Now that we’ve looked at the internal oblique, it’s time to add the external oblique muscle to the picture. Here’s the external oblique muscle, the outermost of the three flat muscles. The fibers of the external oblique spiral downwards and forwards at the side, downwards and medially in front.
The external oblique arises from a broad area on the outside of the rib cage, all the way from here on the twelfth rib, to here on the fifth rib. The zig-zag line of origin of the external oblique fits in with the line of origin of serratus anterior. Though it’s all one continuous muscle, we’ll look at the external oblique in two parts, a posterior part that arises from the twelfth to the tenth ribs, and an anterior part that arises from the ninth to the sixth rib.
The anterior part of the external oblique ends in this external oblique aponeurosis. This fuses with the combined aponeuroses of the other two flat muscles, to form the anterior rectus sheath.
The external oblique aponeurosis has a long free lower border between the anterior superior iliac spine, and the pubic tubercle. As we’ve seen already, this free lower border is the inguinal ligament, which we'll see in detail shortly.
The part of the external oblique that arises from the tenth to the twelfth ribs remains fleshy from its origin to its insertion. It inserts along the outer edge of the anterior half of the iliac crest. Here at the back, the external oblique has a short free border, between the twelfth rib and the iliac crest.
Now that we’ve looked at all three of the flat muscles, we’ll look at their actions. When the three flat muscles on both sides contract together, as they usually do, they raise the pressure inside the abdominal cavity.
When our airway is open the rise in intra-abdominal pressure pushes the diaphragm upwards, causing air to leave the lungs, as we saw in the last section. When we hold our breath by closing our larynx, the flat muscles provide the pressure that’s needed to expel the contents of either the rectum, the bladder, or the uterus, through their respective openings.
When the oblique muscles contract individually, they also play a part in producing lateral flexion of the lumbar spine, and rotation of the thoracic spine.