Pelvis & perineum
Second module — the basin below the abdomen, and its floorThe pelvis is the hardest region of the first semester to SEE: everything is packed into a bowl you view from above, the dissection is deep, and half the structures differ by sex. The organizing trick is layers — bony ring, then the levator ani floor slung across it, then viscera resting on the floor in one fixed order (urinary in front, genital in the middle, rectum behind), then the perineum below the floor as its own two-triangle compartment. Nearly every clinical question is one of three stories: something herniates or prolapses through the floor, something is cut or blocked near the ischial spine, or the ureter meets the uterine artery.
This guide is the map. Drop this region's lecture slides on the dashboard to get flashcards and questions from your lecture, group them into a module, and run a timed module exam the week before. The gross anatomy playbook has the weekly loop.
AI-authored study notes, not faculty-reviewed and not medical advice. Verify every structure against your atlas, your dissector, and your lab instructors; programs differ in what they tag.
High-Yield Pearl
The ischial spine is the region's crossroads: the sacrospinous ligament attaches to it, the pudendal nerve hooks around it, the interspinous distance is the tightest point of the birth canal, and it is the landmark for the pudendal block — one bony point, four exam questions.
What the written exam asks
Relationships, supply, levels, and what happens when each fails.
- The bony pelvis and its ligaments. Two hip bones (ilium + ischium + pubis, fused at the acetabulum) joined to the sacrum at the sacroiliac joints and to each other at the pubic symphysis. The pelvic brim (sacral promontory → arcuate line → pectineal line → pubic crest) separates the false (greater) pelvis above from the true (lesser) pelvis below. The sacrospinous ligament (sacrum → ischial spine) and sacrotuberous ligament (sacrum → ischial tuberosity) convert the sciatic notches into the greater and lesser sciatic foramina — the doorways for everything leaving the pelvis for the gluteal region and perineum.
- Male vs female pelvis. The female pelvis is built for passage: wider subpubic angle (>80°, vs <70° in the male), oval inlet (vs heart-shaped), straighter and shorter sacrum, everted ischial tuberosities, larger outlet. The obstetric conjugate (sacral promontory → the thickest point of the symphysis) is the narrowest fixed diameter of the birth canal; the interspinous distance (between the ischial spines) is the narrowest point of the midpelvis — the reason fetal station is measured at the spines.
- The pelvic floor: levator ani. Levator ani (puborectalis, pubococcygeus, iliococcygeus — front to back) plus coccygeus form the pelvic diaphragm, slung from the pubis, the tendinous arch over obturator internus, and the ischial spine. Puborectalis loops behind the anorectal junction and pulls it forward — the sling that maintains continence and the angle that must relax to defecate. Innervation: the nerve to levator ani (S4) and branches of the pudendal nerve. The urogenital hiatus is the gap the urethra (and vagina) pass through — the weak point every prolapse exploits.
- Viscera in order, and the pouches between them. Front to back in both sexes: bladder → genital organs → rectum. Peritoneum drapes over the tops and forms the low points of the abdominal cavity: in the female the vesicouterine pouch and the deeper rectouterine pouch (of Douglas) — the lowest point of the female peritoneal cavity, reachable through the posterior vaginal fornix; in the male the rectovesical pouch. Free fluid (blood, pus, a ruptured ectopic) collects there first.
- Uterus, its supports, and "water under the bridge". The uterus is normally anteverted (tipped forward over the bladder) and anteflexed. The broad ligament is draped peritoneum (mesometrium, mesosalpinx, mesovarium); the round ligament runs through the inguinal canal to the labium majus (the gubernaculum's path); the ovary hangs between the ovarian ligament (to the uterus) and the suspensory ligament (to the pelvic wall — carrying the ovarian vessels). Real support is the cardinal (transverse cervical) and uterosacral ligaments plus the pelvic floor. The URETER passes UNDER the uterine artery about 2 cm lateral to the cervix — water under the bridge — the classic injury in a hysterectomy.
- Male pathway of the ductus deferens. The ductus deferens re-enters the pelvis at the deep inguinal ring, crosses OVER the ureter ("bridge over water" — the one exception), and joins the seminal vesicle's duct to form the ejaculatory duct, which pierces the prostate to open at the seminal colliculus. The prostate sits between bladder neck and urogenital diaphragm: its posterior surface is what a digital rectal exam feels; the peripheral zone (posterior) is where carcinoma arises, and the periurethral transition zone is where BPH squeezes the urethra.
- Blood supply: the internal iliac. Nearly everything runs on the internal iliac artery. High-yield branches: superior vesical (from the patent part of the umbilical artery), inferior vesical (male) / vaginal (female), uterine, middle rectal, obturator, internal pudendal (the perineum's artery — out the greater sciatic foramen, around the spine, in through the lesser), and the superior and inferior gluteals. The ovarian and testicular arteries come from the abdominal aorta at L2 — the gonads bring their supply down with them.
