Formation of the Walls Above the Arcuate Line
Throughout the entire extent from the costal margins down to a level approximately 4–5 centimeters below the umbilicus, the rectus sheath possesses well-developed, thick anterior and posterior walls. This transition border is called the arcuate line (linea arcuata).
Above the linea arcuata, the walls of the sheath are formed as follows:
- Anterior layer: Formed by the aponeurosis of the external oblique muscle (m. obliquus externus abdominis), tightly fused with the superficial lamina of the split internal oblique aponeurosis.
- Posterior layer: Formed by the deep lamina of the internal oblique aponeurosis, which fuses with the aponeurosis of the transversus abdominis muscle (m. transversus abdominis).
Deep to the muscle and the posterior aponeurotic layer lie the transversalis fascia (fascia transversalis) and parietal peritoneum (peritoneum parietale).
Structural Changes Below the Arcuate Line
At about 4–5 cm below the umbilicus (inferior to the linea arcuata), the anatomy of the tendinous sheath changes abruptly. In this region, the aponeuroses of all three lateral abdominal muscles (external oblique, internal oblique, and transversus abdominis) pass anterior to the rectus abdominis muscle, forming an exceptionally strong anterior wall.
As a result of this redistribution of tendinous fibers, no aponeurosis remains posterior to the rectus abdominis. In the lower abdomen, the posterior wall is represented exclusively by the thin transversalis fascia (fascia transversalis) and the parietal peritoneum. Because of this lack of a strong posterior tendinous backing, this region is a zone of relative anatomical weakness, predisposing to abdominal wall hernias.
Contents of the Rectus Sheath
The tendinous sheath serves as a compartment for several important anatomical structures. The primary contents filling this space are the rectus abdominis muscle (m. rectus abdominis).
Additionally, the rectus sheath contains:
- Pyramidalis muscle (m. pyramidalis) — a small muscle frequently present in the lower part of the sheath, inserting into the linea alba.
- Neurovascular bundles, specifically the epigastric vessels (vasa epigastrica), which supply blood to the tissues of this region.
Linea Alba and Linea Semilunaris
The aponeuroses of the lateral abdominal muscles enclosing the right and left rectus muscles interlace in the midline to form the linea alba.
The main features of the linea alba include:
- It is a dense fibrous band stretching from the xiphoid process of the sternum to the pubic symphysis.
- Its width is about 1–2 cm. Near the umbilicus, the linea alba becomes thinner and wider, containing small cleft-like gaps for the passage of vessels and nerves.
- On the surface of the anterior abdominal wall, it is visible as a longitudinal groove.
- The umbilicus (umbilicus), a retracted scar, is located along the linea alba.
- Clinically, this is a common site for rectus muscle separation (diastasis) and serves as the standard site for midline surgical incisions.
The lateral border of the rectus sheath forms the semilunar line (linea semilunaris), also known as the Spiegel line. This zone is also a potential weak spot vulnerable to Spigelian hernias.