of vaginal delivery [25]. After passing through the pelvic floor and perineal membrane, it ends at the external urethral orifice. The urethra is fixed at its origin by the pubovesical ligaments, throughout its length by the anterior wall of the vagina, and as it enters the perineum by the urogenital diaphragm. The external urethral orifice lies in the midline of the vestibule between the vagina and the clitoris. The orifice is easily seen, and on occasions there may be bright red projections of prolapsed urethral mucosa herniating out. These urethral caruncles are most commonly seen in post‐menopausal women and are thought to be due to reduced oestrogen (Figure 2.10).
Figure 2.10 Urethral caruncle in a patient with lichen sclerosus.
Skene’s glands (paraurethral glands) are paired glands with their ducts opening on each side of the urethral orifice. Evidence suggests that they are analogous to the male prostate gland [26].
Associated structures
The vagina
The vagina is a fibromuscular tube about 7–10 cm long. It opens on the vulval vestibule and extends upwards and backwards, to be attached just above the lower margin of the uterine cervix. As the long axis of the vagina forms a right angle with the long axis of the normal anteverted uterus, the cervix projects downwards and backwards into the upper vagina. The vaginal fold around the periphery of the cervix is divided into anterior, posterior, and lateral fornices. The posterior wall of the vagina is about 2 cm longer than the anterior wall, but they are in contact with each other in the un‐distended vagina. This gives the vagina a crescentic or H‐shaped appearance in cross‐section. The outer wall of the vagina contains its vascular, lymphatic, and nerve supply.
The vagina is related anteriorly to the base of the bladder and to the urethra, which is embedded in its anterior wall. Posteriorly, the upper part of the vaginal wall is covered with peritoneum, and below the rectouterine pouch it is directly related to the ampulla of the rectum. In the perineum, it is separated from the anal canal by the perineal body (Figure 2.11). The upper vagina gives attachment to the uterosacral ligaments posteriorly, the cardinal or transverse ligaments laterally, and the base of the bladder anteriorly, which itself is supported by the pubovesical ligaments. As the vagina passes through the pelvic floor, the most medial fibres of the pubococcygeus blend with its walls to form a supporting muscular sling. Below the pelvic floor the vagina is supported by the urogenital diaphragm, the perineal body, and the perineal musculature. Thus, the vagina has three compartments:
Figure 2.11 Midline section through pelvis and perineum.
upper, above the pelvic floor and related to the rectum
middle, which traverses the pelvic floor and urogenital diaphragm
lower in the perineum.
The perineum
The perineum is the outer diamond‐shaped area inferior to the sheet of muscle forming the pelvic floor, and is bounded by the symphysis pubis anteriorly, the ischial tuberosities laterally, and the coccyx posteriorly. It is an embryological junctional zone derived from the body wall ectoderm, hindgut endoderm, and the intervening mesoderm that surrounded the original cloacal membrane. It is further divided into an anterior urogenital triangle and posterior anal triangle. The vulva lies mainly within the anterior urogenital triangle but then extends anteriorly to the pubic symphysis. The anal canal and ischiorectal fossa occupy the posterior anal triangle. The perineal body is a fibromuscular mass lying between the upper half of the anterior anal wall and the entire posterior portion of the vagina. It is the central point where muscles attach to the ischial tuberosities.
The urogenital triangle
The urogenital triangle is contained within the subpubic arch and is divided into superficial and deep perineal pouches by the urogenital diaphragm.
The urogenital diaphragm
The urogenital diaphragm (also known as perineal membrane or triangular ligament) is a tough fibrous membrane attached to the pubic rami (Figure 2.12). It has three midline breaks, one at the apex of the triangle just below the pubic symphysis where the clitoral vessels and nerves pass from the deep to the superficial perineal pouch, and two more posteriorly for the entrances of the urethra and vagina.
Deep perineal pouch
The deep perineal pouch is limited above by the pelvic floor and pubovesical ligaments and below by the urogenital diaphragm. It is continuous with the ischiorectal fossae posteriorly. The urethra and vagina pass through the deep perineal pouch in the midline. Vessels and nerves run alongside the urethra and vagina, and their clitoral branches ascend through the apical opening in the urogenital diaphragm. The deep pouch also contains voluntary muscle fibres, some of which surround the urethra and vagina while others run transversely into the perineal body behind the vagina.
Superficial perineal pouch
This lies below the urogenital triangle and contains the structural elements of the female external genitalia, which, with their skin covering, exhibit the external appearance characteristic of the vulva. The clitoral bulbs lie in the superficial perineal pouch adjacent to the lateral wall of the vagina and are attached to the inferior surface of the urogenital diaphragm by the overlying bulbospongiosus muscle. Just behind the clitoral bulbs, also lying on the urogenital diaphragm, are Bartholin’s glands. The superficial transverse perineal muscles lie transversely across the base of the urogenital triangle at the posterior margin of the superficial perineal pouch (Figure 2.13).
Figure 2.12 The urogenital diaphragm.
Figure 2.13 The superficial structures of the perineum.
Anal triangle
The anal triangle is the triangular area bounded by the ischial tuberosities and the coccyx. The anal canal lies within this area. The ischiorectal fossae lie laterally to the anal canal (Figure 2.14). These are pyramid‐shaped areas bounded by ischium and obturator internus laterally, levator ani medially, and the perianal skin inferiorly. The fossae
