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The Four Layers of Bone

x-rays of hand bones

Our bones are made of five main layers. Moving from outside the bone to inside the bone, here are the layers:

  1. Periosteum
  2. Cortical, or Hard Bone
  3. Cancellous, or Spongy Bone
  4. Bone Marrow

Let’s take a closer look.

Periosteum

The periosteum is a soft outer covering over the bone’s surface. It provides blood flow to the bone which lets a bone heal, grow, fight infection, and stay healthy. This layer is very thick in children and gets thinner as we get older.

Cortical Bone

This layer is hard and thick. When you see a skeleton, you’re looking at mostly cortical bone. You can think of it like a hard shell, like a turtle has. It’s job is to protect body parts underneath it and hold up muscles around it.

Cancellous Bone

Cancellous bone is a spongy type of bone inside the cortical bone. It’s not as dense as the outer cortical bone. Some bones have a lot of it, and some bones have less. A broken bone may heal faster if it has more cancellous bone inside.

Bone Marrow

Your bone marrow is deep inside your bones – inside the middle of the cortical and spongy bones. It’s job is to make all the blood cells inside your body – red cells, white (infection-fighting) cells, and platelets that help you stop bleeding when you get a cut.

A picture of the four bone layers

This diagram will show you the layers of bone and more information. Just click on the name of the layer for more information!