The Vagina and Vulva: A Canal, Four Fornices and a Window Into the Abdomen
Most of the abdomen can only be reached by cutting. There is one exception, and it is not an incision at all. Behind the cervix, at the top of a canal seven to nine centimetres long, the vaginal wall thins to almost nothing and lies directly against the peritoneum of the lowest point of the abdominal cavity. A finger placed there is one membrane away from the space where blood, pus and free fluid inevitably collect. That single anatomical accident explains why a gynaecologist can diagnose a ruptured ectopic pregnancy with two fingers, why a pelvic abscess can be drained without a single skin incision, and why the ultrasound probe that sees the ovaries best is not placed on the abdomen at all. The vagina is a canal — but it is also a window.
A twenty-eight-year-old woman is brought to the emergency department pale and clammy, with lower abdominal pain that began six hours earlier and a period she thinks is about two weeks late. Her pulse is 118 and her blood pressure is falling. The registrar does not reach first for a scanner. She places two gloved fingers in the vagina and one hand on the abdomen, and then — gently — moves the cervix from side to side. The patient cries out. That sign has a name, cervical excitation, and it is pure anatomy: blood escaping from a ruptured tubal pregnancy has run downhill into the rectouterine pouch, the lowest point of the peritoneal cavity, and is lying against the peritoneum that covers the back of the cervix and the top of the vagina. Moving the cervix drags that inflamed peritoneum across the pooled blood. Nothing has been imaged, nothing has been cut, and the diagnosis is already made — because a canal in the perineum ends a millimetre away from the floor of the abdomen.
A canal with a direction
It is not a vertical tube, and drawing it vertically is the commonest error in a student's sketch. The vagina is a distensible fibromuscular canal extending from the VESTIBULE of the vulva below to the CERVIX of the uterus above. Its anterior wall measures about 7 cm and its posterior wall about 9 cm, the difference being made by the cervix, which projects into its upper end from in front rather than sitting on top of it. From the vestibule it runs UPWARDS AND BACKWARDS at roughly 45 degrees to the horizontal, so that its axis makes an angle of about 90 degrees with the axis of the anteverted uterus described in the uterus and uterine tubes. That direction is not a detail: it is why a speculum or an examining finger is introduced pointing towards the sacrum and not towards the umbilicus, and it is the direction the fetal head must follow through the birth canal. At rest the anterior and posterior walls are in contact, and the lateral walls are held apart by the pelvic fascia, so a cross-section is not a circle but an H-shaped slit. The lining is thrown into transverse ridges — the RUGAE — with a longitudinal ridge on each of the anterior and posterior walls, the vaginal columns. Those rugae are the reason a canal a few centimetres wide can accommodate a fetal head: they unfold rather than tear, in the same way a concertina opens.
The four fornices — and the one that matters
Because the cervix pushes into the upper vagina rather than opening at its summit, the vaginal wall is reflected around it as a continuous circular gutter. The gutter is described in four parts: an ANTERIOR fornix, two LATERAL fornices, and a POSTERIOR fornix, which is the deepest of the four because the cervix leans forwards. Everything clinically important about the top of the vagina follows from the depth of that posterior recess. Behind it lies not muscle, not fat, but the RECTOUTERINE POUCH OF DOUGLAS — the peritoneal cul-de-sac between the back of the uterus and the front of the rectum, and, as established in the perineum and in the peritoneal chapters, the LOWEST POINT of the female peritoneal cavity in the upright and semi-recumbent positions. The separation is a thin vaginal wall and a layer of peritoneum, and nothing else. Free fluid in the abdomen — blood, pus, ascites, the contents of a perforated viscus — runs downhill under gravity and collects there. So the posterior fornix is the one place in the body where the peritoneal cavity can be palpated, punctured, drained and imaged from outside without an incision. It is a window into the abdomen that happens to open in the perineum.
