The Rectus sheath is formed by the aponeuroses of the Obliqui

Obliqui

Obliqui may refer to:* Abdominal internal oblique muscle* Abdominal external oblique muscle...

 and Transversus

Transversus abdominis muscle

The transversus abdominis muscle, also known as the transverse abdominus, transversalis muscle and transverse abdominal muscle, is a muscle layer of the anterior and lateral abdominal wall which is deep to the internal oblique muscle...

. It contains the Rectus abdominis

Rectus abdominis muscle

The rectus abdominis muscle, also known as the "six pack", is a paired muscle running vertically on each side of the anterior wall of the human abdomen . There are two parallel muscles, separated by a midline band of connective tissue called the linea alba...

 and Pyramidalis

Pyramidalis muscle

The pyramidalis is a small and triangular muscle, anterior to the Rectus abdominis, and contained in the rectus sheath.-Attachments and actions:...

 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 two layers:

1. Camper's fascia (anterior part of the Superficial fascia)
2. Scarpa's fascia (posterior part of the Superficial fascia)

Within the sheath, the layers vary:

Region

Illustration

Description >- | Above the arcuate line

one of which passes in front of the Rectus, blending with the aponeurosis of the Obliquus externus. the other, behind it, blending with the aponeurosis of the Transversus, and these, joining again at the medial border of the Rectus, are inserted into the linea alba Linea alba The linea alba is a fibrous structure that runs down the midline of the abdomen in humans and other vertebrates. The name means white line and the linea alba is indeed white, being composed mostly of collagen connective tissue.... . >- | Below the arcuate line

Below this level, the aponeuroses of all three muscles (including the internus) pass in front of the Rectus.

Below the sheath are the following three layers:

1. transversalis fascia
   Transversalis fascia
   The transversalis fascia is a thin aponeurotic membrane which lies between the inner surface of the Transversus abdominis and the extraperitoneal fascia....
   
   
2. extraperitoneal fat
   Extraperitoneal fat
   Between the inner surface of the general layer of the fascia which lines the interior of the abdominal and pelvic cavities, and the peritoneum, there is a considerable amount of connective tissue, termed the extraperitoneal fat or subperitoneal connective tissue.-Parietal portion:The parietal...
   
   
3. parietal peritoneum

The Rectus, in the situation where its sheath is deficient below, is separated from the peritoneum

Peritoneum

The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

 only by the transversalis fascia

Transversalis fascia

The transversalis fascia is a thin aponeurotic membrane which lies between the inner surface of the Transversus abdominis and the extraperitoneal 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

Costal margin

The costal margin, sometimes referred to as the costal arch, is the medial margin formed by the false ribs and one true rib -- specifically, from the seventh rib to the tenth rib....

, 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.

External links

- "Incisions and the contents of the rectus sheath." - "Anterior Abdominal Wall: The Rectus Abdominis Muscle" - anterior layer - posterior layer above arcuate line - posterior layer above arcuate line - "The Rectus Sheath, Anterior View & Transverse Section"