On the surface, human bones---especially dead ones---seem to be made of rigid material. However, the make-up of bones is quite complex--they are alive, growing and changing connectors in the human body. Bones are not completely solid, but rather a mixture of elements that range from soft to hard.
The periosteum is the outer surface of bones. It contains the nerves and blood vessels that provide nourishment to bones.
The next layer is made up of compact bone. It is a smooth but hard quality, with minimal gaps and spaces. This layer gives bones their white and solid appearance, and it is the part that you see when you look at a skeleton. Compact bone accounts for about 75 to 80 percent of a skeleton's bone mass.
Cancellous bone is concentrated as a network of several layers within the compact bone. Cancellous bone has a sponge-like quality, and it is not as hard as compact bone. Thus, cancellous bone gives the overall bone a lighter feel than it would have had. It accounts for the remaining 20 to 25 percent of a skeleton's bone mass.
Bone marrow is the softest part of the bone. It resembles a sort of jelly. Its job is to make blood cells called osteocytes, and layers of cancellous bone protect it.
In addition to the previously mentioned osteocytes, there are two other major bone cells: osteoblasts and osteoclasts. Osteoblasts deposit bone and are used to measure level of bone activity. Osteoclasts absorb bone and are important for bone renewal. Osteocytes are essentially osteoblasts that get trapped in the hardened part of the bone.
Bones are mostly made up of the bone matrix, which is a mixture of inorganic and organic parts. The inorganic part mostly comprises crystalline mineral salts and calcium, while the organic part is mainly composed of collagen.