 All right, our next layer. I'm going to draw a dictionary. Let's do this. Are you ready? My next layer looks a little something, something, something, something like this. Hmm. What kind of tissue are you going to guess that this is? I know you can do it. That is smooth muscle. We've seen it. I'm going to draw it in this color, smooth muscle. And this is actually, okay, this is really cool. This layer is called muscularis externa. Here, we had muscularis mucosic, because it's part of the mucosa. Here, we have muscularis externa, because it's part of the external portion of my tube. It's made out of smooth muscle. Oh, I already wrote that part. Let's write muscularis, muscularis externa. That says muscularis externa. What was I going to tell you about it? Muscularis externa is generally thicker than muscularis mucosic, and there's two layers of it. Now, watch and be amazed. Muscle tissue, muscle cells, are longer than other cells. They tend to be really elongated cells, and smooth muscle kind of takes a shape of, I don't know, I'm going to draw you a smooth muscle cell. They kind of look like this, kind of long and thin. They've got their little, as opposed to like a circular cell, or even a cuboidal cell. A smooth muscle cell kind of has a long, I can't remember the word they used to describe that shape. But imagine that if we zoomed in to look at these cells, we're going to see them being kind of long, like this, and almost kind of skinny and flat, but lots of them, and they're grouped together, and they literally look smooth. We're not looking at our other types of muscle tissue, but the sooner you get comfortable identifying smooth muscle, the happier you're going to be, because it makes identifying these different layers really easy. So, in the muscularis externa, because the smooth muscle cells are kind of long, can you imagine that we could put them all in a row? Like, we could orient them all in the same direction. And in fact, that's what happens. In multiple layers of muscularis externa, one layer is all oriented in one direction, and then there's another layer. The second layer is oriented in another direction. And I'm drawing this... I'm drawing my other direction with a bunch of circles, because do you agree that if I cut this cell right down the middle of it, and then looked at the end of it, it would look like a little circle with possibly a dot in it if I caught the nucleus? So, this just indicates that it's the same tissue type, but one part, one layer of muscularis externa, has the cells oriented in a circular fashion around the tube. Do you see that? When these guys shorten, when they contract, they're going to shorten along this line, they're basically going to decrease the diameter of the tube when they contract. Do you agree with that? Because muscle tissue, its entire function is based on the fact that it is contractile tissue. So, it does shorten, and when it is oriented in this kind of long, circular way, it's going to shorten and decrease the diameter. How are these guys oriented? Okay, I'm making some green, smooth muscle cells. How are they oriented? Think about this for a second. They're oriented like this way in my tube. So, when they shorten, they're going to shorten this way, and they're actually going to shorten the whole tube. Why? Why would you need to have these layers, one of them going like this, and one of them going like this, so that your tube is actually shortening and lengthening, and decreasing in diameter and increasing in diameter. Really? Why would we have that happening? One of the functions of the digestive system is to move stuff through. You have to move your food through from the start to the finish, and in order to get your stuff to move, your muscle tissue, your smooth muscle has to contract. It's the process of peristalsis, and you can look up peristalsis on YouTube and entertain yourself a lot, because it's really fantastic how the tube literally squishes the food forward. How does it do it? It does it because of muscularis external. I think that's it. Let's move on to the next layer.