Three procedures and one physical sign all live in the posterior fornix, and they are the same fact stated four ways. CULDOCENTESIS: a needle passed through the posterior fornix into the pouch of Douglas; non-clotting blood confirms haemoperitoneum, and before ultrasound was universal this was how a ruptured ectopic pregnancy was proven at the bedside. TRANSVAGINAL DRAINAGE: a pelvic abscess sitting in the pouch — after appendicitis, diverticulitis or pelvic inflammatory disease — is drained through the same fornix, because pus that is pointing into the vagina should not be reached through the abdominal wall. TRANSVAGINAL ULTRASOUND: the probe sits in the fornix a couple of centimetres from the ovaries and the pouch, so it uses high frequency at short range and sees what the abdominal probe cannot. And CERVICAL EXCITATION: blood or pus lying against the pelvic peritoneum makes any movement of the cervix agonising, because that movement drags the inflamed peritoneum. One recess, one membrane, four clinical uses.
What lies around it
Learn the relations as four walls, because every one of them is a route by which the vagina is injured or examined. ANTERIORLY, the upper part of the anterior wall is related to the BASE OF THE BLADDER, and the lower part to the URETHRA, which is not merely nearby but embedded in the vaginal wall itself — the two share a fascial plane, which is why the female urethra described in the urethra can be supported by a sling placed vaginally, and why obstructed labour that crushes the anterior wall produces a vesicovaginal fistula. POSTERIORLY, there are three relations from above downwards: the pouch of Douglas at the level of the posterior fornix, then the AMPULLA OF THE RECTUM in the middle third, and then the PERINEAL BODY in the lower third, which separates the vagina from the anal canal. LATERALLY lie the URETERS, crossing beneath the uterine arteries and running about 2 cm lateral to the cervix — close enough that they are palpable in the lateral fornix in a thin patient, and close enough to be the classic injury of hysterectomy. Below the ureters the lateral relations are the levator ani muscles of the pelvic floor, the pelvic fascia and the CARDINAL (transverse cervical) LIGAMENTS. These lateral supports matter more than any other single structure for the position of the vagina, because when they fail the canal descends.
A wall with no glands, and an acid that protects
The vaginal wall has three coats. The MUCOSA is STRATIFIED SQUAMOUS NON-KERATINISED epithelium on a lamina propria rich in elastic fibres and thin-walled veins — and it contains NO GLANDS AT ALL. This is one of the most frequently misremembered facts in pelvic anatomy. The canal is kept moist by cervical mucus from above and by a transudate that seeps between the epithelial cells from the venous plexus beneath, supplemented by the greater vestibular glands at the entrance; the vagina itself secretes nothing. Outside the mucosa is a MUSCULAR coat of inner circular and outer longitudinal smooth muscle, continuous below with the striated bulbospongiosus acting as a voluntary sphincter, and outside that an ADVENTITIA of dense connective tissue that binds the canal to its neighbours and carries the vessels and nerves. Under the influence of OESTROGEN the epithelium becomes thick and loaded with GLYCOGEN. Commensal LACTOBACILLI metabolise that glycogen to LACTIC ACID, and the result is a vaginal pH of roughly 3.8 to 4.5 — acid enough to suppress most pathogens. The chain is worth reciting in order, because every link can break: oestrogen → thick epithelium → glycogen → lactobacilli → lactic acid → low pH → protection.
Think of the vaginal ecosystem as a fruit orchard tended by one crop of bees. Oestrogen is the season: while it lasts, the trees are thick with fruit — glycogen. The lactobacilli are the bees that ferment the fruit and, as a by-product, keep the whole orchard slightly sour, and it is that sourness rather than any wall or guard that keeps other insects out. Now break the chain at any point and the same thing happens. Take away the season and the trees thin out: after the menopause, with oestrogen gone, the epithelium becomes thin and pale, glycogen disappears, the lactobacilli starve, the pH climbs, and the tissue becomes dry, fragile and easily infected. Or kill the bees while the fruit remains: a course of broad-spectrum antibiotics wipes out the lactobacilli, the sugar-rich orchard is left unguarded, and candida — which loves both sugar and an acid pH — overgrows. Or let a different colony move in: in bacterial vaginosis the lactobacilli are displaced by anaerobes, the acid is lost and the pH rises above 4.5, which is exactly what the bedside pH paper is testing for.
