 The anterior layer is derived from the external oblique aponeurosis and the anterior lamina of the internal oblique aponeurosis.The posterior layer is made up of the posterior lamina of the internal oblique aponeurosis and the transversus abdominis aponeurosis.Inferior to the arcuate line, all three muscle aponeuroses make up the rectus sheath, that is now only anterior to the rectus abdominis and not posterior to it at all.