rectus sheath

1. ARCUATE LINE OF RECTUS SHEATH

2. The arcuate line of the abdomen, linea semicircularis
or Douglas' line 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 perforate the rectus
abdominis.
ARCUATE LINE

3. Superior to the arcuate line, the internal oblique
aponeurosis splits to envelop the rectus abdominis
muscle both anteriorly and posteriorly.
Inferior to the arcuate line, the internal oblique and
transversus abdominis aponeuroses merge and pass
superficial (i.e. anteriorly) to the rectus muscle.

4. The arcuate line occurs about 1/2 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
layer of the rectus sheath and a posterior layer.
The anterior layer is derived from the external oblique aponeurosis and
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.
Therefore, inferior to the arcuate line, the rectus abdominis rests directly
on the transversalis fascia.