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MUSCLES of the PELVIS Pelvic Walls: Obturator internus Origin: Most of the internal surface of the anterolateral wall of the pelvis minor (pelvic surfaces of the ilium, ischium, and obturator membrane). Insertion: Greater trochanter of femur (medial surface) via strong tendon which passes through the lesser sciatic foramen. Pelvic Walls: Obturator internus Action: Lateral thigh rotation (when hip is flexed); helps to hold femoral head in acetabulum. Nerve: Nerve to obturator internus (L5-S1). Pelvic Walls: Piriformis Origin: Anterior aspect of the second through the fourth lateral masses of the sacrum and the sacrotuberous ligament. Insertion: Greater trochanter of the femur–superior border. Pelvic Walls: Piriformis Action: Lateral rotation of thigh (when hip is extended); thigh abduction (when hip is flexed); helps to hold femoral head in acetabulum; forms the muscular bed for the sacral plexus; leaves the pelvis via the greater sciatic foramen. Nerve: Ventral rami of S1-2. Pelvic Diaphragm: Levator Ani Composed of three muscles: Puborectalis. Pubococcygeus. Iliococcygeus. Figure 3.39 Pelvic Diaphragm: Levator Ani Puborectalis: Origin: Insertion: Action: Nerve: Pectin pubis. Rectum and anal canal. Maintain anorectal flexure. Compress abdominal viscera. Compress pelvic contents. Perineal nerve. Pelvic Diaphragm: Levator Ani Pubococcygeus: Origin: Insertion: Action: Nerve: Pectin pubis. Coccyx. Raise pelvic floor. Compress abdominal viscera Compress pelvic contents. Perineal nerve. Pelvic Diaphragm: Levator Ani Iliococcygeus: Origin: Insertion: Action: Nerve: Obturator fascia and ischial spine. Coccyx and anococcygeal ligament. Same as for pubococcygeus. Perineal nerve. Pelvic Diaphragm: Coccygeus Part of pelvic diaphragm. Not part of levator ani. Origin: Ischial spine. Insertion: Inferior sacrum and coccyx. Action: Support pelvic viscera. Flex coccyx. Nerve: S4, S5