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🦴 Complete Anatomy of the Pelvis and Perineum

Gray's Anatomy for Students & BDC — Medical Student Reference

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PART I: CONCEPTUAL OVERVIEW

1. Contain and support the terminal parts of the GI tract (rectum, anal canal), urinary tract (bladder, urethra), and reproductive tracts
2. Anchor the roots of the external genitalia (penis/clitoris) to the bony pelvic outlet via the perineal membrane
3. Provide a birth canal in females — the fetus passes from the uterus, through the pelvic inlet, and out through the pelvic outlet

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PART II: BONY PELVIS

Bones

• Two pelvic (hip) bones — each formed by fusion of ilium, ischium, and pubis (fusion occurs at the triradiate cartilage, complete by age 15–17)
• Sacrum (S1–S5 fused vertebrae)
• Coccyx (3–4 rudimentary vertebrae)

• Anteriorly — at the pubic symphysis (secondary cartilaginous joint / amphiarthrosis)
• Posteriorly — at the sacroiliac joints (synovial + syndesmosis)

Pelvic Inlet (Superior Pelvic Aperture)

🔑 USMLE Pearl: The obstetric conjugate (anteroposterior diameter of inlet) is the shortest fixed diameter (~11 cm in females) — measured from sacral promontory to posterosuperior margin of pubic symphysis. The true conjugate (anatomical) = promontory to top of symphysis (~11.5 cm). The diagonal conjugate (~12.5 cm) is the only one measurable clinically (subtract 1.5 cm to estimate obstetric conjugate).

Pelvic Outlet (Inferior Pelvic Aperture)

• Anterior midline: Inferior border of pubic symphysis
• Anterolateral: Inferior pubic ramus → ischial ramus → ischial tuberosity (forming the pubic arch)
• Posterolateral: Sacrotuberous ligament
• Posterior midline: Coccyx

🔑 USMLE Pearl: The subpubic angle (pubic arch): ~80–85° in females (wider), ~50–60° in males (narrower). Wider angle favors vaginal delivery.

Greater vs. True (Minor) Pelvis

• Greater (false) pelvis — above the pelvic inlet; bounded by iliac wings; contains lower abdominal organs (cecum, sigmoid, ileal loops, uterus when pregnant)
• True (lesser) pelvis — below the pelvic inlet; contains pelvic viscera

🔑 USMLE Pearl: The linea terminalis (terminal line) separates greater from true pelvis. It runs from sacral promontory → arcuate line → pectineal line → pubic crest.

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PART III: PELVIC WALLS

Anterolateral Wall

• The obturator foramen (largest foramen in the body), covered by the obturator membrane except at its superior angle (the obturator canal)
• Obturator internus muscle — lines the anterolateral pelvic wall; its tendon exits through the lesser sciatic foramen, makes a 90° turn around the ischium, then inserts on the greater trochanter

Posterolateral Wall

• Piriformis muscle — arises from the anterior aspect of the sacrum (between the anterior sacral foramina S2–S4), exits through the greater sciatic foramen, inserts on the greater trochanter

• Suprapiriform space → Superior gluteal nerve and vessels
• Infrapiriform space → Inferior gluteal nerve/vessels, sciatic nerve, pudendal nerve, internal pudendal vessels, posterior femoral cutaneous nerve, nerve to obturator internus, nerve to quadratus femoris

Three Apertures in the Lateral Pelvic Wall

🔑 USMLE Pearl: The pudendal nerve and internal pudendal vessels exit via the greater sciatic foramen (below piriformis), loop around the ischial spine and sacrospinous ligament, and re-enter the perineum via the lesser sciatic foramen. This is why pudendal nerve block is performed at the ischial spine.

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PART IV: LIGAMENTS OF THE PELVIS

🔑 USMLE Pearl: The cardinal ligament (Mackenrodt's ligament) is the most important for preventing uterine prolapse. It contains the uterine artery and ureter (ureter runs under the uterine artery — "water under the bridge"). Injury during hysterectomy causes ureteral ligation.

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PART V: PELVIC FLOOR (PELVIC DIAPHRAGM)

• Supports pelvic viscera against intra-abdominal pressure
• Controls the openings of the urethra, vagina, and anal canal

Components

• Origin: Ischial spine
• Insertion: Coccyx and lower sacrum
• Innervation: S4–S5
• Functionally similar to sacrospinous ligament

🔑 USMLE Pearl: Weakening of the puborectalis (e.g., after difficult vaginal delivery) leads to loss of the anorectal angle, contributing to fecal incontinence. Damage to the pelvic floor may cause cystocele, rectocele, or uterine prolapse.

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PART VI: PERINEAL MEMBRANE AND DEEP PERINEAL POUCH

Perineal Membrane

• A thick fibrous sheet that fills the urogenital triangle
• Free posterior border anchored to the perineal body
• Lateral attachments to the ischiopubic rami (pubic arch)
• The urethra (and vagina in females) pierce this membrane

Deep Perineal Pouch (Space)

🔑 USMLE Pearl: Cowper's (bulbourethral) glands are in the deep perineal pouch. Their ducts open into the bulbous urethra. The glands of Littre (urethral glands) line the spongy urethra. Both produce mucus during sexual arousal.

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PART VII: PELVIC FASCIAE

Parietal Pelvic Fascia

Visceral Pelvic Fascia

• Rectovaginal septum — separates posterior vagina from rectum
• Vesicovaginal septum — separates anterior vagina from bladder
• Urogenital diaphragm — older term for deep perineal pouch contents

Endopelvic Fascia Condensations (In Women)

• Pubocervical ligament — cervix → pubis (anterior)
• Cardinal (Mackenrodt's) ligament — cervix → lateral pelvic wall
• Uterosacral ligament — cervix/isthmus → sacrum (posterior)

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PART VIII: PERITONEUM IN THE PELVIS

In Males

• Rectovesical pouch — between bladder (and seminal vesicles) and rectum; the most dependent point of the male peritoneal cavity

In Females

• Vesicouterine pouch — between bladder and uterus; relatively shallow
• Recto-uterine pouch (Pouch of Douglas / Douglas' cul-de-sac) — between uterus and rectum; the most dependent point of the female peritoneal cavity

• Uterine tube (in the free edge)
• Round ligament of uterus
• Ligament of ovary
• Uterine and ovarian vessels, lymphatics, nerves

🔑 USMLE Pearl: Free fluid (blood, pus) collects in the pouch of Douglas in females — accessible by culdocentesis (needle through the posterior vaginal fornix). In ruptured ectopic pregnancy or PID with pelvic abscess, this pouch fills with blood/pus.

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PART IX: PELVIC VISCERA

A. Urinary Bladder

• Apex — points anterosuperiorly; connected to umbilicus by the median umbilical ligament (remnant of urachus)
• Base (fundus) — posteroinferior; related to seminal vesicles + ductus deferentes (males) or vagina (females)
• Superior surface — covered by peritoneum
• Two inferolateral surfaces — rest on pelvic floor muscles
• Neck — where the two inferolateral surfaces and base converge; most fixed part; anchored by pubovesical (females) or puboprostatic (males) ligaments

• Trigone — triangular smooth area on the posterior wall; bounded by the two ureteric orifices (superolateral) and the internal urethral orifice (inferior)
• Interureteric fold (ridge) — connects the two ureteric orifices; raised ridge of mucosa
• The trigone is derived from the mesonephric (Wolffian) duct — this is why it looks different (smoother) from the rest of the bladder mucosa
• Uvula vesicae (in males) — a slight elevation just above the internal urethral orifice due to the underlying median lobe of the prostate

• Detrusor muscle: Parasympathetic (S2–S4 via pelvic splanchnic nerves) → contraction/voiding
• Internal urethral sphincter (smooth): Sympathetic (T11–L2 via hypogastric nerve) → maintains tone during ejaculation (prevents retrograde ejaculation); alpha-1 receptors
• External urethral sphincter (skeletal): Somatic pudendal nerve (S2–S4)

🔑 USMLE Pearl: Alpha-1 blockers (tamsulosin) relax the internal urethral sphincter and prostate smooth muscle → treat BPH. Anticholinergics (oxybutynin) block detrusor (M3 receptors) → treat overactive bladder. Spinal cord transection above the sacral micturition center → upper motor neuron bladder (spastic, reflexic, small capacity). Damage to the sacral center (S2–S4, conus medullaris) → lower motor neuron bladder (flaccid, areflexic, overflow incontinence).

