Pectoralis major muscle

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. 

Summary

• origin: medial clavicle, sternum, costal cartilages, and external oblique aponeurosis

• insertion: lateral lip of the bicipital groove of the humerus, anterior lip of deltoid tuberosity, and deep fascia of the arm

• innervation: medial and lateral pectoral nerves

• arterial supply: thoracoacromial artery (main supplier), intercostal perforators from internal thoracic artery

• action: adduction and internal rotation, and flexion of the humerus at the shoulder joint

Gross anatomy

Pectoralis major is a large, fan-shaped muscle. It covers much of the anterior chest wall. It is composed of two or three parts (depending on texts) 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. Some texts consider this a part of the sternocostal head¹².

The fibers 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 tendon is trilaminar:

• anterior lamina arises from fibers of the clavicular head, and also inserts onto the anterior lip of deltoid tuberosity and deep fascia of the arm

• intermediate lamina arises from the manubrial fibers (superior to the sternal angle) of the sternocostal head

• posterior lamina arises from sternocostal fibers (including abdominal part) inferior to the sternal angle^9,12.

Pectoralis major is covered between the two layers of deep fascia (known as pectoralis fascia). The deep layer of the pectoralis fascia covers the clavipectoral fascia ¹¹. Meanwhile, the clavipectoral fascia covers subclavius and pectoralis minor muscles ¹⁰.

Arterial supply

• thoracoacromial artery provides its major blood supply

• 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

Innervation

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

C5-6 segments supply the clavicular part, and C7-T1 segments supply the sternocostal part¹².

Action

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.

Variant anatomy

• some authors consider sternalis muscle a variant of pectoralis major muscle rather than of the rectus abdominis muscle ⁷

  • sternalis muscle may give some superior muscular slips which blend into pectoralis major

  • present in 5% of dissections¹²

• 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 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.

Related pathology

• ginkgo leaf sign of subcutaneous emphysema

• pectoralis major injury