The 3-month-old fetus has an early skeleton-like framework composed of cartilage and connective tissue membrane. As the fetus matures, the cartilage and connective tissue change into bone. the formation of bones is called ossification. Ossification occurs in different ways in flat and long bones.

Bones come in many shapes and sizes, from the pea sized bones in the wrist to the 24 inch long femur in the thigh. The size and shape of the bone reflects its function.

Long bones. Long bones are longer than they are wide. They are found in the arms and legs.

Short bones. Short bones are shaped like cubes and are found primarily in the wrist and ankles.

Flat bones. Flat bones are thin, flat, and curved. They form the ribs, breastbone, and skull.

Irregular bones. Irregular bones are different shaped and are not classified as long, short, or flat. They include the hip bones, vertebrae, and various bones in the skull.

Sesamoid bones. Sesamoid bones are small round bony masses embedded in certain tendons that may be subjected to compression and tension. The largest sesamoid bone is the patella, which is embedded in the tendon of the quadriceps femoris at the knee.

Short, flat, and irregular bones are all made of spongy bone covered with a thin layer of compact bone. Red bone marrow is found within the spongy bone.

The joint surfaces of bones are covered with articular cartilage, which provides a smooth surface. Covering the rest of the bone is the periosteum.