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B. Ureters in the Pelvis

🔑 USMLE Pearl — Three sites of ureteral narrowing (where stones get stuck):

1. Ureteropelvic junction (UPJ) — renal pelvis–ureter junction
2. Pelvic brim — where ureter crosses over iliac vessels
3. Ureterovesical junction (UVJ) — where ureter enters bladder wall (most common obstruction site)

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C. Male Reproductive Organs in the Pelvis

Prostate

• Fibromuscular glandular organ; roughly chestnut-shaped
• Lies inferior to bladder neck, posterior to pubic symphysis, anterior to rectum
• Surrounds the prostatic urethra
• Divided into zones (McNeal):
  • Peripheral zone (~70%) — posterior and lateral; site of most prostate cancers; palpable on DRE
  • Central zone (~25%) — surrounds ejaculatory ducts
  • Transition zone (~5%) — surrounds proximal urethra; site of BPH
  • Anterior fibromuscular stroma — no glandular tissue
• Lobes (classic description): Anterior, posterior, median, two lateral lobes
• Fascia: Prostatic capsule (fibromuscular) + visceral pelvic fascia; neurovascular bundles lie posterolateral to the prostate (site of erection nerves — cavernous nerves from pelvic plexus)
• Relations: Ejaculatory ducts pierce the posterior surface to open into the prostatic urethra at the seminal colliculus (verumontanum)

🔑 USMLE Pearl: PSA (prostate-specific antigen) is made only by prostate epithelium. BPH causes obstructive symptoms (hesitancy, weak stream, nocturia) from transition zone enlargement. Prostate cancer is most common in the peripheral zone — detected by DRE (hard, nodular) or elevated PSA. The nerve-sparing prostatectomy preserves the neurovascular bundles to prevent erectile dysfunction.

Seminal Vesicles

• Paired sacculated glands posterior to the bladder base and superior to the prostate
• Produce ~60% of semen volume (fructose-rich, alkaline fluid)
• Each joins the ampulla of ductus deferens to form the ejaculatory duct
• Ejaculatory ducts pass through the prostate to open at the verumontanum (seminal colliculus) in the prostatic urethra

Ductus Deferens (Vas Deferens)

• Thick-walled muscular tube; continuation of the epididymis
• Course: Epididymis → passes through the inguinal canal → crosses external iliac vessels → arches over ureter → ampulla → joins seminal vesicle duct → ejaculatory duct
• Vasectomy = most common site of ligation is at the scrotal neck (accessible part)

🔑 USMLE Pearl: The ductus deferens crosses over the ureter near the posterior bladder — "the ductus deferens crosses the ureter from lateral to medial." During prostatectomy, the ureter may be injured at this crossing.

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D. Female Reproductive Organs in the Pelvis

Uterus

• Thick-walled muscular organ; piriform shape; ~7.5 × 5 × 2.5 cm (nulliparous)
• Parts: Fundus (above uterine tube origins), body (corpus), isthmus, cervix
• Walls: Endometrium (innermost, shed monthly), myometrium (middle, thick smooth muscle), perimetrium (serosa)
• Normal position: Anteversion (~90°, angle between long axis of uterus and vagina) + Anteflexion (~170°, angle between corpus and cervix)
• Retroversion = common normal variant (~20% of women); only clinically significant if retroverted AND retroflexed

• Anterior: Vesicouterine pouch → bladder
• Posterior: Recto-uterine pouch (Douglas) → rectum
• Lateral: Broad ligament, uterine vessels, ureter

1. Pelvic floor (levator ani) — primary
2. Cardinal ligament — most important ligament; prevents uterine descent
3. Uterosacral ligaments — maintain anteversion; contain autonomic nerve fibers
4. Round ligaments — maintain anteversion; NO supporting function against prolapse (they stretch); vestigial remnant of gubernaculum

🔑 USMLE Pearl: The round ligament travels through the inguinal canal to reach the labium majus — homologous to the gubernaculum testis in males. It is accompanied by the ilioinguinal nerve in the inguinal canal.

Cervix

• Lower cylindrical part of uterus; projects into vaginal vault
• External os — opening into vaginal vault (visible on speculum exam)
• Internal os — junction of cervical canal and uterine cavity
• Transformation zone (squamocolumnar junction) — where columnar epithelium of endocervix meets squamous epithelium of ectocervix; site of origin of cervical carcinoma (HPV)
• Cervical mucus becomes thin and watery at ovulation (sperm-friendly), thick under progesterone influence

🔑 USMLE Pearl: Cervical incompetence causes second-trimester pregnancy loss (painless dilation of cervix). Treated with cerclage. HPV types 16 and 18 cause ~70% of cervical cancers; HPV 6 and 11 cause condylomata.

Uterine (Fallopian) Tubes

• Parts: Intramural (within uterine wall), isthmus (narrowest), ampulla (widest, commonest site of fertilization and ectopic pregnancy), infundibulum (fimbriated end, opens into peritoneal cavity)
• Cilia move oocyte toward uterus; smooth muscle peristalsis also contributes

🔑 USMLE Pearl: ~95% of ectopic pregnancies occur in the ampulla. Risk factors: PID, prior tubal surgery, IUD, endometriosis, prior ectopic. Presents with amenorrhea + pelvic pain + vaginal bleeding + positive β-hCG. A ruptured ectopic is a surgical emergency.

Ovaries

• Paired almond-shaped gonads (~4 × 2 × 1 cm) on the posterior surface of the broad ligament
• Attached to: Posterior broad ligament (mesovarium), uterus (ovarian ligament), pelvic wall (suspensory/infundibulopelvic ligament)
• Suspensory (infundibulopelvic) ligament carries the ovarian artery, vein, and lymphatics
• Blood supply: Ovarian artery (from aorta at L2) → drains via ovarian vein (right → IVC directly; left → left renal vein)

🔑 USMLE Pearl: Lymphatic drainage of the ovary and testis follows the gonadal vessels to para-aortic (lumbar) lymph nodes at L2 — NOT to inguinal nodes (inguinal LNs drain the scrotum/labium majus skin). This is why ovarian and testicular cancers metastasize to para-aortic nodes.

Vagina

• Fibromuscular tube; courses posterosuperiorly from vestibule to cervix
• Fornices: Posterior (deepest, related to pouch of Douglas), anterior (related to bladder base), two lateral
• Posterior fornix is directly related to the pouch of Douglas (separated only by peritoneum + vaginal wall)
• Normal epithelium: Stratified squamous non-keratinizing (estrogen-maintained)
• Lubricated by transudation (not glands); greater vestibular (Bartholin's) glands provide additional lubrication

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E. Rectum and Anal Canal

Rectum

• 12–15 cm long; begins at S3 (rectosigmoid junction), ends at anorectal junction
• Three lateral curvatures with corresponding transverse folds (Houston's valves / plicae transversales recti): upper right, middle left (most prominent), lower right
• Ampulla — the wide lower part that stores feces
• Relations:
  • Posterior: Sacrum, coccyx, piriformis, lower sacral plexus, median sacral vessels
  • Anterior (males): Rectovesical pouch, seminal vesicles, prostate, membranous and prostatic urethra
  • Anterior (females): Recto-uterine pouch, posterior vaginal wall, uterus

Anal Canal

• 3–4 cm long; extends from anorectal junction to anal verge (skin)
• Dentate (pectinate) line — key surgical and anatomical landmark at the level of the anal valves (at the midpoint of the anal canal)
• Embryological significance: Marks the junction of endoderm (cloacal membrane/hindgut) above and ectoderm (proctodeum) below

🔑 USMLE Pearl: Porto-systemic anastomosis occurs at the rectum (superior rectal/portal ↔ inferior rectal/systemic). In portal hypertension → anorectal varices. The anal columns of Morgagni are longitudinal folds above the dentate line; between them lie the anal sinuses where anal glands open.

• Internal anal sphincter (IAS): Smooth muscle; continuation of inner circular layer of rectal wall; involuntary; maintained in tonic contraction by sympathetic tone (L1–L2); relaxed by parasympathetics
• External anal sphincter (EAS): Skeletal muscle; 3 parts (subcutaneous, superficial, deep); voluntary; innervated by inferior rectal nerve (branch of pudendal, S2–S4) and perineal branch of S4; the deep part is continuous with puborectalis

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PART X: NERVES OF THE PELVIS

Lumbosacral Plexus

Autonomic Nervous System in the Pelvis

• Superior hypogastric plexus — descends from the aortic plexus over L5 vertebra; bifurcates into the two hypogastric nerves
• Inferior hypogastric plexus (pelvic plexus) — formed by hypogastric nerves (sympathetic, T10–L2) + pelvic splanchnic nerves (parasympathetic, S2–S4); lies lateral to the rectum, vagina, prostate

• Pelvic splanchnic nerves (nervi erigentes) — arise directly from S2–S4; join the pelvic plexus → supply pelvic viscera and contribute to erection

🔑 USMLE Pearl — Erection vs. Ejaculation:

• Erection = PARASYMPATHETIC (S2–S4; "Point" = parasympathetic); NO (nitric oxide) mediates vasodilation; the cavernous nerves (branches of pelvic plexus) innervate the penile/clitoral erectile tissue
• Emission (movement of semen into posterior urethra) = SYMPATHETIC (T10–L2)
• Ejaculation (propulsion) = SOMATIC (pudendal nerve, S2–S4)

Mnemonic: "Point and Shoot" — Parasympathetic = erection (point); Sympathetic = emission/ejaculation (shoot)

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PART XI: BLOOD VESSELS OF THE PELVIS

Arteries

• Branches from common iliac artery at the pelvic brim (sacroiliac joint level)
• Divides into anterior and posterior divisions

• Uterine artery — runs in the base of the broad ligament; crosses above the ureter ("ureter runs under the bridge") 2 cm lateral to cervix
• Ovarian artery — directly from abdominal aorta at L2
• Superior rectal artery — terminal branch of IMA → supplies upper rectum and joins with middle rectal
• Median sacral artery — from aortic bifurcation; runs on anterior sacrum

🔑 USMLE Pearl: In ectopic pregnancy surgery, ligation of the uterine artery is key. During total hysterectomy, the ureter may be ligated when tying the uterine artery — check for ureteral peristalsis before cutting. Postpartum hemorrhage from uterine atony can be managed by uterine artery ligation or hypogastric (internal iliac) artery ligation.

Veins

• Correspond to the arteries (paired)
• Form the pelvic venous plexuses around each organ (vesical plexus, prostatic plexus, uterovaginal plexus, rectal plexus)
• Prostatic venous plexus (of Santorini) communicates with the Batson vertebral venous plexus — route of prostate cancer metastasis to lumbar vertebrae (osteoblastic metastases — increased PSA)

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PART XII: LYMPHATICS OF THE PELVIS

🔑 USMLE Pearl Lymph Drainage Summary:

• Testis/ovary → para-aortic (L2) — NOT inguinal
• Scrotum/labium majus skin → superficial inguinal
• Glans penis/clitoris → deep inguinal
• Anal canal above dentate → internal iliac
• Anal canal below dentate → superficial inguinal
• Uterine body → external/internal iliac + para-aortic (via ovarian vessels at fundus)
• Cervix → internal and external iliac nodes (primarily)

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PART XIII: THE PERINEUM

Boundaries and Divisions

• Anterior apex: Inferior border of pubic symphysis
• Posterior apex: Coccyx tip
• Lateral points: Ischial tuberosities
• Anterior boundary (sides): Ischiopubic rami
• Posterior boundary (sides): Sacrotuberous ligaments

• Urogenital triangle (anterior) — horizontal plane; associated with urinary and reproductive openings; contains roots of external genitalia
• Anal triangle (posterior) — tilted slightly posteriorly; contains anus and ischio-anal fossae

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Ischio-Anal (Ischiorectal) Fossae

• Medial wall: External anal sphincter + levator ani
• Lateral wall: Obturator internus covered by obturator fascia
• Roof: Levator ani (pelvic diaphragm)
• Floor: Skin of perianal region
• Anteriorly: Communicates with the anterior recess (extends above the perineal membrane, below the levator ani on either side of the urethra and vagina)
• Posteriorly: Limited by the sacrotuberous ligament and gluteus maximus

• Fat (important cushion for defecation and childbirth)
• Pudendal canal (Alcock's canal) — within the obturator fascia on the lateral wall; contains the pudendal nerve and internal pudendal vessels

🔑 USMLE Pearl: Perianal/ischio-anal abscess — the most common perianal infection; starts in anal glands (at dentate line); tracking of pus gives rise to superficial, intersphincteric, ischio-anal, or supralevator abscesses. Ischio-anal abscesses may communicate across the posterior midline (through the posterior ischio-anal fossa connecting across behind the anal canal — horseshoe abscess).

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Anal Triangle

1. Anal canal — with internal and external anal sphincters
2. Ischio-anal fossae — bilateral fat-filled spaces
3. Anococcygeal body (ligament) — fibromuscular mass between anal canal and coccyx
4. Terminal branches of inferior rectal nerve and inferior rectal vessels

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Urogenital Triangle

Superficial Perineal Pouch

🔑 USMLE Pearl: Bartholin's gland cyst/abscess occurs when the duct becomes blocked, forming a cyst; if infected (usually by Neisseria gonorrhoeae or S. aureus), an abscess forms at the posterior lateral vestibule (4 or 8 o'clock position). Treated by marsupializaton.

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Perineal Body (Central Tendon of the Perineum)

• A fibromuscular mass in the midline at the junction of the urogenital and anal triangles; ~3–4 cm anterior to the anal verge
• Attachment point for 8 muscles:
  • External anal sphincter (EAS)
  • External urethral sphincter (EUS)
  • Bulbospongiosus (bilateral)
  • Superficial transverse perineal (bilateral)
  • Deep transverse perineal (bilateral)
  • Levator ani (puborectalis fibers)

🔑 USMLE Pearl: The perineal body is critically important in obstetrics. A 3rd degree perineal tear involves the external anal sphincter; a 4th degree tear involves the rectal mucosa. Damage to the perineal body weakens pelvic floor support and may cause fecal incontinence, rectocele, or posterior vaginal wall prolapse. Episiotomy is a deliberate cut to enlarge the vaginal orifice during delivery.

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Male Urethra (4 Parts)

🔑 USMLE Pearl: The membranous urethra is the most vulnerable segment in pelvic fractures (straddle injuries in males). When the membranous urethra is injured, blood may extravasate into the perineum and scrotum (limited by Colles' fascia — superficial perineal fascia). Colles' fascia is continuous with Scarpa's fascia on the abdomen — blood can track up to the abdomen but NOT down the thigh (Colles' fascia fuses with fascia lata at the thigh).

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Female Urethra

• Only ~4 cm long (much shorter than male)
• Runs from the bladder neck through the deep perineal pouch and perineal membrane, opens in the vestibule anterior to the vaginal orifice
• Surrounded by the external urethral sphincter in the deep perineal pouch
• Lined by: Transitional epithelium superiorly → pseudostratified columnar → stratified squamous distally

🔑 USMLE Pearl: The short female urethra explains why UTIs are more common in women — bacteria from the perineum colonize the urethra more easily. Urethral caruncle (painful red protrusion at urethral meatus in postmenopausal women) is the most common benign lesion of the female urethra.

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PART XIV: PUDENDAL NERVE — THE NERVE OF THE PERINEUM

1. Exits pelvis via greater sciatic foramen (below piriformis)
2. Crosses the ischial spine and sacrospinous ligament
3. Re-enters perineum via lesser sciatic foramen
4. Travels in Alcock's (pudendal) canal on the lateral wall of the ischio-anal fossa
5. Divides into three terminal branches:

🔑 USMLE Pearl: Pudendal nerve block — needle directed at the ischial spine (transvaginally or transperineally); used for obstetric analgesia during second-stage labor and perineal repairs. Pudendal neuralgia causes chronic perineal pain; can result from cycling (cyclist's syndrome) or childbirth trauma. The pudendal nerve is the most commonly blocked nerve in obstetrics.

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PART XV: KEY USMLE HIGH-YIELD CLINICAL CORRELATES

Pelvic Fractures

• Posterior urethral injury (membranous urethra) in males → blood at urethral meatus, scrotal hematoma, "high-riding" prostate on DRE → retrograde urethrogram before catheterization
• Open book fracture (symphysis diastasis) → massive pelvic bleeding from pelvic venous plexuses; emergency stabilization with pelvic binder

Ectopic Pregnancy

• 95% in ampulla of uterine tube; rupture → hemoperitoneum → blood in pouch of Douglas → culdocentesis positive; β-hCG positive without intrauterine pregnancy on US

Uterine Prolapse

• Due to weakness of cardinal and uterosacral ligaments + pelvic floor; graded I–III (or using POP-Q system); cystocele (anterior wall prolapse), rectocele (posterior wall prolapse), enterocele (pouch of Douglas prolapse)

Iatrogenic Ureteral Injury

• Commonest sites: (1) Ligation with uterine artery during hysterectomy (2 cm lateral to cervix), (2) Infundibulopelvic ligament ligation (ovarian vessels), (3) Pelvic brim crossing
• Presents postoperatively as urinary leak, hydronephrosis, or hematuria

Prostate Cancer Metastasis

• Via Batson's paravertebral venous plexus to lumbar vertebrae → osteoblastic (sclerotic) bone metastases → elevated PSA + elevated alkaline phosphatase
• Lymphatic spread first to obturator/internal iliac nodes then to common iliac → para-aortic

Ovarian Torsion

• Twisting of the ovary on the suspensory (infundibulopelvic) ligament → ischemia; sudden onset severe pelvic pain, nausea/vomiting; US shows enlarged ovary ± absent Doppler flow

Portal Hypertension and Rectal Varices

• Porto-systemic anastomosis at rectum; esophageal varices (left gastric ↔ azygos), rectal varices (superior rectal ↔ inferior rectal), periumbilical veins (caput medusae), retroperitoneal (veins of Retzius and Treitz)

Sciatic Nerve Compression

• Compression by uterus (pregnancy), pelvic mass, or piriformis (piriformis syndrome); pain radiating down posterior thigh and below knee; foot drop (peroneal division, L4–L5)

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QUICK REFERENCE: EMBRYOLOGICAL DERIVATIVES

🔑 USMLE Pearl: Müllerian agenesis (MRKH syndrome) = congenital absence of uterus and upper vagina (karyotype 46,XX); primary amenorrhea with normal secondary sexual characteristics and normal ovaries. Androgen Insensitivity Syndrome (AIS) = 46,XY, female phenotype; absent uterus/tubes (testes present; Müllerian inhibiting factor still produced), normal breasts but absent/sparse pubic hair.

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ANATOMICAL IMAGES FROM GRAY'S ANATOMY FOR STUDENTS

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MASTER SUMMARY TABLE: MUSCLES OF THE PELVIC FLOOR AND PERINEUM

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What's Included (15 Major Sections):

🦴 COMPLETE ANATOMY OF THE PELVIS AND PERINEUM

Gray's Anatomy for Students (Drake et al.) + BDC (Moore, Dalley & Agur) — Complete MBBS Reference

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PART 1: THE PELVIS

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SECTION 1.1 — CONCEPTUAL OVERVIEW

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SECTION 1.2 — BONY PELVIS

Bones Forming the Pelvis

1. Two pelvic (hip) bones — each formed by fusion of ilium, ischium, and pubis
2. Sacrum (S1–S5 fused vertebrae)
3. Coccyx (3–4 rudimentary vertebrae)

• Anteriorly: Pubic symphysis (secondary cartilaginous joint)
• Posteriorly: Two sacroiliac joints (synovial + syndesmosis)

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The Pelvic Bone (Hip Bone)

ILIUM (most superior)

• Upper part = lateral wall of false pelvis
• Lower part = lateral wall of true pelvis
• Arcuate line — rounded anterior ridge separating upper/lower parts; contributes to pelvic inlet margin (linea terminalis)
• Iliac crest — superior border; ASIS and AIIS are important landmarks
• Sacral surface — L-shaped facet + roughened area for strong sacroiliac ligaments
• Iliac fossa — concave inner surface; site of origin of iliacus muscle
• Gluteal surface — 3 lines (anterior, posterior, inferior) for gluteal muscle attachment

ISCHIUM (posteroinferior)

• Body — forms posterolateral acetabulum wall
• Ischial tuberosity — large, rough; we sit on these; attachment of hamstrings
• Ischial spine — projects posteromedially between greater and lesser sciatic notches; landmark for pudendal nerve block
• Greater sciatic notch — above ischial spine
• Lesser sciatic notch — below ischial spine
• Ramus of ischium — joins inferior pubic ramus to form ischiopubic ramus

PUBIS (anterior)

• Body — forms anterior acetabulum
• Superior pubic ramus — extends from body to ilium; has pectineal line (pecten pubis) on superior border
• Inferior pubic ramus — joins ischial ramus → ischiopubic ramus
• Pubic tubercle — small projection on anterior body; attachment of inguinal ligament
• Pubic crest — medial border of superior ramus
• Obturator foramen — largest foramen in the body; bounded by ischium and pubis; mostly closed by obturator membrane

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Sacrum

• 5 fused sacral vertebrae (S1–S5)
• Triangular, concave anteriorly
• Sacral promontory — anterior projection of S1 body into pelvic inlet
• Alae (wings) — lateral parts of S1; contribute to pelvic inlet
• Anterior sacral foramina (4 pairs) — passage of anterior rami of S1–S4
• Posterior sacral foramina (4 pairs) — passage of posterior rami
• Median sacral crest — fused spinous processes
• Sacral hiatus — inferior opening of sacral canal; used for caudal epidural block
• Sacral canal — continuation of vertebral canal; contains sacral nerve roots + filum terminale

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Coccyx

• 3–4 rudimentary fused vertebrae
• Tip — attachment of sacrotuberous ligament, levator ani, sphincter ani externus, gluteus maximus
• Coccydynia = pain at coccyx (common after falls or difficult childbirth)

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SECTION 1.3 — PELVIC JOINTS AND LIGAMENTS

Pubic Symphysis

• Type: Secondary cartilaginous joint (symphysis)
• United by fibrocartilaginous interpubic disc
• Very little movement; increases slightly during pregnancy (relaxin hormone)
• Diastasis symphysis pubis — widening > 10 mm on X-ray; can occur after childbirth

Sacroiliac Joints

• Type: Synovial anteriorly + syndesmosis posteriorly (strongest joint in body)
• Very little movement (primarily shock absorption)
• Supported by:
  • Anterior sacroiliac ligament (weaker)
  • Posterior sacroiliac ligament (extremely strong)
  • Interosseous sacroiliac ligament (strongest — fills the space between the bones)
  • Iliolumbar ligament (L4–L5 transverse processes → iliac crest)

Sacrotuberous Ligament

• From: Posterior ilium, sacrum, coccyx → ischial tuberosity
• Converts the greater and lesser sciatic notches into foramina
• Resists forward rotation (nutation) of sacrum
• Forms posterior wall of lesser sciatic foramen

Sacrospinous Ligament

• From: Sacrum/coccyx → ischial spine
• Lies anterior to sacrotuberous ligament
• Converts greater sciatic notch into foramen
• Separates greater from lesser sciatic foramen
• Site of pudendal nerve block (at ischial spine, just medial to ligament)

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SECTION 1.4 — FALSE vs. TRUE PELVIS

• Sacral promontory
• Alae of sacrum
• Arcuate line of ilium
• Pectineal line (pecten pubis)
• Pubic crest
• Pubic symphysis

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SECTION 1.5 — PELVIC INLET AND OUTLET

Pelvic Inlet (Superior Aperture)

• Posterior: Sacral promontory + alae
• Lateral: Arcuate line + pectineal line
• Anterior: Pubic crest + pubic symphysis

Pelvic Outlet (Inferior Aperture)

• Anterior apex: Inferior border of pubic symphysis
• Anterolateral: Inferior pubic ramus + ischial ramus + ischial tuberosity → pubic arch
• Posterolateral: Sacrotuberous ligament
• Posterior apex: Coccyx tip

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SECTION 1.6 — MALE vs. FEMALE PELVIS

Viva point: Four pelvic types — Gynecoid (50%, ideal for delivery), Android (male-type), Anthropoid (AP elongated), Platypelloid (transversely wide, AP narrow).

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SECTION 1.7 — PELVIC WALLS

Anterolateral Wall

• Origin: Inner surface of obturator membrane + surrounding bone
• Course: Passes laterally, exits through lesser sciatic foramen (makes a 90° turn around ischium between ischial spine and tuberosity)
• Insertion: Greater trochanter (medial surface)
• Action: Lateral rotation of hip; abduction when hip is flexed
• Innervation: Nerve to obturator internus (L5, S1, S2)
• Forms the largest part of anterolateral pelvic wall

• Contents: Obturator nerve (L2–L4) + obturator vessels

Posterolateral Wall

• Origin: Anterior surface of sacrum (between anterior sacral foramina S2–S4)
• Course: Passes laterally through greater sciatic foramen
• Insertion: Greater trochanter (tip/apex)
• Action: Lateral rotation of hip; abduction when hip is flexed
• Innervation: S1, S2 directly
• Forms large part of posterolateral pelvic wall
• Divides greater sciatic foramen into suprapiriform and infrapiriform spaces

Three Major Apertures in Lateral Pelvic Wall

Key concept: The pudendal nerve exits through the greater sciatic foramen (below piriformis), loops around the ischial spine/sacrospinous ligament, then re-enters the perineum through the lesser sciatic foramen → enters Alcock's (pudendal) canal on lateral wall of ischio-anal fossa.

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SECTION 1.8 — PELVIC FLOOR (PELVIC DIAPHRAGM)

Components:

1. Levator ani (primary, paired)
2. Coccygeus (ischiococcygeus) (posterior)
3. Anteriorly completed by: perineal membrane + deep perineal pouch muscles

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Levator Ani — Three Parts

• Primarily from nerve to levator ani (S3, S4) on its superior (pelvic) surface
• Also by branch of pudendal nerve (inferior rectal branch) on its inferior surface in some individuals

• In both sexes: Urethra
• In females additionally: Vagina

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Coccygeus (Ischiococcygeus)

• Origin: Ischial spine
• Insertion: Lateral sacrum + coccyx
• Innervation: S4, S5
• Functionally resembles the sacrospinous ligament (overlies it)
• Flexes coccyx after defecation/parturition

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SECTION 1.9 — PELVIC VISCERA

A. URINARY SYSTEM IN THE PELVIS

Ureters (Pelvic Part)

• In males: Crossed superiorly by the ductus deferens (ductus deferens arches over the ureter)
• In females: Crossed superiorly by the uterine artery ("water under the bridge" — ureter passes UNDER the uterine artery, ~2 cm lateral to the cervix)

1. Ureteropelvic junction (UPJ) — where renal pelvis meets ureter
2. Pelvic brim — where ureter crosses over iliac vessels
3. Ureterovesical junction (UVJ) — where ureter enters the bladder wall (narrowest, most common obstruction)

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Urinary Bladder

• Smooth triangular area on posterior bladder wall
• Bounded by: 2 ureteric orifices (superolateral) + 1 internal urethral orifice (inferior)
• Interureteric fold (ridge) — raised mucosal fold connecting the two ureteric orifices
• Embryological origin: Mesonephric (Wolffian) duct — explains its smooth appearance vs. trabeculated rest of bladder
• In males: Uvula vesicae — small elevation above internal urethral orifice from underlying median lobe of prostate

• Anchored by: Pubovesical ligaments (females) / Puboprostatic ligaments (males)

Micturition reflex: Bladder fills → stretch receptors activated → signal via pelvic splanchnics → pontine micturition center → voiding response. The pontine micturition center (PMC) coordinates relaxation of EUS with detrusor contraction.

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Urethra

• Runs from bladder neck → through deep perineal pouch and perineal membrane → opens in vestibule (anterior to vaginal orifice, posterior to clitoris)
• Surrounded by external urethral sphincter in deep perineal pouch
• Lined by: Transitional → pseudostratified columnar → stratified squamous distally

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B. MALE REPRODUCTIVE ORGANS IN PELVIS

Prostate

• Superior: Bladder neck (puboprostatic ligaments)
• Inferior: Pelvic diaphragm (urogenital hiatus)
• Anterior: Pubic symphysis (separated by retropubic space of Retzius)
• Posterior: Rectum (separated by rectovesical fascia of Denonvilliers)
• Posterolateral: Neurovascular bundles (cavernous nerves + prostatic venous plexus)

• Anterior lobe — in front of urethra; no glands
• Posterior lobe — behind urethra, below ejaculatory ducts; palpable on DRE
• Median (middle) lobe — between urethra and ejaculatory ducts; can compress urethra in BPH
• Two lateral lobes — largest; on each side of urethra

• True fibromuscular capsule surrounds prostate
• Visceral pelvic fascia surrounds capsule
• Prostatic venous plexus (of Santorini) — between capsule and fascia; communicates with Batson's paravertebral venous plexus → route of prostate cancer to lumbar vertebrae (osteoblastic mets)

Seminal Vesicles

• Paired sacculated pouches on posterior bladder base, superior to prostate
• Secrete ~60% of seminal volume (fructose-rich, alkaline, prostaglandins)
• Each unites with ampulla of ductus deferens → ejaculatory duct
• Ejaculatory ducts pierce posterior prostate → open at verumontanum in prostatic urethra

Ductus (Vas) Deferens

1. Epididymis (tail) → ascends in scrotum as part of spermatic cord
2. Passes through inguinal canal → deep inguinal ring
3. Arches over pelvic brim lateral to inferior epigastric vessels
4. Crosses external iliac vessels → arches over ureter medially
5. Runs posteromedially along lateral pelvic wall
6. Dilates → ampulla (behind bladder)
7. Joins seminal vesicle duct → ejaculatory duct

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C. FEMALE REPRODUCTIVE ORGANS IN PELVIS

Uterus

• Endometrium — inner mucosa; shed cyclically; stratum functionalis + basalis
• Myometrium — thick smooth muscle; largest layer; responsible for labour contractions
• Perimetrium — outer peritoneal serosa

• Anteversion — uterus angled ~90° anteriorly on the vagina (angle at cervicovaginal junction)
• Anteflexion — body angled ~170° anteriorly on cervix (angle at isthmus)
• Retroversion + retroflexion — common variant (~20% women); usually no symptoms

• Anterior: Vesicouterine pouch → bladder base
• Posterior: Recto-uterine pouch (Douglas) → rectum
• Lateral: Broad ligament, uterine artery, ureter (~2 cm lateral to cervix)
• Above: Uterine tubes project laterally; free peritoneum
• Below: Vaginal vault

Viva pearl: "The round ligament maintains anteversion but cannot prevent prolapse."

• External os — visible on speculum; circular in nulliparous (oval/fish-mouth in multiparous)
• Internal os — junction of cervical canal and uterine cavity
• Cervical canal — contains cervical glands secreting mucus
• Transformation zone (squamocolumnar junction) — junction of columnar endocervical epithelium and squamous ectocervical epithelium; most common site of cervical carcinoma (HPV 16, 18)
• Cervical incompetence — painless dilatation of cervix in second trimester → habitual abortion; treated by cerclage

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Uterine (Fallopian) Tubes

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Ovaries

• Mesovarium — short fold of peritoneum attaching ovary to posterior broad ligament
• Ligament of ovary (ovarian ligament) — ovary → uterus posteriorly; remnant of gubernaculum
• Suspensory (infundibulopelvic) ligament — ovary → pelvic wall; contains ovarian artery, vein, lymphatics, nerves (important to ligate during salpingo-oophorectomy)

• Ovarian artery — directly from abdominal aorta at L2 (gonadal artery)
• Ovarian vein — right drains to IVC; left drains to left renal vein (explains left-sided varicocele more common)

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Vagina

• Posterior fornix — deepest; directly related to pouch of Douglas through thin peritoneum + vaginal wall; used for culdocentesis
• Anterior fornix — related to bladder base
• Two lateral fornices — related to parametrium + uterine vessels + ureter

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D. RECTUM AND ANAL CANAL

Rectum

• Upper (right)
• Middle (left) — most prominent; at S3–S4 level; corresponds to peritoneal reflection
• Lower (right)

• Upper 1/3: Covered on front and sides (intraperitoneal)
• Middle 1/3: Covered on front only
• Lower 1/3: Completely extraperitoneal (below peritoneal reflection)

• Posterior: Sacrum, coccyx, piriformis, sacrospinous ligament, sacral plexus, sympathetic trunks, median sacral vessels
• Anterior (males): Rectovesical pouch, seminal vesicles, ductus deferens, prostate, membranous urethra
• Anterior (females): Recto-uterine pouch, posterior vaginal wall, uterovaginal fascia

• Superior rectal artery — terminal branch of IMA (most important supply)
• Middle rectal artery — from internal iliac
• Inferior rectal artery — branch of internal pudendal (from internal iliac)
• Median sacral artery — from aortic bifurcation

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Anal Canal

DENTATE (PECTINATE) LINE — THE MOST IMPORTANT LANDMARK

• Anal columns of Morgagni — 8–10 vertical mucosal folds above the dentate line; correspond to underlying internal hemorrhoidal plexuses
• Anal sinuses — pockets between anal columns; contain anal glands (source of anorectal abscesses)
• Anal valves — semilunar folds connecting the bases of adjacent anal columns

DIFFERENCES ABOVE vs. BELOW DENTATE LINE (MOST IMPORTANT TABLE IN PROCTOLOGY)

Anal Sphincters

• Smooth muscle; thickening of inner circular layer of rectal wall
• Involuntary control
• Maintained in tonic contraction by sympathetic activity (L1–L2, via hypogastric nerve)
• Relaxes reflexly when rectum fills (recto-anal inhibitory reflex — RAIR)
• Provides ~85% of resting anal canal pressure
• Damaged by lateral internal sphincterotomy (for anal fissure)

• Skeletal (striated) muscle; 3 parts: subcutaneous, superficial, deep
• Voluntary control
• Innervated by: Inferior rectal nerve (branch of pudendal, S2–S4) + perineal branch of S4
• Deep part continuous with puborectalis (levator ani)
• Provides ~15% of resting pressure, but major component of voluntary squeeze pressure

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SECTION 1.10 — PELVIC FASCIA

Types of Pelvic Fascia

• Septa: Rectovaginal septum (between vagina and rectum), Vesicovaginal septum (between bladder and vagina)
• Ligaments (condensations):
  • Cardinal (Mackenrodt's/transverse cervical) ligament — most important; cervix → lateral pelvic wall; contains uterine artery + ureter
  • Uterosacral ligaments — cervix → sacrum; contain autonomic nerve fibers
  • Pubocervical ligament — cervix → posterior pubic symphysis
  • Pubovesical ligaments (females) / Puboprostatic ligaments (males) — anchors bladder neck

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SECTION 1.11 — PERITONEUM IN THE PELVIS

Male Peritoneum

1. Covers anterior bladder wall and superior bladder surface
2. Reflects posteriorly over seminal vesicles + posterior bladder
3. Descends to form rectovesical pouch between bladder and rectum
4. Ascends over rectum

Female Peritoneum

1. Covers uterovesical junction → forms vesicouterine pouch (shallow; between bladder and uterus)
2. Covers anterior, top, and posterior uterus
3. Descends to form recto-uterine pouch (Pouch of Douglas / cul-de-sac) — between posterior uterus/cervix/upper vagina and rectum
4. Pouch of Douglas = most dependent peritoneal recess in females → free fluid (blood, pus) collects here → accessible by culdocentesis via posterior vaginal fornix

Broad Ligament

• Double peritoneal fold draping over uterus laterally
• Contents:
  • Free edge (mesosalpinx): Uterine tube
  • Mesovarium: Attaches ovary to posterior broad ligament
  • Mesometrium: Largest part; covers uterine body
  • Round ligament (in anterior layer)
  • Ligament of ovary (in posterior layer)
  • Uterine artery + vein
  • Autonomic nerves, lymphatics

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SECTION 1.12 — BLOOD VESSELS OF THE PELVIS

Internal Iliac Artery (Hypogastric Artery)

• Arises from common iliac artery at pelvic brim (sacroiliac joint level)
• Descends into pelvis and divides into anterior and posterior divisions

Uterine Artery

• Branch of anterior division of internal iliac
• Runs in the base of the broad ligament to reach the uterus
• At 2 cm lateral to the cervix → crosses OVER the ureter ("water under the bridge")
• Ascends lateral margin of uterus → anastomoses with ovarian artery
• Ligated during hysterectomy → risk of ureteral injury at this point

Corona Mortis ("Crown of Death")

• Anastomosis between obturator artery and inferior epigastric artery across the superior pubic ramus
• Can be injured during repair of inguinal hernia or pelvic fracture → potentially fatal hemorrhage

Ovarian Artery

• Direct branch of abdominal aorta at L2
• Runs in suspensory (infundibulopelvic) ligament to reach ovary
• Anastomoses with uterine artery

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Veins of the Pelvis

• Mirror the arteries as venous plexuses around each organ:
  • Vesical venous plexus — around bladder
  • Prostatic venous plexus (Santorini) — between prostate and pubic symphysis; communicates with Batson's plexus → route for prostate cancer vertebral metastasis
  • Uterovaginal plexus — alongside uterus and vagina
  • Rectal venous plexus — porto-systemic anastomosis

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SECTION 1.13 — LYMPHATICS OF THE PELVIS

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SECTION 1.14 — NERVES OF THE PELVIS

Lumbosacral Plexus (L1–S4)

Autonomic Plexuses

• Lies in front of L5 vertebra/sacral promontory; bifurcates into right and left hypogastric nerves
• Contains sympathetic fibers (from T10–L2) + some visceral afferents
• Division during pelvic surgery → retrograde ejaculation (sympathetic damage)

• Formed by: Hypogastric nerves (sympathetic) + Pelvic splanchnic nerves (parasympathetic S2–S4)
• Lies on lateral wall of rectum/vagina/prostate
• Gives off: Branches to bladder, rectum, prostate/uterus, and cavernous nerves

• Arise from inferior hypogastric plexus
• Pass posterolateral to prostate (in neurovascular bundles)
• Enter corpus cavernosum → mediate erection via NO release
• Damaged during radical prostatectomy → erectile dysfunction

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Erection vs. Ejaculation vs. Emission

Mnemonic: "Point and Shoot" — Parasympathetic = erection (Point); Sympathetic = emission (Shoot)

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PART 2: THE PERINEUM

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SECTION 2.1 — OVERVIEW OF THE PERINEUM

• Anterior apex: Inferior margin of pubic symphysis
• Posterior apex: Coccyx tip
• Lateral points: Ischial tuberosities
• Anterolateral sides: Ischiopubic rami
• Posterolateral sides: Sacrotuberous ligaments

