Mons Pubis, Labia, and Clitoris
The pudenda—commonly designated the vulva—includes all structures visible externally from the symphysis pubis to the perineal body. This includes the mons pubis, labia majora and minora, clitoris, hymen, vestibule, urethral opening, greater vestibular or Bartholin glands, minor vestibular glands, and paraurethral glands (Fig. 2-3). The embryology of the external genitalia is discussed in Chapter 7 (Development of Genitalia), and its innervations and vascular support are described with the pudendal nerve (Anal Sphincter Complex).
Vulvar structures and subcutaneous layer of the anterior perineal triangle. Note the continuity of Colles and Scarpa fasciae. Inset: Vestibule boundaries and openings onto the vestibule. (From Corton, 2012, with permission.)
The mons pubis, also called the mons veneris, is a fat-filled cushion overlying the symphysis pubis. After puberty, the mons pubis skin is covered by curly hair that forms the escutcheon. In women, hair is distributed in a triangle, whose base covers the upper margin of the symphysis pubis and whose tip ends at the clitoris. In men and some hirsute women, the escutcheon is not so well circumscribed and extends onto the anterior abdominal wall toward the umbilicus.
Embryologically, the labia majora are homologous with the male scrotum. Labia vary somewhat in appearance, principally according to the amount of fat they contain. They are 7 to 8 cm in length, 2 to 3 cm in depth, and 1 to 1.5 cm in thickness. They are continuous directly with the mons pubis superiorly, and the round ligaments terminate at their upper borders. Posteriorly, the labia majora taper and merge into the area overlying the perineal body to form the posterior commissure.
Hair covers the labia majora outer surface but is absent on their inner surface. In addition, apocrine, eccrine, and sebaceous glands are abundant. Beneath the skin, there is a dense connective tissue layer, which is nearly void of muscular elements but is rich in elastic fibers and adipose tissue. This mass of fat provides bulk to the labia majora and is supplied with a rich venous plexus. During pregnancy, this vasculature commonly develops varicosities, especially in parous women, from increased venous pressure created by the enlarging uterus. They appear as engorged tortuous veins or as small grapelike clusters, but they are typically asymptomatic.
Each labium minus is a thin tissue fold that lies medial to each labium majus. In males, its homologue forms the ventral shaft of the penis. The labia minora extend superiorly, where each divides into two lamellae. From each side, the lower lamellae fuse to form the frenulum of the clitoris, and the upper merge to form the prepuce. Inferiorly, the labia minora extend to approach the midline as low ridges of tissue that join to form the fourchette. The size of the labia minora varies greatly among individuals, with lengths from 2 to 10 cm and widths from 1 to 5 cm (Lloyd, 2005).
Structurally, the labia minora are composed of connective tissue with numerous vessels, elastin fibers, and very few smooth muscle fibers. They are supplied with many nerve endings and are extremely sensitive (Ginger, 2011a). The epithelia of the labia minora vary with location. Thinly keratinized stratified squamous epithelium covers the outer surface of each labium. On their inner surface, the lateral portion is covered by this same epithelium up to a demarcating line—Hart line. Medial to this line, each labium is covered by squamous epithelium that is nonkeratinized. The labia minora lack hair follicles, eccrine glands, and apocrine glands. However, there are many sebaceous glands (Wilkinson, 2011).
The clitoris is the principal female erogenous organ and is the erectile homologue of the penis. It is located beneath the prepuce, above the frenulum and urethra, and projects downward and inward toward the vaginal opening. The clitoris rarely exceeds 2 cm in length and is composed of a glans, a corpus or body, and two crura (Verkauf, 1992). The glans is usually less than 0.5 cm in diameter, is covered by stratified squamous epithelium, and is richly innervated. The clitoral body contains two corpora cavernosa. Extending from the clitoral body, each corpus cavernosum diverges laterally to form a long, narrow crus. Each crus lies along the inferior surface of its respective ischiopubic ramus and deep to the ischiocavernosus muscle. The clitoral blood supply stems from branches of the internal pudendal artery. Specifically, the deep artery of the clitoris supplies the clitoral body, whereas the dorsal artery of the clitoris supplies the glans and prepuce.
