Medical School · Year 1 · Anatomy Pelvis Head Neck · includes a quiz and discussion video
Lecture 2: Pelvic Floor and Perineum
Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the muscles and fascia of the pelvic floor (pelvic diaphragm)
- Identify the boundaries and divisions of the perineum
- Describe the anatomy of the urogenital triangle
- Describe the anatomy of the anal triangle
- Explain the innervation and blood supply of the pelvic floor and perineum
- Correlate pelvic floor anatomy with clinical conditions
Overview of the Pelvic Floor
The pelvic floor, also termed the pelvic diaphragm, represents a muscular and fascial structure that closes the inferior pelvic aperture, separating the pelvic cavity above from the perineum below. This structure functions not as a passive barrier but as a dynamic system that supports the pelvic viscera against the forces of gravity and intra-abdominal pressure, maintains urinary and fecal continence through tonic contraction and coordinated relaxation, permits passage of the urethra, vagina (in females), and rectum through carefully positioned openings, actively assists in increasing intra-abdominal pressure during coughing, sneezing, and straining, and participates in sexual function.
The pelvic floor comprises two paired muscles—the levator ani and the coccygeus—enveloped by superior and inferior layers of fascia. Together, these muscles form a funnel-shaped structure that slopes downward from the pelvic walls toward a midline opening, the levator hiatus, through which the urogenital and anal structures pass.
<image>Panel A: Superior view of pelvic cavity showing funnel-shaped pelvic floor muscular diaphragm. Panel B: Levator ani forming anterior and lateral portions (light red) with coccygeus posteriorly (darker red). Panel C: Central levator hiatus as elongated opening for urethra, vagina, and rectum passage. Panel D: Muscles attaching to pelvic walls with sacrum, coccyx posteriorly and pubic symphysis anteriorly.</image>
Levator Ani Muscle
The levator ani forms the majority of the pelvic diaphragm as paired muscles that unite in the midline, creating a hammock or sling that supports the pelvic organs while permitting passage of critical structures. The muscle comprises two main portions—the pubococcygeus and iliococcygeus—each with distinct origins and fiber orientations.
Pubococcygeus
The pubococcygeus is the larger, more medial component, arising from the posterior surface of the pubic body and the anterior part of the arcus tendineus levator ani (tendinous arch of the levator ani). Its fibers pass posteriorly, partially inserting into the structures they pass and ultimately reaching the coccyx and anococcygeal ligament (raphe).
The pubococcygeus contains several specialized components named for their relationships: the pubovaginalis (in females) or puboprostaticus (in males) consists of fibers that insert into the vaginal wall or prostate, respectively, providing support to these structures. The puboanalis comprises fibers that insert into the external anal sphincter and intersphincteric groove. Most importantly, the puborectalis forms the most medial and inferior portion as a thick U-shaped sling that loops around the anorectal junction without attaching to the coccyx. This muscular sling creates the anorectal angle of approximately 80-90 degrees, a critical mechanism for fecal continence. Relaxation of the puborectalis straightens this angle, facilitating defecation.
Iliococcygeus
The iliococcygeus is the thinner, more lateral and posterior component, arising from the arcus tendineus levator ani along its entire length from the ischial spine to the pubis. Its fibers pass medially and posteriorly to insert into the coccyx and anococcygeal ligament, where they interdigitate with fibers from the opposite side.
Arcus Tendineus Levator Ani
The arcus tendineus levator ani represents a thickening of the obturator fascia extending from the pubic body to the ischial spine. This tendinous arch provides a continuous origin for the iliococcygeus and part of the pubococcygeus, bridging between the bony attachments.
Levator Hiatus
The levator hiatus is the U-shaped opening in the levator ani through which the urethra, vagina (in females), and rectum pass from the pelvis to the perineum. The size and integrity of this opening are clinically significant; excessive widening contributes to pelvic organ prolapse.
