[TA] sheath of the rectus abdominis, formed by the aponeuroses of the three anterolateral muscles of the abdominal wall that split to enclose the rectus and fuse medially to form the linea alba; it consists of an anterior lamina and a posterior lamina, the latter being absent below the arcuate line.aponeurosis of external oblique (muscle), aponeurosis of internal oblique muscle. SYN: vagina musculi recti abdominis [TA].