External intercostal muscles lift ribs upward and outward during inhalation.
The external intercostal muscles are the most superficial layer of the intercostal muscle group, lying between the ribs in each intercostal space. These muscles are primarily responsible for elevating the ribs during inspiration, contributing to the expansion of the thoracic cavity and facilitating airflow into the lungs. They are essential for maintaining the structural integrity of the thoracic wall and play an important role in the mechanics of breathing.
External intercostal muscles occupy the eleven intercostal spaces on each side of the thoracic cage. They extend from the rib tubercles posteriorly to the costochondral junctions anteriorly, where they are replaced by a thin fibrous sheet known as the external intercostal membrane. The muscles lie deep to the serratus anterior and superficial to the internal intercostal muscles.
Each muscle runs obliquely downward and forward, forming a slanted, diagonal pattern that resembles the direction of placing hands into one's pockets. This fiber orientation contributes to rib elevation during contraction.
The muscle fibers run in an inferomedial direction—downward and toward the midline—at approximately a 45-degree angle. This direction is opposite to that of the internal intercostal muscles, which run inferolaterally.
The external intercostal membrane continues anteriorly from the muscle fibers toward the sternum and contributes to the overall elasticity of the thoracic wall.
The external intercostal muscles are the outermost layer in the intercostal space. From superficial to deep, the arrangement is:
The intercostal neurovascular bundle (vein, artery, nerve) runs between the internal and innermost layers within the costal groove.
The primary function of the external intercostal muscles is to elevate the ribs during inspiration. This action increases the anteroposterior and transverse dimensions of the thoracic cavity, reducing intrathoracic pressure and allowing air to flow into the lungs.
During breathing, the coordinated contraction of the external intercostals with the diaphragm ensures smooth expansion of the thoracic volume, optimizing ventilatory efficiency.
External intercostals act synergistically with other respiratory muscles:
The upward and outward movement of the ribs they produce contributes to the "bucket-handle" and "pump-handle" mechanisms of rib motion.
Each intercostal nerve provides motor supply to the corresponding intercostal muscles and sensory fibers to the overlying skin and parietal pleura.
External intercostal muscles develop from the myotomes of the paraxial mesoderm, specifically from thoracic somites. Their segmental innervation reflects this origin.
Together, all three layers help regulate thoracic pressure changes during respiration and support thoracic wall tension.