Sectional Anatomy of the Pelvis
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Direct answer
Reading an axial pelvic slice works outside-in: the bony ring (ilium, ischium, pubis with the acetabular roof or obturator canal depending on height), then the muscular wall (piriformis and obturator internus filling the greater sciatic and lateral pelvic walls, levator ani slinging the floor), then the viscera midline — bladder anterior, rectum posterior, and the genital tract between. In males the seminal vesicles sit behind the bladder with their characteristic septated "bow-tie" look, the prostate below the bladder at pubic symphysis level; in females the anteverted uterus and ovaries vary wildly in position, flanked by the broad ligament. The rectouterine pouch of Douglas is the most dependent peritoneal space in either sex, which is why fluid, blood and dropped metastases collect there first — the anatomical fact behind a positive rectovesical or rectovaginal examination.
What you must remember
- Order at any mid-pelvic slice: bladder (with the urachus remnant heading to the umbilicus), genital viscera, rectum, then sacrum — "BGR" from front to back.
- Prostate zones on MRI: peripheral zone (posterior and lateral, site of about 70-75% of cancers), central zone (cone around ejaculatory ducts), transition zone (enlarges in benign prostatic hyperplasia) — the zonal map that separates BPH from carcinoma on one image.
- Seminal vesicles: behind and below the bladder base, cystic and septated; their angle to the bladder is a rectal-ultrasound and MRI landmark for staging.
- Pouch of Douglas: rectouterine (female) or rectovesical (male) — most dependent peritoneal recess; free fluid, pus and Krukenberg deposits localise here.
- Obturator canal: oblique tunnel in obturator internus, carrying the obturator nerve and vessels from pelvis to thigh — the target of an obturator block in transurethral bladder tumour resection to prevent adductor spasm.
- Corona mortis: anastomosis between inferior epigastric/external iliac and obturator vessels behind the superior pubic ramus — a vascular variant that can bleed dangerously during pelvic fracture and hernia repair fixation.
- Greater versus lesser sciatic foramina: carved by piriformis and the sacrospinous ligament; piriformis, sciatic nerve and pudendal bundle exit below piriformis — the cross-sectional substrate of deep gluteal syndrome.
- Pelvic floor: levator ani (puborectalis, pubococcygeus, iliococcygeus) plus coccygeus; defects visible on MRI as lateral descent in prolapse and incontinence.
From slice to surgical decision
Follow a 60-year-old man undergoing staging for prostate cancer. The radiologist reports a lesion in the right peripheral zone with extracapsular extension at the neurovascular bundle. Every phrase is sectional anatomy: the peripheral zone is the posterolateral outer gland the examining finger actually feels in digital rectal examination; the neurovascular bundle runs along the posterolateral capsule, so extension there threatens the cavernous nerves and dictates whether nerve-sparing radical prostatectomy is feasible. One slice above, the seminal vesicle angle tells the surgeon if the dissection must extend higher; one slice below, the external urethral sphincter at the apex determines continence planning. In the female pelvis the same logic governs a prolapse MRI: levator avulsion from the pubic arch, widening of the levator hiatus, descent of the bladder neck (cystocele) or rectum (rectocele) measured relative to the pubococcygeal line. Sectional anatomy is not decoration for these operations — the measurements ARE the operation's map.
Where students slip
Two errors recur in practicals. First, expecting the ovaries in a fixed spot: on CT they may lie anywhere from the pelvic brim to the pouch of Douglas, and the follices, not position, identify them — candidates who hunt "beside the uterus" miss an ovary pulled by adhesions. Second, confusing the seminal vesicles with bowel loops or the rectum's ampullary bulk; the septated cystic texture behind the bladder is unmistakable once learned, and the "bow-tie" description is worth quoting in the viva. Indian examiners also relish the applied question: why is a Douglas abscess drained through the posterior vaginal fornix? Because the fornix is the closest peritoneal reach and the pouch is the most dependent space — a question that fuses sectional, peritoneal and clinical anatomy in one answer.
Frequently asked questions
Which prostate zone gives rise to most carcinomas?
The peripheral zone, about 70-75% of cancers, which is also the zone palpable on digital rectal examination; BPH arises instead in the transition zone.
What is the corona mortis?
An aberrant arterial (sometimes venous) anastomosis behind the superior pubic ramus between the inferior epigastric and obturator systems, at risk during pelvic surgery and fracture fixation.
Why does free fluid collect in the pouch of Douglas?
It is the most dependent peritoneal recess in the erect and supine pelvis, so blood, pus and malignant deposits gravitate there and are sampled by culdocentesis or seen on CT.
What passes through the obturator canal?
The obturator nerve with the obturator artery and vein, passing from the lateral pelvic wall to the medial thigh — the target of an obturator nerve block in bladder tumour surgery.
Which structures bound the greater sciatic foramen?
The greater sciatic foramen is bounded by the greater sciatic notch of the hip bone, the sacrotuberous ligament below and the sacrospinous ligament in front, with piriformis dividing it into suprapiriformis and infrapiriformis parts.