The lower urinary tract

nerves. The obturator nerve descends medial to the psoas muscle, runs along the lateral pelvic w a ll and through the upper part of the obturator foramen into the thigh. It is posteromedial to the common iliac vein in the pelvis. The femoral nerve, surrounded by fat, descends between the psoas and iliacus muscles, passing under the inguinal ligament into the thigh. The sciatic nerve is the largest in the body. It is formed from the lumbosacral plexus on the anterior surface of the sacrum and piriformis. It passes through the greater sciatic foramen into the gluteal region. Nerves to the organs of the pelvis are derived from the lumbosacral plexus, which lies on the sacrum and p i r iformis muscle.

THE LOWER URINARY TRACT

The pelvic ureters

The distal ureters enter the pelvis anterior to the psoas muscle. They pass anterior to the bifurcation of the iliac vessels and run on the lateral wall of the pelvis. Just in front of the ischial spine they turn medially to enter the posterolateral aspect of the bladder. The distal ureter lies above the seminal vesicles in the male and is crossed by the vas deferens. The distal ureter passes above the lateral vaginal fornix in the female, lateral to the cervix and under the uterine vessels in the broad ligament. The intravesical ureter tunnels obliquely through the bladder wall. The bladder muscle exerts a sphincter-like action on the lower ureter.

The bladder (see Fig. 6.12) This is a pyramidal muscular organ when empty. It has a triangular-shaped base posteriorly. The ureters enter the posterolateral angles and the urethra leaves inferiorly at the narrow neck, which is surrounded by the (involuntary) internal urethral sphincter. It has one superior and two inferolateral walls, which meet at an apex behind the pubic symphysis.

The urethra is separated from the pubic symphysis by the retropubic fatty space (of Retzius), which contains a venous plexus as well as nerves and lymphatics. Perivesical fat surrounds the bladder. The obturator internus muscle is anterolateral and the levator ani muscle inferolateral to this. Superiorly it is loosely covered by peritoneum, which separates it from loops of small bowel and sigmoid colon.

In the female, the body of the uterus rests on its postero¬ superior surface and the cervix and vagina are posterior, w i th the rectum behind. In the male, the termination of the vasa and the seminal vesicles are between the posterior bladder and the rectum. In the female, the bladder neck rests on the pelvic fascia. In the male it is fused w i th the prostate. The trigone is the triangular inner wall of the bladder between the ureteric and urethral orifices, as seen on cystoscopy. This part of the wall is smooth; the remainder of the bladder wall is coarsely trabeculated by criss-cross muscle fibres.

The bladder is an extraperitoneal structure. As it fills it becomes ovoid and rises into the abdomen, stripping the loose peritoneum off the anterior abdominal wall. The peritoneum is loose over the bladder, except posteriorly where the ureters enter. The bladder is relatively fixed inferiorly via condensations of pelvic fascia, which attach it to the back of the pubis, the lateral walls of the pelvis and the rectum. Its continuity w i th the prostate in the male makes it even more immobile at this point owing to the strong puboprostatic ligaments.

Peritoneal reflections

In the female, the peritoneum is reflected from the superior surface of the bladder to the anterior surface of the uterus. The peritoneum is further reflected from the posterior part of the uterus to the rectum, and this fold is known as the pouch of Douglas or cul-de-sac. In the male, the peritoneum is reflected off the bladder to the rectum, forming the rectovesical pouch or cul-de-sac. These peritoneal pouches usually contain loops of bowel.

Fig. 6.12 Male bladder and urethra: (a) sagittal section; (b) diagram of prostatic urethra to show ducts.