<image>Panel A: Superior view of levator ani showing origin from pubis and arcus tendineus levator ani (curved white line). Panel B: Pubococcygeus medially with puborectalis sling around rectum creating anorectal angle (inset lateral diagram). Panel C: Iliococcygeus laterally arising from tendinous arch in different shade. Panel D: Inferior view showing converging fibers at anococcygeal raphe with levator hiatus and passing structures marked.</image>
Coccygeus (Ischiococcygeus)
The coccygeus muscle forms the posterior portion of the pelvic floor, lying on the pelvic surface of the sacrospinous ligament. This triangular muscle arises from the ischial spine and fans out to insert into the lower sacrum (S4-S5) and upper coccyx.
Functionally, the coccygeus supports the pelvic viscera alongside the levator ani and can flex the coccyx slightly, pulling it forward. The nerve supply derives from branches of S4-S5, entering the pelvic surface of the muscle.
The close relationship between the coccygeus and the sacrospinous ligament has led some authorities to consider them as musculoligamentous variants of the same structure, with the ligament representing a degenerated or fibrous portion of the muscle.
<image>Panel A: Coccygeus muscle from within pelvis showing triangular shape from ischial spine to lower sacrum and coccyx. Panel B: Sacrospinous ligament depicted deep to or continuous with the muscle. Panel C: Relationship to adjacent levator ani with coccygeus forming posterior pelvic floor. Panel D: Ischial spine as common attachment point for muscle, sacrospinous ligament, and levator ani.</image>
Innervation and Blood Supply of the Pelvic Floor
The levator ani receives motor innervation from direct branches arising from the anterior rami of S3-S4 on the superior (pelvic) surface of the muscle. These nerves are sometimes called the nerve to levator ani. The pudendal nerve may contribute additional motor fibers to the external components, though this remains debated. The coccygeus receives innervation from branches of S4-S5.
Blood supply to the pelvic floor derives from multiple sources, reflecting its position at the junction of pelvic and perineal vascular territories. The inferior gluteal artery, a branch of the internal iliac artery, supplies the posterior portions. The internal pudendal artery contributes to the inferior and lateral aspects. Branches from the obturator artery supply portions of the lateral pelvic floor. Venous drainage parallels the arterial supply, ultimately reaching the internal iliac veins.
<image>Panel A: Schematic of sacral plexus on posterior pelvic wall with S3-S4 branches to levator ani (yellow nerve fibers). Panel B: Internal iliac artery with inferior gluteal branch through greater sciatic foramen. Panel C: Internal pudendal artery course around ischial spine. Panel D: Obturator artery through obturator canal with colored zones showing arterial distribution to pelvic floor.</image>
Pelvic Fascia
The pelvic fascia forms a continuous connective tissue system that lines the walls, covers the floor, and invests the organs of the pelvis. Understanding its organization clarifies the anatomy of ligamentous supports and potential spaces for pathological processes.
Parietal pelvic fascia lines the inner surface of the pelvic walls, investing the muscles and forming a continuous sheet. It takes regional names based on the underlying muscle: obturator fascia over the obturator internus, piriformis fascia over the piriformis, and so forth. Thickenings within this fascia form important structures, including the arcus tendineus levator ani (from which the levator ani arises) and the arcus tendineus fasciae pelvis (providing lateral attachment for paravaginal support).
Visceral pelvic fascia (endopelvic fascia) surrounds the pelvic organs, containing loose connective tissue, smooth muscle, blood vessels, lymphatics, and nerves within its substance. Condensations of this fascia form the pelvic ligaments that support the organs: the cardinal (transverse cervical) ligament extending from the cervix and vagina to the lateral pelvic wall provides the primary support for the uterus and contains the uterine vessels. The uterosacral ligament passes from the cervix posteriorly to the sacrum, maintaining uterine position. The pubovesical ligaments (females) and puboprostatic ligaments (males) extend from the bladder neck to the posterior pubis.
The superior fascia of the pelvic diaphragm covers the superior surface of the levator ani and coccygeus, continuous with the obturator fascia laterally. The inferior fascia of the pelvic diaphragm covers the inferior surface, forming the medial wall of the ischioanal fossa and continuous with the fascial components of the perineum.
