Acting together, the two heads of the pectoralis major flex, adduct, and medially rotate the arm at the glenohumeral joint. The clavicular head flexes the arm from an extended position, whereas the sternocostal head extends the arm from a flexed position, particularly against resistance.
The pectoralis major is innervated by the lateral and medial pectoral nerves, which originate from the brachial plexus in the axilla.
The subclavius muscle is a small muscle that lies deep to the pectoralis major muscle and passes between the clavicle and rib I (Fig. 7.42). It originates medially, as a tendon, from rib I at the junction between the rib and its costal cartilage. It passes laterally and superiorly to insert via a muscular attachment into an elongate shallow groove on the inferior surface of the middle third of the clavicle.
The function of the subclavius is not entirely clear, but it may act to pull the shoulder down by depressing the clavicle and may also stabilize the sternoclavicular joint by pulling the clavicle medially.
The subclavius muscle is innervated by a small branch from the superior trunk of the brachial plexus.
The pectoralis minor muscle is a small triangular-shaped muscle that lies deep to the pectoralis major muscle and passes from the thoracic wall to the coracoid process of the scapula (Fig. 7.42). It originates as three muscular slips from the anterior surfaces and upper margins of ribs III to V and from the fascia overlying muscles of the related intercostal spaces. The muscle fibers pass superiorly and laterally to insert into the medial and upper aspects of the coracoid process.
The pectoralis minor muscle protracts the scapula (by pulling the scapula anteriorly on the thoracic wall) and depresses the lateral angle of the scapula.
The pectoralis minor is innervated by the medial pectoral nerve, which originates from the brachial plexus in the axilla.
The clavipectoral fascia is a thick sheet of connective tissue that connects the clavicle to the floor of the axilla (Fig. 7.42). It encloses the subclavius and pectoralis minor muscles and spans the gap between them.
Structures travel between the axilla and the anterior wall of the axilla by passing through the clavipectoral fascia either between the pectoralis minor and subclavius muscles or inferior to the pectoralis minor muscle.
Important structures that pass between the subclavius and pectoralis minor muscles include the cephalic vein, the thoraco-acromial artery, and the lateral pectoral nerve.
The lateral thoracic artery leaves the axilla by passing through the fascia inferior to the pectoralis minor muscle.
The medial pectoral nerve leaves the axilla by penetrating directly through the pectoralis minor muscle to supply this muscle and to reach the pectoralis major muscle.
Occasionally, branches of the medial pectoral nerve pass around the lower margin of the pectoralis minor to reach and innervate the overlying pectoralis major muscle.
The medial wall of the axilla consists of the upper thoracic wall (the ribs and related intercostal tissues) and the serratus anterior muscle (Fig. 7.43 and Table 7.4, and see Fig. 7.40).
The serratus anterior muscle originates as a number of muscular slips from the lateral surfaces of ribs I to IX and the intervening deep fascia overlying the related intercostal spaces (Fig. 7.43). The muscle forms a flattened sheet, which passes posteriorly around the thoracic wall to insert primarily on the costal surface of the medial border of the scapula.
The serratus anterior pulls the scapula forward over the thoracic wall and facilitates scapular rotation. It also keeps the costal surface of the scapula closely opposed to the thoracic wall.