Human skeleton. Computer artwork of the human skeletal system seen from the side. The 206 bones of the skeleton provide protection and support, and their joints allow locomotion. The skull (at top) protects the brain. The ribs of the chest (at upper centre) enclose the heart and lungs. The pelvis (behind arm, centre) protects the organs of the lower abdomen. The flexible backbone runs from the skull to the pelvis and protects the spinal cord. The long leg and arm bones provide support. The humerus (upper-arm bone) articulates with the radius and ulna (lower-arm bones) at the elbow. The femur (thigh bone) articulates with the fibula and tibia (lower-leg bones) at the knee.