Posterior Inferior Serratus

Description

Running obliquely across the lower posterior thorax, the serratus posterior inferior appears as a thin, quadrilateral muscle sheet arising from the midline aponeurosis over the lower thoracic and upper lumbar spinous processes (classically T11 to L2) and fanning laterally to the inferior borders of ribs 9 to 12 near their angles. Its superior margin lies deep to the latissimus dorsi, while the erector spinae mass remains medial and posterior to its costal attachments. Inferiorly, the muscle blends toward the thoracolumbar fascia and approaches the iliac crest region without inserting on the pelvis. Bony landmarks include the scapulae superiorly, the vertebral column centrally, and the posterior rib cage framing the muscle’s distal slips. Posterior dissection teaching often skips this layer or conflates it with thoracolumbar fascia, yet its relationship to the lower ribs makes it a helpful landmark when explaining rib mechanics and the transition from thoracic to abdominal wall. Clinically, pain over the posterolateral lower ribs is frequently attributed to intercostal strain or costovertebral joint irritation, and this view helps distinguish a muscular source from deeper pleural or renal causes by localizing the serratus posterior inferior to ribs 9 to 12 and the paraspinal gutter. It also clarifies why the muscle is encountered when reflecting latissimus dorsi during posterior approaches and flap planning. Use this artwork in gross anatomy labs to orient learners during back dissections, in respiratory biomechanics lectures to map muscle layers over the thorax, and in surgical education materials discussing posterior thoracolumbar exposure and donor-site anatomy for latissimus dorsi flaps. Suitable for atlas plates and board-style review figures where posterior rib numbering and muscle layering must read clearly. Anatomical accuracy verified by SciePro's Medical Advisory Board.