The vulva, named precisely
Vulva is the collective term for the female external genitalia; each of its parts has an exact name and an exact male homologue. The MONS PUBIS is the fatty pad over the pubic symphysis. From it two folds run backwards: the LABIA MAJORA, hair-bearing folds of skin containing fat, smooth muscle and the terminal fibres of the ROUND LIGAMENT of the uterus — which reaches them after traversing the inguinal canal — and which are the female homologue of the SCROTUM. Within them lie the LABIA MINORA, hairless folds that split anteriorly to form the prepuce and the frenulum of the clitoris. The space they enclose is the VESTIBULE, into which open the EXTERNAL URETHRAL ORIFICE in front and the VAGINAL ORIFICE behind. The CLITORIS is an erectile organ with the same three-part construction as the penis: two CRURA attached to the ischiopubic rami, a BODY formed by their union, and a GLANS, richly supplied with sensory endings; its crura are homologous with the corpora cavernosa, and the paired VESTIBULAR BULBS lying on either side of the vaginal orifice are homologous with the bulb and corpus spongiosum of the penis. Each crus is covered by ISCHIOCAVERNOSUS and each bulb by BULBOSPONGIOSUS, the superficial perineal muscles whose contraction compresses the erectile tissue and its draining veins. Posterolateral to the vaginal orifice, deep to the bulbs, sit the GREATER VESTIBULAR (BARTHOLIN'S) GLANDS — pea-sized mucus glands whose ducts open into the vestibule between the hymen and the labium minus, the homologue of the male bulbourethral glands. Scattered LESSER VESTIBULAR GLANDS open around the vestibule, and the PARAURETHRAL (SKENE'S) GLANDS, the female homologue of the prostate, open beside the external urethral orifice. The HYMEN is a thin fold of mucosa partly closing the vaginal orifice in the virgin, persisting after rupture as small tags called the carunculae hymenales.
Blood, lymph and nerve — each split at a different level
The ARTERIAL supply is a chain running down the canal. The VAGINAL ARTERY, usually from the internal iliac artery directly or from the uterine artery, supplies most of it; branches of the UTERINE artery supply the upper part, the MIDDLE RECTAL artery contributes behind, and the INTERNAL PUDENDAL artery supplies the lower part — all of them branches of the internal iliac artery. The veins form a rich VAGINAL VENOUS PLEXUS along the sides, communicating with the uterine, vesical and rectal plexuses, and draining to the internal iliac veins. The vulva is supplied mainly by the INTERNAL PUDENDAL artery, with the superficial and deep EXTERNAL PUDENDAL arteries from the femoral artery contributing anteriorly. The LYMPHATIC drainage divides by level, and the division is the whole basis of staging and of surgery: the UPPER vagina drains to the INTERNAL and EXTERNAL ILIAC nodes, the MIDDLE vagina to the INTERNAL ILIAC nodes, and the LOWER vagina — below the hymen — together with the whole VULVA drains to the SUPERFICIAL INGUINAL nodes. This is why a woman with vulval carcinoma has her groin nodes examined and sampled, while a tumour of the upper vagina spreads to nodes deep in the pelvis that no hand can feel. The NERVE supply splits at almost the same level, but for a different reason: the UPPER vagina is supplied by the UTEROVAGINAL (autonomic) plexus, an extension of the inferior hypogastric plexus, and carries visceral afferents only — it is relatively insensitive to touch, cutting and burning, which is why an intrauterine device can be inserted or a cervical biopsy taken with little or no anaesthesia. The LOWER vagina, below the hymen, and the whole of the vulva are supplied by the PUDENDAL NERVE (S2–S4) and its branches, and are somatically innervated and exquisitely sensitive.
