Posterior wall of abdomen proper. Cross‐section of the lumbar region of abdomen illustrating the posterior wall of abdomen proper (PWAP)   consisting of the quadratus lumborum [QL], psoas muscles (only the psoas major [PS] shown in this cross section) and the body and transverse process of the lumbar vertebra. The posterior boundary of the PWAP is constituted by the middle layer of the thoracolumbar fascia and the lateral raphe, separating it from the lumbar region of back and its constitutional parts including the erector spinae. The mid‐axillary line separates the PWAP from the anterior and lateral abdominal wall. The cross section is from the Visible Human Project (slice afv1588c, courtesy of the U.S. National Library of Medicine)