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Body Surface

The primary surface landmarks of the pelvis and perineum are associated with the innominate (iliac crest, pubic symphysis and ischial tuberosity) and coccyx. The skin overlying the pelvis and perineum is innervated by cutaneous branches of dorsal rami of lumbar and sacral spinal nerves, ilioinguinal, genitofemoral and pudendal nerves, and posterior cutaneous nerve of the thigh.

Skeleton and Joints

The bones associated with the pelvis and perineum are the innominate, sacrum and coccyx. The two innominate bones articulate along the midline at the pubic symphysis, supported by the superior and arcuate pubic ligaments. The sacrum and innominate bones articulate (gliding or plane) at the sacroiliac joint and are supported by the sacroiliac and iliolumbar ligaments.

Organization

The pelvic region is subdivided into the greater (false) pelvis, lesser (true) pelvis, and perineum. The greater pelvis contains abdominal viscera. The lesser (true) pelvis is located between the pelvic inlet (from the promentory of the sacrum to the the pubic symphysis along the arcuate line of the ilium) and the pelvic outlet (pubic symphysis, ischiopubic rami and sacrotuberous ligaments). The lateral pelvic wall is closed by two muscles of the lower extremity, the piriformis and obturator internus muscles and their associated fasciae. The pelvic outlet is closed by the pelvic diaphragm and to a lesser extent by the urogenital diaphragm.

The perineum is the diamond-shaped region between the abducted thighs extending from the pubic symphysis to the ischial tuberosity and finally the coccyx. The perineum is subdivided into two triangles by the superficial transverse perineus muscles running from the ischial tuberosities to the perineal body (central tendon) along the midline. The posteriorly positioned anal triangle is traversed by the anus and its associated external anal sphincter. The anteriorly placed urogenital triangle is traversed by the urethra and vagina.

The pelvic diaphragm is comprised of the levator ani and its superior and inferior fascae and the coccygeus muscle. The levator ani includes the puborectalis, pubococcygeus, iliococcygeus and ischiococcygeus muscles, named for their attachments. The muscles of the pelvic diaphragm function to support and compress the pelvic viscera (rectum, uterus and bladder).

The urogenital triangle includes the superficial and deep perineal spaces (pouches). The superficial perineal space is bounded superiorly by the perineal membrane and inferiorly by the perineal (Colle's) fascia. The perineal fascia is equivalent to and continuous with the membranous (Scarpa’s) fascia of the anterior abdominal wall. It contains the bulb of the vestibule, crura of the clitoris, bulbospongiosus, ishiocavernousus and superficial transverse perineus muscles, greater vestibular glands, and branches of the pudendal nerve and internal pudendal vessels. The muscles of the superficial perineal space function to fix the perineal body, compress the vagina and contribute to erection of the clitoris. The deep perineal space is bounded superiorly by the inferior fascia of the pelvic diaphragm and inferiorly by the perineal membrane. It contains the deep transverse perineus (smooth muscle in female), urethral sphincter (inferior aspect), compressor urethrae, and urethrovaginal sphincter muscles, urethra, anterior recesses of the ischiorectal fossa, and branches of the pudendal nerve and internal pudendal vessels. The muscles of the deep perineal space function to compress the vagina and urethra.

Viscera

The pelvic region and perineum include viscera associated with the digestive, urinary and reproductive systems. The rectum is a retroperitoneal pelvic structure with a dilated lumen, the ampulla. It terminates at the anus with a well-defined external sphincter that blends with the adjacent levator ani (puborectalis) muscle. The luminal wall of the anus is characterized by the presence of columns, valves and sinuses.

The urinary bladder is a preperitoneal structure positioned posterior to the pubic symphysis when empty. It has a posterior-inferior fundus and a superior surface with an anterior apex that is continuous with the urachus (an embryonic remnant). The inferior neck is continuous with the urethra. Internally the fundus is characterized by a smooth appearing trigone with the orifices of the ureters and urethra at its angles.

The internal reproductive viscera include the uterus, uterine tubes and ovaries all surrounded by folds of peritoneum. The uterus is positioned between the bladder and rectum, and has a fundus, corpus and cervix. The cervix extends into the superior aspect of the vagina. The uterine tubes extend laterally from the uterus near the fundus. The uterine tubes have a short, narrow isthmus adjacent to the uterus, gradually expand as an ampulla and end at a dilated infundibulum with an associated fimbria. The ovaries are positioned posterior to the uterine tubes suspended by the ovarian (to the uterus) and suspensory (to the pelvic wall) ligaments. The broad ligament and its subparts (mesosalpinx – uterine tube, mesovarium – ovary and mesometrium – cervix) surround the internal reproductive viscera.

The external genitilia are comprised of the labia majora, labia minora, clitoris and vagina. The clitoris has a glans, corpus and two crura attached to the adjacent inferior pubic rami. The ischiocavernosus muscles surround the crura. The vagina is positioned between the external urethral meatus and the perineal body (central tendon), and surrounded by the two vestibular bulbs. The bulbospongiosus muscles surround the vestibular bulbs and vagina. The greater vestibular glands drain into the lower aspect of the vagina, and the fornices are the spaces between the vaginal wall and the cervix.

Innervation

Splanchnic nerves, ganglia and plexuses arising from the lumbar and sacral sympathetic chains and ventral rami S2-4 (preganglionic parasympathetic) innervate (sensory, sympathetic and parasympathetic) the pelvic viscera. The postganglionic parasympathetic neurons are associated with the wall of the viscera. Branches of the pudendal nerve innervate (sensory, motor, postganglionic sympathetic and preganglionic parasympathetic) the external genitilia (skin, erectile tissues and associated muscles).

Blood Supply

Branches of the inferior mesenteric and internal iliac arteries are the primary blood supply to the rectum. Tributaries of the inferior mesenteric and internal iliac veins drain the rectum. Branches of the internal iliac arteries are the primary blood supply of the bladder, distal ureters and urethra. Tributaries of the corresponding veins drain the same structures.

Branches of the internal iliac (uterine and vaginal) and ovarian arteries are the primary blood supply of the uterus, uterine tubes and ovaries. Tributaries of the corresponding veins drain the same structures. Branches of the internal pudendal arteries and veins are the primary blood supply and venous drainage of the external genitilia.

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