Chapter 12. Pelvis and Perineum

Big Picture

The pelvic diaphragm forms the floor of the pelvis and serves as a bed for the pelvic organs.

Pelvic Diaphragm

The pelvic diaphragm is formed by the union of the levator ani and the coccygeus muscles. A layer of fascia lines the superior and inferior aspects of the pelvic diaphragm (Figures 12-1A). The levator ani muscle consists of three separate muscles: pubococcygeus, puborectalis, and iliococcygeus.

The pelvic diaphragm circumferentially attaches along the pubis, lateral pelvic walls, and coccyx. The rectum pierces the center of the pelvic diaphragm, giving the appearance of a funnel suspended within the pelvis. In addition to the rectum, the urethra and the vagina (in females) and the urethra (in males) pierce the pelvic diaphragm.

Functions of the pelvic diaphragm are as follows:

• Closes the pelvic outlet.
• Supports the abdominopelvic viscera.
• Resists increases in intra-abdominal pressure.
• Controls the openings of the rectum, urethra, and vagina (e.g., helps retain or release feces during a bowel movement).
• Marks the boundary between the rectum and the anal canal.

Other Muscles of the Pelvic Floor

• Obturator internus muscle. Covers and lines most of the lateral wall of the pelvis. The obturator nerves and vessels and other branches of the internal iliac vessels course along the medial surface of the obturator internus muscle. The obturator internus muscle exits the pelvis through the lesser sciatic foramen and inserts on the greater trochanter of the femur and performs external hip rotation.
• Piriformis muscle. Covers most of the posterior wall of the pelvis. The piriformis muscle exits the pelvis through the greater sciatic foramen and inserts on the greater trochanter of the femur and performs external hip rotation. The sacral plexus of nerves is medial to the piriformis muscle.

Big Picture

The perineum is the diamond-shaped region inferior to the pelvic diaphragm. The pubic symphysis, pubic arches, ischial tuberosities, and coccyx bound the perineum. An imaginary line between the ischial tuberosities divides the perineum into an anterior (urogenital) triangle and a posterior (ischioanal) triangle (Figure 12-2A). The urogenital triangle extends between the paired ischiopubic rami. The ischioanal triangle is the fat-filled area surrounding the anal canal.

Urogenital Triangle

The urogenital triangle consists of a superficial and a deep layer of fascia (Figure 12-2A–D), with a middle layer of skeletal muscle called the urogenital diaphragm.

Deep Perineal Space

The deep perineal space is filled by the muscles that comprise the urogenital diaphragm. The deep perineal space contains the following structures (Figure 12-2B and D):

• External urethral sphincter. This skeletal muscle voluntarily prevents urine from spilling from the bladder.
• Deep transverse perineal muscle.
• Bulbourethral (Cowper's) gland. In males, this gland produces and secretes mucus, which lines the lumen of the urethra during ejaculation.

Superficial Perineal Space

The superficial perineal space is the region inferior to the urogenital diaphragm and its inferior fascia, and is enclosed by the superficial perineal (Colles') fascia (Figure 12-2A and C). The superficial perineal space contains erectile tissue, muscles, and neurovascular structures associated with the external genitalia. These structures will be covered in more detail in Chapters 13 and 14.

Ischioanal Triangle

The ischioanal triangle contains a horseshoe-shaped fossa filled with fat and is known as the ischioanal fossa. The anal canal and its associated sphincters are in the center of the triangle.

The boundaries of the ischioanal fossa are as follows:

• Anterior. Posterior border of the urogenital triangle.
• Posterior. Gluteus maximus muscle and the sacrotuberous ligament.
• Lateral. Deep fascia of the obturator internus muscle, which condenses around the pudendal nerve (S2–S4) and the internal pudendal artery and vein, forming the pudendal (Alcock's) canal. The inferior rectal vessels and nerves cross the fossa from the pudendal canal and supply the inferior portion of the rectum. The pudendal nerve and internal pudendal artery and vein exit the pudendal canal and course into the urogenital triangle to supply the structures in the triangle and the external genitalia.
• Floor. Deep fascia of the levator ani muscle.

