The Vagina

Form

The opening of the vagina in virgins is partly closed by a thin sheet of tissue, the hymen. The vagina is a thin walled muscular tube lying inferior to the bladder and anterior to the anus and rectum. Its superior end is invaginated by the cervix of the uterus forming the fornices. The anterior and lateral fornices are shallow but the posterior fornix is deep. The posterior fornix meets the fascia of the peritoneum of the recto-uterine pouch. The vagina receives its blood supply from the vaginal artery, a branch of either the internal iliac or uterine arteries. The venous drainage is to the internal iliac vein. The innervation is from the inferior hypogastric plexus and the pelvic splanchnic nerves along the vaginal arteries. The lower vagina is supplied by the pudendal nerve.

Histology

The vagina is made up of four layers. It is lined by stratified squamous epithelium sitting on a lamina propria which is rich in elastic fibres. Below the lamina propria is a fibromuscular layer composed of circular and longitudinal smooth muscle fibres. Close to the hymen some striated muscle is also present. An outer adventitial coat is present in which run nerves and blood vessels. The loose construction of the vaginal wall and the presence of elastin allows for distention. The lamina propria contains extensive thin-walled veins and vennules which allow fluid to pass through to the vaginal lumen. The breakdown of epithelial surface cells which store glycogen results in an acid environment, deterring the proliferation of bacterial pathogens and fungi.

The Perineum

The external genitalia lie inferior to the urogenital diaphragm. The urogenital diaphragm fills-in the triangle below the inferior pubic rami. This fascial sheet encloses the sphincter urethrae muscle and the deep transverse perinei muscles. The urogenital diaphragm lies inferior to the anterior defect in the pelvic diaphragm. On the inferior pubic rami lie the erectile tissues of the corpora cavernosa. The erectile tissue of the bulb of the vestibule lies on the urogenital diaphragm passing around the opening for the vagina. The erectile tissues pass superiorly to form the clitoris. The corpora cavernosa are covered by a thin layer of muscle, the ischiocavernosus, while the bulb is covered by the bulbospongiosus muscle. Paired glands, the greater vestibular or Bartholin's glands lie on the urogenital diaphragm deep to the bulb. These glands open into the vestibule. The skin covering these perineal structures is formed into two pairs of folds. The labia majora are the outer larger folds enclosing fat pads and bearing hair (LM in the image).  The perineal body (P) lies posteriorly, anterior to the anus. The inner smaller labia minora are smooth lips which fuse posteriorly as the fourchette and anteriorly over the clitoris. The labia minora enclose the vestibule. The urethra and the vagina open into the vestibule.

Blood Supply

The blood supply to these perineal structures is by the internal pudendal artery. The artery arises from the internal iliac artery and passes inferiorly and laterally to leave the pelvis through the greater sciatic foramen. The artery then passes around the ischial spine and re-enters the pelvis through the lesser sciatic foramen. The artery accompanied by the vein and nerve travels along the ischial ramus in a fascial canal. It gives a branch to the rectum and anus, the labia and muscles, the erectile tissue and the clitoris. The venous drainage follows the plan of the arterial supply and drains to the internal iliac veins. The inferior rectal vein anastomoses with the superior rectal vein in the wall of the rectum, allowing communication between the portal and systemic venous systems.

Lymphatic Drainage

The lymphatic drainage of the perineum is to superficial inguinal and internal iliac nodes.

Innervation

The innervation of the perineum is by the pudendal nerve, S2,3,4. The nerve follows the route taken by the artery to innervate the anal sphincters, the sphincter urethrae, transverse perinei, ischiocavernosus and bulbospongiosus muscles and the clitoris. The erectile tissue receives its autonomic innervation from the inferior hypogastric plexus and pelvic splanchnic nerves through nerves which travel with the vaginal artery along the vagina.