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The sphincter urethra muscle lies more anterior and attaches at either side to the ischiopubic rami. Part of this muscle encircles the membranous urethra. The deep transverse perineal muscle attaches laterally to the ischiopubic rami, and forms the posterior border of the urogenital diaphragm.
Links and References:
Grant's: 3.79
Netter (1ed.): 355, 356 (2ed.): 351, 352
Rohen/Yokochi: 341