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Pelvis Anatomy and Physiology


Pelvis Anatomy and Physiology
The pelvic cavity is a basin-shaped region below and behind the pelvic inlet (Fig. 5.1). It is surrounded by the bones of the pelvic girdle supplemented by muscles, ligaments and fascia. The anterior wall, near the pubic symphysis, is shallow while the posterior wall, the sacrum, is deep and concave. Each lateral wall is lined by the obturator internus, a broad muscle covered on its medial surface by fascia. Above this muscle are two apertures providing access for nerves and vessels entering the lower limb. The greater sciatic foramen leads into the gluteal region, the obturator canal into the thigh.


The organs in the pelvis include the bladder and lower ureters, the rectum and possibly coils of small intestine and the sigmoid colon. The male pelvis also contains the prostate (inferior to the bladder) and seminal vesicles (posterior to the bladder) and parts of the ductus deferentes (Fig. 5.5). In the female, the reproductive organs are interposed between the bladder and rectum and include the vagina, the uterus, uterine tubes and ovaries (Fig. 5.6). In the infant, the pelvic cavity is comparatively shallow and therefore parts of the bladder and uterus may lie above the pelvic brim. In the adult, however, the bladder and uterus often lie below the pelvic inlet (brim), though they rise into the abdomen when distended with urine or enlarged by pregnancy.
The peritoneum of the greater sac covers the superior parts of the pelvic organs. In the male it dips into a single pouch between the bladder and rectum while in the female it forms two deeper pouches, anterior and posterior to the uterus. The peritoneum that drapes over each uterine tube is called the broad ligament and the ovary attaches to its posterior aspect (Fig. 5.8).
The arterial supply to most of the pelvic organs is provided by the internal iliac artery (Fig. 5.2), but the rectum and ovaries are supplied by the inferior mesenteric artery and the ovarian arteries, respectively, from the abdominal aorta. Similarly, most venous blood passes to the internal iliac veins but the rectum drains to the portal venous system via the inferior mesenteric vein, while the ovarian veins enter the inferior vena cava and left renal vein. Much of the lymph from the pelvic organs passes to the internal iliac nodes, but the rectum and ovaries drain to aortic nodes in the abdomen.
Many of the nerves in the pelvis, including the sacral plexuses, are applied to the pelvic walls and are merely in transit to the lower limb. The pelvic organs themselves receive autonomic innervation via the left and right pelvic plexuses, which surround the branches of the internal iliac arteries. The parasympathetic contribution to these plexuses comes from the pelvic splanchnic nerves, branches of the second, third and fourth sacral spinal nerves, while the sympathetic innervation is provided by the hypogastric plexus, which descends from around the aortic bifurcation.
The gutter-shaped pelvic floor is formed largely by the left and right levatorani muscles. They arise from the pelvic wall, chiefly from the fascia covering the obturator internus muscles, slope downwards and fuse in the midline. There is a narrow midline gap near the pubic symphysis traversed by the urethra and vagina. The central part of the pelvic floor is pierced by the rectum, turning downwards and backwards to become the anal canal.

Perineum Anatomy and Physiology
The perineum is the shallow region that includes the anal canal and the external genitalia. It is bounded by the pelvic outlet: the inferior margins of the bones of the pelvis and their associated ligaments (Fig. 5.3). The region extends forwards to the pubic symphysis, backwards to the coccyx and laterally to the ischial tuberosities. The roof of the perineum is formed by the two levatorani muscles, the floor is the skin and each lateral wall is similar to that of the pelvis: the pubis and ischium are covered by the obturator internus muscle. The posterior half of the perineum, the anal triangle, contains the anal canal, its sphincters and a fat-filled space on each side, the ischioanal (ischiorectal) fossa. Each fossa communicates with the gluteal region via a small aperture, the lesser sciatic foramen. The anterior half of the perineum, the urogenital triangle, includes the external genitalia. In the female, the lower parts of the vagina and urethra are surrounded by the vulva (labia majora and minora and the clitoris), while in the male the distal part of the urethra is enclosed by the penis. Below the root of the penis is the scrotum. Although the scrotum is part of the perineum it is described with the testis and inguinal canal in Chapter 4 (p. 149).

Most structures in the perineum, including the scrotum but excluding the testes, are supplied by the pudendal nerve and the internal pudendal vessels (Fig. 5.2). The neurovascular bundle arises in the pelvis but does not pierce the levatorani to reach the perineum. Instead, it traverses the greater sciatic foramen, the gluteal region and the lesser sciatic foramen. The nerve and vessels then run forward through the perineum, giving branches to the anal canal, the scrotum or labia and the penis or clitoris. Most of the lymph from these structures passes to the inguinal nodes, which are also the main site of drainage for the lower parts of the vagina and anal canal, but the testes drain to aortic lymph nodes in the abdomen.

The pudendal nerve gives motor branches to many striated muscles in the perineum, including the external anal and urethral sphincters. In addition, it supplies sensory branches to the anal canal, vagina, urethra and most of the perineal skin.