 Hey you guys, so we're back today to talk about the other half of the digestive system. In the last lecture, we totally went through gross anatomy of the digestive system. We looked at the tube, we named all the parts of the tube, we named the things that dump stuff into the tube, and today we're going to go microscopic and we're going to look at the layers of the tube and the histology of the tube. So the histology is actually significant. Like, what do you know? Structure determines function and the structure of the tissues that are used to build the tube in various places in your body determine the function of that part of the tube. There's lots of really cool hints about where you are when you are looking at the histology of the digestive tubing, and there's also some really cool similarities between all parts of the tube. So I'm going to start us out by drawing a picture of a generalized tube for you. And I want you to remember that we are looking at a tube, and I'm going to draw a flat version of this thing first just so you can see the layers that we're going to be looking at. And then we'll correlate the layers to this image from the OpenStacks textbook. Okay, so first of all, it's a tube, doggies. So what do we know is present in our tube? This thing right here, we know we have a lumen, right? I'm going to put the lumen at the top of this little page here. And I know there's settings that I can fix on this thing that will let it do what I want it to do. Okay, what do you didn't expect? We've talked about this before. We've talked about different kinds of membranes. What kind of membrane is found lining the digestive tube? We know that it's a mucous membrane, right? And a mucous membrane is made up of a layer of epithelial tissue. And I'll go ahead and just make my epithelial tissue black, outlined in black. And you can guess what kind of epithelium it is. The different parts of the tube, the kind of epithelium you're going to find, often tell you where you are, can tell you where you are. I'm going to make a structure next that looks a little something, something like that. What is that structure, y'all? Shout it out loud and proud. That would be the basement membrane, yeah. So you know that the epithelial tissue is connected to connective tissue deep to the basement membrane, deep is the wrong word when we're talking about tubes, basal to the basement membrane. And I'm going to make my next layer, connective tissue, this color. This is a layer of loose connective tissue or a realer connective tissue, and it has a name. Do you know this name, my friends? Yes, this is the lamina propria. And you know that epithelial tissue bound to connective tissue via a basement membrane lining an external space is often a mucous membrane. The mucous membrane in this digestive tubing is really attached to another layer, a really thin layer, a smooth layer that is called muscularis mucosi. Muscularis mucosi is smooth. What do you think it's made out of? What kind of tissue do you think it's made out of? It is made out of smooth muscle. The propria was made out of loose connective tissue or a realer connective tissue. And epithelial tissue is made out of epithelial tissue. Most epithelial layers in the digestive tubing are actually simple columnar epithelium. This whole thing is one layer. This whole thing is the mucosa. That's the first layer of my digestive tubing. That's the luminal layer of my digestive tubing. Now, there is another layer of... I'm going to make it a little bit lighter. There's another big old layer, big. It depends on where you are. Sometimes it's big. Another layer of what am I drawing? Connective tissue madness. And this layer is called the submucosa. It's made out of either a realer connective tissue or dense regular connective tissue. But it's definitely connective tissue. And it's definitely basal to muscularis mucosi. Basal to submucosa is another... What am I drawing? I actually can't remember if this layer comes next or if this layer comes next. What kind of tissue am I drawing? I'm giving you a hint. I've already drawn it once. I've drawn this kind of tissue once. I haven't drawn this layer yet. This layer is usually two layers, same kind of tissue. And it's called muscularis hint hint externa. Muscularis externa usually is two layers and usually it is made out of smooth muscle also, usually. I think I've sort of... Oh, we've got more. I'm going to do this last layer kind of vague. And the last layer is either the adventitia or the serosa. And it depends on where you are, whether you are going to be an adventitia or a serosa. Now, look, this whole thing is your digestive tubing. I'm going to layer it, I mean label it, on this tubular diagram for you. So check it out. We've got our lumen. You can see that lining the lumen is epithelial tissue. So this edge is epithelium. What was basal to the epithelium? That would be laminopropria. And you can totally see that, right? It looks like a connective tissue, like who knows what. And then basal to that was muscularis mucosii, right? And that's made out of smooth muscle most of the time. Next up, we have a layer basal to muscularis mucosii. This whole thing, remember, made up the mucosa. So the next layer in line is the submucosa, right? That thing looks like a little connective tissue, fun times, whatever. Then it looks like we've got two layers here, this one and this one, of smooth muscle, and that's muscularis externa. And then we actually have the, I think this one is cirrhosa, but it could be adventitia. And it's actually, I'm going to give you a little tiny hint. It's simple squamous epithelium with a tiny thin layer of a real or connective tissue. Do you know what that is if it's cirrhosa? That's actually the aceris membrane. So that'll be an easy way to remember that. I also want to draw your attention to this little look. Here's my cirrhosa and it makes a little sandwich with veins and nerves inside of it. This is actually a mesentery. I just don't want you to forget all the things that you did last time. And as long as you are remembering that, we'll be good to go. Okay, so here's your generalized tube. Every tube you look at is going to have layers. And as long as you don't freak out, don't freak out. As long as you don't freak out, you can identify those layers and it's not that hard. Just look in, stay far away and look for those layers. And don't freak out. His toe. It's fun. Okay, I'll be right back and we'll talk about some specific things that some specific ways that this tube can be arranged to increase the surface area in various places in the tube.