<image>Panel A: Coronal section of pelvis with parietal fascia lining lateral walls over obturator internus (light gray). Panel B: Arcus tendineus levator ani and arcus tendineus fasciae pelvis as white thickenings. Panel C: Visceral fascia (yellow-green) surrounding bladder, uterus/vagina, rectum with cardinal and uterosacral ligaments. Panel D: Superior and inferior fasciae of pelvic diaphragm sandwiching levator ani muscle.</image>
The Perineum: Overview and Boundaries
The perineum is the diamond-shaped region below the pelvic floor, extending from the pubic symphysis anteriorly to the tip of the coccyx posteriorly, and between the ischial tuberosities laterally. Though superficially this region appears simple, it contains the external genitalia, terminal portions of the urogenital and gastrointestinal tracts, and complex muscular, fascial, and neurovascular structures.
The boundaries form a diamond with the following margins: anteriorly, the pubic symphysis and arcuate pubic ligament; anterolaterally, the inferior pubic rami and ischial rami; posterolaterally, the sacrotuberous ligaments; posteriorly, the tip of the coccyx; and at the lateral corners, the ischial tuberosities.
An imaginary transverse line connecting the anterior portions of the ischial tuberosities divides the perineum into two triangles: the urogenital triangle anteriorly, containing the external genitalia and the openings of the urogenital tract, and the anal triangle posteriorly, containing the anal canal and the ischioanal fossae.
<image>Panel A: Lithographic perineum view in lithotomy position showing diamond-shaped outline. Panel B: Labeled boundaries with pubic symphysis (12 o'clock), ischial tuberosities (3 and 9 o'clock), coccyx (6 o'clock). Panel C: Urogenital triangle anteriorly (light blue) containing external genitalia. Panel D: Anal triangle posteriorly (light pink) with centered anus, divided by horizontal dashed line.</image>
The Anal Triangle
The anal triangle occupies the posterior half of the perineum, extending from the transverse line between the ischial tuberosities to the coccyx. Its principal contents are the anal canal with its sphincteric apparatus and the bilateral ischioanal fossae.
Anal Canal
The anal canal represents the terminal 4 centimeters of the gastrointestinal tract, extending from the anorectal junction (where the rectum passes through the pelvic floor at the puborectalis sling) to the anus (the external opening). The canal is surrounded by two muscular sphincters that control defecation.
The internal anal sphincter is a thickened continuation of the circular smooth muscle layer of the rectum. This involuntary sphincter maintains constant resting tone, contributing approximately 70% of resting anal pressure. It is controlled by the autonomic nervous system, with sympathetic fibers causing contraction and parasympathetic fibers causing relaxation.
The external anal sphincter is a voluntary skeletal muscle encircling the anal canal. Classically described in three parts—subcutaneous, superficial, and deep—these merge into a continuous muscular mass. The deep part blends superiorly with the puborectalis to form a functional continence unit. Innervation derives from the inferior rectal nerve (branch of pudendal, S2-S4), providing voluntary control over defecation.
Ischioanal (Ischiorectal) Fossa
The ischioanal fossae are wedge-shaped, fat-filled spaces located on either side of the anal canal. Their unique geometry and contents make them clinically significant.
The boundaries of each fossa include: medially, the external anal sphincter and the sloping inferior surface of the levator ani; laterally, the obturator internus muscle (covered by its fascia) above and the ischial tuberosity below; superiorly (apex), where the levator ani meets the obturator fascia; and inferiorly (base), the perianal skin.
The fossa contains primarily fat (the ischioanal fat pad), which allows for expansion of the anal canal during defecation. Traversing the lateral wall within a fascial tunnel is the pudendal canal, containing the pudendal nerve and internal pudendal vessels. The inferior rectal nerve and vessels cross the fossa from the pudendal canal to the anal canal.
The bilateral ischioanal fossae communicate posteriorly behind the anal canal through the post-anal space (deep postanal space), allowing infections to spread from one side to the other (horseshoe abscess).
