Now we’ll move on, to look at the muscles of the pelvic diaphragm. These muscles form a sling, which closes off the inferior pelvic aperture, and supports the organs that lie within the pelvic cavity.
On each side, the pelvic diaphragm is formed by two most unequal muscles, the small coccygeus muscle behind, and the much larger and more important levator ani muscle in front.
Here's the coccygeus muscle. It runs from the ischial spine, to the edge of the lower sacrum and coccyx. Coccygeus is a vestigial muscle, with no demonstrable function.
Now, we’ll add the levator ani muscle to the picture. Here’s the levator ani. The levator ani has a line of origin that’s partly bone and partly fascia. In front, it arises from the body of the pubis. Behind, it arises from the ischial spine. Between these two bony origins, it arises from the tendinous arch in the fascia that overlies obturator internus.
The fibers of levator ani pass downwards, backwards and medially, to meet in the midline with those of the opposite side, as we'll see shortly.
Let's go round to the back, to see the underside of levator ani. Here's the ischial tuberosity, here's the sacro-tuberous ligament. The space between these structures and the underside of the levator ani is called the ischio-rectal fossa. In the living body it's filled with fat.
The levator ani is described as having a number of parts, which are named as though they were separate muscles. Unfortunately the names of these parts are somewhat irrational. This part of levator ani is known as ilio-coccygeus. Ilio-coccygeus is very thin. This part is pubo-coccygeus. It's much more substantial. Pubo-coccygeus is sub-didivided further, in ways that we won't go into.
Now that we've looked at one levator ani muscle, let's look at the two of them together. We're looking from above. Here's the upper part of obturator internus, here's the tendinous arch. The ischial spines are here, here's the tip of the coccyx.
Here are the coccygeus muscles, here are the two levator ani muscles. Between them, in front, there's a gap, the urogenital hiatus, though which pass the rectum, the urethra, and in the female the vagina..
The fibers of levator ani which arise more posteriorly unite in the midline with this fibrous band, the ano-coccygeal ligament.
The fibers which arise more anteriorly form a loop which passes around the back of the urogenital hiatus. Some fibers along the edge of the hiatus attach to the sides of the rectum, the urethra, and in the female the vagina.
We'll add the urethra and the rectum to the picture. Here's the lowest part of the rectum, here's the urethra, with the lowest part of the prostate in front of it. We'll see these structures in Volume Five of this atlas.
The levator ani and coccygeus muscles are covered over by this dense layer of pelvic fascia, which completes the pelvic diaphragm on the inside.
The pelvic diaphragm supports the pelvic organs and closes off the pelvic outlet, while allowing passage for the rectum, vagina and urethra. When we're upright, the levator ani muscles are in a constant state of tonic contraction, which becomes greater or less in response to changes in abdominal pressure.
The main action of the levator ani muscles is to keep a set of downwardly mobile structures, the pelvic organs, constantly in one place. In addition, vigorous contraction of the levator ani muscles pulls the lower end of the rectum upwards and forwards.