The origin of this muscle is inferiorly and therefore the fibres run superiorly and anteriorly in contrast to those of external oblique. The origin is from the anterior aspect of the iliac crest, lateral aspect of the inguinal ligament and thoracolumbar fascia. The insertion is at the linea alba, the pubis and the lower three ribs.

Axial CT image demonstrating the internal oblique muscle (arrow) and its aponeurosis (arrowhead) which splits to cover rectus abdominis anteriorly and posteriorly.