The rectus sheath is formed by the aponeuroses of the three lateral abdominal wall muscles (external oblique, internal oblique, transverse abdominis) enclosing the rectus abdominis, with its layered composition differing above versus below the umbilicus. Above the umbilicus, the internal oblique aponeurosis splits into anterior and posterior laminae, so the rectus abdominis is sandwiched between one and a half layers anteriorly (external oblique plus the anterior lamina of internal oblique) and one and a half layers posteriorly (posterior lamina of internal oblique plus transverse abdominis, followed by transversalis fascia and peritoneum); below the umbilicus, all three aponeuroses pass anterior to the rectus abdominis, leaving only transversalis fascia and peritoneum posteriorly. This regional variation in aponeurotic arrangement is a key structural principle in abdominal wall anatomy, with the aponeuroses converging in the midline to form the linea alba.
The rectus sheath is formed by the aponeuroses of the three lateral abdominal wall muscles (external oblique, internal oblique, transverse abdominis) enclosing the rectus abdominis, with its layered …