Sheath of rectus abdominis Posterior layer Above arcuate line 9 Formed by fusion of post. leaf of aponeurosis of obliquus internus abdominis and aponeurosis of transversus abdominis Post layer of Absent in about 4-5cm below the the sheath umbilicus, where aponeuroses of all three muscles form ant layer the lower free border named arcuate line Below this line rectus abdominis Below arcuate line in contact with transverse fasciaPosterior layer  Formed by fusion of post. leaf of aponeurosis of obliquus internus abdominis and aponeurosis of transversus abdominis  Absent in about 4-5cm below the umbilicus, where aponeuroses of all three muscles form ant. layer the lower free border named arcuate line  Below this line rectus abdominis in contact with transverse fascia Sheath of rectus abdominis Above arcuate line Below arcuate line Post layer of the sheath