According to the BBC Science and Nature, the human arm consists of three long bones, known as the humerus, in the upper arm, and the ulna and the radius in the lower arm. Of these, the humerus is the thickest.
The BBC describes how the upper and lower bones are joined at the elbow by a large hinge joint. As well as bones in the arm itself, the clavicle and scapula, which make up the shoulder, act as scaffolding and support for the arms, allowing them to hang and move properly. If any of these bones become injured or broken, using the arm for anything becomes a painful process.