pectoralis major muscle
- origin: medial clavicle, sternum, costal cartilages, and external oblique aponeurosis
- insertion: bicipital groove of the humerus
- innervation: medial and lateral pectoral nerves
- arterial supply: thoracoacromial artery (main supplier), intercostal perforators from internal thoracic artery
- action: flexion, adduction and internal rotation of the humerus at the shoulder joint
Pectoralis major is a large, fan-shaped muscle. It covers much of the front upper chest. It is composed of three parts named for their origins :
- 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 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 to flex, adduct, and rotate the arm medially.
- some authors consider sternalis muscle a variant of pectoralis major muscle rather than of the rectus abdominis muscle
- 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 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.