Big Picture

The common iliac arteries divide at the sacroiliac joints and become the external and internal iliac arteries. The external iliac arteries mainly serve the lower limb. The internal iliac arteries distribute blood to the pelvic walls and viscera (i.e., rectum, bladder, prostate, ductus deferens, uterus, uterine tubes, and ovaries). Additionally, these arteries distribute blood to the gluteal region, the perineum, and the medial compartment of the thigh.

Branches of the External Iliac Artery

The external iliac artery courses deep to the inguinal ligament, where it becomes the femoral artery (Figure 12-3A). The femoral artery is the principal blood supplier to the lower limb. Before it exits the pelvis, the external iliac artery gives rise to the inferior epigastric artery, which ascends vertically along the internal surface of the anterior abdominal wall.

Branches of the Internal Iliac Artery

The internal iliac artery divides into an anterior and a posterior trunk near the greater sciatic foramen (Figure 12-3A and B).

Anterior Trunk

Branches from the anterior trunk supply the pelvic viscera, perineum, gluteal region, and medial compartment of the thigh. The branches off the anterior trunk of the internal iliac artery are as follows:

• Umbilical artery. Is usually the first branch off the anterior trunk and ascends out of the pelvis along the internal surface of the anterior abdominal wall to terminate at the umbilicus. In the fetus, the umbilical artery transports blood from the fetus through the umbilical cord to the placenta. After birth, the vessel collapses after the umbilical cord is cut. The superior vesical artery is a branch of the umbilical artery, which supplies the bladder and the ductus deferens.
• Obturator artery. Courses along the medial surface of the obturator internus muscle and exits the pelvis through the obturator canal, along with the obturator nerve and vein. The obturator artery supplies the medial compartment of the thigh.
• Uterine artery. In females, the uterine artery courses within the base of the broad ligament, superior to the ureter before reaching the cervix. At the cervix, the uterine artery ascends along the lateral margin of the uterus to the uterine tube and forms an anastomosis with the ovarian artery. During pregnancy, the uterine artery enlarges significantly to supply blood to the uterus, ovaries, and vaginal walls. This artery is not present in males.
• Inferior vesical and vaginal artery. In males, the inferior vesical artery supplies the bladder, ureter, seminal vesical, and prostate. In females, the vaginal artery is the equivalent of the inferior vesical artery in males and, as its name implies, it supplies the vagina as well as parts of the bladder.
• Middle rectal artery. Supplies the rectum and forms anastomoses with the superior rectal artery (branch of inferior mesenteric artery) and the inferior rectal artery (branch of the internal pudendal artery).
• Internal pudendal artery. Exits the pelvis through the greater sciatic foramen, inferior to the piriformis muscle. Along with the pudendal nerve, the internal pudendal artery courses along the pelvis lateral to the ischial spine and then passes through the lesser sciatic foramen to enter the urogenital triangle. The internal pudendal artery supplies the perineum, including the erectile tissues of the penis and clitoris.
• Inferior gluteal artery. The terminal branch of the anterior division of the internal iliac artery. The inferior gluteal artery courses between the ventral rami of S1 and S2 of the sacral plexus and exits the pelvis through the greater sciatic foramen. The inferior gluteal artery supplies the gluteal region.

Posterior Trunk

Branches from the posterior trunk of the internal iliac artery serve the lower portion of the posterior abdominal and pelvic walls and the gluteal region.

• Iliolumbar artery. Gives risk to branches that supply the posterior portion of the abdominal and pelvic walls.
• Lateral sacral artery. Gives rise to branches that traverse the anterior sacral foramina to supply the posterior sacrum and overlying muscles.
• Superior gluteal artery. The terminal branch of the posterior trunk. The superior gluteal artery courses between the lumbosacral trunk and the ventral ramus of the S1 spinal nerve. This artery supplies the gluteal region, with anastomotic overlap with the inferior gluteal artery.

