Pectoralis major muscle

Origin and insertion

The pectoralis major muscle is a fan-shaped muscle that consists of three parts that originate from three different sites: 

• The clavicular part originates from the anterior surface of the medial half of the clavicle.
• The sternocostal part originates from the anterior surface of sternum and the anterior aspects of the costal cartilages of ribs 1-6. 
• The smallest, abdominal part originates from the anterior layer of the rectus sheath.

The muscle fibers from all three parts run laterally, converging towards the proximal humerus. They give off a broad tendon that inserts along the crest of the greater tubercle of humerus.

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Relations 

The pectoralis major muscle is a broad superficial muscle found superficially in the anterior chest wall. In males, it is covered by the deep layer of fascia, subcutaneous tissue and the adjacent skin. In females, it is covered by the breast. The deep surface of the muscle covers the pectoralis minor and serratus anterior muscles and the anterior surface of the upper six ribs. 

The triangular depression between the pectoralis major muscle, deltoid muscle and clavicle is called infraclavicular fossa (Mohrenheim’s fossa) which serves as an important landmark in the surgical procedures on the subclavian artery.

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Innervation 

The pectoralis major muscle is innervated by the lateral and medial pectoral nerves (root value C5-T1), which stem from the brachial plexus.

Blood supply

The pectoralis major muscle is vascularized by the pectoral branches of thoracoacromial artery and the perforating branches of internal thoracic artery.

Function

When the arm is in the anatomical position, the pectoralis major acts as a strong adductor and internal rotator of the humerus at the shoulder joint. Acting independently, the clavicular portion of the muscle flexes the humerus up to 90 degrees in a horizontal plane. The sternocostal portion of the muscle can produce the antagonistic movement and extend the humerus back to the anatomical position. 

Acting together with the latissimus dorsi muscle, the pectoralis major muscle pulls the trunk forwards or upwards when its humeral attachment is fixed. This action is important in activities such as climbing. When acting from the humeral attachment, the pectoralis major muscle also facilitates the act of inspiration. This is particularly important during forced breathing in physical distress.

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