Pectoralis Tertius
Chest & Trunk
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Think of it as a rare ‘third pec.’ It can run through the axilla, so it may matter clinically because it could confuse surgical access to the axillary region.
Function
Variable and mostly inferred. Depending on attachment, it may help pull the arm/shoulder region forward, adduct or medially rotate the humerus, or reinforce the anterior axillary wall.
Origin
Usually the lower ribs, especially the external surface of the 6th rib and nearby costal/cartilage regions; some descriptions extend toward the 7th rib or adjacent external oblique fibers.
Insertion
Variable; reported insertions include the coracoid process, the crest of the greater tubercle/intertubercular sulcus of the humerus, fascia near pectoralis minor, or near the insertion of pectoralis major.
Innervation
Reported innervation includes branches of the lateral pectoral nerve reaching the deep surface of the muscle.