Now that we've introduced the gross anatomy of the stomach and we've identified the major regions of the stomach, we want to zoom in on the stomach wall and identify the layers of that stomach wall. So we're going to just start by saying that the stomach wall contains the same four layers as the rest of the GI tract, but with certain modifications. So remember from your esophagus to your anus, the structure of that alimentary canal is basically the same, but there are some unique things along the way, and here we want to identify those unique things in the stomach. Now before we do, let's just orient ourselves to our image here. We have a cross-section of that stomach wall. It's not exactly to scale, but it's drawn so that we can see the different layers clearly. Now in this drawing, the lumen would be sort of up here at the top, so that's sort of the inside of the stomach. You'd then go down through the wall 1, 2, 3, 4 layers there, each shown in a different color. And then the outside of the stomach, that would be down at the bottom.
The first layer we want to talk about, at the top of our drawing here, is that innermost layer, the mucosa, and we'll just note the mucosa, that's the part that's closest to the lumen. So the stomach acid, the kind that's in the stomach, is going to be up against this mucosa. Alright. So the mucosa is going to be made of mucus cells. That's going to be that stomach epithelium, and it is composed of simple columnar cells. And simple columnar cells make up that epithelium of the entire alimentary canal, and we can see those in our drawing here. Those are these cells sort of lining the tops there in pink. But what makes it unique in the stomach is we're going to have this mucosal barrier, this thick alkaline mucus that is there for protection. Alright. Alkaline, that means basic or high pH because remember, the stomach is filled with acid. This really corrosive low pH liquid. So the stomach lining has this thick mucus barrier that's alkaline to protect the cells underneath. You don't want that acid digesting or breaking down those cells underneath it. And if there's a break in that mucosal barrier, well, then you will have damage to the stomach lining, and that's what we call an ulcer. And you don't want that.
Now we have that mucosal barrier, but also between all the cells, we're going to have tight junctions. Those tight junctions are also going to prevent the leakage of the gastric juice. Alright. Now it's not a smooth lining as you can see here. Well, remember, we have the rugae of the stomach, but here we're talking even at a smaller level. At an even smaller level, you can see all along here these sorts of holes. And these holes, well, you can see they lead down to glands that are down here deeper in the mucosa. But right now we're just talking about these holes and the starts of these tunnels leading into the mucosa. Those are called the gastric pits. The gastric pits are these openings to the glands or the gastric glands. Now you can see these gastric glands here sort of deep in the mucosa there. We're not going to talk about them here because we're going to go into a lot more detail about those in a future video coming up.
As we move down from the mucosa, we reach the submucosa, and that's this layer here. The submucosa is connective tissue with arteries, veins, and lymph vessels. That's basically what it is throughout the entire alimentary canal. There's nothing really unique that you need to know about the submucosa in the stomach, so we're not going to spend much time on it. Moving down from there, we reach the muscularis externa, and that's this section here, the muscular wall of the stomach. Now the stomach is going to have 3 layers of muscle. Now if you know your alimentary canal and your muscularis externa, that may have sort of your ears may have heard something funny there because the rest of the alimentary canal has only 2 layers. So in the stomach, we have this oblique layer, and that's going to be the innermost layer. So in our image here, it's this layer, this top layer here. You can see we sort of try to draw the cells there cut at a bit of an angle. So oblique means at an angle. So it means, if you think of the sort of length of the stomach, this layer is running sort of at an angle to that length. It's the innermost layer, and we are going to say here it is only in the stomach. This stomach has an extra layer of muscle, meaning it is an extra muscular portion of this alimentary canal. Outside of the oblique layer, we have the circular layer. That's that middle layer that you see here in this drawing. Here, we're sort of trying to show those cells cut in cross-section there. That circular layer, like the rest of the alimentary canal, the cells are running sort of perpendicular to the length of the alimentary canal. So when they squeeze in, they squeeze in a skirt circular way. Then we have the longitudinal layer. That's this layer on the bottom. That's the outermost layer, and the longitudinal layer—those cells are running lengthwise sort of parallel to the length of the alimentary canal. So when they contract sort of the whole alimentary canal, the stomach would sort of almost shrink in length.
Finally, we have the serosa, and here we see the serosa in green on the outside. And remember, in the serosa here, that's the peritoneum. And since it's attached to the stomach wall, it would be the visceral peritoneum. Now remember the peritoneum, it's covering really the majority of these digestive organs, so it's a really complex membrane. In this section on the stomach, we just want to note that it's going to be continuous with the omentum. So coming off, sort of, the lesser curve and the greater curve of the stomach is going to be this peritoneum, and it goes into these portions where the peritoneum doesn't directly attach to other organs, and it makes sort of like a sheet. And those sheets are called the lesser and the greater omentums. Those are really noticeable, especially if you're doing a dissection. So especially if you're doing a dissection class, there's a good chance you're going to have to identify those. Alright. So that's the stomach. We're going to dive into those gastric glands and see what's being made there in a lot more detail coming up. But first, we have example and practice problems. You should give them a try.