A woman in the second stage of labour needs an instrumental delivery and an episiotomy, but no epidural is in place and there is no time for one. The obstetrician passes two fingers into the vagina and finds the ISCHIAL SPINE through the lateral vaginal wall — the same bony point used to assess descent of the head. A needle is guided along the fingers to a point just medial and inferior to the spine, where the pudendal nerve hooks around the sacrospinous ligament on its way from the greater sciatic foramen into the lesser, and local anaesthetic is injected on each side. Within minutes the lower vagina, the whole vulva and the perineum are numb, while the upper vagina, uterus and cervix — supplied by autonomic fibres that the block never touches — are unaffected, and the woman retains the abdominal expulsive power she needs. The forceps are applied and the episiotomy cut without pain. The whole technique, set out in the pudendal nerve, works only because the nerve supply of the birth canal is split at the hymen, and because that split is reachable through a vaginal wall.
- The vagina is a fibromuscular canal from the vestibule to the cervix, anterior wall ~7 cm and posterior wall ~9 cm, directed UPWARDS AND BACKWARDS at about 45°, at roughly 90° to the axis of the anteverted uterus.
- Anterior and posterior walls are normally in contact — H-shaped in cross-section — with transverse RUGAE that unfold to allow distension in intercourse and childbirth.
- The cervix projects into the upper vagina, creating FOUR FORNICES: anterior, posterior and two lateral. The POSTERIOR fornix is the deepest.
- Only a thin vaginal wall and peritoneum separate the posterior fornix from the RECTOUTERINE POUCH OF DOUGLAS — the lowest point of the female peritoneal cavity — hence culdocentesis, transvaginal drainage of pelvic abscess, transvaginal ultrasound and cervical excitation in ruptured ectopic pregnancy.
- Relations: ANTERIORLY bladder base and the urethra embedded in the anterior wall; POSTERIORLY pouch of Douglas, then rectum, then perineal body; LATERALLY the ureters about 2 cm lateral to the cervix, the levator ani and the cardinal ligaments.
- The wall is stratified squamous non-keratinised epithelium with NO GLANDS, a smooth muscle coat and an adventitia; lubrication comes from cervical mucus and transudate, not from vaginal glands.
- Oestrogen → glycogen-rich epithelium → lactobacilli → lactic acid → pH about 3.8–4.5. Break any link (menopause, antibiotics, anaerobic overgrowth) and infection follows.
- The vulva comprises the mons pubis, labia majora (homologue of the scrotum, receiving the round ligament), labia minora, vestibule, clitoris (glans, body, two crura) and vestibular bulbs, with ischiocavernosus and bulbospongiosus covering the erectile tissue.
- Glands of the vestibule: greater vestibular (Bartholin's) posterolaterally — homologue of the bulbourethral glands; lesser vestibular around the vestibule; paraurethral (Skene's) beside the urethral orifice — homologue of the prostate.
- Arteries: vaginal (from internal iliac or uterine), uterine, middle rectal and internal pudendal; the vulva mainly internal pudendal with external pudendal contributions from the femoral.
- Lymph BY LEVEL: upper vagina → internal AND external iliac nodes; middle → internal iliac; lower vagina and the whole VULVA → SUPERFICIAL INGUINAL nodes. This division decides cancer staging and groin node surgery.
- Nerve BY LEVEL: upper vagina = uterovaginal (autonomic) plexus, relatively insensitive; lower vagina and whole vulva = PUDENDAL nerve (S2–S4), highly sensitive — the basis of the pudendal block for delivery.
The vagina in the clinic
Almost every gynaecological procedure is an application of one of the four walls. On BIMANUAL EXAMINATION the cervix is felt as a firm rounded projection, the uterus is trapped between the internal fingers and the abdominal hand, the ovaries and any adnexal mass are sought in the lateral fornices, and a full pouch of Douglas bulges into the posterior fornix. A BARTHOLIN'S CYST is a tense swelling in the posterior third of the labium majus, produced when the duct blocks; infected, it becomes an abscess, and it is treated by marsupialisation rather than simple incision so that the duct is left permanently open. The PERINEAL BODY between the vagina and anal canal is the point at which many perineal muscles converge, and it is what an EPISIOTOMY deliberately cuts — mediolaterally, away from the anal sphincter — and what a third-degree tear destroys, taking continence with it, as continence and pelvic organ prolapse describes. After the MENOPAUSE the whole oestrogen chain fails: the epithelium thins, glycogen and lactobacilli disappear, the pH rises, and the result is atrophic vaginitis with dryness, dyspareunia, bleeding on contact and recurrent urinary infection — a problem treated logically with topical oestrogen, and covered pharmacologically in menopause and HRT. Prolonged OBSTRUCTED LABOUR crushes the vaginal wall between the fetal head and the pelvic bones until it necroses, producing a VESICOVAGINAL fistula (continuous urinary leakage) or a RECTOVAGINAL fistula (faecal leakage) — the great preventable injury of childbirth wherever emergency obstetric care is out of reach. After HYSTERECTOMY the upper vagina is closed as a blind VAULT, and if the cardinal and uterosacral ligaments are not reattached to it that vault can descend and prolapse, sometimes turning the canal inside out. Finally, the vaginal wall's rich venous plexus and thin epithelium make it an efficient ABSORPTIVE surface, used deliberately for local and systemic drug delivery: pessaries for prostaglandin induction of labour, antifungals, topical oestrogen, and contraceptive rings.
