The ulna, the forearm bone that hooks your elbow into place
Straighten your arm fully and it stops dead rather than bending backwards. The brake is a hook of bone called the olecranon, the tip of the ulna, which slots into a pocket on the upper arm. It is also the hard point you lean on when resting your elbow on a table.
The forearm holds two long bones side by side, running from elbow to wrist. The ulna sits on the little finger side. Although it is longer than its partner, the radius, it is also thinner, so anatomists count it as the smaller of the pair. Its twin in the leg is the fibula. The ulna is wide near the elbow and tapers towards the wrist, the reverse of the radius.
Its upper end is shaped like a wrench. The olecranon curls over the back of the humerus, and a forward-pointing ledge called the coronoid process catches the front, so that between them they wrap a deep notch around a spool of bone on the humerus called the trochlea. That arrangement makes a hinge: the elbow folds and unfolds but cannot twist. Twisting is left to the radius, which pivots in a small notch on the side of the ulna, letting it roll across its neighbour when you turn your palm down. A sheet of tissue, the interosseous membrane, binds the two bones along their whole length.
Down at the wrist, the ulna ends in a rounded head and a small spike, the styloid process, which you can feel as the knob on the outer side of the wrist. The head does not touch the wrist bones directly; a triangular pad of cartilage sits in between.
The bone starts hardening around the eighth week in the womb, but its ends stay cartilage at birth. Extra growth centres appear at about age four near the wrist and ten near the elbow, and the ends only fuse fully around sixteen and twenty. In many four-legged mammals the ulna matters mainly for muscle attachment and may be partly fused to the radius; even in horses, where fusion is extreme, the olecranon survives as a spur on the radius.
Source: Ulna
