Now, with our focus still on muscles, we’ll move on to look at the four muscles which form the great expanse of the lateral and anterior abdominal wall.
These muscles fill in the huge gap that’s created by the costal margin above, the edge of the thoraco-lumbar fascia behind, and the iliac crest, the inguinal ligament, and the pubic crest below.
First we’ll look at the rectus abdominis, which runs vertically next to the midline, from the lower anterior ribs to the pubis; then we’ll look at the three large flat muscles, transversus abdominis, internal oblique, and external oblique.
There’s a time-honored word that’s used in describing the tendons of the flat muscles: aponeurosis. Aponeurosis is a word that’s used to describe a tendon that’s in the form of a sheet.
We’ll look at the rectus abdominis muscle first. Here’s the rectus abdominis, together with the innermost of the flat muscles, transversus. The rectus abdominis is a very long muscle, wide at the top, and tapering to a more narrow attachment at the bottom. It arises from the fifth, sixth and seventh costal cartilages. It’s inserted on the pubic crest.
The rectus is divided on the front by these bands of tendon, called tendinous intersections. Sometimes there are three of them, sometimes four, as in this case. The intersections go about halfway way through the muscle. The action of the rectus muscles is to produce flexion of the lumbar spine. The rectus muscles act in opposition to the erector spinae muscles.
Besides producing active flexion, the rectus muscles have an important static effect. They keep the the lumbar spine straight at times when the force of gravity tends to extend it.
In the intact body the rectus abdominis is enclosed on the front, and on the back, by a tendinous envelope, that’s formed by the aponeuroses of the three flat muscles. This is the posterior layer of the rectus sheath. The posterior layer ends here, about three-quarters of the way down the muscle. Sometimes it ends gradually, sometimes abruptly, with a distinct lower border known as the arcuate line. Below here there’s just a little loose fascia between the back of the rectus and the peritoneum.
Now we’ll add the anterior layer of the rectus sheath to the picture. The anterior layer extends all the way from the costal margin, to the pubis.
If we incise the anterior rectus sheath and try to retract it, we can see that it’s firmly attached to the tendinous intersections. There’s one here, another one here. Here we’ve divided those adhesions so that we can retract the anterior rectus sheath medially.
The two layers of the rectus sheath come together near the midline, here’s the posterior layer. Both layers insert into this dense mid-line band of tendinous tissue, the linea alba. The linea alba extends from the xiphoid process to the pubis.
We’ll see more of the rectus sheath, as we look at the aponeuroses of the three flat muscles.