Stretching from the ribs on each side, down to the margins of the pelvis, the abdominal contents are protected and retained in place by a dense layer of fibrous tissue, the abdominal fascia, which is effectively also a broad tendon, called aponeurosis. Into this fascia, and forming the aponeurosis, are inserted several muscles, but the specific names of the muscles should not detract from the importance of the fascia.
Visually prominent, and known to the body conscious public as a “6-pack” are the longitudinal centrally placed rectus abdominis muscles. These muscles are attached to the crest of the pubis inferiorly and to the 5th, 6th, & 7th costal cartilages above; they are encased in an envelope of deep fascia or aponeurosis (deficient posteriorly in the lower abdomen). The space between them is formed by the fascia and termed linea alba; there are transverse tendinous intersections which divide the muscle apparently into its 6 components.
The external oblique muscle has its upper bony attachment to ribs 4-12, its multiple slips fuse into a single sheet of muscle as they sweep downwards and forward to fuse into the aponeurosis that forms the sheath around rectus abdominis; the lowermost muscle fibers insert into the iliac crest of the pelvis and along its upper margin at the pubis.
The internal oblique muscle lies deep to the external oblique; its fibers run in what seems to be the opposite direction, from the lowest three ribs and crest of the pelvis, upwards and inwards to form part of the aponeurosis around the rectus muscle.
The transversus abdominis is attached posteriorly to the dense lumbar fascia, the muscle fibers run transversely around the side of the abdomen to join the aponeurosis that forms the posterior aspect of the rectus sheath.