The Perineum: Two Triangles and the Spaces Between Them

A diamond with four bony corners

The perineum is everything below the pelvic floor — the floor is its roof. Anatomically the perineum is the diamond-shaped region lying BELOW the pelvic diaphragm, between the thighs. Its roof is the levator ani and coccygeus described in the pelvic floor; its floor is the skin. Its outline is drawn on bone and ligament, and the four corners are easy to find on a skeleton or on a living patient in the lithotomy position: ANTERIORLY the pubic symphysis; ANTEROLATERALLY the ischiopubic rami running back on each side to the ISCHIAL TUBEROSITIES, the two prominences you sit on; POSTEROLATERALLY the SACROTUBEROUS LIGAMENTS sweeping back from the tuberosities to the sacrum; and POSTERIORLY the tip of the COCCYX. Those landmarks are laid out in the hip bone and the bony pelvis, and every one of them can be palpated — which is why the perineum, alone among the deep regions, is a place the examining finger can reach directly. The diamond is not flat: it is pitched like a shallow tent, sloping upward from the skin towards the pelvic floor above, and its two halves lie in slightly different planes.

One line, two triangles

Draw a line transversely between the two ischial tuberosities and the diamond splits into two triangles, and this single line organises the entire region. In front of it lies the UROGENITAL TRIANGLE, containing the external genitalia, the outlet of the urinary tract and, in the female, the vaginal opening. Behind it lies the ANAL TRIANGLE, containing the anal canal and its sphincters. The division is not a textbook convenience — the two triangles are built differently. The anal triangle is a simple space: a canal in the middle and fat on either side, with no fascial floor beneath it. The urogenital triangle is closed off by a tough sheet, the perineal membrane, which stretches between the ischiopubic rami and turns the front half of the perineum into a sandwich of two distinct compartments. Everything clinical about the perineum comes from that structural asymmetry: infection behaves one way behind the line and a completely different way in front of it.

The anal triangle and its two fatty wedges

In the middle, a canal; on each side, a wedge of fat doing an unglamorous but essential job. The centre of the anal triangle is occupied by the anal canal and the EXTERNAL ANAL SPHINCTER wrapped around it, a somatic, voluntary muscle described with the rest of that canal in the anal canal. On either side lies the ISCHIOANAL FOSSA — still widely called the ischiorectal fossa — a wedge-shaped space packed with coarse fat and crossed by fibrous strands. Its boundaries are worth memorising as a shape rather than a list: MEDIALLY the sloping undersurface of levator ani and the external anal sphincter, which form the roof and the medial wall together, meeting the lateral wall at a sharp apex above; LATERALLY the vertical fascia over obturator internus and the ischial tuberosity below it; POSTERIORLY the sacrotuberous ligament and the gluteus maximus; ANTERIORLY the posterior edge of the perineal membrane; and INFERIORLY the perineal skin. Duplicated in the fascia of the lateral wall is the PUDENDAL CANAL, ALCOCK'S CANAL, a sleeve carrying the internal pudendal artery and vein and the pudendal nerve forwards along the ischial ramus. The fat is not padding for its own sake: it is soft, mobile, compressible tissue that allows the anal canal to distend hugely during defaecation and allows the whole perineum to be flattened by the descending fetal head without tearing the levator plate.

Two features of these fossae generate almost all the surgery done in the region. The first is that the two fossae COMMUNICATE WITH EACH OTHER posteriorly, behind the anal canal and deep to the anococcygeal body — so they are not two sealed compartments but one horseshoe-shaped space with a narrow bridge at the back. The second is that each fossa sends an ANTERIOR RECESS forwards, above the perineal membrane and below levator ani, into the urogenital triangle, where pus can hide out of reach of a finger placed in the anal triangle. Add the fact that fat is poorly vascularised and therefore poorly defended, and the consequences write themselves: infection of an anal gland at the level of the pectinate line tracks into the fat and forms a PERIANAL or ISCHIOANAL ABSCESS; if it is neglected it crosses the posterior bridge and becomes a HORSESHOE ABSCESS involving both sides, which cannot be cured by draining one side alone; and once the abscess has discharged, the epithelialised track that remains between the anal canal and the perineal skin is a FISTULA-IN-ANO, a problem measured in years rather than days. Whenever a perianal abscess recurs after apparently adequate drainage, the anatomy is telling you that the other limb of the horseshoe, or an anterior recess, was never opened.

The urogenital triangle: one membrane, two pouches

Everything in the front half of the perineum is described as being either above the membrane or below it. The PERINEAL MEMBRANE is a tough triangular sheet of dense fibrous tissue stretched horizontally between the two ischiopubic rami, with its apex at the pubic arch and its free posterior border running between the ischial tuberosities. It is perforated by the urethra in both sexes and additionally by the vagina in the female. Above it lies the DEEP PERINEAL POUCH, a thin compartment between the membrane below and the fascia of levator ani above. Its contents are the ones that matter most functionally: the EXTERNAL URETHRAL SPHINCTER, a circular skeletal muscle under voluntary control that grips the membranous urethra and is the muscle a patient uses to interrupt the stream; the MEMBRANOUS URETHRA itself, the narrowest and least forgiving segment of the male urethra as set out in the urethra; the DEEP TRANSVERSE PERINEAL muscles running transversely to steady the perineal body; the BULBOURETHRAL (COWPER'S) GLANDS in the male, sitting within the sphincter and sending their long ducts down into the spongy urethra below; and the internal pudendal vessels with the dorsal nerve and artery of the penis or clitoris travelling forwards to reach the erectile tissue. In the female the corresponding pouch contains the sphincter urethrae complex and the vessels, but the greater vestibular glands lie below the membrane, not above it.

