Pelvis and perineum

Pelvis

The bony framework of the pelvis is called the pelvic girdle. It is composed of the two hip bones and the sacrum. Pelvic bones are held together by the two main joints of the pelvis; the pubic symphysis and the sacroiliac joint, and reinforced by pelvic muscles. The pelvic cavity opens superiorly to, and is continuous with, the abdominal cavity through the pelvic inlet. Whilst the pelvic outlet is enclosed by the pelvic floor, made of the pelvic diaphragm. There are two parts of the pelvis; greater (false) pelvis and lesser (true) pelvis. The greater pelvis is found superior to the pelvic inlet and contains the inferior parts of the abdominal organs. The lesser pelvis is located between the pelvic inlet and pelvic outlet, and it includes the distal urinary organs, internal reproductive organs and the perineum.
 

Pelvic floor

The pelvic floor is formed by the funnel-shaped pelvic diaphragm. The pelvic diaphragm comprises of the two paired muscles and their fasciae; levator ani muscle and the coccygeus muscle. The function of the pelvic diaphragm is to support the pelvic organs and prevent them from prolapse. 

Levator ani muscle is the main guy here. It consists of three parts; puborectalis, pubococcygeus, and the iliococcygeus. The puborectalis part makes a U-turn around the anorectal junction (puborectal sling), forming the urogenital hiatus. Besides the rectum and urethra in both sexes, in females, this hiatus also transmits the vagina.

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Perineum

The perineum is the part of the pelvis which contains the external genitalia and anus. It is inferior to the pelvic diaphragm. Regarding the surface anatomy, the perineal area is the region between the thighs, extending from the pubic symphysis anteriorly to the gluteal folds posteriorly.  

The perineum is diamond shaped, and the corners of that diamond are the:

• Pubic symphysis anteriorly
• Sacrum and coccyx posteriorly
• Ischial tuberosities on each lateral side
• The pelvic diaphragm is the roof, while the perineal skin forms the floor.

An imaginary line connecting the ischial tuberosities divides the perineum into the urogenital triangle (UGT) anteriorly, and the anal triangle posteriorly.  The anal triangle includes the rectum and the two ischioanal fossae. These fossae are filled with fat tissue which supports the movements of the pelvic diaphragm. They communicate with the UGT through their anterior recesses.

The urogenital triangle contains the perineal membrane and superficial fascia of Colles. They bound two anatomical spaces within the UGT; superficial perineal pouch and deep perineal pouch. Among sexes, there are anatomical differences only within the contents of the urogenital triangle as it contains the external genitals. A mass of connective, skeletal and smooth muscle tissues called the perineal body is found between the triangles. It is the central mass of the perineum which anchors most of the perineal muscles.

Female pelvis

The female pelvis is broader and larger than the male pelvis to provide a comfortable environment for fetus development. The greater pelvis is a part of the abdomen study section, we won’t spend too long on that here. Just note that the terminal ileum, cecum and sigmoid colon are found within the greater pelvis in both sexes. 

The lesser pelvis in females contains: 

• Distal parts of the urinary and digestive systems: ureter, urinary bladder, urethra and rectum.
• Internal genitalia: ovaries, uterine tubes, uterus and vagina 
• External genitalia, contained by the perineum 

Let’s look at the anatomical relationships for the female pelvic viscera.

In the medial view, from anterior to posterior we see the rectum just anterior to the coccyx, urinary bladder posterior to the pubic symphysis, and the uterus sandwiched between the two of them. Peritoneum overlays the superior surfaces of these organs, creating two peritoneal pouches: the rectouterine pouch of Douglas between the rectum and the uterus, and the vesicouterine pouch between the uterus and the bladder. These three organs communicate with the outside of the body by extending through the perineum: 

• The urethra extends from the inferior surface of the bladder and opens at the external urethral orifice 
• The vagina extends from the cervix of the uterus and opens at the ostium of the vagina 
• The rectum continues as the anal canal which opens at the anus 

Now, where are the uterine tubes and the ovaries? In the peritoneal cavity suspended within their respective peritoneal folds, mesovarium and mesosalpinx. The uterine tubes extend from each lateral side of the body of the uterus and open near the ovaries. Learn more about the female pelvis with Kenhub’s study materials.

