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Medical Definition of Rectus sheath
1. 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. See: aponeurosis of external abdominal oblique muscle, aponeurosis of internal abdominal oblique muscle. Synonym: vagina musculi recti abdominis. (05 Mar 2000)