The posterior compartment of the arm contains triceps brachii, the radial nerve accompanied by the profunda brachii artery with its venae comitantes, and the ulnar nerve.
Proximally, this muscle attaches to the scapula and the humerus by three heads. The tendon of the long head (Fig. 3.65) attaches to the infraglenoid tubercle of the scapula, and the lateral head attaches to the posterior aspect of the shaft of the humerus above the radial groove (spiral groove, sulcus of the radial nerve) (Fig. 3.67). Both the long and lateral heads lie superficial to the medial head (Fig. 3.66), which has an extensive origin from the lateral and medial intermuscular septa and from the shaft of the humerus below the radial groove. Dis- tally, the three heads fuse, and triceps attaches by a single tendon to the olecranon process of the ulna. Frequently, a bursa lies deep to the tendon, separating it from the capsule of the elbow joint. The three heads are supplied separately by branches of the radial nerve. Triceps is a powerful extensor of the elbow joint. The long head alone spans the shoulder joint and may assist in stabilizing that joint, particularly in full abduction.
Vessels and nerves
The radial nerve, a terminal branch of the posterior cord, leaves the axilla by passing below teres major and between the humerus and the long head of triceps (Fig. 3.64). In the posterior compartment, the nerve passes between the medial and lateral heads of triceps and in the radial groove is intimately related to the shaft of the humerus (Fig. 3.68). It then leaves the posterior compartment by piercing the lateral intermuscular septum to reach the lateral part of the cubital fossa in front of the elbow joint (Fig. 3.29). In the arm, the radial nerve gives muscular branches to the medial and lateral heads of triceps and to brachioradialis and extensor carpi radialis longus, and cutaneous branches to the lateral aspect of the arm and the posterior aspect of the forearm. The branch to the long head of triceps usually arises in the axilla. The radial nerve may be damaged in the radial groove by fracture of the shaft of the humerus (p. 113), but the branches to triceps, which arise more proximally, are usually spared.
The profunda brachii artery, a proximal branch of the brachial artery (Fig. 3.18), supplies the posterior compartment and accompanies the radial nerve in the radial groove of the humerus (Fig. 3.68). It gives muscular branches and contributes to the anastomosis around the elbow joint. The venae comitantes of the profunda brachii artery drain into the axillary vein.
The ulnar nerve passes from the anterior to the posterior compartment by piercing the medial intermuscular septum at about midarm level (Fig. 3.24) and descends between the septum and the medial head of triceps. At the elbow it passes posterior to the medial epicondyle of the humerus, medial to the joint, and enters the forearm between the humeral and ulnar heads of flexor carpi ulnaris. The nerve gives no branches in the arm.