Pectoralis major muscle

The pectoralis major is a large, fan-shaped, multipennate muscle of the anterior thoracic wall that forms the bulk of the chest musculature. It is a powerful adductor and medial rotator of the humerus, with additional roles in flexion and extension. Structurally and functionally, it is divided into three parts: the clavicular head, sternocostal head, and abdominal part, which converge into a common tendon.

Origin and fibre orientation

1. Clavicular head

• Origin: Anterior surface of the medial half of the clavicle

• Fibre direction: Inferolateral

• Primary action: Flexion of the humerus (especially from an extended position)

2. Sternocostal head

• Origin: Anterior sternum and costal cartilages of ribs 16

• Fibre direction: Horizontal to superolateral

• Primary actions: Adduction and medial rotation; contributes to extension of a flexed humerus

3. Abdominal part

• Origin: Aponeurosis of the external oblique muscle

• Fibre direction: Superolateral, blending with inferior sternocostal fibres

• Primary action: Assists in adduction and medial rotation

Insertion

All fibres of pectoralis major muscle converge into a bilaminar tendon inserting into the lateral lip of the intertubercular sulcus of the humerus. The tendon is twisted, such that inferior (abdominal) fibres insert superiorly, an arrangement that contributes to the muscles complex functional range.

Innervation

• Lateral pectoral nerve (C5 to C7): Predominantly supplies the clavicular head

• Medial pectoral nerve (C8 to T1): Supplies the sternocostal and abdominal parts

Blood Supply

• Pectoral branch of the thoracoacromial artery (primary supply)

• Contributions from the lateral thoracic artery

Function

• Primary actions: Adduction and medial rotation of the humerus

• Clavicular head: Flexion of the humerus

• Sternocostal head: Extension from a flexed position

• Acts as an accessory muscle of respiration, assisting in forced inspiration when the upper limb is fixed

Clinical Relevance

• Forms the anterior axillary fold, which is an important surgical landmark in breast and axillary procedures

• Nerve injury (pectoral nerves) leads to weakness in adduction and medial rotation

• Muscle rupture, particularly of the sternocostal head, is seen in athletic injuries (e.g., weightlifting)

• Its robust vascularity and size make it useful in reconstructive flap surgery