Rectus sheath is a long fibrous sheath in the center of the anterior abdominal wall. It encloses the rectus abdominis and pyramidalis muscles and contains the anterior rami of the lower six thoracic nerves, superior and inferior epigastric vessels and lymph vessels. It is formed by the aponeuroses of the three lateral abdominal muscles namely external oblique, internal oblique and transversus abdominis.
For description of rectus sheath, it is considered at three different levels, presenting different features at each of them.
1) Rectus sheath above the costal margin:
Above the costal margin, the anterior wall is formed by the aponeurosis of external oblique. The posterior wall at this level is formed by the thoracic wall, containing the fifth, sixth and seventh costal cartilages and the respective intercostal spaces.
2) Rectus sheath between the costal margin and the level of anterior superior iliac spine:
At this level, the aponeurosis of internal oblique splits to enclose the rectus abdominis muscle. The aponeurosis of external oblique is directed in front of it while the aponeurosis of transversus abdominis is directed behind it.
3) Rectus sheath between the level of anterior superior iliac spine and pubis:
At this level the aponeuroses of all three muscles form the anterior wall of the rectus sheath. The posterior wall is absent and consequently the rectus abdominis lies in contact with the fascia transversalis.
Where the aponeuroses forming the posterior wall of the rectus sheath pass to the front, at the level of anterior superior iliac spine, the posterior wall has a free border known as the arcuate line. It is here that the inferior epigastric vessels enter the rectus sheath and pass upward to anastomose with the superior epigastric vessels.
The rectus sheath of one side is separated from its fellow on the opposite side by a fibrous band called the linea alba. It extends from the xiphoid process to the symphysis pubis. It is formed by the fusion of the aponeuroses of muscles of two sides. Above the level of umbilicus it is wider, but below it narrows down to be attached to the symphysis pubis.