Male pelvis

In comparison to the female pelvis, the male pelvis is narrower. The lesser pelvis in males contains: 

• Distal parts of the urinary and digestive systems: ureter, urinary bladder, urethra and rectum.
• Internal genitalia: testes, epididymides, ductus deferens, seminal glands, ejaculatory ducts, prostate, and bulbourethral glands
• External genitalia within the perineum

The rectum is found anterior to the coccyx. Anteriorly is the urinary bladder. Between the rectum and bladder are the seminal glands superiorly and the prostate inferiorly. The rectum continues as the anal canal and then opens through the anus. 

The male urethra extends from the inferior wall of the bladder, penetrates the prostate and then enters the perineum. It passes through the penis and opens at the external urethral orifice. Each seminal gland also has its own canal called the ejaculatory duct. These ducts also penetrate the prostate where they open into the ureter. So this is where the male reproductive and urinary systems meet.

Testes and epididymides are found in the scrotum. They are considered to be internal genitalia because of their development in the abdominopelvic cavity. Ductus deferens is the continuation of the epididymis which joins the ducts of the seminal gland in forming the ejaculatory duct. Finally, bulbourethral glands are small glands just inferiorly to the prostate, which opens into the penile urethra.

Blood vessels

There are four main arteries of the pelvis:

• Paired internal iliac arteries, which supply the pelvic viscera
• Paired gonadal arteries (testicular and ovarian), which supply the internal genitalia outside of the pelvic cavity (testis, epididymis; ovaries, uterine tubes)
• A single median sacral artery that supplies sacrum and coccyx
• A single superior rectal artery that supplies the rectum  

Most significant is the internal iliac artery. Its branches are grouped into the anterior and posterior divisions. The true pelvic viscera are supplied by the many branches of the anterior division. The internal pudendal branch is the main artery of the perineum. The remainder include the umbilical, obturator, inferior vesical (males), uterine (females), middle rectal and inferior gluteal arteries. The posterior division of the internal iliac artery supplies the pelvic and gluteal muscles.

Venous blood from the pelvis is drained by the venous plexuses that surround the pelvic organs. They include the rectal, vesical, prostatic, uterine and vaginal venous plexuses. Most of them empty into the internal iliac vein, which is a tributary to the inferior vena cava. Other than the vena cava, some portion of the venous blood flows into the inferior mesenteric vein and then into the hepatic portal system. If you came here confused about the pelvic vasculature, we have a study set to help you learn this topic efficiently.

Innervation

There are four major nervous structures found in the pelvis:

• Lumbosacral trunk
• Sacral plexus 
• Coccygeal plexus 
• Autonomic pelvic nerves

These nerves supply the pelvic viscera, muscles of the pelvic floor and perineum, gluteal region and the lower limb.

The lumbosacral trunk is a nerve bundle formed by the anterior rami of L4-L5 lumbar nerves. It is a root which contributes to the sacral plexus. The lumbosacral trunk and anterior rami of S1-S4 interconnect to form the sacral plexus. Whilst the anterior rami of S4, S5 and Co (coccygeal nerve) unite to form the coccygeal plexus.Concerning the autonomic pelvic nerves, there are sympathetic and parasympathetic inputs. They are given by the lumbar, sacral and pelvic splanchnic nerves. Lumbar and sacral splanchnic nerves provide the pelvis with sympathetic innervation, whereas the parasympathetic supply is given by the pelvic splanchnic nerves. Splanchnic nerves contribute to forming additional pelvic plexuses, such as the inferior hypogastric plexus. This plexus is the source for all subsequent plexuses that innervate the pelvic viscera: prostatic plexus (males), uterovaginal plexus (females) and middle rectal plexus. 

Lumbar and sacral plexus

The lumbar plexus (L1-L4) is formed by the anterior rami L1-L4 and a contribution from the anterior ramus of T12. The plexus spreads over the anterior surface of the psoas major muscle. Forming 6 main branches, it provides innervation for the muscles of the posterior abdominal wall and thigh, as well as for the skin of the scrotum, labiae, inguinum, and thigh.

Sacral plexus is formed by the lumbosacral trunk (L4, L5), anterior rami of S1-S4 and a part of the anterior ramus of S5. The plexus is found inferiorly to the lumbar plexus, lying on the anterior surface of the piriformis muscle. Most of its branches supply the gluteal and lower limb muscles. The perineum is supplied by the pudendal nerve.

Related articles

Articles within this topic:

• Female reproductive organs
• Male reproductive organs
• Pelvic girdle and floor
• Perineal region
• Urinary bladder and urethra
• Veins, lymphatics and nerves of the pelvis