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.
Triceps brachii
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
Radial nerve
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.
