Welcome back to Anatomy and Physiology on Catalyst University. My name is Kevin Tokoff. Please make sure to like this video and subscribe to my channel for future videos and notifications. In this video, we're going to continue our discussion of the anterior abdominal wall and talk specifically about a structure called the rectus sheath. Now when we think of a sheath, like for a sword let's say, you know you've seen movies, Lord of the Rings would be a great example, where when somebody's not using their sword they have it in a protective case.
usually they're held by their side or sometimes you'll see it on the back. That would be the sheath of the sword and so the rectus sheath is going to be a protective covering for the rectus abdominis muscle which we see right here and depending on where you are on the rectus abdominis that rectus sheath can be in front of the rectus abdominis in which case we would call it the anterior layer of the rectus sheath or it can be behind the muscles and that would be the posterior layer of the rectus sheath. Okay, so we're actually going to see some variation in where the rectus sheath lies depending on where we are on the muscle.
Now recall that lateral to the rectus abdominis, so in these regions over here, we're going to have several muscles and the superficial one is going to be the external abdominal oblique and then deep to that we would have the internal abdominal oblique and then the deepest of them would be the transverse abdominis. All three of those muscles on both sides are going to have with them an associated aponeurosis. Remember, aponeuroses are basically sheet-like tendons that allow the muscle to attach to either its origin or its insertion. And it turns out that the fusion of the aponeuroses of all three of these is going to create the rectus sheath.
So that's an important point. The rectus sheath is really just the fusion of the aponeuroses. of these three muscles, the external and internal obliques, and then the transverse abdominis.
Now when we're determining the nature of the rectus sheath, it's important to consider this point which is called the arcuate line. And when we look at the rectus abdominis, there's on each side left and right, there's actually four regions. On top we have these three quadrats.
Quadrat just means kind of a square rectangular shape. They're pretty close to squaring. rectangular and then this much larger one at the bottom.
And somewhere kind of bisecting this large region of the rectus abdominis at the bottom, we have what's called the arcuate line. And so the nature of the rectus sheath actually differs depending on if you're above the arcuate line or below it. And that's what we're going to explore now. So we want to consider what composes or comprises the anterior rectus sheath. And the anterior rectus sheath is really just the portion of the rectus sheath that lies anterior to or we could say superficial to the rectus abdominis.
So let's take a look at this figure right here. Okay so we're first going to look at above the arcuate line. Okay so this region in blue.
Right so here are some muscles right here. These are the muscles lateral to the rectus abdominis. Okay so right here is the superficial one.
This is the external oblique and here's its aponeurosis and we have the internal oblique, that's the intermediate one, here's its aponeurosis, and then right here is the transverse abdominis, and here's its aponeurosis. Now here is one half of the rectus abdominis, and if we look carefully at these aponeurosis as they're going toward the rectus abdominis, we see that the one associated with the external oblique, that's this one, actually goes completely in front of the rectus abdominis. If we look at the one associated with the transverse abdominis, the deepest one, this one actually goes completely posterior to the rectus abdominis. But something interesting happens with the internal oblique aponeurosis. As we go toward the rectus abdominis, it actually divides or bifurcates.
And so half of it goes in front of the muscle and then half of it goes behind. And so if we look at the aponeurosis that are in front of the rectus abdominis, these would be the anterior layer of the rectus sheath. And what we would say is that it's composed of all of the external oblique aponeurosis and half of the internal oblique aponeurosis, because half of the internal one is actually going in front, but the other half is going behind.
Now, if we want to talk about the posterior layer of the rectus sheath, this would, of course, contain the other half of the internal oblique aponeurosis and actually... all of the transverse abdominis, aponeurosis. And so when we're at the rectus abdominis above the arcuate line, Okay, we have both an anterior and a posterior layer of the rectus sheath.
And so it's important to know what actually comprises each of them. While we're here, let's still follow the anterior rectus sheath and the posterior rectus sheath as we go toward the midline between the two halves of the rectus abdominis. And we see that the anterior and posterior rectus sheaths actually fuse.
And this point of fusion between the left and right halves of the rectus abdominis This is actually what's called the linea alba. So if we zoom back out, the linea alba is really just this midline region between the halves of the rectus abdominis quadrats, okay? It goes all the way down, okay?
And this is your linea alba. And all the linea alba is, is it's really just the fusion of all three aponeuroses of all three of these muscles, external and internal obliques and the transverse abdominis. But note that if we're above the arcuate line, okay, the external oblique aponeurosis goes in front of the rectus abdominis, the transverse abdominis aponeurosis goes behind the rectus abdominis, and then the internal oblique, actually half of it goes in front and half of it goes behind.
But when we get to the midline, all three of them still fuse into the linea alba, okay? And the same thing would occur on the other side. All right, now... Something interesting is going to happen when we drop below the arcuate line.
Okay, when we drop below the arcuate line, it turns out that all three aponeuroses, so the one associated with the external oblique, internal oblique, and transverse abdominis, all three of them are actually going to move in front of the rectus abdominis. So if we actually zoom in down here at the bottom, so what comprises the anterior rectus sheath below the arcuate line? Well actually it's just going to be all three aponeurosis.
So notice here we have our external oblique muscle, here's our internal oblique, and here's our transverse abdominis. Notice all three of the aponeuroses are actually crossing in front of the rectus abdominis. And the same thing's true on the other side.
And then notice all we have behind the rectus abdominis muscle is really just this fascia right here called the transversalis fascia. And the purpose of the transversalis fascia is really to, first of all, prevent the rectus abdominis from crashing into all the abdominal viscera and contents that are back here. Part of the function of the rectus abdominis in addition to trunk flexion is to compress the abdominal contents, but you don't want these muscles actually bumping into and crashing into the contents of the abdominal cavity right there. So that's actually prevented by the transversalis fascia. But the bottom line here in this video is to understand that whether or not you're above or below the arcuate line.
dictates the nature of the rectus sheath. If you're above the arcuate line, then you actually have an anterior and a posterior layer, and that's because different aponeuroses either go in front of or behind the rectus abdominis. However, if you're below the arcuate line, all three aponeuroses of these lateral muscles, they actually go in front of the rectus abdominis.
Okay, and so hopefully this makes sense. Please make sure to like this video and subscribe to my channel for future videos and notifications. Thank you very much