 Now, it makes sense that we're going to have one more layer that we're going to like wrap the whole tube in this final layer and it's going to get a little bit weird because the layer, I'm going to make it really simple and thin right here and you'll see why in just a second. And here's the deal. If the tube is retroperitoneal, no, let me do that another way. If the tube is in the peritoneal cavity, in peritoneal cavity, then this outer layer, which that literally is my layer right there, it's called the serosa. And here's the fantastic scoop. The serosa is visceral peritoneum. Can you imagine that? It's visceral peritoneum. Because remember in the peritoneal cavity, remember how we covered our organs with the visceral peritoneum. It's just a serous membrane. Why do we have to call it something else? Because this is human anatomy and you don't have enough structures to learn. So it is called the serosa. It is visceral peritoneum because it's in the peritoneal cavity. Now, are all parts of your tube in the peritoneal cavity? Dog pounds, no. Sometimes the tube is either in the thoracic cavity like the esophagus, most of the esophagus, or it is retroperitoneal like the duodenum. Remember the duodenum like hung out behind the peritoneum? And if it is retroperitoneal and it's not in the peritoneal cavity, then it's called adventitia. Structurally, we're not going to be able to see a huge difference. For our intents and purposes, I'm sure someone somewhere can actually tell the difference based on the structure of the tube. Actually, I have no idea if you can tell the difference. But for our purposes, the outermost layer of our tube, you're going to know whether it's serosa or adventitia based on what part of the tube it is in. If you're looking at some esophagus, then you pretty much can guarantee that you're looking at adventitia as the outermost layer. If you're looking at stomach, then you can guarantee that you're looking at serosa as the outermost layer. Duodenum, adventitia. Jejunum, serosa. That's it. Now, let's look at what is it going to look like? We looked at structures last time that were strategies to increase the surface area of our digestive tubing. And let's see what those structures are going to look like from a histological perspective.