Part 2 - Perineal Membrane and Deep Perineal Pouch
This is a tutorial on the pelvic floor. What we're looking at here is a superior view into the pelvis. You can see these muscles which make up the pelvic floor at the bottom of the pelvis.
I've got the femurs in here because I've included the muscles of the walls of the pelvis. You’ve got the piriformis muscles attaching to the sacrum and to the greater trochanter of the femur. And you've got the obturator internus muscles which you can see here.
If I rotate around to the back, you can see the tendon of the obturator internus muscle attaching to the femur. Those two muscles make up part of the walls of the pelvis.
The pelvic floor separates the pelvic cavity above from the perineum below. It consists of the pelvic diaphragm. And then you've got the perineal membrane and the deep perineal pouch.
The word 'pelvic diaphragm' is often used interchangeably for pelvic floor, but in this tutorial, I'm going to talk about the pelvic diaphragm in relation to two specific muscles. And then I'll go on to tell you about the perineal membrane and the deep perineal pouch. These three structures combined make up the pelvic floor.
To begin with, I'm going to talk about the pelvic diaphragm. The pelvic diaphragm is this dome-shaped set of structures, which we're looking down at. It consists of the levator ani muscles on either side. You’ve got this midline raft, this ligamentous midline where the two halves of the levator ani muscle attach. You've got the coccygeus muscle, which is this muscle here.
I've just isolated the pelvic diaphragm muscle. You can see the shape of them here. It's like this bowl-shape of muscles.
Ignore this extension up here. The muscle doesn't actually extend this far up. Just bear that in mind. It has its anterior attachment on the posterior surface of the pubis here. And then it attaches along the fascia of the obturator internus muscle.
And then at the back, it attaches to the coccyx. And it meets in the midline to form this midline raft. This is where the levator ani meets in the midline posterior to the anus, which is this aperture here.
Anteriorly, you can see that the levator ani muscle has this defect. It's got this u-shaped defect. This is called urogenital hiatus. This allows the urogenital apparatus to pass through the pelvic floor into the perineum below. In males, you've got the passage of the urethra. In females, you've got passage of the urethra and the vagina through this urogenital hiatus.
And as you can see, the muscle consists of various different fibers. You’ve got these loops of fibers which loop around various structures.
The levator ani muscle is typically thought of in terms of three sets of fibers. You've got the pubococcygeus, which attaches from the bony bit of the pubis and extends back to the coccyx. You’ve got the coccyx at back here.
And then the anterior fibers of the pubococcygeus actually loop around the prostate in muscles and the vagina in females. You’ve got these anterior fibers which are divided and loop around the prostate in males forming the levator prostatae or the puboprostaticus and in females, it loops around the vagina forming the pubovaginalis.
And then in the midline as I’ve mentioned before, connecting from the coccyx down to the anus. Remember, this is the aperture for the anus. Connecting from the coccyx to the anus, you've got this midline raft, this ligament, which is called the anococcygeal ligament or anococcygeal body.
And then the next part of the levator ani muscle is this puborectalis muscle. I'm going to draw this on in green, outline in green. This forms a sling around the distal end of the gastrointestinal tract, so around the sort of anus and rectum, around the anorectal junction. You’ve got this sling of muscle from the levator ani forming around the anorectal junction.
These are the intermediate fibers of the levator ani. They again originate on the pubis. They have the important function of maintaining this anorectal angle. They keep this angle of 90° which closes off the anal canal. And I'll come on to talk about this in a moment.
And then we've got the posterior fibers of the levator ani muscle. These are called iliococcygeus muscles or fibers. You’ve got these which I'm outlining in purple.
Those are the three collections of fibers which make up the levator ani muscle. This muscle forms the bulk of the pelvic diaphragm.
Just to quickly recap, the levator ani is composed of these three collections of muscle fibers. If we rotate the model around, you can see the origin of the levator ani on the posterior surface of the pubis and then it's got this origin along the border of the obturator internus muscle.
Covering the obturator internus is this fascia and you've got this thickening. You can see this white thickening. This is a tendinous thickening called the tendinous arch and this is the thickening of the fascia where the levator ani takes part of its origin.
And then over here, we've got the ischial spine. Along the spine, from the body of the pubis, along this tendinous arch to the spine of the ischium, the ischial spine, the levator ani takes its origin and it inserts on the coccyx and in the midline at this anococcygeal ligament.
If we just rotate to an inferior view, you can see these muscles taking their attachment on that little coccyx and it's got this perineal body which is a fibromuscular connective tissue node which joins the perineum and the pelvic floor and you've got some convergence of the levator ani muscles on this node.
The function of the levator ani muscle is to support the pelvic viscera and it keeps the rectum and vagina closed. It has this kind of sphincter closing action on the rectum and the vagina. And importantly, it resists, rises in intrapelvic pressure during any straining. For example, during coughing, when the abdominal muscles increase the intrapelvic pressure, it resists this rise and prevents anything being evacuated from the digestive tract.
One thing I mentioned was the puborectalis maintains the anorectal angle. You’ve got this angle between the rectum and the anal canal. The puborectalis loops around this and it keeps this angle. By maintaining this angle, it forms this valve which stops the anal canal filling with feces from the rectum.
When this muscle relaxes and releases its tension on this angle, the angle between the rectum and the anal canal increases and it becomes more like this, so then you don’t get this pinching off of the anal canal and feces can flow from the rectum into the anal canal.
This is important in defecation. You need to be able to relax the pelvic diaphragm muscles, in particular the puborectalis portion of this muscle in order to relax this anorectal angle and prevent shutting off of the anal canal.