Big Picture

Somatic and autonomic nerves contribute to pelvic innervation. The obturator nerve and the sacral plexus provide innervation to skeletal muscles and skin in the pelvis and lower limbs. All autonomics of the pelvis and perineum pass through the inferior hypogastric plexus. Sympathetic and parasympathetic nerves contribute to the inferior hypogastric plexus through the sacral splanchnics and pelvic splanchnics, respectively.

Obturator Nerve

The obturator nerve originates from the ventral rami of spinal nerves L2–L4 and courses along the obturator internus muscle, superior to the obturator artery and vein. The neurovascular bundle exits the pelvis through the obturator canal and supplies the medial compartment of the thigh.

Sacral Plexus

The sacral plexus is formed by the lumbosacral trunk (ventral rami of spinal nerves L4–L5) and the ventral rami of spinal nerves S1–S4 (Figure 12-4A and B). The sacral plexus lies on the anterior surface of the piriformis muscle. The following nerves branch from the sacral plexus:

• Superior gluteal nerve (L4–S1). Exits the pelvis superior to the piriformis muscle, through the greater sciatic notch. The superior gluteal nerve supplies the gluteus medius and minimus muscles and the tensor fascia lata muscle.
• Inferior gluteal nerve (L5–S2). Exits the pelvis inferior to the piriformis muscle, through the greater sciatic notch. The inferior gluteal nerve innervates the gluteus maximus muscle.
• Pudendal nerve (S2–S4). Exits the pelvis inferior to the piriformis muscle and enters the perineum through the lesser sciatic foramen, where the pudendal nerve enters the pudendal canal along the lateral wall of the ischioanal fossa.

Sciatic nerve (L4–S3). The largest peripheral nerve in the body. The sciatic nerve is comprised of the tibial and common peroneal nerve and exits the pelvis inferior to the piriformis muscle, between the ischial tuberosity and the greater trochanter of the femur.

Sacral Sympathetic Trunk

The sacral sympathetic trunk crosses the pelvic brim posterior to the common iliac vessels (Figure 12-4A). Four ganglia are present along the trunk. The trunks of the two sides unite in front of the coccyx at a small swelling called the ganglion impar. The sacral sympathetic trunk contributes sympathetic nerves to the somatic branches of the sacral nerves (targeting the skin) and contributes visceral branches to the inferior hypogastric plexus (targeting the pelvic viscera and perineum).

Pelvic Splanchnic Nerves

The pelvic splanchnic nerves are the only splanchnic nerves that carry parasympathetic fibers (Figure 12-4A). All other splanchnic nerves, such as the greater splanchnic nerve, carry only sympathetic fibers.

Preganglionic parasympathetic fibers originate from the S2–S4 spinal cord levels. The fibers course through the ventral rami of spinal nerves S2–S4 and then through the pelvic splanchnic nerves. Pelvic splanchnic nerves carry the parasympathetic and visceral afferent fibers to and from the inferior hypogastric plexus. These nerves supply the distal portion of the transverse colon, descending colon, sigmoid colon, and rectum, as well as organs of the pelvis and perineum.

Inferior Hypogastric Plexus

The inferior hypogastric plexus is formed by the union of nerves from the superior hypogastric plexus, sacral splanchnic nerves, and pelvic splanchnic nerves (Figure 12-4A). The inferior hypogastric plexus is located diffusely around the lateral walls of the rectum, bladder, and vagina. The plexus contains ganglia in which both sympathetic and parasympathetic preganglionic fibers synapse. Therefore, the inferior hypogastric plexus consists of preganglionic and postganglionic sympathetic and parasympathetic fibers, as well as visceral sensory fibers. The inferior hypogastric plexus gives rise to many other smaller plexuses that provide innervation to organs involved with urination, defecation, erection, ejaculation, and orgasm.

Big Picture

The colon terminates at the rectum, which in turn terminates at the anus. The rectum is located superior to the pelvic diaphragm in the pelvic cavity. The anus is located inferior to the pelvic diaphragm in the ischioanal fossa. The terminal end of the gastrointestinal tract is significant because it is a junction of two embryologic origins of tissue: endoderm and ectoderm.

