Pectoralis Quartus
Chest & Trunk
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Pectoralis quartus is basically an “extra pec.” It is usually found during cadaver dissection or surgery, and because it can run toward the axilla it may confuse surgeons or look like an unusual soft-tissue band in the armpit/chest region. One case report described it from the 5th costal cartilage inserting into fascia over coracobrachialis; other reports describe slightly different attachments.
Function
No single independent function is firmly established (rare variant). By location it probably assists the pectoral complex with humeral adduction/medial rotation or tensions fascia near the anterior axilla; if it blends toward pectoralis minor or axillary fascia it may slightly influence scapular/axillary mechanics.
Origin
Variable. Commonly the lower ribs/costal cartilages, especially around the 5th costochondral region; some descriptions include the rectus sheath or the lateral border of pectoralis major.
Insertion
Variable. Reported insertions include fascia over coracobrachialis, upper arm fascia, the bicipital groove region, or blending with nearby pectoral/axillary structures.
Innervation
Usually the medial pectoral nerve; some reports describe contribution from the 4th intercostal nerve, or both.