External Intercostal Muscles

Description

Running obliquely infero-anterior between adjacent ribs, the external intercostal muscles occupy the intercostal spaces from the tubercles of the ribs posteriorly toward the costal cartilages anteriorly, where their fibers continue as the external intercostal membrane. Deep to them, slips of internal intercostal muscle are partially exposed, with fiber direction crossing almost at right angles, and the osseous ribs and costal cartilages form the rigid and semi-rigid boundaries of each space. Portions of pectoralis major lie anteriorly on the thoracic wall, while superiorly the deltoid caps the shoulder and proximal arm musculature is included for orientation. Fiber direction is the teaching point. External intercostals elevate ribs during inspiration and help maintain intercostal space rigidity during negative intrathoracic pressure, so their infero-anterior “hands-in-pockets” orientation is a practical landmark when distinguishing them from internal intercostals in dissection and cross-sectional anatomy. Clinically, their relationship to the intercostal neurovascular bundle, which travels in the costal groove along the inferior border of each rib between internal and innermost intercostals, frames safe technique for thoracostomy and intercostal nerve blocks: enter just superior to the rib below to avoid the artery, vein, and nerve. Small errors here matter. Use this plate in gross anatomy and respiratory mechanics teaching to connect rib cage kinematics with layered thoracic wall anatomy, or in surgical education materials on chest tube insertion, thoracotomy planning, and management of rib fractures with intercostal muscle disruption. It also suits publisher layouts that need clear differentiation of external versus internal intercostal layers without losing surrounding shoulder, pectoral, and upper rectus abdominis landmarks. Anatomical accuracy verified by SciePro's Medical Advisory Board.