The abdominal wall consists partly of bone but mainly of muscle (Fig. 4.5). The skeletal elements of the wall (Fig. 4.5A) are: the five lumbar vertebrae and their intervening intervertebral discs, the superior expanded parts of the pelvic bones, and bony components of the inferior thoracic wall, including the costal margin, rib XII, the end of rib XI, and the xiphoid process.