Pectoralis major muscle

Last revised by Assoc Prof Craig Hacking on 15 Aug 2021

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

Pectoralis major is a large, fan-shaped muscle. It covers much of the anterior chest wall. It is composed of three parts named for their origins, with the sternocostal head forming the bulk (~80%) of the total muscle volume 8,9:

  • clavicular head (pars clavicularis): originating from the anterior aspect of the medial half of the clavicle
  • sternocostal head (pars sternocostalis): originating from the anterior part of the sternum and the costal cartilages of most of the true ribs (often excluding the first and/or seventh ribs)
  • abdominal part (pars abdominalis): originating from the aponeurosis of the external oblique muscle

The heads of the pectoralis major converge on the humerus where it inserts into the lateral lip of the bicipital groove as a U-shaped tendon measuring 40-60 mm wide and 3-5 mm thick. The inferior sternal fibers form a deeper layer to the clavicular and superior sternal fibers 9

  • the thoracoacromial artery provides its major blood supply
  • the intercostal perforators arising from the internal thoracic artery provide a segmental blood supply. 
  • the blood supply that provides circulation to this muscle perforates through to the breast, thus also providing its blood supply

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

The function of this muscle is to bring the humerus across the chest, allowing it to adduct and rotate the shoulder medially (primary) as well as forward flexion (secondary) 9.

The breast lies over the musculature that encases the chest wall. These muscles involved include pectoralis major, serratus anteriorexternal 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 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 an adequate mammogram, it is recommended that pectoralis major is seen through to the level of the nipple.

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Cases and figures

  • Figure 1: pec muscles (Gray's illustrations)
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  • Figure 2: anterior arm anatomy
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  • Case 1: normal appearance on mammography
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