Pelvic Floor Muscles and Perineum Anatomy
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Direct answer
The pelvic floor is a layered musculofascial sling that closes the pelvic outlet, and the perineum is the diamond-shaped region beneath it, divided into urogenital and anal triangles by a line joining the ischial tuberosities. The levator ani forms the main support, made of pubococcygeus, puborectalis and iliococcygeus, while the pudendal nerve supplies the perineal skin and external anal sphincter. Knowing these layers explains episiotomy, obstetric tears and prolapse.
What you must remember
- Perineal boundaries: anteriorly the pubic symphysis, posteriorly the coccyx, and laterally the ischial tuberosities, with the perineal body as the central tendinous point.
- Levator ani: the main pelvic floor muscle, innervated mainly by the nerve to levator ani from S3 and S4, with the perineal branch of the pudendal nerve added in some textbooks.
- Urogenital triangle: contains the external urethral sphincter, the deep and superficial transverse perineal muscles and the vestibular bulbs.
- Anal triangle: contains the external anal sphincter, the ischioanal fossae and the anococcygeal ligament.
- Pudendal nerve: arises from S2 to S4, leaves the pelvis through the greater sciatic foramen, re-enters through the lesser sciatic foramen and runs in the Alcock canal with the internal pudendal vessels.
- Perineal body: a fibromuscular node between the vagina and anal canal where several muscles converge, and injury to it weakens the perineum after deep tears.
- Episiotomy choice: a mediolateral cut is preferred over a midline incision, because a midline cut extends more readily into the anal sphincter.
Mapping the triangles and what runs through them
A vaginal delivery stretches the perineum from front to back, so the structures in each region decide which cut or tear matters most. The table compares the two triangles, the levator hiatus and the perineal body, which are the regions that matter most in exam vignettes.
| Region | Key contents | Clinical link |
|---|---|---|
| Urogenital triangle | Vestibular bulbs, external urethral sphincter, deep transverse perineal muscle | Episiotomy and urethral injury |
| Anal triangle | External anal sphincter, ischioanal fossa, anococcygeal ligament | Third and fourth degree tears |
| Levator hiatus | Urethra, vagina and rectum pass through | Prolapse and levator avulsion |
| Perineal body | Converging muscles between vagina and anal canal | Deep perineal tears |
The external anal sphincter lies in the posterior triangle, so a tear that extends through the perineal body threatens continence. Levator avulsion during labour removes the muscular sling that holds the pelvic organs up. Surgeons therefore check the sphincter and perineal body at every repair, since a missed deep tear can cause later faecal incontinence.
Where the exam tries to catch you
Vignettes about delivery and prolapse hinge on a few predictable confusions.
- Trap: Crediting the pudendal nerve with supplying the levator ani. Correct: The levator ani is supplied mainly by the nerve to levator ani (S3 and S4; some books add the perineal branch of the pudendal), while the pudendal nerve supplies the perineal skin and external anal sphincter.
- Trap: Choosing a midline episiotomy because it is easier to repair. Correct: It extends into the anal sphincter more readily, so the mediolateral cut is the textbook choice for a lower risk of deep extension.
- Trap: Confining the perineal body to one triangle. Correct: It sits where the urogenital and anal triangles meet, so injury to it affects both compartments.
Frequently asked questions
Which muscle provides the main support of the pelvic floor?
The levator ani is the main support of the pelvic floor, made of pubococcygeus, puborectalis and iliococcygeus. Its injury during childbirth is a major cause of pelvic organ prolapse.
Which nerve supplies the perineal skin and external anal sphincter?
The pudendal nerve, arising from S2 to S4, supplies the perineal skin and external anal sphincter. It runs through the Alcock canal, where it can be stretched or compressed during a prolonged second stage.
What is the perineal body?
The perineal body is a fibromuscular node between the vagina and anal canal where several perineal muscles converge. Deep perineal tears damage it, which weakens support of the lower vagina and anus.
Why is a mediolateral episiotomy preferred to a midline incision?
A midline incision extends more readily into the anal sphincter and rectum, causing third- or fourth-degree tears. A mediolateral cut carries less risk of that deep extension, although its repair is somewhat harder.