This muscle is composed of two portions, one spread over the upper, and the other over the lower surface of the membranous portion of the urethra. At either side the two layers unite together, and are attached right and left to the inner lateral wall of the pelvis.

When the two layers of this muscle contract, the walls of the urethra are brought together, and the escape of urine from the bladder is prevented. By the same means it prevents the semen from entering the bladder when it emerges from the vesiculae seminales in the act of copulation.

Two other muscles, the "ischio-urethral" and the "transverse perinei", are also concerned in acting upon the urethra.