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Anatomy of the Pelvis and Perineum

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1. The Bony Pelvis

• Two hip (coxal) bones — each comprising the ilium, ischium, and pubis, fused at the acetabulum
• The sacrum — formed by the 5 fused sacral vertebrae
• The coccyx — 4 fused coccygeal vertebrae

Key Bony Landmarks

Pelvic Inlet and Outlet

• Pelvic inlet (superior pelvic aperture): bounded by the sacral promontory posteriorly, the arcuate line of the ilium laterally, and the pubic symphysis anteriorly. The inlet faces anterosuperiorly.
• Pelvic outlet (inferior pelvic aperture): bounded by the coccyx posteriorly, ischial tuberosities laterally, and pubic symphysis anteriorly. It is closed by the pelvic floor.

True vs. False Pelvis

• False (greater) pelvis: above the pelvic inlet; part of the abdominal cavity, contains lower abdominal viscera
• True (lesser) pelvis: below the pelvic inlet; contains the pelvic viscera (bladder, rectum, reproductive organs)

Sexual Dimorphism

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2. Joints of the Pelvis

• Pubic symphysis: secondary cartilaginous (fibrocartilaginous) joint; slightly mobile, especially in pregnancy
• Sacroiliac (SI) joints: synovial joints anteriorly, syndesmosis posteriorly; transmit trunk weight to the lower limbs
• Sacrococcygeal joint: fibrocartilaginous; slight flexion/extension

Ligaments

• Sacrotuberous ligament: sacrum/coccyx → ischial tuberosity; converts the greater and lesser sciatic notches into foramina
• Sacrospinous ligament: sacrum → ischial spine; landmark for pudendal nerve block
• Iliolumbar ligament: L4–L5 transverse processes → iliac crest; stabilises the lumbosacral junction

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3. Pelvic Walls and Floor

Lateral Wall Muscles

Pelvic Diaphragm (Pelvic Floor)

• Puborectalis: origin — superior pubic ramus on both sides; insertion — loops around the anorectal junction, forming the anorectal angle (~90°); critical for fecal continence; innervation — pudendal nerve and sacral plexus (S2–S4)
• Pubococcygeus: pubis → anococcygeal ligament and coccyx; innervation — pudendal nerve (S2–S4)
• Iliococcygeus: tendinous arch of the obturator fascia → iliococcygeal raphe and coccyx; innervation — pudendal nerve (S2–S4)

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4. Pelvic Viscera

Urinary Bladder

• Lies in the retropubic space (space of Retzius) posterosuperior to the pubic symphysis
• Has four parts: apex, body, fundus, and neck
• The neck is continuous with the urethra; in males, it is closely related to the prostate
• Peritoneum covers the superior surface; the posterior surface is separated from the rectum (in males) by the rectovesical pouch or from the uterus (in females) by the vesicouterine pouch

Urethra

• Female urethra: ~4 cm long; short, straight course inferiorly through the pelvic floor directly into the perineum (vestibule)
• Male urethra: ~20 cm long; divided into prostatic, membranous, and spongy portions; passes through the prostate, the deep perineal pouch/perineal membrane, and then through the erectile tissues of the penis. Has a fixed anterior angle at the perineal membrane and a mobile angle in the penile shaft

Rectum and Anal Canal

• The rectum begins at the rectosigmoid junction (S3 level) and follows the sacral curve
• Three lateral flexures create the Houston's valves
• The rectum becomes the anal canal as it passes through the pelvic floor; the anorectal angle (~90°) maintained by puborectalis is key for continence
• The anal canal has an internal anal sphincter (smooth muscle; involuntary) and external anal sphincter (striated; voluntary; innervated by pudendal nerve S2–S4)

Reproductive Organs

• Prostate: sits at the neck of the bladder, encircles the urethra; ~3 cm long; has three zones (peripheral, central, transitional); surrounded by a fascial capsule; closely related to the rectum posteriorly (basis of digital rectal exam)
• Seminal vesicles: lie posterior to the bladder, superior to the prostate; join with the vas deferens to form the ejaculatory ducts which pass through the prostate to open into the prostatic urethra at the verumontanum
• Vas deferens: enters the pelvis via the deep inguinal ring, crosses the ureter ("water under the bridge"), and joins the seminal vesicle

• Uterus: lies in the lesser pelvis between the bladder and rectum; consists of fundus, body, isthmus, and cervix; normally anteverted and anteflexed; supported by the broad, round, uterosacral, and cardinal ligaments
• Broad ligament: a double layer of peritoneum flanking the uterus; contains the fallopian tube (in the mesosalpinx), the round ligament (in the mesometrium), and the ovary (attached by the mesovarium)
• Uterosacral ligaments: uterine cervix → sacrum; maintain cervical position
• Cardinal ligaments (Mackenrodt's): cervix/upper vagina → lateral pelvic wall; main support against uterine prolapse
• Ovaries: lie in the ovarian fossa (lateral pelvic wall, below the bifurcation of the common iliac artery); attached to the uterus by the ovarian ligament and to the pelvic wall by the suspensory ligament of the ovary (infundibulopelvic ligament), which carries the ovarian vessels

