Anatomy

Origin

The Sphincter urethrae membranaceae surrounds the whole length of the membranous portion of the urethra, and is enclosed in the fasciae of the urogenital diaphragm. Its external fibers arise from the junction of the inferior rami of the pubis and ischium to the extent of 1.25 to 2 cm., and from the neighboring fasciae. They arch across the front of the urethra and bulbourethral glands, pass around the urethra, and behind it unite with the muscle of the opposite side, by means of a tendinous raphe. Its innermost fibers form a continuous circular investment for the membranous urethra.

Nerve

The perineal branch of the pudendal nerve supplies this group of muscles.

Actions

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