Rectum

The rectum is a continuation of the sigmoid colon and is located superior to the pelvic diaphragm in the pelvic cavity. The rectum has a dilated region called the ampulla, which stores feces. As feces collect in the rectum, increasing pressure causes the walls to bulge. Stretch receptors in the rectal wall relay messages to the brain for the need to defecate.

Peritoneum covers the anterior and lateral surfaces of the proximal portion of the rectum; however, there is no peritoneal covering for the distal portion of the rectum. The vascular and lymphatic supply of the rectum is as follows:

• Arteries. The rectum receives its blood supply from the superior rectal artery (a branch of the inferior mesenteric artery), the middle rectal artery (a branch of the internal iliac artery), and the inferior rectal artery (a branch of the internal pudendal artery) (Figure 12-5A).
• Veins. Venous return is through the superior rectal vein (a branch of the inferior mesenteric vein of the portal system), the middle rectal vein (the internal iliac vein), and the inferior rectal vein (the internal pudendal vein) (Figure 12-5B).
• Lymphatics. Lymphatic drainage of the rectum is through three principal directions. Lymph from the superior region of the rectum drains into the inferior mesenteric nodes, from the middle of the rectum into the internal iliac nodes, and from the inferior part of the rectum into the superficial inguinal nodes.
• Innervation. Parasympathetic innervation via the pelvic splanchnic nerves is through the inferior hypogastric plexus.

Anal Canal

The anal canal is located inferior to the pelvic diaphragm in the ischioanal fossa. The anal canal is divided into an upper two-thirds (visceral portion), which is part of the large intestine, and a lower one-third (somatic portion), which is part of the perineum. The pectinate line (anorectal junction) is an important landmark in that it divides the anal canal into upper and lower portions. Developmentally, the pectinate line is the junction between the development of the hindgut (gut tube) and the proctodeum (body wall). The pectineal line is an important anatomic landmark in that it distinguishes the vascular, nerve, and lymphatic supplies as follows:

• Superior to the pectinate line. The vascular supply is from the superior and middle rectal arteries and veins. Visceral motor and sensory innervation is via the inferior hypogastric plexus. Lymph drainage is to the internal iliac, inferior mesenteric, and pararectal lymph nodes. The epithelium is simple columnar, as is the remainder of the small and large intestines, which reflects the endodermal origin of this part of the anal canal.
• Inferior to the pectineal line. The vascular supply is from the inferior rectal arteries and veins. Somatic motor and sensory innervation is via the inferior rectal nerves. Lymph drainage is to the superficial inguinal lymph nodes. The epithelium is stratified squamous keratinized epithelium, similar to the skin, which reflects the ectodermal origin of this part of the anal canal.

Anal Sphincters

The following two sphincters are responsible for regulating passage of fecal matter:

• Internal anal sphincter. The internal anal sphincter is a continuation of the smooth muscle layer of the remainder of the intestine. Sympathetic innervation (lower lumbar splanchnic nerves) causes contraction of the internal anal sphincter, whereas parasympathetic innervation (pelvic splanchnic nerves) causes relaxation.
• External anal sphincter. The external anal sphincter is a voluntary skeletal muscle that encircles the distal portion of the anus and enables voluntary control of defecation. The paired inferior rectal nerves from the pudendal nerve (S2–S4) innervate the external anal sphincter. The muscle is voluntarily contracted or relaxed. The external anal sphincter has a fascial attachment to the coccyx (anococcygeal ligament). Anterior to this sphincter is the perineal body, a strong tendon into which many of the perineal muscles insert, including the urogenital diaphragm, the levator ani muscle, and the external anal sphincter.

Defecation

The process of defecation occurs as follows:

1. Distension of the rectal ampulla occurs from feces passing from the sigmoid colon.

2. The puborectalis portion of the levator ani muscle relaxes, thereby decreasing the angle between the rectal ampulla and the upper portion of the anal canal.

