The outer surface of the bones is called periosteum. It is a thin solid film that holds nerves and blood vessels that feeds the bones.  The next layer is made up of solid bone. This part is smooth and very hard. It's the part you see when you look at a skeleton. 

Inside the solid bone are many layers of cancellous bones, which looks a bit like a sponge. Cancellous bone is not as hard as solid bone, but it is still very strong. 

In many bones, the cancellous bone protects the deepest part of the bone, the bone core. The bone core is sort of like a thick jelly, and its job is to make blood cells. 

"Your Bones".https://kidshealth.org/en/kids/bones.htm.