- Nerves: pudendal and the pelvic autonomics. The pudendal nerve (S2–S4) exits the greater sciatic foramen, hooks around the ischial spine/sacrospinous ligament, and re-enters through the lesser sciatic foramen into the pudendal (Alcock's) canal on the lateral wall of the ischioanal fossa — branches: inferior rectal, perineal, and the dorsal nerve of the penis/clitoris. It is the somatic nerve of the perineum: both external sphincters and perineal skin. Autonomics: pelvic splanchnics (S2–S4, parasympathetic — erection, bladder emptying, hindgut motility) vs the sympathetic hypogastric plexuses (emission, bladder-neck closure). "Point and Shoot": Parasympathetic erection, Sympathetic emission; somatic (pudendal) ejaculation.
- The perineum: two triangles, two pouches. A diamond between pubis, ischial tuberosities, and coccyx, split by a line between the tuberosities into the urogenital triangle (front) and anal triangle (back). The urogenital triangle is floored by the perineal membrane: below it the superficial pouch (crura + ischiocavernosus, bulb + bulbospongiosus, superficial transverse perineal muscles, greater vestibular glands in the female); above it the deep pouch (external urethral sphincter, deep transverse perineal muscle, membranous urethra). The anal triangle holds the anal canal, external anal sphincter, and the fat-filled ischioanal fossae. The perineal body is the central tendon everything anchors to.
What the practical tags · 37 structures
In the order a dissection meets them — each with the one relationship that identifies it. Your lab's study list is the authority; use this to check yours against.
- 1Ilium, ischium, and pubis — the three bones fused at the acetabulum
- 2Ischial spine — the sacrospinous ligament attaches here; the pudendal nerve hooks around it
- 3Ischial tuberosity — you sit on it; sacrotuberous ligament and hamstrings attach
- 4Sacrospinous ligament — sacrum to ischial spine; separates the two sciatic foramina
- 5Sacrotuberous ligament — sacrum to ischial tuberosity; the longer, more superficial one
- 6Greater sciatic foramen — piriformis fills it; gluteal vessels and the sciatic nerve exit here
- 7Lesser sciatic foramen — the pudendal bundle and obturator internus tendon pass through
- 8Obturator internus — lines the lateral pelvic wall; its tendon turns out the lesser foramen
- 9Piriformis — from the anterior sacrum out the greater sciatic foramen
- 10Obturator foramen and canal — the obturator nerve and vessels leave the pelvis here
- 11Levator ani — the pelvic diaphragm's main sheet: puborectalis, pubococcygeus, iliococcygeus
- 12Puborectalis — the sling behind the anorectal junction; the continence muscle
- 13Coccygeus — ischial spine to coccyx, lying on the sacrospinous ligament
- 14Tendinous arch of levator ani — the thickened obturator fascia the floor hangs from
- 15Perineal membrane — the fascial floor of the urogenital triangle
- 16Perineal body — the central tendon between the vagina or bulb and the anal canal
- 17External anal sphincter — voluntary, pudendal-innervated, encircling the anal canal
- 18Ischioanal fossa — the fat-filled wedge beside the anal canal; abscesses live here
- 19Pudendal (Alcock's) canal — the fascial tunnel on the fossa's lateral wall
- 20Pudendal nerve — S2–S4; rounds the ischial spine into the lesser foramen
- 21Internal pudendal artery — travels with the pudendal nerve; supplies the perineum
- 22Internal iliac artery — the pelvic trunk; be ready to name its visceral branches
- 23Uterine artery — crosses OVER the ureter about 2 cm lateral to the cervix
- 24Ureter (pelvic part) — passes under the uterine artery; peristalses when pinched
- 25Bladder and trigone — the smooth triangle between the ureteric orifices and urethra
- 26Uterus — body, fundus, cervix; normally anteverted and anteflexed
- 27Broad ligament — the peritoneal drape: mesometrium, mesosalpinx, mesovarium
- 28Round ligament of the uterus — into the deep inguinal ring; the gubernaculum's path
- 29Uterine tube — infundibulum with fimbriae, ampulla (where fertilization happens), isthmus
- 30Ovary and suspensory ligament — the vessel-carrying fold down from the pelvic brim
- 31Rectouterine pouch (of Douglas) — the lowest point of the female peritoneal cavity
- 32Ductus deferens — crosses over the ureter on its way to the seminal vesicle
- 33Seminal vesicle — posterior to the bladder, lateral to the ampulla of the ductus
- 34Prostate — around the urethra below the bladder; its posterior surface faces the rectum
- 35Rectum and anal canal — the pectinate line divides visceral from somatic territory
- 36Ischiocavernosus and bulbospongiosus — the superficial-pouch muscles over crura and bulb
- 37Greater vestibular (Bartholin) gland — deep to the bulb, beside the vaginal orifice
Practice it
Active recall over the practical list — flip and claim, match the pairs, label from a bank, or fill in the diagram.
