How do the surgical approaches to the abdomen sometimes necessitate a midline incision between the two rectus sheaths?

Linea alba-the linea alba is an aponeurotic band on the midline of the anterior abdominal wall, which extends from the xiphoid process to the pubic symphysis. it is formed by the combined abdominal muscle aponeuroses. because there are no major arteries or nerves running in the linea alba, it provides a useful site for a midline incision in the abdomen. the linea aspera is a vertical ridge on posterior surface of the femur. the arcuate line is the point at which the posterior lamina of the rectus sheath ends, and transversalis fascia lines the inner surface of rectus abdominis. the semilunar line is the lateral margin of the rectus abdominus, formed by the fused aponeuroses of the abdominal wall muscles. the iliopectineal line is a line on the pelvic bones, formed by the arcuate line of the ilium and the pectineal line of the pubis. (note-the arcuate line of the ilium is totally different than the arcuate line of the rectus sheath!) this line is important because it marks the boundary between the abdominal cavity and the pelvic cavity.