Most bones consist of a central marrow cavity surrounded by a sheath of compact bone. Short bones, such as the bones of the rib cage and skull, contain red marrow which is involved in red blood cells production. Long bones, such as the bones of the thigh and upper arm, contain primarily fatty yellow marrow with some red marrow at their ends. The difference between the two types of marrow is not very distinct, and even red marrow contains seventy percent fat.
The bones of the skeleton are categorized into two groups: the axial and the appendicular skeletons. The bones of the axial skeleton include the skull, vertebral column, and rib cage. The bones of the appendicular skeleton include the humerus, radius, ulna, carpals, metacarpals, and phalanges of each arm and hand; the femur, patella, tibia, fibula, tarsals, and metatarsals, and phalanges of each leg and foot; the clavicles and scapulae of the pectoral girdle; and the pelvic girdle. Bones are joined together in two ways. The numerous small bones that form the skull are held together by immovable sutures. Most bones, however, are held together by moveable joints called ligaments.
