Both Radius and the Ulna are connect the humorous bone of the upper arm the elbow joint. The elbow provides rotation of the lower arm through twisting of the Radius and Ulna. Normally the Radius and Ulna are parallel to each other but during pro nation (the rotation of the lower arm) the radius rolls around the Ulna at both the wrist and elbow. So in this position the Radius and Ulna appear crossed.
The muscles that connected to the Ulna and Radius are the Triceps/
Your Ulna and Radius are more commonly known together, as the forearm. Your forearm can crack slightly or shatter into many different pieces. The broken bone could stay straight or could break as to where some shards of the of the bone stick out through the skin. This is called on open fracture and it needs immediate medical attention or it can became infected. It takes a lot of force to break the Radius or Ulna and is more commonly broken in adults than children. When you brake your Ulna or Radius it usually a force directly to the foresrm The Ulna is typically broken more than a the Radius and it usually takes a direct blow to do that. It takes about 6-8 weeks to heal.