Anatomy

Origin

The Sphincter urethrae membranaceae (Constrictor urethrae), like the corresponding muscle on the male, consists of external and internal fibers. The external fibers arise on either side from the margin of the inferior ramus of the pubis. They are directed across the pubic arch in front of the urethra, and pass around it to blend with the muscular fibers of the opposite side, between the urethra and vagina. The innermost fibers encircle the lower end of the urethra.

Nerve

Perineal branch of the pudendal nerve.

Actions

The muscles of both sides act together as a sphincter, compressing the membranous portion of the urethra. During urination, they are relaxed, and only come into action at the end of the process to eject the last drops of urine.