JODDYS

the muscles of the anterolateral abdominal wall (external oblique, internal oblique, transverse abdominis and rectus abdominis muscles).

The Rectus Sheath

The rectus sheath is the strong, incomplete fibrous compartment for the rectus abdominis muscle.

It forms by the fusion and separation of the aponeurosis of the flat abdominal muscles.

At its lateral margin, the internal oblique aponeurosis splits into two layers, one passing anterior to the rectus muscle and the other passing posterior to it.

The anterior layer joints with the aponeurosis of the external oblique to form the anterior wall of the rectus sheath.

The posterior layer joins with the aponeurosis of the transversus abdominis muscle to form the posterior wall of the rectus sheath.

The fibres of the anterior and posterior wall of the rectus sheath interlace in the medial line to form a complex tendinous raphe, called the linea alba, which is an intermixture of the aponeurotic fibres of the oblique and transverse abdominal muscles.

It is narrow inferior to the umbilicus, but is wide superior to it. The groove is visible in the skin superficial to it in thin muscular persons.

The linea alba lies between the two parts of the rectus abdominis muscle; the umbilicus is located just inferior to its midpoint.

Superior to the costal margin, the posterior wall of the rectus sheath is deficient because the transversus abdominis muscle passes internal to the costal cartilages and the internal oblique muscle is attached to the costal margin. Hence, superior to the costal margin, the rectus muscle lies directly on the thoracic wall.

The inferior 1/4 of the rectus sheath is also deficient because the internal oblique aponeurosis does not split to enclose the rectus muscle.

A crescentic border called the arcuate line marks the inferior limit of the posterior wall of the rectus sheath.

The position of this line is usually midway between the umbilicus and the pubic crest.

Inferior to the arcuate line, the aponeuroses of the three flat muscles pass anterior to the rectus muscle to form the anterior layer of the rectus sheath.

"Actually, all the lateral abdominal muscles have bilaminar aponeuroses which blend and cross each other in a complex fashion."