The ulna is the medial bone of the forearm, and is homologous with the fibula of the lower limb. This along with other bone called radius connects elbow with radius. It has upper and lower ends a shaft.
The Upper End
The upper end presents the olecranon and coronoid processes, and the trochlear and radial notches.
1. The olecranon process projects upwards from the shaft. It ahs superior, anterior, posterior, medial and lateral surfaces. The anterior surface is articular: it forms the upper part of the trochlear notch. The posterior surface forms a triangular subcutaneous area which is separated from the skin by a bursa.
Inferiorly it is continous with the posterior border of the shaft of the ulna. The upper part forms the point of the elbow. The medial surface of the shaft. The lateral surface is continous inferiorly with the posterior surface of the shaft.
2. The coronoid process projects forwards from the shaft just below the olecranon and has four surfaces, superior, anterior, medial and latera. The supeiro surface forms the lower part of the trochlear notch. The anterior surface is triangular and rough. Its lower corner forms the ulnar tuberosity.
The upper part of its lateral surface is marked by the radial notch for the head of the radius. The annular ligament is attached to the anterior and posterior margins of the notch. The lower part of the lateral surface forms a depressed area (to accommodate the radial tuberosity). It is limited behind by a ridge called the supinator crest.
3. The trochlear notch forms an articular surface that articulates with the trochlea of the humerus to form the elbow joint.
4. The radial notch articulates with the head of the radius to form the superior radioulnar joint.
The Shaft has three borders and three surfaces.
1. The interosseus (or lateral) border is sharpest in its middle two fourths. Inferiorly it can be traced to the lateral side of the head. Superiorly it is continuous with the supinator crest.
2. The anterior border is thick and rounded. It begins above on the medial side of the ulnar tuberosity, passes backwards in its lower one third, and terminates at the medial side of the styloid process.
Text adapted from: Human Anatomy by BD Chaurasia
Image Credit: Wikipedia