Pelvis part 2

Second last post for ANHB2212 (and for the entire semester, unless I write some extras for funsies!)

This post is about all of the stuff that's inside the pelvis. I find the pelvis kinda tricky, because there's so many things in such a small space!

The pelvic floor
Innervation of the perineum 

The pelvic floor, to my understanding, are the muscles that make up the base of the true pelvis. These are the levator ani muscles, which are divided into the iliococcygeus and pubococcygeus, which form a massive U-shaped loop around the pelvic floor. (There's a large spot in the anterior pelvic floor where the levator ani are not present because the urethra and all that are there. This is the weakest part of the pelvic floor.) Another muscle that makes up the pelvic floor is the closely-related ischiococcygeus, or coccygeus for short, which is more posterior.

Where do the levator ani attach? Well, to answer that, I'll have to explain some of the other bits and pieces around the pelvis. The two holes at the front of the pelvis are obturator foraminae. The obturator membrane usually fills this space. On either side of the obturator membrane are the obturator internus and obturator externus muscles. The obturator fascia covers the obturator internus, and is where the levator ani attach. The point at which the levator ani attach to the obturator fascia is called the white line, or tendinous arch. There are also other points of attachment, such as the tendinous perineal body which is kinda in the middle, and the anococcygeal raphe towards the back. (A "raphe" is a place where fibres from two sides meet up.)

Why is the pelvic floor important? Well, part of the pubococcygeus muscle is the puborectalis. These are the fibres of the pubococcygeus that don't attach to the coccyx (the rest of it does, hence pubo = pubis, coccygeus = coccyx). The puborectalis forms a sling around the rectum (and by extension everything in front of it). When it contracts, it maintains a bendy anorectal angle which isn't exactly conducive to poo falling out. Hence, this muscle is important in maintaining continence.

Another important feature to take note of, even though it's not part of the pelvic floor per se, is the pudendal canal. The pudendal canal is a canal that runs through the obturator fascia. The pudendal nerve and artery, which supply all the "down below" bits with a nerve and blood supply, pass through here. The pudendal nerve branches out into the rectal nerve and the posterior labial/scrotal nerve, as does the pudendal artery (though obviously the artery branches out into arteries). If you go further back, the pudendal nerve originally arises from the ventral rami of the pelvic splanchnic nerves (S234), and the pudendal artery from the internal iliac artery.

Perineum
Urogenital region
Ischiorectal fossa

The perineum is basically the bit between your legs. It can be divided up into two roughly triangular-shaped bits (the dividing line is an imaginary line between the ischial tuberosities). The anterior part is the urogenital region, and the posterior part is the anal region.

First, I'll start with the urogenital region. The urogenital region has to be kinda tough to compensate for the fact that there's no levator ani in front. There are two main regions of the urogenital region (yup, subregions of regions).

The top region of the urogenital diaphragm is the deep perineal pouch, and has three layers (layers of subregions of regions... totally not confusing at all!). The topmost/deepest layer is a thin fascia, and then the next layer is made up of muscles, such as the sphincter urethrae surrounding the urethra (in females, it surrounds the vagina as well). Other muscles here are the transverse perineal muscles, which as you can guess run transversely across the perineum. Below the muscular layer is the thick perineal membrane.

Below the deep perineal pouch is the superficial perineal pouch. The external genitalia are here, anchored onto the perineal membrane. The genitalia are made up of two main erectile tissues: the bulb and the crus, which are paired. In the male, the two crura eventually become the corpora cavernosa of the penis, whereas the bulbs fuse and become the corpus spongiosum. In the female, the two crura become fuse at the front to become the clitoris, and the bulbs become the labia minora.

There isn't as much to say about the anal region, aside from that it doesn't have this membrane. It does have a space between the anus and the ischial tuberosities, which is usually filled with fat. This space is called the ischiorectal/ischioanal fossa.

Disposition of pelvic organs
Pelvic organs and the peritoneum 

I'm fairly sure you have an idea of what the pelvic organs are- the bladder, uterus (in females), the rectum etc. I'm not going to go into too much detail, except for note their relation to the pelvic floor. All of their tubey parts (urethra, vagina etc.) are sticking through the pelvic floor, which acts as an extra sphincter to whatever other sphincters the organs themselves may have. Hence, if the pelvic floor is weak, as can happen with age, straining, obesity or damage, you can become incontinent. If the pelvic floor is very weak, then organs may even prolapse (i.e. come out of the body) giving you inverted buttholes and so forth.