serratus posterior inferior muscle serratus posterior inferior muscle the serratus posterior inferior muscle, also known as the posterior serratus muscle, is a muscle of the human body. serratus posterior inferior musclemuscles connecting the upper extremity to the vertebral column (serratus posterior inferior labeled at center right).serratus posterior inferior (red) seen from back.detailsoriginvertebrae: spinous processes of t11 - l2insertionthe inferior borders of the 9th through 12th ribsarteryintercostal arteriesnerveintercostal nerves t9 through t12actionsdepress the lower ribs 9-12, aiding in expirationidentifierslatinmusculus serratus posterior inferiorta98a04.3.01.010ta22235fma13402anatomical terms of muscle structure the muscle is situated at the junction of the thoracic and lumbar regions. it has an irregularly quadrilateral form, broader than the serratus posterior superior muscle, and separated from it by a wide interval. it arises by a thin aponeurosis from the spinous processes of the lower two thoracic and upper two or three lumbar vertebrae. passing obliquely upward and lateralward, it becomes fleshy, and divides into four flat digitations. these are inserted into the inferior borders of the lower four ribs, a little beyond their angles. the thin aponeurosis of origin is intimately blended with the thoracolumbar fascia, and aponeurosis of the latissimus dorsi muscle. function the serratus posterior inferior draws the lower ribs backward and downward to assist in rotation and extension of the trunk. this movement of the ribs may also contribute to inhalation and forced expiration of air from the lungs. additional images position of the serratus posterior inferior (shown in red). animation. close up. the muscle arises from the vertebrae t11 through l2 and inserted into lower border of the 9th through 12th ribs. lumbar triangle