This is the functionally mature female structure derived from the embryonic urogenital membrane. In adult women, it is an almond-shaped area that is enclosed by Hart line laterally, the external surface of the hymen medially, the clitoral frenulum anteriorly, and the fourchette posteriorly. The vestibule usually is perforated by six openings: the urethra, the vagina, two Bartholin gland ducts, and at times, two ducts of the largest paraurethral glands—the Skene glands. The posterior portion of the vestibule between the fourchette and the vaginal opening is called the fossa navicularis. It is usually observed only in nulliparas.
The bilateral Bartholin glands, also termed greater vestibular glands, are major glands that measure 0.5 to 1 cm in diameter. On their respective side, each lies inferior to the vascular vestibular bulb and deep to the inferior end of the bulbocavernosus muscle. The duct from each measures 1.5 to 2 cm long and opens distal to the hymeneal ring—one at 5 and the other at 7 o’clock on the vestibule. Following trauma or infection, either duct may swell and obstruct to form a cyst or, if infected, an abscess. In contrast, the minor vestibular glands are shallow glands lined by simple mucin-secreting epithelium and open along Hart line.
The paraurethral glands are a collective arborization of glands whose multiple small ducts open predominantly along the entire inferior aspect of the urethra. The two largest are called Skene glands, and their ducts typically lie distally and near the urethral meatus. Clinically, inflammation and duct obstruction of any of the paraurethral glands can lead to urethral diverticulum formation.
The lower two thirds of the urethra lie immediately above the anterior vaginal wall. The urethral opening or meatus is in the midline of the vestibule, 1 to 1.5 cm below the pubic arch, and a short distance above the vaginal opening.
In adult women, the hymen is a membrane of varying thickness that surrounds the vaginal opening more or less completely. It is composed mainly of elastic and collagenous connective tissue, and both outer and inner surfaces are covered by nonkeratinized stratified squamous epithelium. The aperture of the intact hymen ranges in diameter from pinpoint to one that admits one or even two fingertips. Imperforate hymen is a rare malformation in which the vaginal orifice is occluded completely, causing retention of menstrual blood (Chap. 3, Bladder and Perineal Abnormalities). As a rule, the hymen is torn at several sites during first coitus. However, identical tears may occur by other penetration, for example, by tampons used during menstruation. The edges of the torn tissue soon reepithelialize. In pregnant women, the hymeneal epithelium is thick and rich in glycogen. Changes produced in the hymen by childbirth are usually readily recognizable. For example, over time, the hymen transforms into several nodules of various sizes, termed hymeneal or myrtiform caruncles.
Proximal to the hymen, the vagina is a musculomembranous tube that extends to the uterus and is interposed lengthwise between the bladder and the rectum (Fig. 2-4). Anteriorly, the vagina is separated from the bladder and urethra by connective tissue—the vesicovaginal septum. Posteriorly, between the lower portion of the vagina and the rectum, there are similar tissues that together form the rectovaginal septum. The upper fourth of the vagina is separated from the rectum by the rectouterine pouch, also called the cul-de-sac or pouch of Douglas.
Vagina and surrounding anatomy. (From Corton, 2012, with permission.)
Normally, the anterior and posterior walls of the vaginal lumen lie in contact, with only a slight space intervening at the lateral margins. Vaginal length varies considerably, but commonly, the anterior wall measures 6 to 8 cm, whereas the posterior vaginal wall is 7 to 10 cm. The upper end of the vaginal vault is subdivided into anterior, posterior, and two lateral fornices by the cervix. These are of considerable clinical importance because the internal pelvic organs usually can be palpated through the thin walls of these fornices. Moreover, the posterior fornix provides surgical access to the peritoneal cavity.
At the midportion of the vagina, its lateral walls are attached to the pelvis by visceral connective tissue. These lateral attachments blend into investing fascia of the levator ani. In doing so, they create the anterior and posterior lateral vaginal sulci. These run the length of the vaginal sidewalls and give the vagina an H shape when viewed in cross section.