<image>Panel A: Coronal section through anal triangle showing central anal canal with internal sphincter (pink, smooth muscle). Panel B: External anal sphincter (red, skeletal muscle) with levator ani sloping from lateral wall and puborectalis sling. Panel C: Ischioanal fossae as triangular spaces with yellow fat, bounded by obturator internus and ischial tuberosity. Panel D: Pudendal canal in lateral wall with neurovascular bundle (nerve yellow, artery red, vein blue) and inferior rectal branches.</image>
The Pudendal Canal (Alcock's Canal)
The pudendal canal is a fascial tunnel located on the lateral wall of the ischioanal fossa, formed by a splitting of the obturator fascia over the obturator internus muscle. This approximately 4-centimeter canal transmits the pudendal nerve and internal pudendal vessels from the lesser sciatic foramen to the perineum.
The pudendal nerve (S2-S4) is the principal nerve of the perineum, providing motor innervation to the external anal sphincter, muscles of the urogenital triangle, and external urethral sphincter, as well as sensory innervation to the perineal skin and external genitalia. After exiting the pelvis through the greater sciatic foramen below the piriformis, the nerve crosses the sacrospinous ligament near the ischial spine and re-enters the perineum through the lesser sciatic foramen to enter the pudendal canal. Within and beyond the canal, it gives branches in sequence: the inferior rectal nerve arises early, crossing the ischioanal fossa to the external sphincter and perianal skin; the perineal nerve emerges to supply the muscles of the urogenital triangle and sensory branches to the posterior scrotum or labia; and the dorsal nerve of the penis or clitoris continues to provide sensation to the glans.
The internal pudendal artery parallels the nerve through the same pathway, giving corresponding branches: inferior rectal, perineal, and terminal branches to the external genitalia.
<image>Panel A: Pudendal nerve and artery course beginning in pelvis, exiting through greater sciatic foramen below piriformis. Panel B: Crossing ischial spine (red dot) and entering perineum through lesser sciatic foramen to pudendal canal. Panel C: Inferior rectal nerves crossing fossa to anal canal and perineal branches to urogenital triangle. Panel D: Dorsal nerve continuing to external genitalia with artery (red) paralleling entire course.</image>
The Urogenital Triangle: General Features
The urogenital triangle occupies the anterior half of the perineum, containing the external genitalia and the urogenital openings. Its structure differs significantly between males and females, but common features provide a framework for understanding both.
Perineal Membrane
The perineal membrane is a strong triangular fascial sheet spanning between the ischiopubic rami, forming the inferior boundary of the deep perineal space. This structure provides a stable attachment platform for the roots of the external genitalia and is pierced by the urethra (in both sexes) and the vagina (in females).
Perineal Body
The perineal body (central tendon of the perineum) is a fibromuscular node located in the midline between the anal canal posteriorly and the bulb of the penis (males) or vaginal orifice (females) anteriorly. Though small (approximately 2-4 cm in diameter), it serves as the central anchor point for multiple muscles: the external anal sphincter posteriorly, the bulbospongiosus anteriorly, the superficial and deep transverse perineal muscles laterally, and the levator ani superiorly. In females, damage to the perineal body during childbirth can compromise pelvic floor integrity.
<image>Panel A: Urogenital triangle from perineal view with perineal membrane as strong triangular sheet (semi-transparent). Panel B: Membrane attached to ischiopubic rami with openings for urethra and vagina. Panel C: Perineal body as dense midline node posterior to membrane. Panel D: Radiating muscle fibers showing convergence with labels for each attaching muscle.</image>
The Urogenital Triangle: Male
Superficial Perineal Space
The superficial perineal space (superficial perineal pouch) lies between the perineal membrane deeply and the superficial perineal fascia (Colles' fascia) superficially. This space contains the root of the penis and associated muscles.
The root of the penis comprises the crura and the bulb. The two crura are attached to the ischiopubic rami and are covered by the ischiocavernosus muscles; they become the corpora cavernosa of the penile shaft. The midline bulb attaches to the perineal membrane and is covered by the bulbospongiosus muscle; it becomes the corpus spongiosum containing the urethra.
