Urethra. The female urethra measures 3 to 4 cm and originates within the bladder trigone (p. 28). he distal two thirds of the urethra are fused with the anterior vaginal wall. The epithelialilining of the urethra changes from transitional epithelium proximally to nonkeratinized stratiied squamous epithelium distally. The walls of the urethra consist of two layers of smooth muscle, an inner longitudinal and an outer circular. This is in turn surrounded by a circular layer of skeletal muscle referred to as the sphincter urethrae or rhabdosphincter (see Fig. 2-6). Approximately at the junction of the middle and lower third of the urethra, and just above or deep to the perineal membrane, two strap skeletal muscles called the urethrovaginal sphincter and compressor urethrae are found. Together with the sphincter urethrae, these constitute the striated urogenital sphincter complex. This complex supplies constant tonus and provides emergency relex contraction to sustain continence.
Distal to the level of the perineal membrane, the walls of the urethra consist of ibrous tissue, serving as the nozzle that directs the urine stream. Here, the urethra has a prominent submucosal layer that is lined by hormonally sensitive stratiied squamous epithelium. Within the submucosal layer on the dorsal (vaginal) surface of the urethra lie the paraurethral glands, described earlier (p. 17).
he urethra receives its blood supply from branches of the inferior vesical, vaginal, or internal pudendal arteries. Although still controversial, the pudendal nerve is believed to innervate the most distal part of the striated urogenital sphincter complex. Somatic eferent branches from 52-54 that course along the inferior hypogastric plexus variably innervate the sphincter urethrae.
Dorsal v. of clitoris Labium minus (cut) Hymeneal ring Sphincter urethrae m.}Striated Compressor urethrae m. urogenital sphincter Urethrovaginal sphincter m. complex Ischiopubic ramus (cut) Perineal membrane Ischiopubic ramus Cut edge of perineal membrane Bulbospongiosus m. (cut) Ischiocavernous m. (cut) Superficial transverse perineal m. External anal sphincter m. Gluteus maximus m. Compressor urethrae m. Urethrovaginal sphincter m. Pubococcygeus }(pubovisceral) m . Pub"ecto'" m. Levator ani mm. . lliococcygeus m."S FIGURE 2-6 Deep space of anterior triangle of the perineum. Structures on the right side of the image can be seen after remova l of the perineal membrane. Also shown are structures that attach to the perineal body: bulbospongiosus, superficial transverse perineal, external anal sphincter, and puboperinealis muscles as well as perineal membrane. (Reproduced with permission from Corton MM: Anatomy. In Hoffman BL, Schorge JO, Bradshaw KD, et al (eds): Gynecology, 3rd ed. New York, McGraw-Hili Education, 2016.) ·m. External anal sphincter m. Longitudinal smooth m. layer Circular smooth m. layer Plicae transversalis recti
Internal anal sphincter m .
FIGURE 2-7 Anal canal and ischioanal fossa. (Reproduced with permission from Corton MM: Anatomy. In Hoffman BL, Schorge JO, Bradshaw KD, et al (eds): Williams Gynecology, 3rd ed. New York, McGraw-Hili Education, 2016.)