This surrounds the rectus abdominis muscle on either side which arises from the pubic symphysis and pubic crest and inserts at the xiphoid process and 5th, 6th and 7th costal cartilages. Also, within the sheath are, amongst other vessels, the inferior epigastric vessels. The anterior margin of the sheath is formed by the external oblique aponeurosis and  the anterior part of the internal oblique aponeurosis. The posterior margin is formed from the posterior part of the internal oblique aponeurosis and the transversus abdominis aponeurosis. The posterior layer ends at the arcuate line or linea semicircularis equidistant from the pubic symphysis and umbilicus.  Additionally, the sheath ends superiorly at the superior insertions of the contributing muscles. The linea semilunaris indicates the most lateral extent of the rectus abdominis.

A midline transverse section US view of the linea alba (arrow) and the rectus abdominis muscles on either side (RA), the anterior (black arrowhead) and posterior (white arrowhead) components of the rectus sheath are shown