The vaginal lining is composed of nonkeratinized stratified squamous epithelium and underlying lamina propria. In premenopausal women, this lining is thrown into numerous thin transverse ridges, known as rugae, which line the anterior and posterior vaginal walls along their length. Deep to this, there is a muscular layer, which contains smooth muscle, collagen, and elastin. Beneath this muscularis lies an adventitial layer consisting of collagen and elastin (Weber, 1997).
There are no vaginal glands. Instead, the vagina is lubricated by a transudate that originates from the vaginal subepithelial capillary plexus and crosses the permeable epithelium (Kim, 2011). Due to increased vascularity during pregnancy, vaginal secretions are notably increased. At times, this may be confused with amnionic fluid leakage, and clinical differentiation of these two is described in Chapter 22 (Admission Procedures).
After birth-related epithelial trauma and healing, fragments of stratified epithelium occasionally are embedded beneath the vaginal surface. Similar to its native tissue, this buried epithelium continues to shed degenerated cells and keratin. As a result, firm epidermal inclusion cysts, which are filled with keratin debris, may form and are a common vaginal cyst.
The vagina has an abundant vascular supply. The proximal portion is supplied by the cervical branch of the uterine artery and by the vaginal artery. The latter may variably arise from the uterine or inferior vesical or directly from the internal iliac artery. The middle rectal artery contributes supply to the posterior vaginal wall, whereas the distal walls receive contributions from the internal pudendal artery. At each level, blood supply from each side forms anastomoses on the anterior and posterior vaginal walls with contralateral corresponding vessels.
An extensive venous plexus immediately surrounds the vagina and follows the course of the arteries. Lymphatics from the lower third, along with those of the vulva, drain primarily into the inguinal lymph nodes. Those from the middle third drain into the internal iliac nodes, and those from the upper third drain into the external, internal, and common iliac nodes.
This diamond-shaped area between the thighs has boundaries that mirror those of the bony pelvic outlet: the pubic symphysis anteriorly, ischiopubic rami and ischial tuberosities anterolaterally, sacrotuberous ligaments posterolaterally, and coccyx posteriorly. An arbitrary line joining the ischial tuberosities divides the perineum into an anterior triangle, also called the urogenital triangle, and a posterior triangle, termed the anal triangle.
The perineal body is a fibromuscular mass found in the midline at the junction between these anterior and posterior triangles (Fig. 2-5). Also called the central tendon of the perineum, the perineal body measures 2 cm tall and wide and 1.5 cm thick. It serves as the junction for several structures and provides significant perineal support (Shafik, 2007; Woodman, 2002). Superficially, the bulbocavernosus, superficial transverse perineal, and external anal sphincter muscles converge on the central tendon. More deeply, the perineal membrane, portions of the pubococcygeus muscle, and internal anal sphincter contribute (Larson, 2010). The perineal body is incised by an episiotomy incision and is torn with second-, third-, and fourth-degree lacerations.
Superficial space of the anterior triangle and posterior perineal triangle. Structures on the left side of the image can be seen after removal of Colles fascia. Those on the right side are noted after removal of the superficial muscles of the anterior triangle. (From Corton, 2012, with permission.)
Superficial Space of the Anterior Triangle
This triangle is bounded by the pubic rami superiorly, the ischial tuberosities laterally, and the superficial transverse perineal muscles posteriorly. It is divided into superficial and deep spaces by the perineal membrane. This membranous partition is a dense fibrous sheet that was previously known as the inferior fascia of the urogenital diaphragm. The perineal membrane attaches laterally to the ischiopubic rami, medially to the distal third of the urethra and vagina, posteriorly to the perineal body, and anteriorly to the arcuate ligament of the pubis.
The superficial space of the anterior triangle is bounded deeply by the perineal membrane and superficially by Colles fascia. As noted earlier, Colles fascia is the continuation of Scarpa fascia onto the perineum. On the perineum, Colles fascia securely attaches laterally to the pubic rami and fascia lata of the thigh, inferiorly to the superficial transverse perineal muscle and inferior border of the perineal membrane, and medially to the urethra, clitoris, and vagina. As such, the superficial space of the anterior triangle is a relatively closed compartment, and expanding infection or hematoma within it may bulge yet remains contained.