The ischiocavernosus muscles compress the crura to maintain erection by impeding venous outflow. The bulbospongiosus muscle compresses the bulb during urination and ejaculation, expelling contents from the urethra. The superficial transverse perineal muscles extend from the ischial tuberosities to the perineal body, stabilizing this central anchor.
Deep Perineal Space
The deep perineal space lies between the perineal membrane inferiorly and the superior fascia of the urogenital diaphragm (or levator ani) superiorly. It contains the external urethral sphincter (sphincter urethrae), which surrounds the membranous urethra and provides voluntary control of urination; the deep transverse perineal muscle, spanning between the ischiopubic rami; the bulbourethral glands (Cowper's glands), which lie posterolateral to the membranous urethra and whose ducts pierce the perineal membrane to open into the spongy urethra; and branches of the internal pudendal vessels and pudendal nerve, including the dorsal nerve of the penis.
<image>Panel A: Male urogenital triangle superficial space with penile root crura attached to ischiopubic rami, covered by ischiocavernosus (red). Panel B: Central bulb covered by bulbospongiosus (lighter red) with superficial transverse perineal muscles to perineal body. Panel C: Perineal membrane with deep space external urethral sphincter around membranous urethra. Panel D: Deep transverse perineal muscle and bulbourethral glands posterolaterally with ducts passing forward.</image>
The Urogenital Triangle: Female
Superficial Perineal Space
The female superficial perineal space contains structures homologous to the male but arranged around the vaginal opening. The clitoris consists of erectile tissue analogous to the penis: paired crura attach to the ischiopubic rami and are covered by ischiocavernosus muscles, uniting to form the body; the glans clitoris, the only externally visible part, is a sensitive erectile structure. The vestibular bulbs are paired masses of erectile tissue flanking the vaginal vestibule deep to the labia minora, covered by bulbospongiosus muscles; they are homologous to the bulb and corpus spongiosum of the male. The greater vestibular glands (Bartholin's glands) lie posterolateral to the vaginal orifice within this space; their ducts open into the vestibule, providing lubricating secretion. The muscles—ischiocavernosus, bulbospongiosus, and superficial transverse perineal—serve analogous functions to their male counterparts.
Deep Perineal Space
The female deep perineal space contains the external urethral sphincter, which surrounds the urethra and also sends fibers to the vagina (compressor urethrae and sphincter urethrovaginalis), the deep transverse perineal muscle, branches of the internal pudendal vessels, and the dorsal nerve of the clitoris.
The Vulva
The external female genitalia (vulva) comprise the mons pubis (the fatty eminence over the pubic symphysis), the labia majora (outer hair-bearing folds homologous to the scrotum), the labia minora (inner hairless folds that meet anteriorly at the clitoris), the clitoris with its prepuce (hood) and glans, the vestibule (the space between the labia minora containing the external urethral orifice and vaginal orifice), and the hymen (a mucous membrane partially covering the vaginal orifice, highly variable in appearance).
<image>Panel A: Female urogenital triangle superficial view with mons pubis, labia majora (outer folds), labia minora meeting at clitoral hood. Panel B: Glans clitoris at apex with vestibule containing urethral and vaginal orifices. Panel C: Deeper dissection showing clitoral crura with ischiocavernosus and vestibular bulbs with bulbospongiosus. Panel D: Bartholin's glands posterolateral to vaginal opening with perineal membrane and deep space structures.</image>
Clinical Correlations
Pelvic Organ Prolapse
Pelvic organ prolapse results from weakening of the pelvic floor musculature and endopelvic fascial supports, allowing pelvic organs to descend into or beyond the vagina. Types include cystocele (anterior vaginal wall prolapse with bladder descent), rectocele (posterior vaginal wall prolapse with rectal bulging), and uterine prolapse (descent of the uterus into the vaginal canal). Risk factors include vaginal childbirth (especially prolonged labor or large babies), aging with estrogen deficiency, chronic increases in intra-abdominal pressure (obesity, chronic cough, heavy lifting), and connective tissue disorders. Treatment ranges from pelvic floor muscle exercises (Kegel exercises) to pessary devices to surgical repair (colporrhaphy, sacrocolpopexy).
