pectoralis

The pectoralis, or pectoral muscles, are a group of chest wall muscles that play a key role in movements of the shoulder girdle and upper limb. The term comes from Latin and means pertaining to the chest. The two principal muscles in this group are the pectoralis major and the pectoralis minor.

Pectoralis major is a large, fan-shaped muscle covering the upper chest. It has two main parts: the clavicular head, arising from the anterior surface of the medial half of the clavicle, and the sternocostal head, arising from the anterior surface of the sternum, the upper six costal cartilages, and the aponeurosis of the external oblique. It inserts onto the lateral lip of the intertubercular sulcus of the humerus. Innervation is via the lateral and medial pectoral nerves, with the clavicular head primarily receiving the lateral pectoral nerve and the sternocostal head primarily receiving the medial pectoral nerve. The muscle receives blood supply from branches of the thoracoacromial trunk, among others. Actions include adduction and medial rotation of the humerus; the clavicular head can flex the arm, while the sternocostal head can extend it from a flexed position.

Pectoralis minor is a smaller, thin muscle located beneath the pectoralis major. It originates from ribs 3–5 and inserts onto the coracoid process of the scapula. Innervation is via the medial pectoral nerve, with possible contributions from the lateral pectoral nerve. Its primary actions are to stabilize the scapula by drawing it anteriorly and inferiorly against the thoracic wall and to assist in deep inspiration when the scapula is fixed.

Together, these muscles contribute to chest movement, shoulder stability, and respiratory mechanics.