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Pectoralis Major is a muscle that forms part of the pectoral girdle and also helps in its movement. There are other pectoral muscles that form the pectoral girdle and these muscles (pectoralis major, pectoralis minor, subclavius, and serratus anterior) will be discussed individually; these four (4) muscles form the anterior axioappendicular (thoracoappendicular or pectoral) muscles and they all move the pectoral girdle. Pectoral major is also called P. Major; the blood supply, nerve supply, action and location of Pectoralis Major muscle will be described below.
Pectoralis Major boundaries and description
Pectoralis major muscles has the shape of a fan and covers the superior part of the thorax. P. major has two heads which are the clavicular head which is attached to the clavicle and the sternocostal head which is attached to both the sternum and the costal ribs. The sternocostal head of P. major is larger and the lateral border forms most of the anterior wall of the axilla. The inferior border of Pectoralis major forms the anterior axillary fold. Together with the adjacent deltoid, pectoralis major forms the narrow deltopectoral groove where the cephalic vein runs.
The clavipectoral triangle (also known as deltopectroal triangle) is formed by the clavicle, deltoid muscle and pectoralis major superiorly. The lower medial part of the pectoralis major muscle is thinner and in danger of being perforated during insertion of a prosthesis such as in breast reconstruction when a subpectoral pocket is created.
Pectoralis major Origin and Insertion
The clavicular head of pectoralis major originiates from the anterior surface of medial half of clavicle while the Sternocostal head originates from the anterior surface of sternum and also from the superior 6 costal cartilages and aponeurosis of external oblique muscle. Pectoralis major insertion is at the lateral lip of intertubercular sulcus (groove) of the humerus.
Pectoralis Major Action
Pectoralis major action is mainly to adducts and medially rotate the humerus. It also draws the scapula anteriorly and inferiorly. When acting alone, the clavicular head flexes humerus and sternocostal head extends the humerus from the flexed position.
Pectoralis Major Innervation (Nerve supply)
Pectoralis major is innervated by the lateral and medial pectoral nerves of the brachial plexus because of their origins from the lateral and medial cords of the brachial plexus. The clavicular head is innervated by C5 and C6 segments while the sternocostal head is innervated by C7, C8, and T1 segments. Pectoralis major is the only muscle in the upper limb to be supplied by all five segments of the brachial plexus. The degree of paralysis of pectoralis major may be helpful in assessing the extent of a brachial plexus injury.
Pectoralis Major Blood supply
Perforating branches of the internal thoracic artery penetrate the deep surface of P. major muscle at the sternal edge and are at risk of being torn during subpectoral dissection. These arterial branches and others from the superior and lateral thoracic arteries supplement the dominant vascular pedicle for pectoralis major from the pectoral branch of the thoracoacromial artery. A musculocutaneous flap based on this dominant pedicle is used in reconstructive procedures after surgical resections for head and neck cancer.