External Intercostal Muscles

Description

Sweeping across the posterolateral thoracic wall, the external intercostal muscles occupy the intercostal spaces between adjacent ribs from the rib tubercles and angles anteriorly toward the costochondral junction. Obliquely oriented fibers run infero-anteriorly from the inferior border of each rib to the superior border of the rib below, forming the familiar “hands-in-pockets” direction on the lateral chest. Near the anterior thorax, the muscle belly gives way to the external intercostal membrane, so the costal cartilages and parasternal region remain defined even as the contractile fibers taper out. Portions of the vertebral column, posterior ribs, and the superior pelvis, including the iliac crest and sacrum, provide bony context for the thoracic cage’s alignment in a male body habitus. Fiber direction is the teaching point here, and it matters clinically. External intercostals elevate the ribs during inspiration, and their oblique orientation helps explain rib flare patterns and why intercostal strain pain often tracks along a specific interspace rather than radiating randomly. The transition to an anterior membrane also sets the stage for procedural anatomy: anteriorly, the intercostal space is thinner, while the intercostal neurovascular bundle still courses in the costal groove along the inferior rib margin, a practical consideration for chest tube placement and intercostal nerve blocks (needle kept just superior to the rib to avoid the vein, artery, and nerve). Anatomy faculty can drop this into thoracic wall modules to contrast external versus internal intercostal fiber direction and to reinforce the costochondral junction as a landmark. It also fits well in surgical education materials covering thoracostomy, pleural procedures, and safe intercostal access, where understanding what becomes membrane versus muscle affects expectations during dissection and palpation. Anatomical accuracy verified by SciePro's Medical Advisory Board.