The serratus anterior is innervated by the long thoracic nerve, which is derived from the roots of the brachial plexus, passes through the axilla along the medial wall, and passes vertically down the serratus anterior muscle on its external surface, just deep to skin and superficial fascia.
The only major structure that passes directly through the medial wall and into the axilla is the intercostobrachial nerve (Fig. 7.43). This nerve is the lateral cutaneous branch of the second intercostal nerve (anterior ramus of T2). It communicates with a branch of the brachial plexus (the medial cutaneous nerve of the arm) in the axilla and supplies skin on the upper posteromedial side of the arm, which is part of the T2 dermatome.
The lateral wall of the axilla is narrow and formed entirely by the intertubercular sulcus of the humerus (Fig. 7.44). The pectoralis major muscle of the anterior wall attaches to the lateral lip of the intertubercular sulcus. The latissimus dorsi and teres major muscles of the posterior wall attach to the floor and medial lip of the intertubercular sulcus, respectively (Table 7.5).
The posterior wall of the axilla is complex (Fig. 7.45 and see Fig. 7.50). Its bone framework is formed by the costal surface of the scapula. Muscles of the wall are: the subscapularis muscle (associated with the costal surface of the scapula), the distal parts of the latissimus dorsi and teres major muscles (which pass into the wall from the back and posterior scapular region), and the proximal part of the long head of the triceps brachii muscle (which passes vertically down the wall and into the arm).
Gaps between the muscles of the posterior wall form apertures through which structures pass between the axilla, posterior scapular region, and posterior compartment of the arm.
The subscapularis muscle forms the largest component of the posterior wall of the axilla. It originates from, and fills, the subscapular fossa and inserts on the lesser tubercle of the humerus (Figs. 7.45 and 7.46). The tendon crosses immediately anterior to the joint capsule of the glenohumeral joint.
Together with three muscles of the posterior scapular region (the supraspinatus, infraspinatus, and teres minor muscles), the subscapularis is a member of the rotator cuff muscle group, which stabilizes the glenohumeral joint.
The subscapularis is innervated by branches of the brachial plexus (the superior and inferior subscapular nerves), which originate in the axilla.
The inferolateral aspect of the posterior wall of the axilla is formed by the terminal part of the teres major muscle and the tendon of the latissimus dorsi muscle (Fig. 7.45). These two structures lie under the posterior axillary fold, which marks the posteroinferior border of the axilla.
The flat tendon of the latissimus dorsi muscle curves around the inferior margin of the teres major muscle on the posterior wall to insert into the floor of the intertubercular sulcus of the humerus, anterior to and slightly above the most distal attachment of the teres major muscle to the medial lip of the intertubercular sulcus. As a consequence, the inferior margin of the teres major muscle defines the inferior limit of the axilla laterally.
The axillary artery becomes the brachial artery of the arm as it crosses the inferior margin of the teres major muscle.
Long head of the triceps brachii
The long head of the triceps brachii muscle passes vertically through the posterior wall of the axilla, and, together with surrounding muscles and adjacent bones, results in the formation of three apertures through which major structures pass through the posterior wall: the quadrangular space, the triangular space, and the triangular interval (Fig. 7.45).