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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 perineal 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.

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 (prostate, bladder and rectum).

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 penis, crura of the penis, bulbospongiosus, ishiocavernousus and superficial transverse perineal muscles, 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 spongy urethra and contribute to erection of the penis. 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 perineal and urethral sphincter muscles, membranous urethra, bulbourethral glands, 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 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 external genitilia are comprised of the scrotum, testes and penis. The scrotum houses the testes. Sperm are transported from the testis by the head, body and tail of the epididymis. The epididymis continues as the ductus deferens, a component of the spermatic cord. The penis has an attached root and a free body. The root is comprised of a single midline bulb, and right and left crura attached to the inferior pubic rami. The ischiocavernosus muscles surround the crura and the bulbospongiosus muscle surrounds the bulb of the penis. The body of the penis has three erectile structures. The two corpora cavernosa are continuous with the cura, and the single corpus spongiosum is continuous with the bulb. The corpus spongiosum ends as the glans of the penis and is traversed by the spongy urethra.

The ductus deferens traverses the inguinal canal and passes in a preperitoneal position along the anterior-lateral pelvic wall to reach the prostate, just inferior to the bladder. The ductus deferens ends as a dilation, the ampulla, and joins the duct of the seminal vesicle to form the ejaculatory duct. The seminal vesicles extend laterally from the ejaculatory ducts. The prostate surrounds the urethra and the two ejaculatory ducts drain into the prostatic urethra.

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 and testicular arteries are the primary blood supply of the testis, ductus deferens, seminal vesicles and prostate. 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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