Rectus sheath

The Rectus sheath is formed by the aponeuroses of the Obliqui and Transversus. It contains the Rectus abdominis and Pyramidalis muscles.

It can be divided into anterior and posterior laminae.

The arrangement of the layers has important variations at different locations in the body.

Below the costal margin
For context, above the sheath are the following three layers:


 * 1) superficial fascia
 * 2) Camper's fascia
 * 3) Scarpa's fascia

Within the sheath, the layers vary:

Below the sheath are the following three layers:
 * 1) transversalis fascia
 * 2) extraperitoneal fat
 * 3) parietal peritoneum

The Rectus, in the situation where its sheath is deficient below, is separated from the peritoneum only by the transversalis fascia, in contrast to the upper layers, where part of the internal oblique also runs beneath the rectus. Because of the thinner layers below, this region is more susceptible to herniation.

Above the costal margin
Since the tendons of the Obliquus internus and Transversus only reach as high as the costal margin, it follows that above this level the sheath of the Rectus is deficient behind, the muscle resting directly on the cartilages of the ribs, and being covered merely by the tendon of the Obliquus externus.