I've been having problems with Blogger so let's hope this works and I can post everything I've done over the past few days. The lower arm has two bones named the ulna and the radius. They run parallel to each other when then hand is in a palm up position (supination). When the hand is palm down (pronation) the bones overlap each other as seen in the bottom picture. The ulna's function is simply flexion and extension. It bends and straightens the arm and that is all. The elbow belongs to the ulna while the radius holds the hand. The radius' function is to rotate the arm.