The other muscle of the pelvic diaphragm is the coccygeus, which you can see here on either side. This muscle lies over the sacrospinus ligament. If I rotate around to the back, you can see this ligament connecting the sacrum to the ischial spine. It lies over the sacrospinus ligament and it forms the posterior part of the pelvic diagram.
It originates on this ischial spine and it inserts laterally on the coccyx in the adjacent margins of the sacrum. If I just rotate around to the back, you can see its insertion along the margins of the sacrum and the coccyx below.
This muscle functions to support the pelvic floor and it's innervated by branches from the anterior rami of S4 and S5.
The levator ani is actually innervated by branches of pudendal nerve from roots S2-S4. You’ve got that useful mnemonic, 'S234 keep shit off the floor'. It describes the function of the levator ani muscle.
We talked about the pelvic diaphragm now in quite a lot of detail. The next part of the pelvic floor is the perineal membrane and the deep perineal pouch.
Perineal Membrane and Deep Perineal Pouch
In this next part of this tutorial, we'll be looking at the deep perineal pouch and the perineal membrane.
Inferior to the pelvic diaphragm, you've got this membrane which is called the perineal membrane. You can see this triangularly-shaped structure here, which I've outlined in green. This is a thick fascial structure which attaches along the pubic arch. You’ve got this attachment along the pubic arch here. At the back, it's got this posterior border, which is free. It's not attached to anything.
This membrane actually provides attachment for the roots of the external genitalia. I'll come on to talk about this in another tutorial on the perineum. But importantly, it's got these two holes to allow structures to pass from the pelvic floor into the perineum. This is a model of a woman, so you've got these two holes here. Anteriorly, you've got the urethral orifice and behind it, you've got the vaginal orifice. In males, obviously, you don’t have this orifice for the vagina. You've just got the anterior urethral orifice in males.
If you remember, I mentioned the urogenital hiatus of the pelvic diaphragm, that u-shaped defect. Above the perineal membrane, you've got this urogenital hiatus. If I just rotate the model anteriorly again, you can see that there's this little gap between the anterior border of the perineal membrane and the pubic symphysis. You’ve got a little gap here. If I rotate back to superior view, you can see the urogenital hiatus here and the parts of the perineal membrane with the two orifices for the vagina and the urethra.
I've just added the female reproductive system in and you can see the vagina entering into the pelvic cavity through this vaginal orifice in the perineal membrane. With the urinary system added, you can see the bladder and you can see the urethra here.
The last part of pelvic floor to mention is the deep perineal pouch. The deep perineal pouch is this fascial capsule which lies above the perineal membrane. This contains various layers of skeletal muscle which differ between men and women. All the muscles in this compartment are innervated by the perineal branches of the pudendal nerve.
What I'm going to do is I'm going to switch to a cross-section to try and illustrate the deep perineal pouch. What I've done here is taken a median sagittal section of this model. If I just rotate inferiorly, you can see that I've sliced in half the perineal membrane. If I just rotate around laterally, I'll show you what we're looking at.
We’ve got the layer of levator ani muscles here, so the pelvic diaphragm above here superiorly. And then just below it, we've got the perineal membrane.
Between the pelvic diaphragm muscles and the perineal membrane, you've got this deep perineal pouch. It lies like this. It's actually opened above. There’s no layer separating the pelvic diaphragm muscles from the perineal pouch. You’ve got this fascial pouch, which I've drawn in green and it's opened superiorly.
Like I've said, you've got a few different structures within this pouch and they're innervated by the perineal branches of the pudendal nerve and they're different in men and women.
What I'm going to do is show you some of the structures that are within this perineal pouch, this deep perineal pouch. Back to the superior view of the pelvic, which you're familiar with, I'm just going to dissect away the pelvic diaphragm muscles, so I've got rid of those muscles and we're looking superiorly at the perineal membrane. The content of the pouch sits on top of this in this fascial pouch.
Remember, we're looking at a model of a woman, so we've got the vaginal opening here and we've got the urethral opening anteriorly.
First, we'll look at the muscles in female because they've got two more muscles than men. We've got this free border of the perineal membrane at the posterior aspect. In the midline, we've got this little body called the perineal body, which is a fibromuscular connective tissue node, which connects the structures of the perineum to the pelvic cavity above, the pelvic floor above.
The first muscle that we have is this deep transverse perineal muscle. It attaches laterally on the ischiopubic ramus and it inserts onto this perineal body in the midline. That’s the deep transverse perineal muscle.
And then over here, you can see this little muscle here. This is the external urethral sphincter. This surrounds the proximal part of the urethra in women and the membranous part of the urethra in males.
And then we've got two muscles which are unique to females. We’ve got the compressor urethrae. And then we've got the sphincter urethrovaginalis.
The compressor urethrae originate from the sides of the ischiopubic ramus. And then it loops in like this and meets anteriorly to the urethra. You’ve got this compressor urethrae muscle and it meets anterior to the urethra. This muscle aids the external urethral sphincter in closing off the urethra.
And then you've got this muscle called the sphincter urethrovaginalis. As the name suggests, it forms this sphincter around the opening of the vagina in the perineal membrane and it also blends with these fibers here. It surrounds the opening of the urethra as well, so urethrovaginalis.
This originates on the perineal body at the back. I'm going to pass this forward surrounding the opening with the vagina and then it blends anteriorly with the muscles surrounding the urethral opening in the perineal membrane.
In men, you've just got the deep transverse perineal muscle and the external urethral sphincter. Something else to remember in males is that within this deep perineal pouch, you've also got these glands called bulbourethral glands, which are also known as the glands of Cowper. They lie within the deep perineal pouch.