This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The lower end of the humerus broadens laterally and is slightly concave on its anterior surface; this causes the articular surfaces to look downward and forward and not backward. It carries two articular surfaces: one, the trochlea, for the ulna, and the other, the capitellum, for the radius. The trochlea, descending lower than the capitellum, causes the line of the joint to incline downward and inward instead of being directly transverse, thus producing the " carrying angle " (Fig. 294). Extending from the edges of the articular surfaces outward, one on each side, are the condyles, medial (internal) and lateral (external).
Chaussier gave the name epicondyle to the condyles. He called the medial condyle the epitrochlea and the lateral (external) condyle the epicondyle. Henle called the internal condyle the epicondylus medialis and the external condyle the epicondylus lateralis. The name epicondyle is now quite generally employed by both surgical and anatomical writers to designate the projecting extra-articular portion of the condyles, so that the terms are practically synonymous (Fig. 293).
From the condyles two ridges run upward. The lateral (external) supracondylar ridge is the more marked of the two and gives origin to the brachioradialis (supinator longus) and the extensor carpi radialis longior muscles, and passes posterior to the deltoid eminence to be continuous with the posterior lip of the radial (musculospiral) groove. The medial (internal) supracondylar ridge is much less prominent than the lateral and soon blends with the shaft of the bone. Above the trochlea and capitellum anteriorly are two fossae, the coronoid and the radial, to receive the coronoid process and head of the radius when the arm is in complete flexion. On the posterior surface there is another depression, the olecranon fossa, to receive the olecranon process in extreme extension. The projecting hook-like shape of the median condyle causes it to be more frequently fractured than the less prominent lateral condyle. The two condyles are readily felt directly beneath the skin and are the only points of the humerus that are really subcutaneous.
The upper extremity of the ulna articulates above with the trochlea of the humerus and on its outer side with the radius. Its upper end is the olecranon process. The posterior portion of this process is called the tip of the olecranon and is continuous with the posterior surface of the ulna, which is subcutaneous. Immediately in front of the olecranon is a large hollow, which receives the trochlea. It is called the greater sigmoid cavity. The anterior margin of the cavity is called the coronoid process. On the outer side of the coronoid process is a hollow called the lesser sigmoid cavity, which receives the head of the radius. Fracture of the bone frequently occurs through the narrow portion of the olecranon process into the greater sigmoid cavity.
Fig. 292. - Anterior view of bones of right elbow.
Fig. 293. - Posterior view of bones of elbow.
The radius ends above in a flat rounded head. The upper surface of this head articulates with the capitellum. The lateral surface articulates internally with the lesser sigmoid cavity of the ulna. The remainder of the circumference of the head is embraced by the orbicular ligament. Immediately below the head is the constricted neck and bicipital tuberosity. To the posterior half of this latter the tendon of the biceps is attached, but its anterior portion is smooth and provided with a bursa. The head of the radius is subcutaneous posteriorly, but the rest is too much covered by muscles to be readily palpated.