The rectus sheath is composed of the aponeuroses of transversus abdominis, external oblique and internal oblique muscles, which form anterior and posterior layers of the sheath that fuse laterally at the linea semilunaris and in the midline at the linea alba.
Only the middle segment of the rectus abdominis is completely enclosed. The anterior layer passes anterior to the rectus abdominis and is formed by the fusion of the external oblique and anterior layer of the internal oblique aponeuroses. The posterior layer passes posterior to the rectus abdominis and is formed by the fusion of the posterior layer of the internal oblique and transversus aponeuroses. The posterior sheath is lacking in parts of the above and below this middle segment:
- superior to the costal margin the aponeuroses are deficient because they either do not extend that far superiorly (internal and external oblique muscles) or attach to the costal margin (transversus abdominis)
- inferior to the arcuate line, the internal oblique aponeurosis passes anterior to the rectus abdominis and since the other two aponeurosis are fused to it, the posterior surface of rectus abdominis is in contact with the transversalis fascia
- rectus abdominis and pyramidalis muscles
- lower 6 thoracic nerves and accompanying branches of the posterior intercostal vessels
- superior and inferior epigastric vessels