Arcuate line

The arcuate line (linea semicircularis) of the sheath of the rectus abdominis is the curved line that marks the lower limit of the posterior layer of the rectus sheath.

The arcuate line is located on the inner surface of the abdominal wall, typically about one-third of the distance from the umbilicus to the pubic symphysis. Above this line, the rectus abdominis muscle is enclosed in a sheath formed by the aponeuroses of the external oblique, internal oblique, and transversus abdominis muscles (anterior and posterior layers). Below the arcuate line, all three aponeuroses pass anterior to the muscle, and only the transversalis fascia lies posterior to it.

While most detailed references to the arcuate line are in human anatomy, domestic animals such as dogs and horses also possess a similar structural transition in the abdominal wall, although its location and prominence can vary between species