We’ll look at the flat muscles next. Flat may not be the best word to decribe them - in the vertical plane, they’re markedly curved, as we’ll see. We’ll look at all three of them, then we’ll look at their actions.
We’ll start with the innermost of the three flat muscles, transversus abdominis. Here’s transversus abdominis. The fibers of transversus all run in the same transverse direction, except the lowest ones, which run obliquely downwards.
Transversus abdominis has a long line of origin, from here, to here. At the top, its fibers arise from the inner aspect of the costal margin, from the sixth rib, to the twelfth. Between the twelfth rib and the ilium, transversus arises from the edge of the thoraco-lumbar fascia. Below, it arises from the inner aspect of the iliac crest.
The lowest fibers of transversus arise from a thickening of the ilio-psoas fascia. Transversus has a short free lower border.
The muscle fibers of transversus end in this broad sheet of tendon, the transversus aponeurosis. The transversus aponeurosis fuses here with the aponeurosis of the overlying internal oblique muscle, to form one aponeurotic layer.
Now we’ll add the internal oblique muscle to the picture. The internal oblique arises from the thoraco-lumbar fascia, and from the iliac crest. The lowest fibers of the internal oblique arise, like those of transversus, from a thickening of the ilio-psoas fascia. Like transversus, the internal oblique has a short free lower border.
The internal oblique fans out, so that its highest and lowest fibers run in markedly different directions. Back here, the fibers of the internal oblique run steeply upward. Here they run less steeply, here they’re transverse, and here towards the inguinal region they run downward.
The highest fibers of the internal oblique insert on the lowest three ribs. Its remaining fibers end in this internal oblique aponeurosis, which, as we noted, fuses on the underside with the transversus aponeurosis. It’s also joined on its outer aspect by the external oblique aponeurosis - this is the cut edge of external oblique.
From here to here, the combined aponeurotic layer divides at the edge of the rectus into two layers, one passing behind, and one in front of the rectus. Below here, the combined layer doesn’t divide, but passes entirely in front of the muscle.