, oh my gosh, I can decide what color to use. Wow, that was a thin line. I need a thicker line than that. That's way too distracting. That inner layer is the endocardium. Okay, if you have an endocardium on the inside, that white pink layer, this pink structure, what is that, what is it? Heart muscle. It's actually cardiac muscle tissue that makes up the, you know, you could eat that. It's just like heart steak and it has a name that starts with myo because it's made of muscle, cardium. The layer of heart muscle itself is the myocardium. So if you want to solve this problem once and for all and be happily, what are you going to call this layer on the outside because you know there's got to be a layer there. And you also know that of course you cannot have it be that thin. It's got to be that thin, oh much better. What is this layer that I'm now adding on the outside? It's not endocardium and it's not myocardium. It is what, oh, what kind of cardium do you think this is? It's epi, it's on the outside. It's the epi-cardium. That's easy, right? What else might you call it? It's covering an organ. What do you think you might call a very thin membrane structure that's covering an organ? This is also known as the visceral layer of the cirrus pericardium. Did you hear that? Visceral layer of the cirrus pericardium. Okay. The cirrus pericardium is equal to the pleural membranes and it's equal to the peritoneal membranes. So the visceral, mmm, and parietal peritoneum, you also have the visceral layer of the cirrus pericardium. If you have a visceral layer of cirrus pericardium, what else are you going to expect to have that is going to line the cavity itself? Not cover the organ, but line the cavity. And you don't even have to think twice. You know that we're going to have a structure that's going to do this. And even, like, do you agree that it's really one cirrus membrane? It just is ballooned in. Remember the balloons? I wish I had some balloons right now because I'd shove my fist into a balloon. And then the part of the balloon that is touching my hand is the visceral layer and the part that is out here is the parietal layer. It's exactly the same thing that we've got going on right here. So you can imagine that I just have this place where my visceral layer of cirrus pericardium becomes the parietal layer of cirrus pericardium. Now it's definitely relevant to think about, okay, where is that? It's right here. Oops, I don't like that size pin. I'm happy that I don't normally play with pin size. This is the parietal layer of the cirrus pericardium. Altogether, those two, and this makes sense also, the green ones, all together. We were to grab just the green things. That's the cirrus pericardium. The cirrus pericardium is made up of the parietal layer of the cirrus pericardium and the visceral layer of the cirrus pericardium, also known as the epicardium. How's that? It's at the end of the story, but no, no, no, no. It cannot be the end of the story because now we're going to add in one more structure and it's a super thick, fibrous layer. And we'll make it almost my favorite color. And it surrounds the whole thing. It's got to be bigger than that, just a little bit bigger. And now I have to try and make it actually touch. Okay, when you do this, make them touch because the parietal layer of the cirrus pericardium, okay, this is going to be bad because it's going to make it look like the cirrus pericardium includes this fibrous layer, but it doesn't. Oh, jeez, there we go. This, now I've got to make my pen small. This yellow piece is the fibrous layer. Okay, let's write that down, the fibrous pericardium. Look at all my pericardiums, look, my cardium and my other cardium. And this is just another pericardium, it's the fibrous pericardium. And then this, look at what else I'm going to grab for you. If you find, hello, if you grab the fibrous pericardium and the parietal layer of the cirrus pericardium, the parietal layer, you grab both of those together, and that's the pericardial sac. Now, where's my pericardial cavity? Because, you know, we got to have some more things to label. Where's the cavity? The cavity is between my cirrus, my visceral layer of the cirrus pericardium and my parietal layer of the cirrus pericardium. So this right there is the pericardial cavity. And then, of course, don't forget the fact that this is all held within, oh my gosh, I have to do this. But of course, the whole thing is held within the mediastinum. How awesome is that? So now you have all your layers of the pericardial cavity, and we can actually see them in our cadavers. So we can go in and flip around and see our layers and see how they will be pinned for you in your quiz because, you know, I got to ask the layer questions. All right, so now you know where your heart lives in the pericardial cavity. Now we know all the pieces that fit into the pericardial cavity. Now we want to look at the specific anatomy of the heart itself.