(3.46)
Now that we've seen the intact pelvic diaphragm from above, let's look at it from behind, and from beneath.
Here are the ischial tuberosities, here's the tip of the coccyx, here are the sacro-tuberous ligaments. Here are the two levator ani muscles. This is the ischio-coccygeus part, this is pubo-coccygeus. Here's the urogenital hiatus. Here's the ano-coccygeal ligament.
The levator ani muscles are continuous on the underside with this cone-shaped sleeve of muscle, the external anal sphincter, which maintains closure of the anus. Here's the opening of the anus.
The external anal sphincter is tethered to the ano-coccygeal ligament by its most posterior fibers. In front, here's the divided urethra. The muscle surrounding it is the bulbo-spongiosus.
Till now we've been looking at the pelvic diaphragm in a dissection of a male body. Here’s a similar dissection of the pelvic diaphragm of a female body. The overall structure of the female pelvic diaphragm is the same as the male, except that the pelvic diaphragm is also traversed by the vagina. Here’s the opening of the urethra.
The whole region between the coccyx, the ischial tuberosities, and the pubic symphysis is called the perineum. The area between the ischio-pubic rami is the urogenital triangle. We'll look at the important structures of the urogenital triangle in Volume Five of this atlas.
We've been looking from behind at an isolated dissection of the pelvic diaphragm, with everything else removed. To get a more complete view of where we are, we'll now add the main surrounding structures to the picture.
To see the pelvic diaphragm clearly we've been looking at an unnaturally empty pair of ischio-rectal fossae. In the living body the ischio-rectal fossa is filled with fat, which is traversed by nerves and vessels, as we'll see. Here are the sacrro-tuberous ligaments, here are the ischial tuberosities. Here are piriformis and obturator internus, going to their insertions on the femur, along with the gemelli and quadratus femoris.
Here's the sciatic nerve, emerging below piriformis. Here's gluteus medius, , here are the origins of the hamstring muscles. Here's the line of origin of gluteus maximus, which we'll add to the picture. The lower edge of gluteus maximus covers up the ischio-rectal fossa, when seen from behind.
On the inside, the walls of the pelvic cavity are covered with a layer of loose connective tissue, which is lined in part by peritoneum. We’ll see this in a minute, when we move on to look at the blood vessels and nerves of the region.