The part of the ulna that fits into the olecranon fossa of the humerus is called the olecranon. This bony prominence is located at the proximal end of the ulna, specifically forming the tip of the elbow.
When the elbow is extended, the olecranon fits perfectly into the olecranon fossa of the humerus, which is a depression found at the back of the humerus. This anatomical relationship is crucial for the joint’s stability and function. The olecranon acts as a lever to enhance the strength of the muscles that extend the forearm, while also allowing for a greater range of motion at the elbow.