This superficial pouch contains several important structures, which include the Bartholin glands, vestibular bulbs, clitoral body and crura, branches of the pudendal vessels and nerve, and the ischiocavernosus, bulbocavernosus, and superficial transverse perineal muscles. Of these muscles, the ischiocavernosus muscles each attach on their respective side to the medial aspect of the ischial tuberosity inferiorly and the ischiopubic ramus laterally. Anteriorly, each attaches to a clitoral crus and may help maintain clitoral erection by compressing the crus to obstruct venous drainage. The bilateral bulbocavernosus muscles overlie the vestibular bulbs and Bartholin glands. They attach to the body of the clitoris anteriorly and the perineal body posteriorly. The muscles constrict the vaginal lumen and aid release of secretions from the Bartholin glands. They also may contribute to clitoral erection by compressing the deep dorsal vein of the clitoris. The bulbocavernosus and ischiocavernosus muscles also pull the clitoris downward. Last, the superficial transverse perineal muscles are narrow strips that attach to the ischial tuberosities laterally and the perineal body medially. They may be attenuated or even absent, but when present, they contribute to the perineal body (Corton, 2012).
Embryologically, the vestibular bulbs correspond to the corpora spongiosa of the penis. These almond-shaped aggregations of veins are 3 to 4 cm long, 1 to 2 cm wide, and 0.5 to 1 cm thick and lie beneath the bulbocavernosus muscle on either side of the vestibule. The bulbs terminate inferiorly at approximately the middle of the vaginal opening and extend upward toward the clitoris. Their anterior extensions merge in the midline, below the clitoral body. During childbirth, veins in the vestibular bulbs may be lacerated or even rupture to create a vulvar hematoma enclosed within the superficial space of the anterior triangle.
Deep Space of the Anterior Triangle
This space lies deep to the perineal membrane and extends up into the pelvis (Fig. 2-6) (Mirilas, 2004). In contrast to the superficial perineal space, the deep space is continuous superiorly with the pelvic cavity (Corton, 2005). It contains portions of urethra and vagina, certain portions of internal pudendal artery branches, and the compressor urethrae and urethrovaginal sphincter muscles, which comprise part of the striated urogenital sphincter complex.
Deep space of the anterior triangle of the perineum. Structures on the right side of the image can be seen after removal of the perineal membrane. Also shown are structures that attach to the perineal body: bulbocavernosus, superficial transverse perineal, external anal sphincter, and puboperinealis muscles as well as the perineal membrane. (From Corton, 2012, with permission.)
Found deep to the anterior and posterior triangles, this broad muscular sling provides substantial support to the pelvic viscera. The pelvic diaphragm is composed of the levator ani and the coccygeus muscle. The levator ani is composed of the pubococcygeus, puborectalis, and iliococcygeus muscles. The pubococcygeus muscle is also termed the pubovisceral muscle and is subdivided based on points of insertion and function. These include the pubovaginalis, puboperinealis, and puboanalis muscles, which insert into the vaginal, perineal body, and anus, respectively (Kearney, 2004).
Vaginal birth conveys significant risk for damage to the levator ani or to its innervation (DeLancey, 2003; Weidner, 2006). Of these muscles, the pubovisceral muscle is more commonly damaged (Lien, 2004; Margulies, 2007). Evidence supports that these injuries may predispose women to greater risk of pelvic organ prolapse or urinary incontinence (DeLancey, 2007a,b; Rortveit, 2003). For this reason, current research efforts are aimed at minimizing these injuries.
This triangle contains the ischioanal fossae, anal canal, and anal sphincter complex, which consists of the internal anal sphincter, external anal sphincter, and puborectalis muscle. Branches of the pudendal nerve and internal pudendal vessels are also found within this triangle.
Also known as ischiorectal fossae, these two fat-filled wedge-shaped spaces are found on either side of the anal canal and comprise the bulk of the posterior triangle (Fig. 2-7). Each fossa has skin as its superficial base, whereas its deep apex is formed by the junction of the levator ani and obturator internus muscle. Other borders include: laterally, the obturator internus muscle fascia and ischial tuberosity; inferomedially, the anal canal and sphincter complex; superomedially, the inferior fascia of the downwardly sloping levator ani; posteriorly, the gluteus maximus muscle and sacrotuberous ligament; and anteriorly, the inferior border of the anterior triangle.