• Anterior = Urogenital triangle (horizontal plane; urinary + reproductive openings; roots of external genitalia)
• Posterior = Anal triangle (tilted slightly posteriorly; anus + ischio-anal fossae)

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SECTION 2.2 — PERINEAL MEMBRANE AND PERINEAL POUCHES

Perineal Membrane (Urogenital Diaphragm — old term)

• Thick fibrous sheet spanning the urogenital triangle
• Posterior border — free; anchored in midline to perineal body
• Lateral attachments — to inferior pubic rami (pubic arch)
• Structures piercing it: Urethra (both sexes) + vagina (females) + ducts of Bartholin's glands (females)

Deep Perineal Pouch (Space)

Superficial Perineal Pouch

• Floor: Colles' fascia (superficial perineal fascia — membranous layer of superficial fascia of perineum; continuation of Scarpa's fascia of abdomen and Dartos fascia of scrotum)
• Roof: Perineal membrane
• Posterior: Closed — Colles' fascia fuses with posterior edge of perineal membrane
• Lateral: Colles' fascia fuses with fascia lata of thigh

• Into the scrotum/labia majora (Dartos)
• Up to the anterior abdominal wall (Scarpa's fascia)
• NOT down the thigh (Colles' fuses with fascia lata) and NOT posteriorly (fused to perineal membrane)

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Contents of Superficial Perineal Pouch

In Males:

In Females:

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SECTION 2.3 — PERINEAL BODY (CENTRAL TENDON OF PERINEUM)

1. External anal sphincter
2. External urethral sphincter
3. Bulbospongiosus (bilateral = 2)
4. Superficial transverse perineal (bilateral = 2)
5. Deep transverse perineal (bilateral = 2)
6. Levator ani (puborectalis fibers)

• Obstetric tears:
  • 1st degree: skin only
  • 2nd degree: skin + superficial perineal muscles
  • 3rd degree: EAS involved
  • 4th degree: EAS + rectal mucosa
• Episiotomy — deliberate incision to enlarge vaginal orifice during delivery
• Damage → rectocele, posterior vaginal prolapse, fecal incontinence

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SECTION 2.4 — ANAL TRIANGLE

Ischio-Anal (Ischiorectal) Fossae

• Medial wall: Levator ani (above) + external anal sphincter (below)
• Lateral wall: Obturator internus covered by obturator fascia (this fascia contains the pudendal canal/Alcock's canal)
• Roof: Inferior fascia of levator ani (levator ani meeting the EAS)
• Floor: Skin of perineum (perianal skin)
• Posterior: Sacrotuberous ligament + gluteus maximus
• Anterior recess: Extends above the posterior edge of perineal membrane, below levator ani, on either side of vagina/urethra (thus communicates between the two sides anteriorly around the anal canal)

• Ischio-anal fat (important for bowel function and childbirth)
• Pudendal canal (Alcock's canal) — fascial canal on lateral wall within obturator fascia; contains:
  • Pudendal nerve
  • Internal pudendal artery + vein
  • (All three entering from the lesser sciatic foramen)

• Perianal abscess (most superficial)
• Intersphincteric abscess (between EAS and IAS)
• Ischio-anal abscess (through EAS into ischio-anal fossa)
• Supralevator abscess (above levator ani)
• Horseshoe abscess — tracking through posterior ischio-anal fossa connecting the two sides

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SECTION 2.5 — PUDENDAL NERVE — NERVE OF THE PERINEUM

1. Exits pelvis through greater sciatic foramen below piriformis (infrapiriform space)
2. Crosses the ischial spine + sacrospinous ligament (lateral to it)
3. Enters perineum through lesser sciatic foramen
4. Travels forward in Alcock's canal (pudendal canal) on lateral wall of ischio-anal fossa
5. Exits pudendal canal → gives terminal branches

• Needle aimed at ischial spine (landmark)
• Local anaesthetic injected just medial/posterior to ischial spine (behind sacrospinous ligament)
• Used for: Obstetric analgesia (second-stage labor), perineal laceration repair
• Approach: Transvaginal (finger in vagina guides to ischial spine) or transperineal

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SECTION 2.6 — MUSCLES OF THE PERINEUM — COMPLETE TABLE

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SECTION 2.7 — EXTERNAL GENITALIA

Female External Genitalia (Vulva)

• Mons pubis — rounded pad of fat over pubic symphysis
• Labia majora — outer skin folds; homologous to scrotum; contain fat + smooth muscle (dartos homologue)
• Labia minora — inner hairless folds; homologous to corpus spongiosum + penile skin; meet anteriorly to form clitoral hood (prepuce)
• Clitoris — homologous to penis; body + glans; composed of two corpora cavernosa (no corpus spongiosum); highly innervated (dorsal clitoral nerve — branch of pudendal)
• Vestibule — cleft between labia minora containing: urethral orifice (anterior), vaginal orifice (posterior), openings of Bartholin's glands (posterolateral), Skene's (paraurethral) glands
• Hymen — mucosal fold partly closing vaginal orifice
• Fourchette (posterior commissure) — where labia minora meet posteriorly

Male External Genitalia

• Scrotum — homologous to labia majora; contains testes + epididymis + lower spermatic cord; midline raphe
• Testes — oval (4 × 3 × 2 cm); within tunica albuginea; attached posterolateral to epididymis; posterolateral to testis = spermatic cord structures
• Epididymis — head (efferent ductules) + body + tail; site of sperm maturation and storage; continues as ductus deferens
• Penis — consists of root (bulb + 2 crura) + body (shaft) + glans
  • 3 erectile compartments: 2 corpora cavernosa (dorsal, paired) + 1 corpus spongiosum (ventral, surrounds urethra, expands to form glans)
  • Coverings of penis (superficial to deep): Skin → dartos → superficial fascia (Colles') → deep (Buck's) fascia → tunica albuginea → erectile tissue

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PART 3: EMBRYOLOGICAL DERIVATIVES

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PART 4: CLINICAL CORRELATES

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PART 5: HIGH-YIELD EXAM & VIVA Q&A

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SECTION 5.1 — BONY PELVIS QUESTIONS

The linea terminalis is the bony ridge that separates the false (greater) pelvis from the true (lesser) pelvis. It is formed by: sacral promontory → alae of sacrum → arcuate line of ilium → pectineal line (pecten pubis) → pubic crest → pubic symphysis.

(1) Anatomical (true) conjugate: promontory to top of symphysis = ~11.5 cm; (2) Obstetric conjugate: promontory to posterior surface of symphysis = ~11 cm (shortest fixed AP diameter — limits fetal head descent); (3) Diagonal conjugate: promontory to inferior margin of symphysis = ~12.5 cm (only one measurable clinically — subtract 1.5 cm to estimate obstetric conjugate).

Female: wider and shallower; pelvic inlet oval/round; subpubic angle 80–90°; greater sciatic notch >90°; ischial spines less prominent, everted; sacrum shorter, wider, less curved. Male: narrow and deep; pelvic inlet heart-shaped with prominent promontory; subpubic angle 50–60°; ischial spines prominent, inverted.

They fuse between 16–18 years at the acetabulum (triradiate cartilage junction of ilium, ischium, and pubis).

Obturator nerve (L2–L4), obturator artery, obturator vein.

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SECTION 5.2 — PELVIC WALLS, FLOOR, AND APERTURES

Above piriformis: Superior gluteal nerve + vessels. Below piriformis: Inferior gluteal nerve + vessels; Sciatic nerve (L4–S3); Pudendal nerve (S2–S4); Internal pudendal vessels; Posterior femoral cutaneous nerve (S1–S3); Nerve to obturator internus (L5, S1); Nerve to quadratus femoris (L4, L5).

Pudendal nerve + internal pudendal vessels (re-entering perineum from gluteal region); Tendon of obturator internus; Nerve to obturator internus.

Puborectalis, pubococcygeus, iliococcygeus. Innervated by the nerve to levator ani (S3, S4) on its superior surface; some contribution from inferior rectal nerve (pudendal branch) inferiorly.

