We now want to spend a little bit of time talking about the structure of lamellae. Remember, the lamellae are these layers of bone matrix, and in the osteon, they're in concentric circles. We're talking about the bone matrix here, and that bone matrix is made of collagen, which gives bone its strength. It's strong but flexible like a rope, and also the hydroxyapatite crystals, which give bone its hardness. The hydroxyapatite crystals, that's where the calcium is in your bone.

It's very hard, but it's also very fragile. So here, we're going to say that the arrangement of the lamellae gives bones strength in multiple directions. And if we're talking about strength, we're talking about the collagen. Let's look at our diagram here real quick. We have an osteon here, but we have the different lamellae pulled away so that we can see each concentric lamella.

So we have 1, 2, 3 lamellae. And if we just look at this first one all the way on the left, we see these lines on it that are sort of spiraling around in parallel. That's going to be the direction of the collagen in the lamellae. The collagen is all running in parallel with those hydroxyapatite crystals sort of lined up between it, and it's lined up spiraling around the lamellae. Now, this means the collagen is all running in the same direction within one lamella.

When it's built that way, that's going to really increase strength. So I'm going to indicate that just by drawing an up arrow. It's going to increase strength, but only in that one direction. Right? If you apply stress from a different direction, it's not going to be as strong.

So when you look between lamellae or between adjacent lamellae here, we look at our 2 lamellae over here on the right, and we can see that we have the spiral going in one direction in the first one, but then the next lamellae, the spiral is going in the opposite direction or the alternate direction, I'll say, here. So that means that in one lamellae, all these collagen fibers are running parallel, but in the next one, it's layered over in an alternate direction. That is going to give strength in multiple directions. And because it's in a spiral and they're spiraling in opposite directions, that means that these are going to be really good at resisting the twist. We're going to resist the twist.

One way to break things, to break something like a rod-like shape, like a bone, right, is to twist it. Well, these collagen fibers running in spirals in opposite directions are going to be really good at resisting twisting. Remember, collagen gives it strength, but the arrangement of the collagen in the lamellae gives it strength in multiple directions. For that, practice problems to follow.

Give them a try.