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Pelvic-perineal muscles of the hypaxial ...
Other Terms: Pelvis
As the trunk of the body tapers to its basin-like caudal end, the girdle bones of the lower limb intrude on the tapering body wall muscles to join with the sacrum and form the pelvic skeleton. As a result of this association with the lower limbs, the basic body wall pattern of the trunk muscles is slightly modified. The ventral muscle terminates on the pelvic brim and does not pass into the pelvis. The subvertebral muscle, continuous from the abdominal wall as the psoas musculature, terminates on the pelvic brim (psoas minor) or passes into the thigh to attach to the femur (psoas major). Along with the iliacus, piriformis, and obturator muscles, the psoas major muscles cross the hip joint where they direct their actions onto the femur. For this reason, we will consider these muscles when we study the lower limb. 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.