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Pectoralis major

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Here is a complete overview of the Pectoralis Major drawn from Gray's Anatomy for Students, the THIEME Atlas of Anatomy, Imaging Anatomy Vol. 3, and Miller's Review of Orthopaedics.

Pectoralis Major

The pectoralis major is the largest and most superficial muscle of the anterior chest wall. It lies directly deep to the breast, separated from it by deep fascia and the loose connective tissue of the retromammary space. Its inferior margin forms the anterior axillary fold.
Pectoralis major - anterior view showing clavicular head and sternocostal part with rib attachments I-VII

Origin (three parts)

PartOrigin
ClavicularMedial half of the clavicle
SternocostalManubrium, body of sternum, and costal cartilages of ribs 2-6 (some sources: 1-7)
AbdominalAnterior layer of the rectus sheath / aponeurosis of external oblique

Insertion

The muscle fibers converge laterally into a bilaminar, U-shaped (horseshoe) tendon that inserts into the lateral lip of the intertubercular sulcus (bicipital groove) of the humerus. The tendon footprint spans approximately 4-6 cm on the humeral diaphysis.
A key anatomical detail: the tendon fibers twist on themselves so that:
  • The clavicular and manubrial fibers insert into the anterior limb (lower on the humerus)
  • The sternocostal fibers insert into the inferior cup
  • The abdominal fibers insert into the posterior limb (highest on the humerus)
This arrangement - analogous to an unfolded hand-held fan - means that as the arm is elevated, the fibers become untwisted and stretched, increasing the force the muscle can exert.

Innervation

Medial and lateral pectoral nerves (brachial plexus roots C5-T1)
  • The lateral pectoral nerve pierces pectoralis minor and ramifies on the deep surface of pectoralis major
  • The medial pectoral nerve supplies both pectoralis minor and pectoralis major
Pectoral region with nerves and vessels - showing medial and lateral pectoral nerves, thoracoacromial and lateral thoracic arteries

Blood Supply

Supplied by two branches of the axillary artery:
  • Thoracoacromial artery (pectoral branch)
  • Lateral thoracic artery
Each segment of the muscle has a distinct neurovascular supply running in the epimysium on its posterior surface.

Actions

ActionPart Responsible
Adduction of the armEntire muscle
Medial (internal) rotation of the armEntire muscle
Flexion of the armClavicular + sternocostal parts
Extension from flexion (against resistance)Sternocostal head specifically
Assists respirationWhen the shoulder girdle is fixed (accessory respiratory muscle)
The clavicular head flexes the arm from an extended position; the sternocostal head extends the arm from a flexed position, particularly against resistance - the two heads therefore have partially antagonistic roles.

Relations

  • Directly posterior (deep) to it: pectoralis minor, subclavius, and the clavipectoral fascia
  • The cephalic vein runs in the deltopectoral groove (clavipectoral triangle) at the lateral border
  • The axillary contents lie posterior to its inferior margin

Variations

  • Poland's syndrome: Rare congenital absence of the sternal portion of pectoralis major, sometimes with ipsilateral symbrachydactyly (short, webbed fingers)
  • Accessory muscles: Several are described, including the pectoralis quartus and chondroepitroclearis, which originate from the caudal pectoralis major or from ribs 5-6

Clinical Relevance: Rupture

  • Typically caused by excessive eccentric load on a maximally contracted muscle; classically occurs in weightlifters (bench press)
  • The shoulder must be in an abducted, extended, and externally rotated position with maximal muscle tension for rupture to occur
  • Most commonly a tendinous avulsion from the humeral insertion
  • Signs: localized swelling, ecchymosis, palpable defect / axillary webbing, weakness of adduction and internal rotation
  • Complete ruptures: surgical repair to bone
  • Partial ruptures / myotendinous tears: conservative (nonoperative) management
  • Ruptures are infrequently reported in women
Sources: Gray's Anatomy for Students (9780323934237) | THIEME Atlas of Anatomy - General Anatomy & Musculoskeletal System (9781626237186) | Imaging Anatomy Vol. 3 (9781626239845) | Miller's Review of Orthopaedics 9th Ed. (9780443112140)
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