Radius and Ulna

Radius
the shortest of the 3 arm bones. It is named for its action, a turning movement about the capitulum of the humerus, which allows thebone to rotate relative to the more fixed ulna.
The radius articulates proximally with the humerus at the capitulum and medially with the ulna on both proximal and distal end.
Distally, the radius articulates with two carpal bones of the wrist.

Ossification
The radius is ossified from three centers:
one for the body, and one for either extremity. That for the body makes its appearance near the center of the bone, during the eighth week of fetal life. About the end of the second year, ossification commences in the lower end; and at the fifth year, in the upper end. The upper epiphysis fuses with the body at the age of seventeen or eighteen years, the lower about the age of twenty. An additional center sometimes found in the radial tuberosity, appears about the fourteenth or fifteenth year.

Ulna
The ulna is the longest, thinest bone of the forearm. It articulate proximally with the trochlea of the humerus and head of the radius. Distally it articulate with the ulnar notch of the radius and with an articulare disk that seprates it from the carpal bones and provides freer rotation of the hand and radius around the ulna than is seen in many other mammals.
Ossification
The ulna is ossified from three centers:
one each for the body, the inferior extremity, and the top of the olecranon. Ossification begins near the middle of the body, about the eighth week of fetal life, and soon extends through the greater part of the bone. At birth the ends are cartilaginous. About the fourth year, a center appears in the middle of the head, and soon extends into the styloid process. About the tenth year, a center appears in the olecranon near its extremity, the chief part of this process being formed by an upward extension of the body. The upper epiphysis joins the body about the sixteenth, the lower about the twentieth year.