The ulna

Ulna

The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...

 is ossified from three centers: one each for the body, the inferior extremity, and the top of the olecranon

Olecranon

The olecranon is a large, thick, curved bony eminence of the forearm that projects behind the elbow.It is situated at the upper end of the ulna, one of the two bones in the forearm...

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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

Styloid process

In anatomy, a styloid process , usually serving as points of attachment for muscles, refers to the slender, pointed process of :* temporal bone of the skull - Temporal styloid process...

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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

Epiphysis

The epiphysis is the rounded end of a long bone, at its joint with adjacent bone. Between the epiphysis and diaphysis lies the metaphysis, including the epiphyseal plate...

joins the body about the sixteenth, the lower about the twentieth year.