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5. Pelvic Blood Supply

Arteries

• Superior vesical artery → upper bladder, proximal ureter
• Inferior vesical artery (males) / vaginal artery (females) → bladder base, prostate/vagina
• Obturator artery → medial thigh muscles
• Internal pudendal artery → perineum (crosses through lesser sciatic foramen, runs in Alcock's canal)
• Inferior gluteal artery → gluteal muscles
• Uterine artery (females) → uterus; crosses the ureter at the base of the broad ligament ("water under the bridge")
• Middle rectal artery → middle rectum

• Iliolumbar artery
• Lateral sacral arteries
• Superior gluteal artery (largest branch)

Venous Drainage

• Pelvic venous plexuses (vesical, prostatic, uterovaginal, rectal) drain into the internal iliac veins
• The prostatic venous plexus (of Santorini) connects with the vertebral venous plexus — a route for prostate cancer metastasis

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6. Pelvic Nerves

Lumbosacral Plexus

• Femoral nerve (L2–L4) — exits through greater sciatic foramen
• Obturator nerve (L2–L4) — exits through obturator canal
• Sciatic nerve (L4–S3) — largest nerve; exits below piriformis through greater sciatic foramen
• Superior gluteal nerve (L4–S1) — above piriformis
• Inferior gluteal nerve (L5–S2) — below piriformis
• Posterior cutaneous nerve of the thigh (S1–S3)

Pudendal Nerve (S2–S4)

1. Exits the pelvic cavity through the greater sciatic foramen (below piriformis)
2. Crosses the ischial spine (landmark for pudendal block)
3. Re-enters through the lesser sciatic foramen
4. Runs in the pudendal canal (Alcock's canal) on the lateral wall of the ischiorectal fossa
5. Branches: inferior rectal nerve, perineal nerve (superficial and deep branches), dorsal nerve of the penis/clitoris

Autonomic Innervation

• Sympathetic: presacral nerve (superior hypogastric plexus) → hypogastric nerves → inferior hypogastric plexus; controls ejaculation, vasoconstriction
• Parasympathetic: pelvic splanchnic nerves (S2–S4, "nervi erigentes") → inferior hypogastric plexus; controls erection, bladder detrusor contraction, rectal emptying
• Inferior hypogastric (pelvic) plexus: paired plexuses on the lateral pelvic wall integrating sympathetic + parasympathetic input to all pelvic viscera

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7. The Perineum

• Anteriorly: pubic symphysis
• Laterally: ischiopubic rami and ischial tuberosities
• Posteriorly: tip of the coccyx

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7a. Anal Triangle

• Boundaries: transverse line between ischial tuberosities (base), coccyx (apex), sacrotuberous ligaments (sides)
• Contents: anal aperture (anal canal), ischioanal (ischiorectal) fossa on each side
• Ischioanal fossa: fat-filled wedge-shaped space bounded medially by the pelvic diaphragm, laterally by the obturator internus fascia. Contains the pudendal canal (with pudendal nerve and internal pudendal vessels). The fat allows expansion of the anal canal during defecation. Infections here → perianal abscess/fistula.

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7b. Urogenital Triangle

• Boundaries: pubic symphysis (apex), ischial tuberosities (base), ischiopubic rami (sides)
• Contains the perineal membrane and the perineal pouches

Perineal Membrane

Superficial Perineal Pouch (below the perineal membrane)

Deep Perineal Pouch (above the perineal membrane, within the urogenital diaphragm)

• Deep transverse perineal muscle: stabilises the perineal body
• Sphincter urethrae (external urethral sphincter): voluntary control of micturition
• In males: bulbourethral (Cowper's) glands open into the spongy urethra
• In females: sphincter urethrovaginalis surrounds both urethra and vagina
• Urethra (and vagina in females) pass through this space

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7c. Perineal Body (Central Tendon of the Perineum)

• Levator ani (pelvic diaphragm)
• Bulbospongiosus
• Superficial and deep transverse perineal muscles
• External anal sphincter
• (in females) perineal fibers of the rectovaginal septum

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7d. External Genitalia

• Penis: composed of two corpora cavernosa (dorsal; erectile) and corpus spongiosum (ventral; contains urethra); covered by skin
• Scrotum: contains testes, epididymides, spermatic cords; temperature regulation via the cremaster muscle
• Root of the penis: anchored in the perineum (crura, attached to ischiopubic rami; bulb, attached to perineal membrane)

• Mons pubis: fatty elevation over the pubic symphysis
• Labia majora: homologous to the male scrotum
• Labia minora: enclose the vestibule
• Vestibule: space into which the urethra and vagina open; contains the openings of the greater vestibular (Bartholin's) glands
• Clitoris: erectile structure (two crura + glans); homologous to the penis; highly innervated by the dorsal nerve of the clitoris

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8. Dissection Image — Superficial Perineal Muscles (Male)

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Summary: Pelvic Floor Muscles at a Glance

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Clinical Correlates

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