Pectoralis major muscle

The pectoralis major muscle is a muscle of the pectoral region, overlying the anterior chest wall but is considered an upper limb muscle due to its function. 

Summary

• origin
  • sternal part: sternum and superiorsix costal cartilages
  • clavicular part: medial half of the clavicle
• insertion: lateral lip of the bicipital groove of the humerus
• innervation: medial and lateral pectoral nerves
• action: flexion, adduction and internal rotation of the shoulder joint

Gross anatomy

Pectoralis major is a large, fan-shaped muscle. It covers much of the front upper chest. It is composed of two parts:

• sternal part beginning at the sternum and the first six costal cartilages
• clavicular part takes origin from the clavicle

The heads of the pectoralis major converges on the humerus where it inserts into the bicipital groove.

Blood supply

The thoraco-acromial artery provides its major blood supply, while the intercostal perforators arising from the internal mammary artery provide a segmental blood supply. The blood supply that provides circulation to these muscles perforates through to the breast parenchyma, thus also supplying blood to the breast.

Innervation

The medial and lateral pectoral (or anterior thoracic) nerves provide innervation for the muscle, entering posteriorly and laterally.

Action

The function of this muscle is to bring the humerus across the chest, allowing to flex, adduct, and rotate the arm medially.

Variant anatomy

• some authors consider sternalis muscle a variant of pectoralis major rather than of the rectus abdominis muscle ⁷
  • sternalis muscle may give some superior muscular slips which blend into pectoralis major
• absence
  • Seen in Poland syndrome

Radiographic features

Mammography

The breast lies over the musculature that encases the chest wall. These muscles involved include pectoralis major, serratus anterior, external oblique, and rectus abdominis fascia. By maintaining continuity with the underlying musculature, the breast tissue remains richly perfused.

Depending on a woman's habitus, pectoral muscles have variable angles of obliquity:

• short, stocky women have more horizontally oriented pectoral muscles
• tall, thin women have them more vertically oriented

This element is very important for proper positioning in the efficacy of the medial lateral oblique (MLO) view in mammography: the compression of a pectoralis major differently oriented over a breast support inclined at 45° will produce a deformed geometric projection of the muscle and abnormal distribution of the quadrants, masking the "milky way" partially.

On adequate mammogram it is recommended that pectoralis major is seen through to the level of the nipple.

Related pathology

• ginkgo leaf sign of subcutaneous emphysema
• pectoralis major rupture