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Lateral muscles of the pelvic-perineal ...
The three inner layers of the four-layered lateral wall continue into the bottom of the trunk to form the muscular floor to the pelvis. The three muscle layers from each side meet in the midline to surround the urethra, vagina, and anus. This three-layered muscle floor at the bottom of the pelvis is called the pelvic diaphragm (internal layer) the perineum (middle and external layers.) The perineal muscles span the diamond shaped pelvic outlet formed anteriorly by the pubic and ischial rami, laterally by the ischial tuberosities, and posteriorly by the sacrotuberous ligament and coccyx. A transverse line between the ischial tuberosities divides the perineum into an anterior urogenital triangle and a posterior anal triangle. These muscles of the pelvic floor have a wide variety of functions. The internal muscle layer, collectively called the pelvic diaphragm, forms a basin shaped floor that supports the pelvic viscera. In the anal triangle the muscles of the middle and outer layers migrate around the anus to form a strong, ring-like sphincter that can close the anal opening. In the urogenital triangle the middle muscle layer forms within a strong fascial pouch. This enclosed space is the deep perineal pouch and the layer is termed the urogenital diaphragm. The outer layer of muscles is closely allied to the erectile tissues of the male and female genitals. The perineal muscles support the pelvic viscera, form important sphincter muscles that surround the urethral and anal orifices, assist in erectile function, and propel the sperm from the male penis during ejaculation.