The Pectoralis Major Muscle
Anatomy and supply
The pectoralis major muscle (green highlighted on the picture) is a strong, fan-shaped muscle of the shoulder joint. It decisively shapes the surface anatomy of the breast. Furthermore it participates in forming the anterior wall of the axilla. The innervation is carried by the medial and lateral pectoral nerves (C5-Th1), direct branches of the brachial plexus. Due to its various origins the muscle is subdivided in three parts:
- Clavicular part: medial half of the clavicle
- Sternocostal part: sternum, 2nd to 7th costal cartilages
- Abdominal part: anterior layer of the rectus sheath
All fibers insert together at the crest of the greater tubercle located at the proximal humeral shaft. Due to the different courses of the muscle fibers the insertion has a recess which is open to the top preventing the muscle from overstretching. The triangular depression between the pectoralis major muscle, deltoid muscle and clavicle is called infraclavicular fossa (Mohrenheim’s fossa). Here the cephalic vein passes through in the subfascia within the deltopectoral groove.
The pectoralis major muscle is the most important muscle for the adduction and anteversion of the shoulder joint which is why it is also known as the “breaststroke muscle”. It rotates the upper arm outwards and makes a powerful stroke movement (retroversion) when the arms are elevated (e.g. in wood-chopping). If the arms are fixed the muscle lifts the trunk which can be helpful in climbing or during inspiration (inspiratory breathing muscle).
Aplasias of the pectoralis major muscle rank among the most common muscular malformations. In Poland syndrome the development of the breast wall remains incomplete during embryogenesis. It is distinguished by a defect of the pectoralis major muscle along with further faulty developments of the upper extremities (e.g. finger malformation). Hereby the chest muscle may be partially or completely missing. Depending on the degree of severity both the adduction and anteversion of the shoulder joint may be hindered (e.g. crossing the elevated arms). Moreover the affected people show visibly disfigured breasts so that plastic surgical reconstruction might be a therapeutic option.