Below the membrane lies the SUPERFICIAL PERINEAL POUCH, and this is the compartment that produces the classic clinical picture. Its roof is the perineal membrane; its floor is COLLES' FASCIA, the membranous layer of superficial perineal fascia. Its contents are the erectile bodies and the muscles that squeeze them. The two CRURA of the penis or clitoris are anchored to the ischiopubic rami and covered by ISCHIOCAVERNOSUS on each side; the BULB of the penis, or the paired vestibular bulbs in the female, lies in the midline covered by BULBOSPONGIOSUS; and the SUPERFICIAL TRANSVERSE PERINEAL muscles run from each ischial tuberosity to meet in the midline behind. Also in this pouch in the female are the GREATER VESTIBULAR (BARTHOLIN'S) GLANDS, whose obstruction produces the familiar Bartholin's cyst or abscess at the posterior part of the vaginal opening. Functionally these muscles are a vascular pump: ischiocavernosus compresses the crus to raise pressure inside the corpus cavernosum and maintain rigidity, and bulbospongiosus compresses the bulb to empty the urethra of its last drops of urine and to expel semen at ejaculation. They are supplied, like everything else here, by the pudendal nerve.

When the urethra tears: two ruptures, two destinations

The male urethra can be torn at two places, and the level of the tear determines where the urine goes with complete reliability. A STRADDLE INJURY — falling astride a bar, a bicycle crossbar, the edge of a manhole — crushes the BULB of the penis and the SPONGY (bulbar) URETHRA within it against the inferior surface of the pubic arch. The tear is therefore below the perineal membrane, inside the superficial perineal pouch, and urine and blood extravasate into that pouch. Constrained by Colles' fascia, they distend the SCROTUM, track along the shaft of the PENIS deep to the dartos, and pass upwards over the pubis deep to Scarpa's fascia onto the ANTERIOR ABDOMINAL WALL — and, because of the fusion of Scarpa's fascia with the fascia lata, they never enter the THIGH. This pattern of swelling is so characteristic that it is a diagnosis by inspection. Contrast the second injury: a PELVIC FRACTURE, typically from a road traffic collision, shears the prostate upwards and ruptures the MEMBRANOUS URETHRA at the prostatomembranous junction. That tear lies ABOVE the perineal membrane, so urine collects in the deep perineal pouch and extraperitoneally around the bladder base, producing no scrotal swelling at all — instead a high-riding prostate on rectal examination, blood at the meatus, and a patient who cannot void. In both cases the rule is the same: blood at the meatus means do NOT pass a urethral catheter until the urethra has been imaged, because a blind catheter converts a partial tear into a complete one.

The perineal body: the keystone in the middle

One small fibromuscular node holds the whole floor together, and childbirth is what threatens it. In the midline, exactly on the intertuberous line, sits the PERINEAL BODY — a pyramidal fibromuscular mass between the anal canal behind and the vaginal vestibule (or the bulb of the penis) in front. It is where an extraordinary number of muscles converge and interlace their fibres: BULBOSPONGIOSUS from in front, the SUPERFICIAL and DEEP TRANSVERSE PERINEAL muscles from each side, the EXTERNAL ANAL SPHINCTER from behind, and fibres of LEVATOR ANI, especially puborectalis and pubococcygeus, from above. It is the mechanical keystone of the perineum: the point through which the pull of every one of those muscles is transmitted, and therefore the point whose failure loosens all of them at once. In the female it is larger, sits directly between vagina and anal canal, and is only about three to four centimetres from the anal margin — which is why it is so exposed during the second stage of labour. A perineal body torn and badly repaired is followed years later by a widened vaginal opening, a descending pelvic floor, and prolapse of the rectum or the posterior vaginal wall, because the anchor has gone.

One artery and one nerve for the whole region

The perineum is unusually simple in its supply: essentially everything here is fed by the INTERNAL PUDENDAL ARTERY and innervated by the PUDENDAL NERVE (S2, S3, S4), described in detail in the pudendal nerve. Both leave the pelvis through the greater sciatic foramen below piriformis, hook forwards around the sacrospinous ligament at the ISCHIAL SPINE, re-enter through the lesser sciatic foramen, and run forwards in the pudendal canal on the lateral wall of the ischioanal fossa. The branches follow the anatomy exactly: the INFERIOR RECTAL nerve and artery cross the fossa to supply the external anal sphincter and the skin around the anus; the PERINEAL branch supplies the muscles of both pouches and the posterior scrotal or labial skin; and the DORSAL NERVE and artery of the penis or clitoris run forward in the deep pouch to reach the erectile tissue. Two practical consequences. First, because the ischial spine is palpable through the vagina or the rectum, a needle placed onto it delivers a PUDENDAL NERVE BLOCK that anaesthetises the whole perineum for an instrumental delivery, a perineal repair or a minor perineal operation. Second, the pudendal supply stops at a line inside the anal canal: above the pectinate line the innervation is autonomic and painless, below it somatic and exquisitely painful — the reason an internal haemorrhoid can be banded in the clinic without anaesthesia while an anal fissure a centimetre lower is one of the most severe pains in surgery.