Origin: Vertebrae T11 - L3
Insertion: The inferior borders of the 9th through 12th ribs
Artery: Intercostal arteries
Nerve: Intercostal nerves
Action: Depress the lower ribs, aiding in expiration
The Serratus posterior inferior (Serratus posticus inferior) is situated at the junction of the thoracic and lumbar regions: it is of an irregularly quadrilateral form, broader than the preceding, 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, and from the supraspinal ligament. Passing obliquely upward and lateralward, it becomes fleshy, and divides into four flat digitations, which 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 lumbodorsal fascia, and aponeurosis of the Latissimus dorsi.