A U-shaped defect in the anterior levator ani through which the urethra (and vagina in females) passes. It is bridged anteriorly by the perineal membrane and deep perineal pouch.

Arises from sacral plexus S2–S4; exits pelvis through greater sciatic foramen (below piriformis); crosses the ischial spine and sacrospinous ligament laterally; re-enters the perineum through the lesser sciatic foramen; travels in Alcock's (pudendal) canal on the lateral wall of the ischio-anal fossa; divides into: inferior rectal nerve, perineal nerve, and dorsal nerve of penis/clitoris.

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SECTION 5.3 — PELVIC VISCERA

The trigone is a smooth triangular area on the posterior bladder wall bounded by the two ureteric orifices (superolateral) and the internal urethral orifice (inferior). It is derived from the mesonephric (Wolffian) duct absorbed into the bladder wall, which explains its smooth appearance (unlike the trabeculated rest of the bladder from cloacal endoderm).

In the female pelvis, the uterine artery crosses OVER the ureter at approximately 2 cm lateral to the cervix. The ureter passes UNDER the uterine artery. During hysterectomy, when the surgeon ligates the uterine artery, the ureter may inadvertently be ligated if care is not taken — "the water (ureter) passes under the bridge (uterine artery)."

(1) Peripheral zone (~70%) — most prostate cancers; palpable posteriorly on DRE; (2) Central zone (~25%) — surrounds ejaculatory ducts; resistant to carcinoma; (3) Transition zone (~5%) — surrounds preprostatic urethra; site of BPH; (4) Anterior fibromuscular stroma — no glands.

Preprostatic → Prostatic → Membranous → Spongy (cavernous/penile). The membranous urethra is the shortest and least dilatable — most commonly injured in pelvic fractures.

In the ampulla of the uterine tube. The ampulla is also the most common site of ectopic pregnancy (~70–75%).

(1) Pelvic floor (levator ani) — primary; (2) Cardinal (Mackenrodt's/transverse cervical) ligament — most important ligament preventing uterine descent; (3) Uterosacral ligaments — maintain anteversion; (4) Pubocervical ligament; (5) Round ligament — maintains anteversion only, does NOT prevent prolapse; (6) Broad ligament — minimal support.

The recto-uterine pouch (pouch of Douglas) is the peritoneal recess between the posterior uterus/cervix/upper vagina and the rectum. It is the most dependent part of the female peritoneal cavity. Clinical significance: free peritoneal fluid (blood in ruptured ectopic pregnancy, pus in pelvic abscess) collects here; it is accessible via culdocentesis (needle inserted through the posterior vaginal fornix).

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SECTION 5.4 — ANAL CANAL AND DENTATE LINE

The dentate line is at the midpoint of the anal canal; it marks the embryological junction between endoderm (above) and ectoderm (below). Above: columnar epithelium, superior rectal artery (IMA/portal), portal venous drainage, internal iliac lymphatics, autonomic innervation (pain-insensitive), internal hemorrhoids. Below: squamous epithelium, inferior rectal artery (internal pudendal/systemic), systemic venous drainage, superficial inguinal lymphatics, somatic innervation (pain-sensitive), external hemorrhoids.

Superior rectal vein (portal system via inferior mesenteric vein) anastomoses with middle and inferior rectal veins (systemic via internal iliac/IVC). In portal hypertension, this anastomosis dilates → anorectal varices.

The inferior rectal nerve (branch of the pudendal nerve, S2–S4) + perineal branch of S4. The external anal sphincter is skeletal muscle under voluntary control.

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SECTION 5.5 — NEUROVASCULAR QUESTIONS

The cavernous nerves (nervi erigentes), arising from the inferior hypogastric (pelvic) plexus (which receives parasympathetic input from pelvic splanchnic nerves S2–S4). They pass posterolateral to the prostate in the neurovascular bundles and enter the corpora cavernosa. They release acetylcholine → endothelial nitric oxide (NO) → smooth muscle relaxation → vasodilation → erection.

Erection = parasympathetic (S2–S4); cavernous nerves; NO-mediated vasodilation. Emission = sympathetic (T10–L2); semen moved into posterior urethra; internal sphincter closes (prevents retrograde ejaculation). Ejaculation = somatic pudendal nerve (S2–S4); bulbospongiosus + ischiocavernosus contraction; propels semen out.

To para-aortic (lumbar) lymph nodes at L2 (not inguinal), because the testis develops in the abdomen and its lymphatics accompany the gonadal vessels. Clinically important: testicular cancer metastasizes to para-aortic nodes, not inguinal nodes (inguinal nodes drain the scrotal skin).

Batson's paravertebral venous plexus is a valveless network of veins alongside the vertebral column. The prostatic venous plexus communicates with Batson's plexus → allows retrograde spread of prostate cancer cells to lumbar vertebrae producing osteoblastic (sclerotic) metastases.

Suprapiriform space: Superior gluteal nerve + vessels. Infrapiriform space: Inferior gluteal nerve + vessels, sciatic nerve, pudendal nerve, internal pudendal vessels, posterior femoral cutaneous nerve, nerve to obturator internus, nerve to quadratus femoris.

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SECTION 5.6 — PERINEUM QUESTIONS

The perineal body (central tendon of the perineum) is a fibromuscular mass in the midline between the urogenital and anal triangles, ~3–4 cm anterior to the anal verge. 8 muscles attach to it: EAS, EUS, bulbospongiosus (×2), superficial transverse perineal (×2), deep transverse perineal (×2), and puborectalis fibers of levator ani.

The pudendal canal (Alcock's canal) lies in the obturator fascia on the lateral wall of the ischio-anal fossa and contains the pudendal nerve, internal pudendal artery, and internal pudendal vein.

Urine collects in the superficial perineal pouch, then tracks into the scrotum (dartos), up the anterior abdominal wall deep to Scarpa's fascia, and into the perineum around the penis. It does NOT go: down the thighs (Colles' fascia fuses with fascia lata at the thigh), posteriorly into the anal triangle (Colles' fascia fuses with the posterior edge of the perineal membrane).

The anorectal angle (~90°) is the angle between the anal canal and the lower rectum. It is maintained by the puborectal sling (puborectalis muscle, part of levator ani). This angle is critical for fecal continence — it creates a flap-valve mechanism. The angle straightens (to ~130°) during defecation when puborectalis relaxes.

External urethral sphincter, deep transverse perineal muscle, bulbourethral (Cowper's) glands in males (and sphincter urethrovaginalis + compressor urethrae in females), internal pudendal vessels, dorsal nerve of penis/clitoris (passing through before exiting at superior margin of perineal membrane).

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SECTION 5.7 — CLINICAL/APPLIED QUESTIONS

At the ischial spine. The needle is directed (transvaginally or transperineally) to deposit local anaesthetic just medial and posterior to the ischial spine, infiltrating the pudendal nerve where it lies just behind the sacrospinous ligament. Anaesthetizes the entire perineum.

Obturator nerve (L2–L4) passes through the obturator canal → enters medial thigh → supplies adductors. Compression by an ovarian cyst → obturator nerve syndrome (Howship–Romberg sign): pain down medial thigh worsened by hip medial rotation; weakness of hip adduction.

The internal urethral sphincter (smooth muscle, alpha-1 receptors) loses tone → retrograde ejaculation (semen travels backward into bladder). Also seen with alpha-1 blockers (tamsulosin) used for BPH.

The membranous urethra (most fixed and least dilatable segment, passing through the deep perineal pouch between the pelvic floor and perineal membrane). Management: Retrograde urethrogram before attempting catheterization; if confirmed injury, suprapubic catheterization and delayed repair.

Damage to the cardinal (Mackenrodt's) ligament (the most important uterine support ligament) leads to uterine prolapse. Surgically, it is the structure through which the uterine artery runs and where ureteral injury is most likely during hysterectomy.

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SECTION 5.8 — IMPORTANT MNEMONICS AND MEMORY AIDS

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SECTION 5.9 — RAPID FIRE VIVA FACTS

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