 I'm going to actually draw you a picture of the histology of the stomach because there are a couple of things about it that are kind of wild. First of all, probably the defining feature of the stomach, its primary role in digestion is mechanical digestion by smashing and smushing, which means that the muscularis externa, I'm in stomach land, is thick. And it's so thick that instead of two layers, there are three. In most parts of the digestive tube, the muscularis externa has two layers, but the stomach has three, and not only does it have three layers, it has thick layers. So that's the first thing. If you can see your three layers of muscularis externa, you're stoked. The other thing that's interesting about the stomach is that you end up, you've got the simple columnar epithelium, and even though this looks like a villus, it is not. It's basically just an increase. The stomach has these folds called rugi, and those make the stomach expandable. Check it out. Between these structures, there are pits, and they're called gastric pits, and the gastric pits, like they're between, I don't know, between these folds, and then they extend down in and become, they extend down into the tissue, and then you have gastric glands. This is a gastric gland, and other types of cells that line these pits, and they secrete substances in. Now, one of the things that you're going to see is that we have two different kinds of cells. We have chief cells and parietal cells, and I have to write both of them down to remember what they do. Parietal cell produces hydrochloric acid. The chief cell produces a substance called pepsinogen, which is an inactive protein enzyme. So what happens is the chief cells are located deep. So I'm going to draw chief cells down here. I'm not actually going to be able to tell the difference in our slides between them, but we know that the chief cells are deep in these gastric pits, and they produce the pepsinogen. I'm going to call it PG that gets barfed out into the tube, and up here is the lumen of our stomach, so your food is up here. The pepsinogen gets dumped by the chief cells, and then higher up, how cool is this? Are other kinds of cells, what kinds are these guys? They're parietal cells, and they are producing hydrochloric acid. Now, here's the magic. This is physio, which is what is so fantastic about physio. When pepsinogen comes in contact with hydrochloric acid, so when pepsinogen is in a very acidic environment, it actually turns into pepsin, and pepsin is an active protein enzyme. So pepsin will actively bust, destroy, chomp, break apart your proteins, and start the process of digestion so you can absorb those proteins. If your body, I mean seriously, like why would we do this? Why bother producing pepsinogen? If you didn't produce pepsinogen, if you just went straight to pepsin, what's pepsin going to do inside your cells that are made of proteins? That's going to yumpsilize your cells. It's a terrible idea, terrible design flaw. So this is a physiological process that enables you to efficiently break down your proteins. I said the three layers of muscularis externa. Stomach tends to just be thicker. It looks like, I mean, this is my epithelial tissue, which means what's deep to that. It's not appropriate, and what's deep to that? Muscularis mucosis. Sometimes my muscularis mucosis is actually thicker in the stomach, sometimes not so much. But then the muscularis externa, you sometimes can see all three layers. All right, let's go see how we can tell the different parts of our small intestine.