Rectus Sheath

00:01 So now let's try and put all of these muscles together in what is quite a complex region known as the rectus sheath.

00:08 And how the rectus sheath is formed really does rely on an understanding of the insertion points of the muscles were spoken about, and actually how they form as they run towards the midline.

00:20 So we can see here we've got rectus abdominis.

00:23 That paired muscle running either side of the midline, running superiorly down from around the inferior aspect of the sternum all the way down to the pubic symphysis and the iliac crest.

00:34 So here we can see and transverse abdominis.

00:37 And its arrangement, here we can see by adding in internal oblique it's arrangement, and then finally, external oblique.

00:44 But that's quite a detailed organization of muscle.

00:47 There's three muscles - transverse abdominis, here, then moving to internal oblique, and then moving to external oblique.

00:54 And it can be quite complicated.

00:55 So we'll look at it in a slightly different way, which helps to make sense of it in a moment or two.

01:00 Here we can see the anterior portion of the rectus sheath though.

01:04 It's covered by the aponeurosis that's primarily coming from external oblique.

01:09 If we can then see that from this diagram.

01:12 You can see how here we're looking at the vertebral column closest to us.

01:17 And then we're looking into the posterior view of the anterolateral abdominal wall.

01:23 So we're looking at it as if the abdominal contents have been removed, and we're looking at it from the inside.

01:28 And here you can see the actual belly of rectus abdominis.

01:34 You can see it's running inferiorly at the lower portion of that region.

01:37 And then above it, we can see the aponeurosis of internal, external, etc.

01:43 But what's actually going on there? Here, we can see that aponeurosis.

01:47 And here, we can see the inferior to it, the rectus abdominis muscle.

01:53 Specifically, this is the posterior aspect of the rectus sheath, and that is located in the upper two thirds.

02:03 So only within the upper two thirds of the rectus sheath, Do we actually have a posterior layer that's covering rectus abdominis.

02:12 It doesn't occur in the bottom third.

02:14 The lower third here, we don't have any aponeurotic covering around rectus sheath.

02:20 So what's actually happening within this region? The demarcation between those two is the arcuate line.

02:27 And we'll be able to depict that much clearer in a moment or two.

02:30 So what's actually occurring here? We've got the arcuate line separating the upper two thirds from the lower third.

02:38 But what's actually going on? This is back to the original image we showed previously.

02:43 So imagine you're looking at this, the patient laying on their bed, they're laying on their back, and you're looking up through their feet towards their abdomen.

02:50 So here we can see most superficially, we can see the external oblique aponeurosis.

02:57 And this is what's occurring above the arcuate line.

03:00 So here we can see the aponeurosis of external oblique, and that is coming from the muscle belly, and it runs entirely anterior to rectus abdominis.

03:11 If we then look at the internal oblique aponeurosis, you can see that there's two layers.

03:18 So whereas the aponeurosis of external oblique stays as one? Here, the internal obliques aponeurosis splits into two.

03:27 An anterior layer, which goes anterior to rectus abdominis, and a posterior layer that goes posterior, or in this image here, underneath rectus abdominis.

03:39 So that single aponeurotic layer that we've spoken about, when it gets to the semilunar line, which is lateral aspect of the rectus abdominis.

03:46 It splits into two.

03:48 Then when we go deeper still, and we look at transverse abdominis, it's aponeurosis doesn't split, but it just remains posterior.

03:57 So what you end up with either side of the rectus abdominis muscle is the formation of the rectus sheath above the arcuate line where essentially you have one and a half layers aponeurosis, either side.

04:11 The full layer from external oblique above the full layer from transverse abdominis below, and then internal oblique splits into two, giving half a layer above and half a layer below.

04:25 So we end up having that arrangement of the rectus sheath.

04:30 Below the arcuate line, it's much simpler.

04:33 Below the arcuate line, we do not have anything running posterior or underneath rectus abdominis.

04:40 Here we can see all of external oblique aponeurosis running above running anterior to rectus abdominis.

04:47 Similarly, for internal oblique no longer does it split into an anterior and posterior layer, all of it runs towards internal oblique aponeurosis as a single layer, and then transverse abdominis, which previously was running posteriorly.

05:03 Now it runs anteriorly.

05:05 And this is occurring below the level of the arcuate line.

05:09 So now let's have a look at this in a little bit more detail and look at it from a slightly different angle, because that helps to conceptualize what we're looking at.

05:17 So let's have a look at the rectus sheath again.

05:20 So this is a slightly different view of the same structure.

05:24 But this time, we're really looking at a sagittal section.

05:26 So it's been cut through the rectus sheath.

05:29 And we can see how we've got various layers of tissue.

05:33 Here we can see external oblique. So we can see external oblique, whether it's above or below the arcuate line.

05:39 And you can see the arcuate line where those muscle layers move anteriorly towards the skin in yellow, that's the level of the arcuate line.

05:49 We can see here in green, external oblique it's aponeurosis is permanently running anterior to the rectus sheath.

05:56 Here we can see internal oblique.

05:59 Now if you remember internal oblique, it gave rise to a layer that was both anterior and posterior to rectus abdominis, when it was above the arcuate line.

06:09 So now in green above the arcuate line, we can see we've got two.

06:14 An anterior layer and a posterior layer.

06:16 But then, at the formation of the arcuate line, you see those two layers merge into one.

06:24 Here we can see rectus abdominis, now really helping us work out those two layers that is sitting either side of it anteriorly and posteriorly, for internal oblique of the rectus abdominis muscle.

06:37 Finally, we can then see transverse abdominis.

06:40 When it's above the arcuate line, you can see it is per situated posteriorly.

06:45 But then beneath the arcuate line, you can see how essentially rectus abdominis has penetrated those layers.

06:53 So we just follow rectus abdominis down inferiorly We've got the first muscle belly, the second muscle belly, the third muscle belly inferiorly.

07:01 And you can see how the second muscle belly has actually penetrated the posterior layer of internal oblique aponeurosis and the aponeurosis of transverse abdominis.

07:11 It's penetrated those layers. So they now reside anteriorly.

07:16 The one structure that we haven't spoken about previously, but mentioned at the beginning of this section was transversalis fascia.

07:22 And that's an important fascial layer.

07:24 We'll come back to in later videos.

07:27 And here we can see it permanently resides posteriorly to the rectus abdominis muscle.

07:33 But above the arcuate line you can see how it had some layers of aponeurosis between it.

07:39 But beneath the arcuate line, you can see how there's no nothing between the transversalis fascia and rectus abdominis.

07:48 So here we can see at the level of the umbilicus approximately where the arcuate line would be.

07:53 We have external oblique anteriorly.

07:55 We have the anterior layer of internal oblique aponeurosis.

07:58 Posteriorly, we have the posterior layer of internal oblique aponeurosis and we have transverse abdominis.

08:04 And we have transversalis fascia.

08:07 Now when we look beneath the arcuate line where the umbilicus is indicated here, we have external oblique, internal oblique, transversus abdominis muscle we have those three muscles, but then just left posteriorly, beneath the arcuate line, we have transversalis fascia.

08:25 So now let's have a look at the linea alba.

08:28 We can see in the midline and then we can see the umbilicus.

08:31 So reminding ourselves on the surface of the abdomen, really the structures and the levels of where the arcuate line will occur here at the umbilicus.

08:39 And just for reminder, we have the linea alba, the midline of where these aponeurotic layer will interdigitate and actually merge together into that connective tissue boundary.