Final answer:
The rectus sheath below the arcuate line is characterized by the absence of aponeurosis in the back of the rectus muscle, leaving the transversal fascia as the only posterior layer.
Step-by-step explanation:
The characteristic feature of the rectus sheath below the arcuate line is the absence of aponeurosis in the back of the rectus muscle. This anatomical detail is significant because it pertains to the structure of the abdominal wall, specifically how the aponeuroses of the abdominal muscles contribute to the formation of the rectus sheath.
Above the arcuate line, the rectus sheath has contributions from the aponeuroses of the transversus abdominis, internal oblique, and external oblique, which split to enclose the rectus abdominis muscle. However, below the arcuate line, all the aponeurotic layers pass anterior (in front) to the rectus muscle, leaving only the transversalis fascia on its posterior side. Without the aponeurotic reinforcements, the lower part of the rectus sheath is thinner and has a different composition compared to above the arcuate line.
This anatomical configuration has clinical implications, especially in the context of herniation or surgical procedures involving the lower abdomen. The rectus abdominis muscles are enclosed by the rectus sheath, which is an important structure during abdominal surgeries. It is essential for healthcare professionals to understand the anatomy of the rectus sheath as it relates to the positioning of the aponeuroses, as this can impact both the vulnerability of certain areas to herniation and the approach for abdominal incisions.