Okay, this is a tutorial on the muscles of the abdominal wall. I’m going to do this in two parts. The first part will be on the muscles of the anterior abdominal wall and the second part will cover the muscles of the posterior abdominal wall.
The muscles of the abdomen have several different functions. They protect the viscera, which are the soften organs contained in the abdominal cavity. They assist in breathing. In inspiration, the abdominal muscles relax and allow the thoracic cavity to expand downwards. And in expiration, these muscles can contract and push the viscera upwards. And also, contraction of these muscles is useful in coughing and throwing up.
Another function is to assist in defecation, urination and giving birth. These are assisted by increasing the intra-abdominal pressure. The pressure inside the abdominal cavities increase by a contraction of these muscles and this can help force things out of the body like urine, feces and babies.
In the anterior abdominal wall, you’ve got five muscles. These are all innervated by the anterior rami of spinal nerves T7-L1. You’ve got the rectus abdominis, the pyramidalis, and the external oblique, internal oblique and transversus abdominis. I’ll just take you through these muscles now.
First, we’ve got this muscle here lying right in the midline. It’s a paired muscle, which are separated by this white line running down the middle. This white line is called the linea alba and this literally means ‘white line’ in Latin. This linea alba is formed by the aponeurosis of the various abdominal muscles. the lateral three muscles (which I’ll come on to talk about), the external and internal obliques and the transversus abdominis form this flat tendinous sheath called aponeurosis which join together in the midline forming this linea alba. This separates the two sides of the rectus abdominis muscle.
I’ve just isolated the muscle and we’ll take a look at the origin and the insertion. The rectus abdominis muscle originates on the pubic symphysis and pubic tubercle. Superiorly, it inserts along the costal margin (so the costal cartilages) and also on the xiphoid process. You can see the xiphoid process here.
What this muscle does is it can flex the vertebral column and it’s also involved in the other functions I mentioned at the start of this tutorial.
A couple of other things to just point out. You can see another white line which lies on the edge of the rectus abdominis muscle on either side. This is called the linea semilunaris. And then you can see these horizontal intersections which separate up the muscle bellies of the rectus abdominis. These are called tendinous intersections. This can be seen on people who have low body fat and a lot of muscle mass in the rectus abdominis.
You can see in this picture the sort of surface anatomy. You can see the linea alba down the midline and you can visualize the tendinous intersections between the muscle bellies of the rectus abdominis.
The second muscle which lies anteriorly in the abdominal wall is called the pyramidalis muscle. This is a tiny little muscle and it isn’t always present. It attaches to the pubis inferiorly and in the midline, it attaches to the linea alba. This muscle is innervated by the anterior ramus of T12, spinal nerve T12.
I’ve just switched over a diagram to show you this. What we’re looking at is the left side. This left side has been dissected away and you can see one-half of the rectus abdominis muscle (so the right side of the rectus abdominis muscle. Just at the bottom – so this is the pubis here and in the midline, you’ve got the linea alba – the pyramidalis is this tiny little muscle here, which attaches from the pubis to the linea alba.
These two muscles, the rectus abdominis and the pyramidalis muscle are actually enclosed in something called the rectus sheath. This is a tendinous sheath formed by the aponeurosis of the three lateral muscles, which I’ll come on to talk about next.
The upper three-quarters of the rectus abdominis – so this area that I’m showing you here. The upper three-quarters are fully enclosed by this rectus sheath, whereas the bottom quarter is actually – so only the anterior surface is covered by the rectus sheath.
What I’m going to do now is I’m going to switch over to a cross-section to show you this rectus sheath.
We’re now looking at a cross-section. This is anterior. You’ve got the rectus muscle (both half of the rectus muscle) and in the middle, you’ve got the linea alba. And then laterally, you’ve got the three muscles, lateral muscles, which I haven’t talked to you about yet. But just briefly, you’ve got the outer muscle called the external oblique. The muscle below that is called the internal oblique. And the innermost muscle is called the transversus abdominis.
As you can see, these muscles form this flat tendon, which is known as an aponeurosis. The aponeurosis of these three muscles enclose the rectus abdominis muscle in what is called the rectus sheath. These aponeurosis meet at the midline to form the linea alba.
This is a cross-section taken from the upper three-quarters. You can see how both surfaces of the rectus abdominis are enclosed. The anterior surface is covered; and so is the posterior surface. You can see how the internal oblique splits. It passes anteriorly and posteriorly behind the rectus abdominis.
This next picture I’m showing you is from the lower quarter, so the bottom quarter of the rectus abdominis muscle. You can see that this sheath is slightly different because it doesn’t go behind the rectus abdominis muscle. All three aponeurosis pass in front along the anterior surface of the rectus abdominis muscle.
