The human skeleton is divided into two distinct parts:
The axial skeleton consists of bones that form the axis of the body and support and protect the organs of the head, neck, and trunk. - The Skull - The Sternum - The Ribs - The Vertebral Column
The appendicular skeleton is composed of bones that anchor the appendages to the axial skeleton. - The Upper Extremities - The Lower Extremities - The Shoulder Girdle - The Pelvic Girdle--(the sacrum and coccyx are considered part of the vertebral column)
Types of Bone
The bones of the body fall into four general categories: long bones, short bones, flat bones, and irregular bones. Long bones are longer than they are wide and work as levers. The bones of the upper and lower extremities (ex. humerus, tibia, femur, ulna, metacarpals, etc.) are of this type. Short bones are short, cube-shaped, and found in the wrists and ankles. Flat bones have broad surfaces for protection of organs and attachment of muscles (ex. ribs, cranial bones, bones of shoulder girdle). Irregular bones are all others that do not fall into the previous categories. They have varied shapes, sizes, and surfaces features and include the bones of the vertebrae and a few in the skull.
Bones are composed of tissue that may take one of two forms. Compact, or dense bone, and spongy, or cancellous, bone. Most bones contain both types. Compact bone is dense, hard, and forms the protective exterior portion of all bones. Spongy bone is inside the compact bone and is very porous (full of tiny holes). Spongy bone occurs in most bones. The bone tissue is composed of several types of bone cells embedded in a web of inorganic salts (mostly calcium and phosphorus) to give the bone strength, and collagenous fibers and ground substance to give the bone flexibility