into the pelvis. You can see these muscles which make up the pelvic floor at the bottom

of the pelvis.

So I've got the femurs in here because I've included the muscles of the walls of the pelvis.

So 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. So those two muscles make up part of the walls of the pelvis.

So 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.

So 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.

So these three structures combined make up the pelvic floor.

To begin with, I'm going to talk about the pelvic diaphragm. So 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. So 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. So 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. So this is where the levator ani meets in the midline posterior to the anus, which

is this aperture here.

So 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. So you've got these

loops of fibers which loop around various structures.

So 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. So 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. So 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. So remember, this is the aperture for the anus. So 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. So 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. So you've got this sling of muscle from the levator ani forming around the anorectal

junction.

So 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. So 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. So you've got these which I'm outlining in purple.

So those are the three collections of fibers which make up the levator ani muscle. This

muscle forms the bulk of the pelvic diaphragm.

So 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.

So covering the obturator internus is this fascia and you've got this thickening. So

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. So 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.

So 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.

So the function of the levator ani muscle is to support the pelvic viscera and it keeps

the rectum and vagina closed. So 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.

So one thing I mentioned was the puborectalis maintains the anorectal angle. So you've got

this angle between the rectum and the anal canal. The puborectalis loops around this

and it keeps this angle. So by maintaining this angle, it forms this valve which stops

the anal canal filling with feces from the rectum.

So 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.

So 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.

So the other muscle of the pelvic diaphragm is the coccygeus, which you can see here on

either side. So this muscle lies over the sacrospinus ligament. So if I rotate around

to the back, you can see this ligament connecting the sacrum to the ischial spine. So 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.

So 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.

So 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.