Every structure on the practical list, one card at a time — the name up front, the identifying relationship on the flip. Claim each card honestly and the deck learns what to lead with next time. Progress lives in this browser only.
What everyone misses
The distinctions that lose points on the practical, year after year.
- Perineal membrane vs perineal body. The membrane is the flat fascial FLOOR of the urogenital triangle (the roof of the superficial pouch); the body is the central tendon KNOT between the urogenital and anal triangles. One is a sheet, the other a point — practicals tag both, and students swap the names.
- Sacrospinous vs sacrotuberous ligament. Spinous → ischial SPINE (shorter, deeper; upper border of the lesser sciatic foramen); tuberous → ischial TUBEROSITY (longer, more superficial — the one your fingers find first). The pudendal bundle rounds the spine between the two.
- Ureter vs uterine artery — and the male exception. Water under the bridge: the URETER passes UNDER the uterine artery. In the male, the ductus deferens crosses OVER the ureter. If the tag is a cord crossing a tube, decide which sex's pelvis you're in before you answer.
- Puborectalis vs external anal sphincter. Both wrap the anorectal region and both are under voluntary control, but puborectalis is part of levator ani and forms a forward SLING with no posterior attachment, while the external sphincter is a complete CIRCLE around the canal below it. The practical loves the sling.
- Ovarian ligament vs suspensory ligament of the ovary. The ovarian ligament ties the ovary to the uterus and carries nothing; the suspensory ligament ties it to the pelvic wall and carries the ovarian vessels — the one that gets clamped, and the reason ovarian torsion is a vascular emergency.
- Pelvic splanchnic vs sacral splanchnic nerves. Pelvic splanchnics = PARASYMPATHETIC, straight off the ventral rami of S2–S4. Sacral splanchnics = SYMPATHETIC, off the chain. Same neighborhood, opposite systems — the written exam's favorite one-word swap.
- Deep pouch contents by sex. Both sexes: external urethral sphincter and deep transverse perineal muscle. The male adds the membranous urethra and the bulbourethral (Cowper's) glands — which drain into the SUPERFICIAL pouch's bulb. The female deep pouch transmits both urethra and vagina.
Clinical correlations
Where this region shows up again — in clinical medicine, on rotations, and on the boards.
- Pudendal nerve block. The ischial spine is the landmark: the needle is walked onto the spine and sacrospinous ligament (usually transvaginally) and anesthetic bathes the nerve as it rounds them — perineal anesthesia for delivery or repair. It numbs the perineum, not the labor contractions, which travel with higher visceral afferents.
- Episiotomy and obstetric tears. A mediolateral cut is angled away from the perineal body; a midline cut or a natural tear can extend through it into the external anal sphincter (third degree) or rectal mucosa (fourth degree) — the anatomy behind incontinence after childbirth, and the reason perineal-body repair matters in every laceration.
- Pelvic organ prolapse and stress incontinence. Levator ani weakness (childbirth, age) widens the urogenital hiatus: cystocele from the anterior vaginal wall, rectocele from the posterior wall, uterine prolapse from the apex. Stress incontinence is the same floor failing to support the urethra — leaking with a cough or laugh. Pelvic-floor exercises train exactly this muscle.
- Ectopic pregnancy and the pouch of Douglas. The ampulla of the uterine tube is the most common ectopic site; rupture bleeds into the rectouterine pouch — the blood pools at the lowest point of the peritoneal cavity, historically sampled by culdocentesis through the posterior fornix and now found on transvaginal ultrasound.
- Hysterectomy and the ureter. Clamping the uterine artery lateral to the cervix can catch the ureter passing beneath it — the most-tested iatrogenic injury in the region. A rising creatinine or new flank pain after pelvic surgery is the ureter until proven otherwise.
- Prostate: BPH, carcinoma, and the rectal exam. BPH grows in the periurethral transition zone — obstructive urinary symptoms early; carcinoma arises in the posterior peripheral zone — palpable on rectal exam, often silent until late, and spreading to the vertebral column through the valveless Batson plexus. The rectovesical relationship is why the DRE examines the prostate at all.
- Pelvic fracture and urethral injury. The pelvic ring breaks in two places, and venous plexus bleeding is the immediate threat. In the male, disruption at the urogenital diaphragm tears the membranous urethra — blood at the meatus, inability to void, a high-riding prostate on exam; a straddle injury tears the spongy urethra instead, with urine extravasating into the superficial perineal space.