Healthy forearm. Coloured X-rays of a healthy forearm and wrist joint. A top view is at left and a side view is at right. There are two bones in the forearm, the radius (right) and the longer ulna (left). The ulna articulates with the humerus (upper arm bone) at the elbow and forms a small part of the wrist (carpus) joint. The radius forms the chief part of the wrist joint and only a small part of the elbow joint. There are eight wrist bones, known collectively as the carpal bones. From the carpal bones the five metacarpals extend into the hand.