Anal canal and ischioanal fossa. (From Corton, 2012, with permission.)
The fat found within each fossa provides support to surrounding organs yet allows rectal distention during defecation and vaginal stretching during delivery. Clinically, injury to vessels in the posterior triangle can lead to hematoma formation in the ischioanal fossa, and the potential for large accumulation in these easily distensible spaces. Moreover, the two fossae communicate dorsally, behind the anal canal. This can be especially important because an episiotomy infection or hematoma may extend from one fossa into the other.
This distal continuation of the rectum begins at the level of levator ani attachment to the rectum and ends at the anal skin. Along this 4- to 5-cm length, the mucosa consists of columnar epithelium in the uppermost portion, but at the dentate or pectinate line, simple stratified squamous epithelium begins and continues to the anal verge. Here, keratin and skin adnexa join the squamous epithelium.
The anal canal has several lateral tissue layers. Inner layers include the anal mucosa, the internal anal sphincter, and an intersphincteric space that contains continuation of the rectum’s longitudinal smooth muscle layer. An outer layer contains the puborectalis muscle as its cephalad component and the external anal sphincter caudally.
Within the anal canal, three highly vascularized submucosal arteriovenous plexuses termed anal cushions aid complete closure of the canal and fecal continence when apposed. Increasing uterine size, excessive straining, and hard stool create increased pressure that ultimately leads to degeneration and subsequent laxity of the cushion’s supportive connective tissue base. These cushions then protrude into and downward through the anal canal. This leads to venous engorgement within the cushions—now termed hemorrhoids. Venous stasis results in inflammation, erosion of the cushion’s epithelium, and then bleeding.
External hemorrhoids are those that arise distal to the pectinate line. They are covered by stratified squamous epithelium and receive sensory innervation from the inferior rectal nerve. Accordingly, pain and a palpable mass are typical complaints. Following resolution, a hemorrhoidal tag may remain and is composed of redundant anal skin and fibrotic tissue. In contrast, internal hemorrhoids are those that form above the dentate line and are covered by insensitive anorectal mucosa. These may prolapse or bleed but rarely become painful unless they undergo thrombosis or necrosis.
Two sphincters surround the anal canal to provide fecal continence—the external and internal anal sphincters. Both lie proximate to the vagina, and one or both may be torn during vaginal delivery. The internal anal sphincter (IAS) is a distal continuation of the rectal circular smooth muscle layer. It receives predominantly parasympathetic fibers, which pass through the pelvic splanchnic nerves. Along its length, this sphincter is supplied by the superior, middle, and inferior rectal arteries. The IAS contributes the bulk of anal canal resting pressure for fecal continence and relaxes prior to defecation. The IAS measures 3 to 4 cm in length, and at its distal margin, it overlaps the external sphincter for 1 to 2 cm (DeLancey, 1997; Rociu, 2000). The distal site at which this overlap ends, called the intersphincteric groove, is palpable on digital examination.
In contrast, the external anal sphincter (EAS) is a striated muscle ring that anteriorly attaches to the perineal body and that posteriorly connects to the coccyx via the anococcygeal ligament. The EAS maintains a constant resting contraction to aid continence, provides additional squeeze pressure when continence is threatened, yet relaxes for defecation. Traditionally, the EAS has been described as three parts, which include the subcutaneous, superficial, and deep portions. However, many consider the deep portion to be composed fully or in part by the puborectalis muscle (Raizada, 2008). The external sphincter receives blood supply from the inferior rectal artery, which is a branch of the internal pudendal artery. Somatic motor fibers from the inferior rectal branch of the pudendal nerve supply innervation. Clinically, the IAS and EAS may be involved in fourth-degree laceration during vaginal delivery, and reunion of these rings is integral to defect repair (Chap. 27, “Fourth Stage” of Labor).
This is formed from the anterior rami of S2–4 spinal nerves (Fig. 2-8). It courses between the piriformis and coccygeus muscles and exits through the greater sciatic foramen at a location posterior to the sacrospinous ligament and just medial to the ischial spine (Barber, 2002). