Muscles of the Male Genitalia
The urethral muscle (m. urethralis) is the striated muscle associated with the pelvic portion of the urethra. It completely encircles the urethra of the goat and horse but surrounds only the ventral and lateral aspects of the bovine, porcine, and ovine urethra.
The urethral muscle forms the voluntary sphincter of the urinary bladder; during ejaculation, waves of peristaltic contractions in it help propel the semen through the urethra.The extrapelvic continuation of the urethral muscle is a striated muscle called the bulbospongiosus muscle. It surrounds the urethra and bulb of the penis. The bulbospongiosus muscle of the horse extends from the penile bulb between the crura of the penis along the entire urethra to the glans penis. In other animals it covers the urethral bulb and extends only a short distance along the penile urethra. The bulbospongiosus muscle continues the action of the urethral muscle in emptying the urethra by peristaltic contractions.
The ischiocavernosus muscles are paired striated muscles that cover the superficial aspect of the crura of the penis. The two muscles converge from their origins on the lateral sides of the ischial arch toward the body of the penis. When these muscles contract, they pull the penis dorsocraniad against the bony pelvis, aiding erection by compressing much of the venous drainage from the penis.
The retractor penis muscles are paired smooth muscles that take origin from the ventral aspect of the first few caudal vertebrae. These muscles pass ventrad on each side of the anal canal, then continue on the midline of the penis superficial to the urethra.