3. Intra-abdominal pressure increases when the diaphragm and the abdominal body wall muscles contract.

4. The internal anal sphincter relaxes, as does the external anal sphincter.

5. Feces pass out of the rectum and anus.

6. After defecation has occurred, the puborectalis muscle contracts once again, increasing the angle between the rectal ampulla and the upper portion of the anal canal, as do the anal sphincters to close the anus.

Big Picture

The ureters connect the kidneys to the urinary bladder, which stores urine until urination occurs. The urethra voids urine from the urinary bladder.

Ureter

The ureter is a retroperitoneal organ that transports urine that is propelled by peristaltic waves from the kidney to the urinary bladder (Figure 12-6A). Three constrictions occur along the length of the urethra: (1) where the renal pelvis becomes the ureter; (2) where the ureter crosses the pelvic brim; and (3) where the ureter enters the bladder (Figure 12-6C). In males, the ureter courses posterior and medial to the ductus deferens and anterior to the seminal vesicle. In females, the ureter courses lateral to the cervix, where the ureter courses inferior to the uterine artery.

Urinary Bladder

The urinary bladder is retroperitoneal, as are the ureters (Figure 12-6A). The urinary bladder is positioned against the pubis between the pelvic diaphragm and the obturator internus muscles. The superior surface of the bladder is dome shaped when the bladder is empty and swells superiorly into the abdomen when the bladder is full. The bladder consists of the following parts:

• Apex. The top of the bladder is located at the top of the pubic symphysis. The apex continues as the median umbilical ligament, which is the embryonic remnant of the urachus.
• Base. The base of the bladder is located inferiorly and posteriorly. The paired ureters enter the bladder at each of the superior corners of the base. Internally, the triangular area between the openings of the ureters is known as the trigone.
• Detrusor muscle. The detrusor muscle consists of bundles of smooth muscle located within the wall of the bladder.
• Neck. The most inferior portion of the bladder is the neck. It surrounds the origin of the urethra where the inferolateral surfaces and base intersect. The neck of the bladder is supported by the pubovesical ligament, fibromuscular bands that attach between the neck and pubic bones.
• Internal urethral sphincter. The internal urethral sphincter is smooth muscle that involuntarily contracts or relaxes, thereby regulating the emptying of the bladder (Figure 12-6B). The constant muscular tone of the internal urethral sphincter is through sympathetic innervation. However, to urinate, the muscle is relaxed via parasympathetic innervation. This is the primary muscle for preventing the release of urine.
• External urethral sphincter. This sphincter is composed of skeletal muscle within the urogenital diaphragm that voluntarily opens and closes the urethra to void urine. The external urethral sphincter is innervated by the pudendal nerve and, therefore, is under voluntary control.

The bladder receives its blood supply via the superior and inferior vesical arteries (branches of the internal iliac artery) and is drained by the vesical plexus of veins (into the internal iliac vein).

Ureter and Bladder Innervation

The inferior portions of the ureter and urinary bladder receive innervation from the vesical and prostatic plexuses (extensions of the inferior hypogastric plexus) (Figure 12-6B). Parasympathetic innervation is from the S2–S4 spinal cord levels, which enter the inferior hypogastric plexus, as do sacral splanchnic nerves for sympathetic innervation. The inferior hypogastric plexus then gives rise to the vesical and prostatic plexuses, which innervate the ureter and urinary bladder.

Urination

The process of urination occurs as follows:

• The bladder fills with urine, causing the bladder to distend.
• Visceral sensory fibers relay to the spinal cord (S2–S4), via the pelvic splanchnic nerves, that the bladder wall is stretched.
• Preganglionic parasympathetic fibers from the S2–S4 spinal cord segments enter the spinal nerves and pelvic splanchnic nerves. The pelvic splanchnic nerves enter the inferior hypogastric plexus; the preganglionic parasympathetic fibers course from the inferior hypogastric plexus to the bladder, where they synapse with postganglionic parasympathetic fibers. Stimulation of these parasympathetic nerves causes the detrusor muscle to contract and the internal urethral sphincter to relax.
• Somatic motor neurons in the pudendal nerve cause relaxation of the external urethral sphincter and contraction of the bulbospongiosus muscles, which expel the last drops of urine from the urethra.