At the lateral border of the rectus abdominis muscle, the aponeurosis of the internal oblique muscle splits into an anterior layer and a posterior layer (this splitting forms a shallow groove - the semilunar line).[3]
An anterior rectus sheath composed of the aponeurosis of the external oblique muscle and anterior portion of the aponeurosis of the internal oblique muscle passes in front of the rectus abdominis muscle.[2][3]
A posterior rectus sheath composed of the posterior portion of the aponeurosis of the internal oblique muscle and the aponeurosis of the transversus abdominis passes behind the rectus abdominis muscle.[2][3]
All aponeuroses of the rectus sheath unite at (and and decussate across) the midline, forming the linea alba.[3]
Below this level, the aponeuroses of all three muscles (including the transversus) pass in front of the rectus.[2][3] The posterior layer of the rectus sheath is thus absent and the rectus abdominis muscle is separated from the peritoneum only by the transversalis fascia.[2] Due to this reason, this region is more susceptible to herniation.[citation needed]
Superficial/anterior to the anterior layer of the rectus sheath are the following two layers:[4]
Since the tendons of the internal oblique and transversus abdominis 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 only by the tendons of the external obliques.[citation needed]
^Te Linde, Richard W. (1977), Rock, John A.; Jones Howard W. (eds.), Te Linde's Operative Gynecology(PDF) (10th ed.), Philadelphia, PA: Lippincott (published 2003), p. 107, retrieved 2018-10-01.
