The pelvic floor is formed by the soft tissues that fill the outlet of pelvis. The most important of these is the strong diaphragm of muscle slung like a hammock from the wall of the pelvis. Through it pass the urethra, the vagina and the anal canal.
Function:
The pelvic floor supports the weight of the abdominal and pelvic organs. Its muscles are responsible for the voluntary control of micturition and defecation and play an important part in sexual intercourse. During childbirth it influences the passive movements of the fetus through the birth canal and relaxes to allow the exit of the fetus from the pelvis.
Muscle layer:
The superficial layer:
This layer is composed of five muscles.
1. The external anal sphincter encircles the anus and is attached behind by few fibers to the coccyx.
2. The transverse perineal muscles pass from the ischial tuberosities to the center of the perineum.
3. The bulbocavernosus muscle pass from the perineum forwards around the vagina to the corpora cavernosa of the clitoris just under the pubic arch.
4. The ischiocavernosus muscles pass from the ischial tuberosities along the pubic arch to the corpora cavernosa.
5. The membranous sphincter of the urethra is composed of muscles fibers passing above and below the urethra and attached to the pubic bones. It is not a true sphincter since it is not circular, but it acts to close the urethra.
The deep layer:
This layer is composed of three pairs of muscles, which together are known as the levator ani muscles (left and right) consist of following:
1. The pubococcygeus muscle passes from the pubic to the coccyx, with a few fibers crossing over in the perineal body to form its deepest part.
2. The iliococcygeus muscle passes from the fascia covering the obturator internus muscle (the white line of the pelvic fascia) to the coccyx.
3. The ischiococcygeus muscle passes from the ischial spine to the coccyx, in front of the sacrospinous ligament.
Between the muscle layers, and also above and below them, there are layers of pelvic fascia. This is loose areolar tissue that is used like packing material in the space. The tissue that fills the triangular space between the bulbocavernosus, the ischiocavernosus and the transverse perineal muscles is known as the triangular ligament.
The perineal body:
This is a pyramid of muscle and fibrous tissue situated between the vagina and the rectum. It is made up of the fibers from the muscles described above. The apex, which is the deepest part, is formed from the fibers of the pubococcygeus muscle, which cross over at this point; the base is formed from the transverse perineal muscles, which meet in the perineum, together with the bulbocavernosus in front and the external anal sphincter behind. The perineal body measures 4 cm in each direction.
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