Pectoralis Major Muscle
The pectoralis major muscle 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-T1), 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 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 inwards (medial rotation) 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).
Aplasia of the pectoralis major muscle is one of the most common muscular malformations. In Poland syndrome there is embryonic malformation of the thoracic wall, which is accompanied by a defect of the pectoralis major muscle in combination with other malformations of the upper extremities (e.g., malformations of the fingers). The muscle can be partly or completely missing. Depending on the degree of severity, the adduction and anteversion of the shoulder joint are made more difficult (for example, the crossing of the raised arms). In addition, affected persons have visibly distorted breasts due to the aplasia of the muscle, which is why a plastic-surgical reconstruction can be used therapeutically.