The arcuate line of the abdomen (or linea semicircularis) is a horizontal line that demarcates the lower limit of the posterior layer of the rectus sheath. It is also where the inferior epigastric vessels perforates the rectus abdominus.
Superior to the arcuate line, the internal oblique aponeurosis splits to envelope the rectus abdominis muscle. Inferior to the arcuate line, the internal oblique and transversus abdominis aponeuroses merge and pass superficial to the rectus muscle.
The arcuate line occurs about 1/3 of the distance from the umbilicus to the pubic crest, but this varies from person to person. Above the arcuate line, the rectus abdominis is surrounded by an anterior layer of the rectus sheath and a posterior layer. The anterior layer is derived from the external oblique aponeurosis and the anterior laminina of the internal oblique aponeurosis. The posterior layer is made up of the posterior lamina of the internal oblique aponeurosis and the tranverse abdominal 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.
Therefore, inferior to the arcuate line, the rectus abdominis rests directly on the transversalis fascia.