Pelvis and Perineum Overview

The expanded portion of the pelvic cavity above the pelvic brim/inlet, bounded by the iliac fossae laterally and the lower lumbar vertebrae posteriorly.

• Functionally part of the abdominal cavity (contains lower abdominal viscera, small bowel loops, sigmoid colon), rather than being a true pelvic compartment.

• The bony basin below the pelvic brim, extending from the pelvic inlet (superior boundary) to the pelvic outlet (inferior boundary), containing the pelvic viscera proper.
• This is the compartment referred to when clinicians speak simply of "the pelvis" in the context of pelvic organs, pelvic floor, and pelvic surgery.

• The superior aperture of the lesser pelvis, bounded by the sacral promontory (posteriorly), the arcuate line of the ilium, pecten pubis, and pubic crest (laterally/anteriorly), and the upper border of the pubic symphysis (anteriorly).
• Also termed the pelvic brim; its dimensions and shape are of major obstetric importance (the transverse and anteroposterior diameters determine the pattern of fetal descent during labour).

• The space within the lesser pelvis, bounded by the pelvic walls and floor, containing the bladder, rectum, and (in females) the internal reproductive organs, along with pelvic vessels, nerves, and extraperitoneal fat/connective tissue.

• The inferior aperture of the lesser pelvis, a diamond-shaped opening bounded by the pubic symphysis (anteriorly), the ischiopubic rami and ischial tuberosities (anterolaterally), the sacrotuberous ligaments (posterolaterally), and the coccyx (posteriorly).
• Closed by the pelvic floor (perineal) structures rather than bone, distinguishing it functionally from the bony pelvic inlet.

• Anterior wall: Pubic bones and pubic symphysis (shortest wall).
• Lateral walls: Hip bones (obturator foramen/membrane region), covered by obturator internus muscle.
• Posterior wall: Sacrum, coccyx, and the piriformis muscles covering the sacral portion.

• The muscular diaphragm (levator ani and coccygeus, collectively the pelvic diaphragm) spanning the pelvic outlet, supporting the pelvic viscera against intra-abdominal pressure and separating the pelvic cavity above from the perineum below.
• Perforated by the urogenital hiatus (anteriorly, for the urethra and, in females, the vagina) and the anal hiatus (posteriorly, for the anorectal junction).

• Shared (both sexes): Bladder, rectum, distal ureters.
• Male-specific: Prostate, seminal vesicles, ductus deferens (pelvic part), ejaculatory ducts.
• Female-specific: Uterus, uterine tubes, ovaries, vagina.

• Internal iliac artery and its extensive branching pattern, the dominant arterial supply to pelvic viscera, walls, and perineum.
• Internal iliac vein and the rich pelvic venous plexuses (vesical, rectal, prostatic/uterovaginal) surrounding the pelvic organs.
• Median sacral vessels: small, direct continuations of the aorta/IVC-adjacent vessels along the sacral midline.

• Sacral plexus (L4-S4), gives rise to the sciatic nerve, pudendal nerve, and the gluteal nerves, among others.
• Autonomic supply: superior and inferior hypogastric plexuses, pelvic splanchnic nerves (parasympathetic, S2-S4), and sympathetic fibres from the lumbar/sacral sympathetic chains, governing pelvic visceral function (bladder, bowel, reproductive organs).

• The pelvic peritoneum drapes over the superior surfaces of the pelvic organs to varying degrees, creating characteristic pouches: rectovesical pouch (males, between bladder and rectum), vesicouterine and rectouterine (Douglas) pouches (females).
• Much of the pelvis is subperitoneal (extraperitoneal), the bladder, most of the rectum, and the lower reproductive organs lie below/outside the peritoneal reflections, surrounded by extraperitoneal (pelvic) fascia and connective tissue rather than peritoneum.

• Bony pelvis: Female pelvis is generally wider, shallower, with a larger, more rounded pelvic inlet and a wider subpubic angle, adaptations for childbirth.
• Peritoneal pouches: Males have a single rectovesical pouch; females have two pouches (vesicouterine and rectouterine) flanking the uterus.
• Organ content: Presence of the uterus, uterine tubes, ovaries, and vagina in females; prostate, seminal vesicles in males.
• Pelvic floor/perineum: Broadly similar muscular architecture, but the urogenital hiatus accommodates an additional structure (the vagina) in females.

• Iliac crests, ASIS, pubic tubercles, pubic symphysis, ischial tuberosities, sacrum, coccyx: palpable landmarks used throughout pelvic clinical examination and surgical planning; full detail in Sections 2 and 12.