(Elbow Bone) 56
The ulna (Figs. 212, 213) is a long bone, prismatic in form, placed at the medial side of the forearm, parallel with the radius. It is divisible into a body and two extremities. Its upper extremity, of great thickness and strength, forms a large part of the elbow-joint; the bone diminishes in size from above downward, its lower extremity being very small,and excluded from the wrist-joint by the interposition of an articular disk. 1
The Upper Extremity (proximal extremity) (Fig. 212).—The upper extremity presents two curved processes, the olecranon and the coronoid process; and two concave, articular cavities, the semilunar and radial notches. 2
The Olecranon (olecranon process).—The olecranon is a large, thick, curved eminence,situated at the upper and back part of the ulna. It is bent forward at the summit so as to present a prominent lip which is received into the olecranon fossa of the humerus in extension of the forearm. Its base is contracted where it joins the body and the narrowest part of the upper end of the ulna. Its posterior surface, directed backward, is triangular, smooth, subcutaneous, and covered by a bursa.Its superior surface is of quadrilateral form, marked behind by a rough impression for the insertion of the Triceps brachii; and in front, near the margin, by a slight transverse groove for the attachment of part of the posterior ligament of the elbow-joint. Its anterior surface is smooth, concave, and forms the upper part of the semilunar notch. Its borders present continuations of the groove onthe margin of the superior surface; they serve for the attachment of ligaments, viz., the back part of the ulnar collateral ligament medially, and the posterior ligament laterally. From the medial border a part of the Flexor carpi ulnaris arises; while to the lateral border the Anconæus is attached. 3
FIG. 212– Upper extremity of left ulna. Lateral aspect. (See enlarged image)The Coronoid Process (processus coronoideus).—The coronoid process is a triangular eminence projecting forward from the upper and front part of the ulna. Its base is continuous with the body of the bone, and of considerable strength. Its apex is pointed, slightly curved upward, and in flexion of the forearm is received into the coronoid fossa of the humerus. Its upper surface is smooth, concave,and forms the lower part of the semilunar notch. Its antero-inferior surface is concave, and marked by a rough impression for the insertion of the Brachialis. At the junction of this surface with the front of the body is a rough eminence, the tuberosity of the ulna, which gives insertion to a part of the Brachialis; to the lateral border of this tuberosity the oblique cord is attached. Itslateral surface presents a narrow, oblong, articular depression, the radial notch. Its medial surface, by its prominent, free margin, serves for the attachment of part of the ulnar collateral ligament. At the front part of this surface is a small rounded eminence for the origin of one head of the Flexor digitorum sublimis; behind the eminence is a depression for part of the origin of the Flexordigitorum profundus; descending from the eminence is a ridge which gives origin to one head of the Pronator teres. Frequently, the Flexor pollicis longus arises from the lower part of the coronoid process by a rounded bundle of muscular fibers. 4
The Semilunar Notch (incisura semilunaris; greater sigmoid cavity).—The semilunar notch is a large depression, formed by the olecranon and the coronoidprocess, and serving for articulation with the trochlea of the humerus. About the middle of either side of this notch is an indentation, which contracts it somewhat, and indicates the junction of the olecranon and the coronoid process. The notch is concave from above downward, and divided into a medial and a lateral portion by a smooth ridge running from the summit of the olecranon to the tip of...