The largest of the fossae, the olecranon fossa, occurs immediately superior to the trochlea on the posterior surface of the distal end of the humerus.
These three fossae accommodate projections from the bones in the forearm during movements of the elbow joint.
Proximal end of the radius
The proximal end of the radius consists of a head, a neck, and the radial tuberosity (Fig. 7.62A,B).
The head of the radius is a thick disc-shaped structure oriented in the horizontal plane. The circular superior surface is concave for articulation with the capitulum of the humerus. The thick margin of the disc is broad medially where it articulates with the radial notch on the proximal end of the ulna.
The neck of the radius is a short and narrow cylinder of bone between the expanded head and the radial tuberosity on the shaft.
The radial tuberosity is a large blunt projection on the medial surface of the radius immediately inferior to the neck. Much of its surface is roughened for the attachment of the biceps brachii tendon. The oblique line of the radius continues diagonally across the shaft of the bone from the inferior margin of the radial tuberosity.
Proximal end of the ulna
The proximal end of the ulna is much larger than the proximal end of the radius and consists of the olecranon, the coronoid process, the trochlear notch, the radial notch, and the tuberosity of the ulna (Fig. 7.63A,B).
The olecranon is a large projection of bone that extends proximally from the ulna. Its anterolateral surface is articular and contributes to the formation of the trochlear notch, which articulates with the trochlea of the humerus. The superior surface is marked by a large roughened impression for the attachment of the triceps brachii muscle. The posterior surface is smooth, shaped somewhat triangularly, and can be palpated as the “tip of the elbow.”
The coronoid process projects anteriorly from the proximal end of the ulna (Fig. 7.63). Its superolateral surface is articular and participates, with the olecranon, in forming the trochlear notch. The lateral surface is marked by the radial notch for articulation with the head of the radius.
Just inferior to the radial notch is a fossa that allows the radial tuberosity to change position during pronation and supination. The posterior margin of this fossa is broadened to form the supinator crest. The anterior surface of the coronoid process is triangular, with the apex directed distally, and has a number of roughenings for muscle attachment. The largest of these roughenings, the tuberosity of the ulna, is at the apex of the anterior surface and is the attachment site for the brachialis muscle.
The anterior compartment of the arm contains three muscles—the coracobrachialis, brachialis, and biceps brachii muscles—which are innervated predominantly by the musculocutaneous nerve.
The posterior compartment contains one muscle—the triceps brachii muscle—which is innervated by the radial nerve.
The coracobrachialis muscle extends from the tip of the coracoid process of the scapula to the medial side of the midshaft of the humerus (Fig. 7.64 and Table 7.8). It passes through the axilla and is penetrated and innervated by the musculocutaneous nerve.
The coracobrachialis muscle flexes the arm.
The biceps brachii muscle has two heads:
The short head of the muscle originates from the coracoid process in conjunction with the coracobrachialis.
The long head originates as a tendon from the supraglenoid tubercle of the scapula (Fig. 7.64 and Table 7.8).