Urinary Incontinence
Stress urinary incontinence, involuntary urine leakage with increased abdominal pressure (coughing, sneezing, exercising), results from inadequate support of the bladder neck and proximal urethra, often due to pelvic floor weakness. Treatment includes pelvic floor muscle training, urethral bulking agents, and surgical procedures such as midurethral slings.
Pudendal Nerve Entrapment
Compression of the pudendal nerve, typically at the ischial spine or within the pudendal canal, produces chronic perineal pain that characteristically worsens with sitting (compressing the nerve) and improves with standing or sitting on a toilet seat (which relieves pressure on the nerve). Diagnosis is clinical, supported by nerve block response, and treatment includes physiotherapy, nerve blocks, and occasionally surgical decompression.
Episiotomy
Episiotomy is a surgical incision of the perineum performed during childbirth to enlarge the vaginal outlet when needed. The mediolateral episiotomy cuts at an angle away from the midline, avoiding the external anal sphincter and perineal body but involving more tissue. The midline episiotomy cuts directly posterior through the perineal body; while it heals more readily, it carries higher risk of extension into the anal sphincter (third-degree tear) or rectum (fourth-degree tear). Current practice favors restrictive episiotomy use.
Ischioanal Abscess and Fistula
Infections in the ischioanal fossa produce abscesses presenting with perianal pain, swelling, and fever. The posterior communication between the bilateral fossae allows spread to the contralateral side (horseshoe abscess). Drainage is required, and persistent cryptoglandular infection may lead to anal fistula—an abnormal tract from the anal canal to the perianal skin that typically requires surgical treatment.
<image>Panel A: Pelvic organ prolapse types in sagittal view showing cystocele, rectocele, and uterine prolapse. Panel B: Stress urinary incontinence mechanics with increased abdominal pressure and lack of urethral support. Panel C: Pudendal nerve entrapment at ischial spine and episiotomy comparison (mediolateral vs midline). Panel D: Ischioanal abscess with postanal space spread and anal fistula tract to perianal skin.</image>
Summary
The pelvic floor (pelvic diaphragm) consists of the levator ani (with its pubococcygeus, puborectalis, and iliococcygeus components) and coccygeus muscles, forming a funnel-shaped muscular sling that supports pelvic organs and maintains continence. The puborectalis creates the anorectal angle critical for fecal continence. The perineum lies below the pelvic floor as a diamond-shaped region divided into the anterior urogenital triangle (containing external genitalia, perineal membrane, and perineal spaces) and the posterior anal triangle (containing the anal canal with its sphincters and the ischioanal fossae). The pudendal nerve (S2-S4) and internal pudendal artery, traveling through the pudendal canal in the lateral wall of the ischioanal fossa, provide the principal neurovascular supply to the perineum. The perineal body serves as the central fibromuscular anchor for multiple perineal muscles. Clinical conditions affecting this region include pelvic organ prolapse, urinary incontinence, pudendal nerve entrapment, and ischioanal infections.
Key Terms
Levator ani: The principal muscle of the pelvic floor, comprising pubococcygeus and iliococcygeus, forming a muscular sling that supports pelvic viscera and maintains continence.
Puborectalis: The medial portion of the levator ani that forms a U-shaped sling around the anorectal junction, creating the anorectal angle essential for fecal continence.
Perineal membrane: A strong triangular fascial sheet spanning between the ischiopubic rami in the urogenital triangle, supporting the external genitalia and pierced by the urethra and vagina.
Perineal body: The central fibromuscular node located between the anal canal and urogenital structures, serving as the anchor point for multiple perineal muscles.
Ischioanal fossa: The fat-filled wedge-shaped space lateral to the anal canal, bounded by the levator ani medially and obturator internus laterally, containing the pudendal canal in its lateral wall.
Pudendal canal (Alcock's canal): The fascial tunnel on the lateral wall of the ischioanal fossa containing the pudendal nerve and internal pudendal vessels, serving as the neurovascular pathway to the perineum.
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