The human skeleton has many bones, usually 206, all having different shapes and sizes. These bones are separated into two main groupings—the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones making up the skull, the vertebral column, the sternum, and the ribs. Their main function is to protect the vital organs of the body. The skull, for example, is comprised of several smaller flat bones that are fused together. This fusion creates a strong plate that serves as a helmet for the brain. The appendicular skeleton is comprised of the bones that make up the appendages, such as the arms and legs. The main functions of the appendicular skeleton include movement and dexterity. For example, the bones found in the fingers and hands are small and are attached to a series of intricate muscles. The movement of these flexor, extensor, and interosseous muscles allows for the playing of a piano or manipulation of a pencil.
There are four main shapes of bones found in the human body. These include flat bones, short bones, long bones, and irregular bones.
- Flat bone: bone that is flat in shape and provides protection or serves as a large area for muscle attachment. Flat bones can also be slightly curved and are often not very thick. They are located in the sternum (breastbone), the ribs, the skull, and the shoulder blades (called scapulae).
- Short bone: among the smallest bones in the body, often functions to reduce friction at a joint or to prevent tearing of tendons. Short bones can be found in the ankles and wrists, as well as within some tendons.
- Long bone: has two knobbed ends with a long shaft between them. Long bones are longer than they are wide and provide strength, structure, and mobility. Other than the wrist, ankle, and kneecap, all bones found in the arms and legs are considered this type. The name long bone is not necessarily descriptive of the size of the bone. There are some bones found within the hands that also have the characteristic long bone shape.
- Irregular bone: does not have a recognizable shape and serves a variety of functions, including protection, support, and points of muscle attachment. Irregular bones may have protrusions sticking out from them, such as with the vertebrae, or they may be rounded in certain areas with large differences in thickness, such as in the pelvis.