Next we’ve got the muscles which sit laterally. You know what these are. There are three muscles – the external oblique, internal oblique and transversus abdominis (from superficial to deep).
First is this muscle here, the external oblique. This muscle originates on ribs 5-12, which you can see here. It inserts inferiorly on the iliac crest, the pubic crest and pubic tubercle in this area here. It inserts on the midline on the linea alba and also on the xiphoid process up here. You can see how this muscle forms this flat tendon, which is called an aponeurosis. And you saw how this aponeurosis surrounded the rectus abdominis muscles to form part of the rectus sheath.
The external oblique on either side joins together in the midline, fuses to form the linea alba, which I showed you earlier.
If both muscles contract, it can flex the trunk. But if one side contracts, it can laterally flex the trunk. It can flex the trunk to the same side.
The fibers of the external oblique pass inferomedially. They pass downwards and towards the midline in this direction.
If we just remove the external oblique, we’ve got the internal oblique which lies below it. These fibers pass in the other direction. They pass superiorly and medially.
One way of remembering the direction that the fibers are oriented is to use the mnemonic ‘hands in pockets, hands on tits’. How does this help you to remember?
If your hands are in your pockets, imagine your hands kind of like this. Pockets has the letters ‘e’ in it for external oblique. The direction your forearm is the direction of fibers, so in this direction. For the external oblique, you’ve got your hands in pockets, pockets has an ‘e’, external oblique. The fibers are in the inferomedial direction.
Hands on tits – sorry for being crude, but that’s the mnemonic – ‘hands on tits’, they’re up in that direction, so the fibers pass superiorly and medially. ‘Tits’ has an ‘i’ in it, so ‘internal’ oblique. The fibers are superomedial.
Just going back to the external oblique muscle, there’s one important thing I forgot to tell you about. The bottom part, the inferior margin of the aponeurosis of the external oblique muscle actually forms the inguinal ligament.
I’ve just isolated it here and you can see the bottom part of the aponeurosis forming this ligament, which runs from the anterosuperior iliac spine here down to the pubic tubercle. That’s important to remember.
Just going back to the internal oblique (and you now know which direction the fibers pass), the internal oblique muscle originates on the iliac crest and also on the lateral part of the inguinal ligament. And it inserts on the lower ribs. You can see these ribs, 10-12 (or bottom four ribs, 9-12) and it also inserts in the midline in the linea alba. It joins the other aponeurosis as you know. And it also inserts in the pubic crest here.
The aponeurosis isn’t shown here, but you know that it extends into the midline and joins the linea alba.
Similar to the external oblique, when both of the internal oblique muscles contract, it flexes the trunk. It brings the truck like this. And if one side, one muscle contracts, it laterally flexes the trunk. It brings the trunk to the same side.
The last muscle we’ve got is the transversus abdominis muscle. If I just remove the internal oblique, we’ve got this muscle that sits underneath it. It’s called the transversus abdominis muscle because the fibers are oriented transversely, so horizontally like this.
Let’s just take a look at the origin. It originates on the iliac crest (so you can see that here) and also on the lateral parts of the inguinal ligament. And also, you can see the origin at the top on the costal cartilages. And then again, it inserts on the linea alba and inferiorly, it inserts on the pubic crest.
You can see its attachments here. Iliac crest. The inguinal ligament is gone, but it attaches on the lateral part of the inguinal ligament over here. It attaches on the costal margin and it attaches in the midline at the linea alba and also inferiorly at the pubic crest.
Those are the muscles of the anterior abdominal wall. It’s pretty easy. You’ve just got five to remember. You’ve got two which sit anteriorly. You’ve got the large rectus abdominis and you’ve got the tiny little pyramidalis. And then you’ve got the three lateral muscles (the external and internal oblique muscles and the transversus abdominis muscle).
- rectus abdominis
- external oblique
- internal oblique
- transversus abdominis
- pubic symphyis/tubercle/crest
- anterior superior iliac spine (ASIS)
- iliac crest
- linea alba
- linea semilunaris
- tendinous intersections
- rectus sheath
- inguinal ligament
“Hands in pockets. Hands on tits.”
When your hands are in your pockets, your forearms point downwards and inwards. “Pockets” has an “e”, therefore the External oblique has it’s fibres oriented downwards and inwards (inferomedially).
When your hands are on your tits (if you have them), your forearms point upwards and inwards. “Tits” has an “i”, therefore the Internal oblique has it’s fibres oriented upwards and inwards (superomedially).
Now for a few questions…
1. Name all five muscles (without looking at the list above!)
2. Which spinal nerve innervates the pyramidalis muscle?
3. True of false: The rectus sheath wraps around both surfaces of the rectus abdominis along it’s entire length.
4. True of false: The fibres of the external abdominal oblique are oriented inferomedially.
5. True of false: The transversus abdominis lies between the external oblique and the internal oblique.