Bone is made mostly of collagen and calcium. This combination works to make the bones strong and flexible at the same time, enabling them to be protective and rebound from stress fractures.
Bones are made up of several complex parts that are all living and growing tissue. Unlike bones that are typically seen in outside environments, human bones are dense, growing and somewhat soft. The three main parts of a bone are the periosteum, compact bone and cancellous. The periosteum is the outer layer of the bone, and it provides the blood vessels that nourish the inner part of the bone. The compact bone gives the bones the strength needed to function and is very hard. Cancellous bone makes up the inside of the bone surrounding the marrow; it is softer and sponge-like, unlike the compact bone.
The bone marrow is located within the inside of the bones. Bone marrow is an important part of bones, but it is not like the rest of the bone. It is jelly like and thick. The rest of the bones work to protect the marrow, but the cancellous layer protects it the most. Bone marrow is responsible for creating blood cells.