Serratus Posterior Inferior Muscle Under the Skin, Posterior View

Description

Posterior trunk integument has been retained as a superficial reference while the deeper muscular layer is brought forward, centering on the serratus posterior inferior (SPI), a thin quadrilateral sheet spanning from the lower thoracic and upper lumbar midline to the inferior borders of ribs 9 to 12. Medially, its aponeurotic origin blends with the thoracolumbar fascia and the spinous processes of T11 to L2, and laterally its serrated slips run inferolaterally to the costal angles. Superior to this field sit the caudal fibers of latissimus dorsi and portions of the lower trapezius, while the posterolateral abdominal wall (including internal oblique) lies more inferior and lateral, approaching the iliac crest. Layering matters here. SPI is often taught as an accessory muscle of respiration, depressing the lower ribs during forced expiration, but its real teaching value is spatial: it marks the transition between posterior thoracic wall and the thoracolumbar fascia overlying quadratus lumborum. That relationship becomes clinically relevant when differentiating posterolateral flank pain sources, planning a posterior approach to the kidney, or explaining why lower rib fractures can refer pain along the subcostal nerve as it courses deep to these muscular planes. Palpation and trigger-point discussions also tend to confuse SPI with latissimus dorsi, and a posterior view clarifies the separation. Use this artwork in gross anatomy dissection guides, regional anatomy lectures on the thorax and posterior abdominal wall, and figure plates for rehabilitation texts describing forced expiration and trunk stabilization. It also fits surgical education materials that map posterior flank incisions and the muscular layers encountered when entering the retroperitoneum. Anatomical accuracy verified by SciePro's Medical Advisory Board.