 Cirrus membranes are inside the body and their job is really, their function is to reduce friction. So they line cavities and there's actually a list of cavities that they line and they cover organs. One cirrus membrane does both of those things because they're actually double layered membranes. And I'm going to draw you a picture to illustrate how they end up with this double layered phenomenon. But the cavities that they are lining and the organs that they are covering, there's actually three of them. And again, the first one we're going to see in the next class period when we do the digestive system. So cirrus membranes line the peritoneal cavity. And do you remember where the peritoneal cavity is? It's inside the abdominal cavity. It's a cavity within the abdominal cavity. And it contains a whole bunch of digestive organs, tubes, which is a little bit awkward. Another cirrus membrane is found lining the two pleural cavities. And so they line the pleural cavity and they cover the organs in the pleural cavity. What are you going to find in the pleural cavities? You're going to find lungs in there. So they line the pleural cavity and they cover the lungs, this two-layered cirrus membrane. The third place we're going to see it is the pericardial cavity. So they line the pericardial cavity in the thoracic cavity within the mediastinum, and they cover the heart. The cavity itself is actually a sort of interesting phenomenon as well. I'm going to draw you a picture of how this is. Before I do that, I feel like, dude, we've got to know what they're doing. The whole purpose of a cirrus membrane, two layers, is to reduce friction. And let's just think for a second. Think about the pericardial cavity. This is where the case is the most convincing, I think. Talk to me about the organ that's in the pericardial cavity or that's surrounded by, I'll show you what it is in a second because it's not actually in the cavity. Just like this, your food isn't actually in your body. We've got all sorts of fun technical details in anatomy. At any rate, the heart is the organ that is somehow associated with the pericardial cavity. And tell me what the heart does all day, every day, for the rest of your life until you die. Dude, it's beating. It's beating like mad. And hopefully not beating too crazy right now because hopefully you're feeling all relaxed about anatomy, like anatomy, piece of cake, easy peasy. So you've got a nice slow heart rate going. The act of your heart beating generates friction. And you can imagine that if you didn't reduce that friction, things might get sore. Like if you did this with your hands and rubbed your hands on carpet for like an hour or forever, like eventually you're going to be like, ah, that is not a fun, like that friction can generate painful sensations. You can have friction in your heart that hurts. And breathing, you can have friction between your lungs and the serious membrane where it's not doing its job of producing that fluid. It produces serious fluid that reduces that friction. So I want to show you the structure of this membrane and how it ends up looking like it does. How it ends up with this double membrane. Okay, you ready? We have a layer of epithelial tissue. And the thing starts out as sort of a balloon, like a bubble of epithelial tissue. So, and the epithelial tissue is simple, squamous epithelium every time. So your serious membrane always has a layer of simple squamous epithelium. There's a basement membrane, but I'm not going to confuse the drawing that I'm going to try and do by adding a line of basement membrane in there. The basement membrane connects us to a layer, a very thin layer of connective tissue that surrounds that or is attached to the epithelial tissue. The connective tissue is very thin and it's made of loose, a realer connective tissue. Okay, and it's super thin. Seriously, if you go in, you could like flick the serious membrane and probably bust it all the heck and gone. Don't do it. This is a very thin thing associated with an organ. We'll do the heart because they're fun to draw. During development, these organs that are associated with serious membranes actually push into these balloons of serious membrane. So look at what's going to happen here. I'm going to draw my realer connective tissue first. This is like the next step. The heart is actually going to sort of squish in to the balloon. Okay, do you see it? What's happening here? My epithelial tissue lining is still present. Of course it is, right? My heart is pushing into this thing. So it's sort of getting surrounded by it. Do you see what's happening? And then my heart, my heart, my heart ends up completely surrounded by this membrane. I have to be really careful how I draw this because this is what happens here. There we go. I did it. I did it. Do you see how it just pushed its way in? I didn't leave very much space here, but you will be patient with me. It actually is not even really considered an actual space. The cavity, it's considered a potential space. Oh, geez. So I'm just illustrating the potential nature of this space. Remember that I'm drawing a simple squamous epithelium green that is attached to my blue aryler connective tissue by a basement membrane that I'm not drawing in here. What I've drawn is a cirrus membrane. Do you see how it has two layers? Yes, say yes, say yes. Okay, here's my double layered cirrus membrane. That says cirrus membrane in yellow. Oh, dear. And I'm capturing both those layers. Now, of course, those layers have names. And the one, the layer that's closest covering the organ, do you see how this one is closest to actually covering the organ? I'm going to go this direction. This is called the visceral layer. The one that is farthest away from the organ that actually covers or lines the cavity is called the parietal layer. The visceral layer plus the parietal layer make up the actual cirrus membrane. Both layers secrete fluid. I'm drawing fluid in this space. This is cirrus fluid. And the cirrus fluid is what is reducing that friction. How do you feel about that? The nice thing is all three of my cavities that are associated with cirrus membranes, all three of them have visceral layers and parietal layers. We have the visceral peritoneum and the parietal peritoneum. We have the visceral pleura and the parietal pleura. And we have the visceral layer of the cirrus pericardium. How's that for fun? And we have the parietal layer of the cirrus pericardium. So once you can sort of visualize like what this cirrus membrane actually is, you understand all of them. So let's see how my heart got in there. The cirrus fluid is actually in the cavity. So do you see why the heart isn't actually in the cavity? The cavity is sort of surrounding the heart. That works. That's a little technicality. It's easy to visualize a heart in that double-layered balloon sack. It's easy to visualize a lung. It's a good thing you can grab and stick it in that double-layered balloon sack. Dude, try and visualize a tube like your whatever, I don't know how long your digestive tubing is, but it's long. And try to imagine that being shoved into a balloon layer. And what's that like? What? That's what we're doing next. The peritoneum is the most conceptually, like spatially challenging to understand. So we'll talk about that with the digestive system, which is coming up in the next lecture. Okay, so we have our mucous membranes. We have our cirrus membranes. The first organ system we're going to talk about deals with cutaneous membranes. So let's get started on skin.