Rectus sheath

The rectus sheath is a strong fibrous sleeve of the anterior abdominal wall that encloses the Rectus abdominis muscle and, when present, the Pyramidalis muscle as well.

The rectus sheath is formed by the aponeuroses of the three flat abdominal muscles the external oblique, internal oblique, and transversus abdominis. The arrangement of these aponeuroses changes at different levels of the abdomen, creating three distinct patterns of formation that are clinically and anatomically important.

Above the costal margin

Above the costal margin, the rectus sheath has only an anterior wall because the posterior aspect of the rectus abdominis lies directly on the costal cartilages and intercostal muscles. At this level, the anterior wall is formed mainly by the aponeurosis of the external oblique muscle. Since no posterior sheath exists here, the muscle is in direct contact with the thoracic wall structures posteriorly.

Between the costal margin and the arcuate line

Between the costal margin and the arcuate line, the rectus sheath has both anterior and posterior walls. This is the most typical arrangement.

• The external oblique aponeurosis passes entirely anterior to the rectus abdominis.

• The internal oblique aponeurosis splits into two layers: an anterior lamina and a posterior lamina.

• The anterior lamina joins the external oblique aponeurosis to form the anterior wall of the rectus sheath, while the posterior lamina joins the transversus abdominis aponeurosis to form the posterior wall.

Thus, at this level, the rectus abdominis is enclosed completely within the rectus sheath.

Below the arcuate line

Below the arcuate line, the arrangement changes significantly. At this level, all three aponeuroses external oblique, internal oblique, and transversus abdominis pass anterior to the rectus abdominis muscle. Consequently, the posterior wall of the rectus sheath is absent below the arcuate line, and the muscle lies directly on the transversalis fascia. This area is relatively weaker and therefore clinically important because it is more susceptible to herniation.

Arcuate line

The arcuate line itself is located approximately midway between the umbilicus and the pubic symphysis and marks the lower limit of the posterior rectus sheath. Within the sheath run the superior and inferior epigastric vessels and the thoracoabdominal nerves.

Functions

Functionally, the rectus sheath provides support and protection to the rectus abdominis muscle, helps maintain abdominal wall strength, and assists in transmitting the forces generated by the abdominal muscles during movements and increases in intraabdominal pressure.