Gateways in the posterior wall (See also “Gateways to the posterior scapular region,” pp. 706–710, and Figs. 7.37 and 7.38.)
The quadrangular space provides a passageway for nerves and vessels passing between the axilla and the more posterior scapular and deltoid regions (Fig. 7.45). When viewed from anteriorly, its boundaries are formed by: the inferior margin of the subscapularis muscle, the surgical neck of the humerus, the superior margin of the teres major muscle, and the lateral margin of the long head of the triceps brachii muscle.
Passing through the quadrangular space are the axillary nerve and the posterior circumflex humeral artery and vein.
The triangular space is an area of communication between the axilla and the posterior scapular region (Fig. 7.45). When viewed from anteriorly, it is formed by: the medial margin of the long head of the triceps brachii muscle, the superior margin of the teres major muscle, and the inferior margin of the subscapularis muscle.
The circumflex scapular artery and vein pass into this space.
This triangular interval is formed by: the lateral margin of the long head of the triceps brachii muscle, the shaft of the humerus, and the inferior margin of the teres major muscle (Fig. 7.45).
The radial nerve passes out of the axilla traveling through this interval to reach the posterior compartment of the arm.
The floor of the axilla is formed by fascia and a dome of skin that spans the distance between the inferior margins of the walls (Fig. 7.47 and see Fig. 7.40B). It is supported by the clavipectoral fascia. On a patient, the anterior axillary fold is more superior in position than is the posterior axillary fold.
Inferiorly, structures pass into and out of the axilla immediately lateral to the floor where the anterior and posterior walls of the axilla converge and where the axilla is continuous with the anterior compartment of the arm.
Contents of the axilla
Passing through the axilla are the major vessels, nerves, and lymphatics of the upper limb. The space also contains the proximal parts of two muscles of the arm, the axillary process of the breast, and collections of lymph nodes, which drain the upper limb, chest wall, and breast.
The proximal parts of the biceps brachii and coracobrachialis muscles pass through the axilla (Table 7.6).
The biceps brachii muscle originates as two heads (Fig. 7.48):
The short head originates from the apex of the coracoid process of the scapula and passes vertically through the axilla and into the arm where it joins the long head.
The long head originates as a tendon from the supraglenoid tubercle of the scapula, passes over the head of the humerus deep to the joint capsule of the glenohumeral joint, and enters the intertubercular sulcus where it is held in position by a ligament, the transverse humeral ligament, which spans the distance between the greater and lesser tubercles; the tendon passes through the axilla in the intertubercular sulcus and forms a muscle belly in the proximal part of the arm.
The long and short heads of the muscle join in distal regions of the arm and primarily insert as a single tendon into the radial tuberosity in the forearm.
The biceps brachii muscle is primarily a powerful flexor of the forearm at the elbow joint and a powerful supinator in the forearm. Because both heads originate from the scapula, the muscle also acts as an accessory flexor of the arm at the glenohumeral joint. In addition, the long head prevents superior movement of the humerus on the glenoid cavity.
The biceps brachii muscle is innervated by the musculocutaneous nerve.