Anterior Layer of Rectus Sheath
The Rectus Sheath contains the rectus abdominis and pyramidalis muscles, the lower six thoracoabdominal intercostal nerves and their accompanying posterior intercostal vessels, and the superior and inferior epigastric arteries. The rectus sheath is composed of contributions of the aponeuroses of the lateral abdominal wall muscles. The rectus sheath has anterior and posterior layers (laminae). Above the costal margin only the external abdominal oblique and its aponeurosis contribute to the rectus sheath (anterior layer). Between the linea semicircularis, umbilicus, and the costal margin the aponeurosis of the internal oblique splits around the rectus abdominis. In this area the external and internal abdominal oblique aponeuroses form the anterior layer and the internal abdominal oblique and transversus aponeuroses form the posterior layer. Below the linea semicircularis (arcuate) all three aponeuroses pass in front of the rectus abdominis muscle.