Your bones contain a matrix, osteoblasts, osteoclasts, osteocytes, and collagen fibers. Bones are hard structures because of the calcium that builds up in the matrix, but they retain a slight amount of elasticity due to the fibers. Repeated stress on a bone from physical activity can stimulate the bone to increase calcification (calcium storage), and inactivity can lead to calcium loss.

On the exterior, all bones are covered with periosteum. Bones themselves are actually composed of two types of bone material:

  • Compact bone: The “shell” of each bone is made up of compact bone, which looks like a solid mass.

  • Cancellous (spongy) bone: Some bones have an interior mass of cancellous bone, which is a network of trabeculae (columns) arranged in such a way to reduce stress and pressure on the bone. The outer layer of cancellous bone is composed of compact bone.

Bones without an interior mass of cancellous bone have a medullary cavity instead. The shaft of the long bones (such as the humerus of the arm or the femur of the thigh) are hollow. This space is filled with adipose tissue and/or red marrow, which forms blood cells. The medullary cavity is lined with endosteum, a thin layer of connective tissue.

At birth, the marrow of all your bones created blood cells. By the time you’re an adult, though, blood cells are only produced in the bones of the skull, thoracic cage, spinal column, pelvic and shoulder girdles, and heads of the humerus and femur.

Bones can be classified by their general shape. Some are long, some are short, some are flat, and a few don’t really match any specific shape. Here’s a look at the shapes of the bones in the body:

  • Long bones: Long bones are found in both upper and lower extremities (arms and legs). The longest long bone is the thigh bone, the femur. It’s substantially longer than the phalanges in the hands and feet, which aren’t very long but are still called long bones. Each long bone has the following parts:

    • The diaphysis is the shaft of the bone, made of compact bone.

    • The epiphysis, the part that forms the enlarged ends of the bone, is made of cancellous bone and is covered with compact bone and hyaline cartilage.

    • The epiphyseal cartilage is between the diaphysis and epiphysis. It’s the site of bone elongation during the growing years, but after you’re done growing, the cartilage is replaced by compact bone.

  • Short bones: Short bones are found in the wrist and the ankle. They’re cube-shaped and made from cancelleous bone with a thin layer of compact bone. They’re also covered with a periosteum and hyaline cartilage.

  • Flat bones: These types of bones resemble a bone sandwich made of two tables, which are thin layers of the compact bone, enclosing a layer of cancellous bone called the diploe. The frontal and parietal bones in the skull are flat, as are the scapulae.

  • Sesamoid bones: These small, round bones form in tendons. The biggest one is the patella in the quadriceps tendon of the knee.

  • Irregular bones: This category includes any named bones that just don’t fit into the long, short, flat, or sesamoid categories. The rest of the bones of the skull, the vertebrae, and the pelvic bones fall into this group. These bones are made up of an irregularly shaped mass of cancellous bone covered in a thin layer of compact bone.

The surfaces of bones have all kinds of bumps, lumps, dips, and ridges:

  • Raised lines are called lines or ridges. Crests are larger ridges.

  • Rounded projections include (from smaller to larger in size) tubercles, protuberances, tuberosities, malleoli, and trochanters.

  • Sharp points may be called spines or processes.

  • Joint expansions at the ends of bones include heads (just one expansion on the end), condyles (two expansions; one on each side), and epicondyles (smaller expansions located just above condyles).

  • Facets are small flat areas found on bones that form joint (articular) surfaces.

  • Depressions include (from smaller to larger) notches, grooves, sulci, and fossae.

  • Holes and openings may be called fissures, foramens, canals, and meatuses (the name of the marking depends on the bone where it’s located).