This image shows a portion of the pelvic girdle with each radius (indicated with red arrows) attached at its proximal end to a humerus.  The other bone of the forearm is the ulna.  The head of the radius articulates with the capitulum of the humerus.  Medially, the head articulates with the radial fossa of the ulna.  Distal to the head on the medial surface of the radius is a large radial tuberosity  where the biceps muscle is attached.  Distally, the radius is expanded and has a medial ulnar notch that articulates with the ulna and a prominent, lateral styloid process. Between these two points, the radius is concave where it articulates with the carpal bones of the wrist.  The radius is the major forearm bone which contributes to the wrist joint – when the radius moves, the hand moves with it!