How Bones Work

Bone Basics

The Bone Market
There is nothing to prevent you from legally purchasing human bones and skulls from a vendor in the United States. Specimens often come from China, where unclaimed remains are sold to bone distributors. Many skulls on the market today are from India, which was once a primary provider of skulls to the world market but banned the practice of exporting human remains in 1987. (Skulls don't come cheap: an average adult skull runs around $500.)

There are 206 bones in the adult body. Bone is a honeycomblike grid of calcium salts located around a network of protein fibers. These protein fibers are called collagen.

When you patch a hole in a piece of drywall, you usually cover it with tape that has a gummy fibrous grid, and then cover that with wall compound mortar. Bone is made in much the same way. Collagen fibers are gummed together by a kind of shock-absorbing glue [source: University of California-Santa Barbara]. Then, all of this is covered and surrounded by calcium phosphate, which hardens everything into place. Not only do bones make use calcium for strength, they also keep some stored in reserve. When other parts of the body need a calcium boost, the bones release the needed amount into the bloodstream.

There are two different types of bone tissue: cortical bone (the outer layer) and cancellous bone (the inner layer). Cortical bone, also known as compact bone, provides external protection for the inner layer against external force. It makes up 80 percent of bone mass and is dense, strong and rigid [source: Hollister].

Cortical bone is covered by a fibrous membrane called the periosteum. Think of the periosteum as a utility vest that fits over the bone -- it has brackets and places for muscles and tendons to attach. The periosteum contains capillaries that are responsible for keeping the bone nourished with blood.

In the case of long bones such as the femur (the upper leg bone), the periosteum covers the central portion of the bone but -- like a sleeveless vest -- stops short of the cartilage tissue that resides on both ends of the bone (we'll discuss this cartilage in a later section).

Cancellous bone, also known as trabecular or spongy bone, is the inner layer of bone and is much less dense than cortical bone. It's formed by trabeculae, which are needlelike structures that create a meshwork. However, instead of a network of bone structure with periodic gaps, cancellous bone is more like a network of connecting spaces with periodic structure. The latticework of tiny chambers is filled either with bone marrow or connective tissue. Within these marrow-filled spaces is where new blood cells are produced.

Though cancellous bone only makes up about 20 percent of the body's bone mass, it plays important roles in body function. It provides structural stability and acts as a kind of shock absorber inside the bone, but without adding too much to the overall weight of the body.

In the next section, we'll learn more about bone marrow.