 Okay, our first membrane is the mucous membrane and a mucous membrane is defined as a epithelial tissue plus connective tissue sandwich structure, a structure built from epithelial tissue of some flavor and connective tissue that lines, that says lines, I'm not lying. I'm not lying, that says lines, okay, fine. It lines a space outside the body. And in order to have this conversation about inside and outside the body, because I'm going to tell you that your cheek, the inner surface of your cheek is lined with a mucous membrane, your stomach is lined with a mucous membrane. Dude, your vagina, if you have one of those, is lined with a mucous membrane. All of us have urethra's, mucous membrane, whatever. There's a bunch of places that have mucous membranes and I'm telling you all those places and you might be like, dude, you just said they line spaces outside the body. You have to make this case. So I'm going to draw you a picture of the body and the body looks like this. This is your mouth. Guess what? Good gracious. That is. Say the word. Say it out loud. Loud and proud, that's your anus. This is a tube, right? Do you think it looks like that? No, it doesn't look exactly like that. There's a lot more curves and like crazy anatomy and oh, guess what? We get to do the anatomy of that tube right there in the next lecture. But my argument for you is that this is actually outside the body and that if this is my body, right, and I've got some limbs hanging off of there and I'm not even going to draw those things because this is a really nice anatomically accurate picture of your body. But check it out. I'm going to draw a heart because I love drawing the heart and I'm going to show you like some blood vessels going through and you know that your blood vessels connect to your heart and it's a tube. Is your blood vessel tube outside the body? Silly. Of course not. Of course your blood and your heart is all inside your body. But the ice cream cone that I really wish I was eating right now, good lord, that's an ice cream cone. Kind of looks like a Star Wars critter. The long nose, like am I imagining that or was that a real Star Wars thing? It's an ice cream cone. I'll put a cherry on top for a yummy factor. The ice cream cone that goes in your mouth isn't really inside your body when it's in that tube. If somehow you do some magic, which if I had an ice cream in my tube, I totally would do some magic and I broke it up and I absorbed it through my mucus membrane and into my blood. Now I'm inside my body. The ice cream is now inside my body. Substances that you ingest, that you then poop out, corn. You know that you've had that experience of like eating corn on the cob and then taking a doo-doo and being like, dude, did I even chew that corn? Like how did that happen? Corn for whatever reason is hard for your system to digest. So sometimes it just goes straight on through. It was never inside your body. Don't eat it because it's like contaminated. So you definitely don't want to have that experience. But it didn't ever go inside your body. If it did, it wouldn't have come out looking like that. Okay, that was really disturbing. But did I make a case about inside or outside the body? In order for something to go inside the body, it actually has to cross a cell membrane. And the cell membrane that it crosses is a mucus membrane. Is the epithelial cell membrane that is part of the layer for a mucus membrane? Do you follow what I just said? Good. Let me tell you what a mucus membrane is because you can actually see it right here. In fact, before I draw anything on here, see if you can guess where I'm going to draw the mucus membrane. Look, I'm going to draw it here. Okay, and this is not going to be perfect because I drew my ice cream cone a little bit close to the surface. All mucus membranes have this epithelial tissue layer. Some mucus membranes, like the one lining your cheek, the inside of your cheek, or the one on your esophagus, some mucus membranes are stratified squamous epithelium. Most of your digestive tract is simple columnar epithelium, but again, it's some layer of epithelium. Look at all those cells I'm drawing. That was super dedicated of me. Epithelium is attached. Look, oh my Lord. I don't know. I'm actually trying to draw a straight line and I feel sort of worried like why can't I draw a straight line? What did I just draw in a rather wavy version? What is that thing? It's my basement membrane. Basement membrane indicates that I have a structure that's going to connect the epithelial tissue to an underlying layer of what? Connective tissue. Connective tissues are always easy to draw and this connective tissue layer has a name. That connective tissue, the name of the structure is the lamina propria and it's made of loose connective tissue or aryler. Those are the same thing. So you have this layer of loose aryler connective tissue attached to some flavor of epithelium by the basement membrane and that is a mucous membrane. That's how we know that we have one. Depending on where you are, the mucous membrane can also be called the mucosa. So if you see the term mucosa, that tells you that, oh, it's a mucous membrane. I'm going to see an epithelial layer and I'm going to see a lamina propria and the lamina propria is going to be messy. Probably going to be blood vessels in the lamina propria. The function of a mucous membrane really is epithelial. The purpose of getting, oh, this stuff here, whatever. The purpose of that mucous membrane is to absorb goodies. We also secrete stuff. So for example, these cells might be secreting digestive enzymes that are going to break down that ice cream cone so that then I can absorb that ice cream cone into my bloodstream. And the other thing, and I mentioned this when I talked about the esophagus or maybe I didn't mention it yet, the cheek. One of the other roles of mucosa is protection. I honestly cannot remember if I said this. I know I said it at one point but I've tried to record this clip a couple of times so if I already said this, I apologize. But I don't want to miss it. So eating something like tortilla chips, they're crunchy, they're pokey. If you had a simple squamous epithelium in your cheeks or in your esophagus as part of your mucosa, that's just going to tear your cheeks and your esophagus all the heck and gone. So that's not a great strategy. You actually have stratified squamous epithelium as the epithelial layer of the mucosa attached to a laminopropria, just like you would expect. Okay, so mucous membranes sort of mediate the movement of stuff from inside to outside in vice versa and we're going to see it anywhere. So we have a tube that has an opening to the external environment. All right, our next membrane is the serous membrane so let's look at that thing.