A spigelian hernia passes through a defect where the linea semi lunaris and linea semicircularis join. These form 2% of anterior abdominal hernias(1).

Figure 1 (above): A spigelian hernia. Note that the intact external oblique aponeurosis can be seen (arrowhead).

1. Dahnert W. Radiology Review Manual. 6th ed. Lippincott Williams & Wilkins,US; 2007.