- Believing the vagina has glands. It has none. It is lubricated by mucus descending from the cervix and by a transudate seeping from the submucosal venous plexus, with the greater vestibular (Bartholin's) glands contributing at the entrance — which is why the discharge of cervicitis and of vaginitis behave so differently.
- Treating the vagina as a single unit for lymph and nerve. Both are split by level: lymph drains upper → iliac nodes but lower vagina and vulva → superficial inguinal nodes, and nerve supply is autonomic above the hymen but pudendal (S2–S4) below it. Forget the split and both cancer staging and regional anaesthesia become nonsense.
- Drawing the vagina as a vertical tube and the uterus as sitting directly on top of it. It runs upwards and backwards at about 45°, the cervix projects into it from in front, and the two axes meet at roughly a right angle — which is exactly why the posterior fornix is deep, why it reaches the pouch of Douglas, and why an instrument is directed towards the sacrum.
A 26-year-old woman presents with syncope, lower abdominal pain and amenorrhoea for seven weeks. She is hypotensive, and moving the cervix on vaginal examination causes severe pain. A needle is passed through the posterior fornix and withdraws non-clotting blood. Which space has the needle entered, and what separates it from the posterior fornix?
- The vagina is a distensible fibromuscular canal from vestibule to cervix — anterior wall ~7 cm, posterior ~9 cm — running upwards and backwards at about 45°, its walls in contact (H-shaped section) and thrown into transverse rugae that unfold for intercourse and childbirth.
- The cervix projects into its upper end making four fornices; the POSTERIOR fornix is separated from the rectouterine pouch of Douglas — the lowest point of the female peritoneal cavity — by only a thin wall and peritoneum, which is the anatomy behind culdocentesis, transvaginal abscess drainage, transvaginal ultrasound and cervical excitation.
- Relations: bladder base and urethra in front, pouch of Douglas then rectum then perineal body behind, ureters (2 cm lateral to the cervix), levator ani and cardinal ligaments at the sides. The wall is glandless stratified squamous epithelium whose oestrogen-driven glycogen feeds lactobacilli to make the protective pH of 3.8–4.5.
- The vulva comprises mons pubis, labia majora and minora, vestibule, clitoris and vestibular bulbs, with Bartholin's, lesser vestibular and Skene's glands and the hymen. Both lymph and nerve split by level: upper vagina → iliac nodes and autonomic uterovaginal plexus (relatively insensitive); lower vagina and vulva → superficial inguinal nodes and pudendal nerve S2–S4 (highly sensitive) — hence the pudendal block for delivery.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Pelvis and Perineum: the vagina, vaginal fornices and the female external genitalia.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — The vagina: relations, fornices, blood supply, lymphatic drainage by level and innervation; the vulva.
- Netter FH. Atlas of Human Anatomy — Female perineum and external genitalia; median section of the female pelvis.
- Last RJ. Last's Anatomy: Regional and Applied — The vagina and the female perineum.
- Snell RS. Clinical Anatomy by Regions — The pouch of Douglas, culdocentesis, Bartholin's cyst and the perineal body.
- TeachMeAnatomy — The Vagina; The Vulva; The Pudendal Nerve.
