Classification of bones
Bones are designed to carry out a variety of different and specific functions. They are classified into five categories, largely according to their shape:
- Long bones: these bones are cylindrical in shape and are found in the limbs of the body. They are formed of compact bone with cancellous bone located at the ends of the shaft. Examples of long bones include: femur, tibia, humerus, ulna, radius and phalanges. Their primary function is to act as levers and, therefore, to generate movement. Another vital function is the production of blood cells deep inside the bone.
- Short bones: these bones are short and compact in nature, and are often equal in width and length. These bones are designed for strength and weightbearing. They consist of entirely cancellous bone surrounded by a thin layer of compact bone. Examples include: carpals (bones of the wrist), tarsals (bones of the ankle) and the calcaneous (heelbone).
- Irregular bones: these are complex, have no definite shape, and have a variety of functions which include protection. Examples include the vertebrae and the bones of the face. Irregular bones consist of two outer layers of compact bone with cancellous bone between them.
- Flat bones: these bones offer protection for the internal organs of the body. Examples include: the bones of the cranium, the sternum, bones of the pelvis, the shoulder blade (scapula) and the ribs. Flat bones also allow suitable sites for muscle attachment, with the origins of muscles often attaching to them, forming a firm, immoveable base against which to pull. The sternum, pelvis and cranium also produce blood cells. Flat bones consist of two outer layers of compact bone with cancellous bone between them.
- Sesamoid bones: these bones ease joint movements and help resist friction and compression. They are usually formed within tendons and are covered with a layer of articular cartilage. They are usually found in joints where bones meet (articulate). They are generally small, but do vary in size. The largest sesamoid bone is the patella (knee cap).