The male urethral sphincter is a striated muscle in contact with the urethra from the base of the bladder to the perineal membrane. The individual muscle fibers are 25 to 30% smaller than fibers of associated muscles and are embedded in connective tissue which obscures the visibility of the whole muscle. The muscle primordium is laid down around the urethra prior to the development of the prostate. Subsequently, the prostate develops as a diverticulum of the urethra and grows into the developing sphincter, thinning the overlying musculature. With the onset of puberty, accelerated growth of the prostate displaces the sphincter, with atrophy of the overlying muscle, resulting in what may appear to be isolated segments of the sphincter muscle distributed around the prostate. The prostate overgrows the anterior portion of the urethra and the associated sphincter muscle. There is no distinct superior fascia of the so-called urogenital diaphragm separating the sphincter muscle from the prostate. The fascia of the sphincter muscle is inseparable from the prostatic sheath, is oriented vertically, and passes through the urogenital hiatus to unite with the fascia of the pudendal canals at the isochiopubic rami. Thus, the sphincter muscle is a component of a bladder-urethra-prostate-sphincter unit which lies within the pelvis, in the urogenital hiatus, and rests upon the perineal membrane. The concept of a urogenital diaphragm is not borne out by this study.
Striated muscle associated with the female urethra and vagina constitute a continuous mass which appropriately may be called the urogenital sphincter. Though continuous, the muscle may be separated into two parts--one that surrounds the urethra, and the other surrounding the urethra and vagina. The individual muscle fibers are small and are embedded in connective tissue and infiltrated with smooth muscle which obscures the visibility of the muscle to gross dissection. Developmentally the muscle primordium is laid down around the urogenital sinus and urethra early, and foreshadows the anatomical arrangement that is maintained in the adult with little change. The urogenital sphincter muscle extends from the base of the bladder where it lies within the pelvic cavity and continues through the urogenital hiatus of the pelvic diaphragm to expand around the vagina in the perineum. Additional fibers attach to the ischiopubic rami and constitute a compressor of the urethra. As a result there is no superior fascia of the so-called "urogenital diaphragm" which closes off a deep perineal compartment or forms a floor of the urogenital hiatus.
The ilioinguinal nerve was dissected in 196 adult human cadavers. A normal pattern of distribution, consistent with modern textual descriptions, was defined in 60% of inguinal regions. In 35% of the inguinal regions a previously undescribed aberrant course for the cutaneous component of the ilioinguinal nerve was observed in which it is incorporated within the genitofemoral nerve, entered the inguinal canal at the deep inguinal ring, coursed within the cremasteric layer and emerged into the subcutaneous tissue at the superficial inguinal ring on the dorsal side of the spermatic cord or the round ligament of the uterus. The cutaneous component of the ilioinguinal nerve is most frequently found on the ventral surface of the spermatic cord or round ligament during inguinal hernia operations thus an unrecognized aberrant course of this nerve within the spermatic cord is particularly susceptible to accidental trauma.
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