We've been talking about the gross anatomy of bone in general, but now we want to take what we've learned and apply it to specific types of bone. We're gonna apply it first to short, flat, and irregular bones, and also at the end of the video, sesamoid bones. You'll see what's missing from this list are the long bones. Long bones have a unique structure that we're going to go into more detail in a future video. We're starting with these bones because what we've learned so far applies pretty much directly to these types of bones, and that is that these bones on the inside have spongy bone.
And to illustrate this, we have a diagram of a vertebrae here. Vertebrae is an irregular bone, and we pulled one vertebrae out of the spine, and we see it right here, a transverse view that we see in cross-section. And if we look first at the body of the vertebrae, the anterior portion of the vertebrae here, we can see that the center of this bone that I'm gonna color in green is filled with all these little holes because the middle of that bone is the spongy bone. Now the posterior portion of the vertebrae, on these processes, it almost looks like a separate bone here. They are connected, but this irregular bone, when you take a cross-section of it, you're going to see two different sections separately here.
But you can see here, even though it's thinner, we still have this thin line of spongy bone that's making up the middle of this bone here. Now that spongy bone is going to be lined with endosteum because, remember, endosteum is the connective tissue that lines the inside of bone. It's that real thin layer that has the bone stem cells in it. Now, in contrast, the outside of the bone is going to be compact bone. And again, we can see this in the diagram.
I'm going to color it in blue just all the way around the outside. There's this thin layer of compact bone. Now remember spongy bone, each individual strut, well, when you put them together, it's quite strong, but each individual strut may not be that strong on its own. So when you wrap them all with compact bone, it gives a nice strong outer layer that can give protection and structure to the bone, and it also just sort of defines the edge of the bone really well. So that outside surface, compact bone, and of course, the outside of it is going to be lined with periosteum.
And periosteum, remember, has two layers, the osteogenic layer, which is right next to the bone, which has the bone stem cells, and then the fibrous layer, which has all that collagen wrapped in the bone. Okay. So that's the pattern that goes for short, flat, and irregular bones. It also is true for sesamoid bones with one exception. Sesamoid bones have no periosteum.
And that's because the sesamoid bone, remember, is entirely enclosed in a tendon, so there's just no place for the periosteum to develop. Now that does down the road create some differences for how sesamoid bones grow and some physiological aspects of them. You usually don't need to worry about that much, though. Just know the sesamoid bone is inside the tendon, and so it doesn't have a periosteum. Again, this does not all apply to long bones.
We're going to talk about that in more detail coming up. And I'll see you there.
