12.2:
Bones of the Upper Limb: Ulna
The ulna and radius are parallel bones that constitute the forearm. These bones are joined at the proximal and distal ends by the radioulnar joints and connected by a flat, flexible interosseous membrane across their length.
The ulna is the longer of the two bones and has two projections at the proximal end — the olecranon and coronoid processes. A deep trochlear notch between the two processes fits the trochlea of the humerus, forming a hinge joint and allowing the elbow to flex.
During flexing, the coronoid process also slides into the humeral coronoid fossa, forming a V-shaped angle between the forearm and arm.
During arm extension, the olecranon fossa of the humerus holds the olecranon of the ulna preventing the bones of the forearm from sliding beyond a certain anatomical point.
The ulnar shaft ends in a disc-shaped ulnar head that joins the wrist bones. The ulnar head is separated from the wrist bones by a triangular fibrocartilage which stabilizes the distal radioulnar joint and supports the carpal bones of the wrist.
12.2:
Bones of the Upper Limb: Ulna
The ulna and radius are parallel bones of the antebrachium or the forearm. The ulna lies medially and consists of a bony tip called the olecranon process at its proximal end. This hook-like projection articulates with the olecranon fossa of the humerus and forms the "hinged" ulnohumeral part of the elbow joint. This joint facilitates forearm extension and flexion while preventing its hyperextension. Similarly, the coronoid process, another bony projection on the proximal/anterior side of the ulna, articulates with the coronoid fossa of the humerus and serves as the insertion point for the muscle brachialis.
Lateral to the coronoid process is a narrow depression called the radial notch that helps attach the radius and forms the proximal radioulnar joint. This joint is essential for the supination and pronation of the forearm. Distally, the ulnar head articulates with the radius forming the distal radioulnar joint. Projecting from the posterior side of the ulnar head is the styloid process of the ulna, a short bony projection. The styloid process attaches triangular fibrocartilage and radioulnar ligaments, stabilizing the wrist joint.
Several muscles attached to the ulna, including the pronator teres, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum profundus, and pronator quadratus, facilitate elbow and wrist joints movement such as flexion and extension, pronation, and supination.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 8.2: Bones of the Upper Limb.