The rectus sheath is made up of two parts, known as the posterior sheath and the anterior sheath. These sheaths are made of fibers of the transversus abdominis, internal abdominal oblique (IAO), and external abdominal oblique (EAO), which are muscles of the abdomen. These muscles come together at the linea alba, which is a tendon-like tissue that runs down the middle of the abdomen. The compositions of the posterior sheath and the anterior sheath differ from one abdominal wall to another.
The anterior sheath is made of the fibers of the EAO that lie over the area where the rib cage ends. The posterior sheath consists of the fibers of the IAO and the transversus abdominis.
The posterior sheath is absent lower down the abdomen, but the anterior sheath is present here as a combination of fibers from all three muscles of the abdomen.
Where the anterior rectus sheath is closest to the sides of the body, the fibers of the IAO segregate into two thin layers of tissue, called lamellae. The one on the front goes to the fibers of the external oblique. The one in back goes to the fibers of the transversus, finally joining with the fibers of the EAO.