The normal origin of the serratus muscle is the lateral surface of the 1st to 8th ribs, and the insertion is on the rib surface of the medial border of the scapula. As the origin of a muscle indicates that's the fixed structure when the muscle contracts, in opposition to the insertion which indicates the moving structure, the structure that moves, when in normal conditions, is the scapula, being rotated, allowing the arm to rise more than what the glenohumeral joint allows.
When in respiratory distress, the need to increase the volume of the thoracic cage is so great that the medial border of the scapula works as the origin of the muscle, becoming fixed, and the muscle's contraction pulls the 1st-8th ribs instead, becoming the ribs the insertion location.
