# Perineum and Pelvic Floor > Perineum and pelvic floor for FMGE Anatomy: levator ani, pudendal nerve course, ischioanal fossa, episiotomy anatomy and obstetric tear grading. - Canonical URL: https://prepelephant.com/topics/fmge/anatomy/perineum-pelvic-floor-fmge - Exam / course: FMGE · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Perineum and Pelvic Floor", PrepElephant, https://prepelephant.com/topics/fmge/anatomy/perineum-pelvic-floor-fmge ## Direct answer "S2, 3, 4 keep the pelvis off the floor" — the mnemonic folds the whole region into one line: the pelvic diaphragm of levator ani and coccygeus, the perineal muscles, the external anal and urethral sphincters and the pudendal nerve all draw on the fourth sacral segment and its neighbours. The perineum is the diamond below the pelvic outlet, split by a line joining the ischial tuberosities into the anterior urogenital triangle and the posterior anal triangle, with the perineal body as the central fibromuscular node between them. The pudendal nerve (S2–S4) leaves the pelvis through the greater sciatic foramen below piriformis, hooks around the sacrospinous ligament medial to the ischial spine, and re-enters through the lesser sciatic foramen to run in Alcock's canal on obturator internus — the entire path surveyed by one landmark, the ischial spine. ## What you must remember - **Levator ani parts and roles:** pubococcygeus (largest, supports viscera), puborectalis (the sling that pulls the anorectal junction forward, maintaining the anorectal angle of continence) and iliococcygeus; innervation S4 fibres via the nerve to levator ani and the pudendal nerve. - **Perineal membrane and pouches:** the membrane stretches across the pubic arch; the deep perineal pouch between membrane and levator ani holds the external urethral sphincter and deep transverse perineal muscles, the superficial pouch the bulbospongiosus, ischiocavernosus and superficial transverse perineal muscles. - **Pudendal nerve course and branches:** S2–S4 roots, greater sciatic foramen below piriformis, around the sacrospinous ligament at the ischial spine, into Alcock's (pudendal) canal on the lateral wall of the ischioanal fossa; branches are inferior rectal, perineal and dorsal nerve of penis or clitoris, all bilaterally blocked at the spine. - **Ischioanal fossa:** a wedge of fat across the anal canal on each side, crossed by the inferior rectal nerve and vessels; its loose fat offers expansion for abscesses, which spread to the contralateral side as a horseshoe collection because the fossae communicate behind the anal canal. - **Anal sphincters:** internal sphincter of involuntary smooth muscle under autonomic control; external sphincter in subcutaneous, superficial and deep parts, supplied by the pudendal nerve — the sphincter torn in third- and fourth-degree obstetric injuries. - **Obstetric anatomy:** an episiotomy is cut mediolaterally to avoid the perineal body and the anal sphincter; tears are graded first to fourth degree, with the third and fourth involving the anal sphincter and anorectal mucosa respectively (obstetric anal sphincter injuries). - **Applied block:** pudendal block, guided by palpating the ischial spine transvaginally, anaesthetises the perineum for forceps delivery — but not uterine contraction pain, which travels with visceral afferents. ## Guiding an episiotomy and a pudendal block Watch the anatomy work in the second stage of labour. As the head crowns, the perineal body — into which bulbospongiosus, the transverse perineal muscles and the external anal sphincter insert — thins to a few millimetres. A midline tear would run straight through it into the sphincter; the mediolateral cut at 60 degrees from the midline deliberately sacrifices skin and vaginal wall while steering away from the sphincter complex and the ischioanal fat. For instrumental delivery, the block is placed where the nerve turns: the ischial spine is palpated transvaginally, the needle passed just below it through the sacrospinous ligament, and local anaesthetic deposited on each side. Because the block acts on the nerve's trunk, it covers the perineal skin, lower vagina and anal sphincter within minutes, while uterine contraction pain continues via visceral afferents to T10–L1 — which is why neuraxial techniques, not pudendal blocks, abolish labour pain. ## Landmark questions that repeat Screening papers fix on three items. The course of the pudendal nerve with the ischial spine as the surface landmark — asked as a block question or as an anatomy list. The content of Alcock's canal — the pudendal nerve with the internal pudendal vessels — distinguished from the fossa it crosses. And the puborectalis, whose angle explains both faecal continence and the dysnergia of dyssynergic defaecation; a stem describing incontinence after sacral injury or obstetric sphincter rupture is testing the same S2–S4 territory the mnemonic guards. ## Frequently asked questions ### What is the course of the pudendal nerve? From S2 to S4 it exits the greater sciatic foramen below piriformis, winds around the sacrospinous ligament medial to the ischial spine, and enters the lesser sciatic foramen to run in Alcock's canal on the lateral wall of the ischioanal fossa. ### Which landmark is used to administer a pudendal nerve block? The ischial spine, palpated transvaginally, where the nerve crosses the sacrospinous ligament — anaesthetic deposited just below and behind it. ### What fills the ischioanal fossa, and which structures cross it? Fat crossed by the inferior rectal nerve and vessels, with the pudendal canal on its lateral wall; infection here spreads readily and may form a horseshoe abscess. ### Which nerve supplies the external anal sphincter? The pudendal nerve (S2–S4), through its inferior rectal branch; the internal sphincter is involuntary smooth muscle under autonomic control. ### Why is an episiotomy cut mediolaterally rather than in the midline? A mediolateral cut avoids the perineal body and the anal sphincter complex, reducing the risk of extension into a third- or fourth-degree tear.