The rectus abdominis runs on both sides of the midline from the rib cage to the pubis.
The three flat abdominal muscles are the external oblique, internal oblique and the transverse abdominis. Mouse over them on the cross sectional image . On the dissection image mouse over the anterior layer of the rectus sheath. This is the aponeuroses of the external (and internal oblique). Mouse over the midline to identify the linea alba.
If we remove the skeletal system and take a posterior view of the abdominal muscles, we can see the posterior rectus sheath. Mouse over the superior part of the rectus to identify the aponeuroses of the transverse abdominis (and internal oblique). Mouse over the inferior part of the rectus to confirm that the sheath is absent. The transition occurs at the linea alba.