The Four Muscles of the Pectoral Region
The pectoral region contains four muscles: pectoralis major, pectoralis minor, subclavius, and serratus anterior.
1. Pectoralis Major
Origin:
- Clavicular head: Anterior surface of the medial half of the clavicle
- Sternocostal head: Anterior surface of the sternum and costal cartilages of ribs 1-6
- Abdominal head: Aponeurosis of the external oblique muscle
Insertion:
- Lateral lip of the intertubercular sulcus (bicipital groove) of the humerus
Nerve Supply:
- Lateral pectoral nerve (supplies the proximal segment and upper two-thirds of the muscle) 1, 2
- Medial pectoral nerve (supplies the lower third/distal segment) 3, 2
- The lateral pectoral nerve is the dominant innervation, with the medial pectoral nerve providing supplementary supply 2
Actions:
- Adduction of the arm
- Medial rotation of the arm
- Flexion of the arm (clavicular head)
- Extension of the arm from flexed position (sternocostal head)
2. Pectoralis Minor
Origin:
- Anterior surfaces of ribs 3,4, and 5 near the costochondral junctions 4
Insertion:
- Medial margin and upper surface of the coracoid process of the scapula 4
Nerve Supply:
- Medial pectoral nerve (pierces through the muscle in 76% as a single trunk, or as dividing branches in 34% of cases) 3
Actions:
- Stabilizes the scapula by drawing it anteroinferiorly against the thoracic wall
- Assists in protraction and depression of the scapula
3. Subclavius
Origin:
- Junction of the first rib and its costal cartilage
Insertion:
- Inferior surface of the middle third of the clavicle
Nerve Supply:
- Nerve to subclavius (from the superior trunk of the brachial plexus, C5-C6)
Actions:
- Anchors and depresses the clavicle
- Protects the subclavian vessels and brachial plexus
4. Serratus Anterior
Origin:
- External surfaces of the lateral parts of ribs 1-8
Insertion:
- Anterior surface of the medial border of the scapula (from superior to inferior angle)
Nerve Supply:
- Long thoracic nerve (C5, C6, C7)
Actions:
- Protraction of the scapula
- Rotation of the scapula for overhead arm movements
- Holds the medial border of the scapula against the thoracic wall
Clinical Relevance
Important anatomical considerations:
- The pectoralis major has separate proximal and distal neurovascular territories, allowing the proximal segment to be safely harvested as a myocutaneous flap 3, 2
- The medial pectoral nerve consistently pierces the pectoralis minor before supplying the lower pectoralis major 3, 5
- During modified radical mastectomy or axillary dissection, removal of pectoralis minor can cause partial denervation of pectoralis major if the medial pectoral nerve is injured 3
- The lateral pectoral nerve runs along the upper border of pectoralis minor in 98% of cases 3