 You remember that we had several ways that we were going to increase the surface area in the intestinal tract or the digestive tract. And one of the ways was just by how long it is. We also had villi. We had microvilli. And then we had these circular folds. And I want to draw you a picture because I can draw you a picture of a part of the tube where you can see all three of these. Are you ready for my amazingness? Okay, you're going to have to be patient for just a second while I figure out how to make this happen. Here, what is the... Okay, here's my lumen. And this is my luminal layer. Okay, so this is my mucosa. And watch the madness. Now, I'm going to draw these finger-like projections. What were my finger-like projections? Do you remember what they were? Those, each one of these little projections is a villis. This, okay, wait a second. This is a villis. How many villi do you have in here? One, two, three, four, five, six, seven, eight, nine, ten, eleven, eleven. Okay, let's do it here. Eleven villi in my picture. You got it? Okay, what is deep to the epithelial layer in the mucosa? We actually are going to have laminopropria that's actually extending up inside of this thing. So this is actually laminopropria in here. Does that work for you? That's one of the ways that you can tell that you have a villis. Now, microvilli, talk to me, what's the scoop? Remember your microvilli? If I take that chunk and I blow it up, because you know, we got to blow things up. So I'm blowing it up right over here. You agree that I'm going to have some kind of epithelium, right? And what is lining the luminal edge of the epithelium? Microvilli. Look at all these microvilli lining my single epithelial cells. This is just a blown-up version. Look, I'm even going to draw my little purple box around it so that you can see that, yeah, I just blew this up, dogs. Because of course you blew it up. And you can see that the microvilli, how many microvilli are in my picture right here? Okay, because, I mean, I know that you think it would be really cool if I sat here and actually counted them all. But we're going to say there's an infinite number of microvilli in here because I am not going to count them. And there's like holy microvilli all over. Do you think that you can see the microvilli on, like in your microscope, can you see the microvilli? Probably not. It might look a little fuzzy. Let's just say fuzzy, fuzzy, fuzzy, fuzzy. That's a good book. I haven't had to read that book in a long time because my humans are getting too old for fuzzy, fuzzy, fuzzy. Lots of little fuzzy microvilli. What's the circular fold? I need, look, we'll make the circular fold this color. This whole thing is one circular fold. There are 11 villi, there are a billion microvilli, and there's one circular fold in this picture. And you can tell because you know this whole thing is a villus. So then we've got this other extension out. Well, that extension is a circular fold. All right. What else do you need to know? Nothing. Now you know what to look for in your slides to see if you can find all of these different structures. Now we're going to look at each specific section of tubing and figure out, like, how are you going to know? How are you going to know if you're in the esophagus, in the stomach, in the small intestine? How are you going to know? I'm about to tell you. Be right back.