Pectoral Region and Axilla

The axilla is a transition area between the neck and upper limb; the bones and muscles we learn in this tutorial contribute to its anatomical boundaries.

We'll learn the following muscles: subclavius, pectorals minor, pectoralis major, and serratus anterior.

These muscles act on the pectoral girdle, which comprises the clavicle, scapula, and humerus.

Innervation

The muscles of the pectoral region and axilla are innervated by spinal nerves from C5-T1 (the brachial plexus).

Subclavius:

• Originates on the first rib/costal cartilage
• Inserts on the inferior surface of the clavicle
• Stabilizes the sternoclavicular joint and depresses the clavicle when contracted.
• Innervated by the nerve to subclavius (C5 and C6 of the brachial plexus).

Pectoralis minor:

• Originates on the third, fourth, and fifth ribs
• Inserts on the coracoid process of the scapula
• Protracts the scapula (it pulls the scapula anteriorly on the thoracic wall) and depresses the lateral angle of the scapula. Pectoralis minor also assists in deep inspiration by elevating the ribs.
• Pectoralis minor overlies key clinically important neurovascular structures.
• Innervated by the medial pectoral nerve (C6, C7, and C8 of the brachial plexus).
• Clavipectoral fascia is a thick layer of connective tissue that encloses subclavius and pectoralis minor.

Pectoralis major (clavicular head) & Pectoralis major (sternocostal head)

• Originates on the medial half of the clavicle, the sternum, and first 6 ribs. Fibers can extend inferiorly and blend with the anterior abdominal wall.
• Inserts on the lateral lip of the intertubercular groove of the humerus
• Clavicular head flexes the arm; the sternocostal head abducts and medially rotates the humerus, and also extends the humerus from a flexed position.
• Comprises the bulk of the anterior thoracic wall.
• The cephalic vein traverses superficially along the superolateral border of pectoralis major in the deltopectoral groove, which is a groove between the deltoid and pectoralis major muscles. vein dives deep at the clavipectoral triangle, piercing through the clavipectoral fascia, to join the axillary vein.

Serratus anterior:

• Originates on the lateral aspect of the first 8 ribs and wraps posteriorly.
• Inserts on the medial border of the scapula.
• Stabilizes, abducts, and protracts the scapula.

Clinical Correlation:

• "Winged" scapula, in which the medial border projects posteriorly, occurs if the long thoracic nerve is damaged.