Pectoralis major muscle
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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
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.
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.
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
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.
sternalis muscle may give some superior muscular slips which blend into pectoralis major
seen in Poland syndrome
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.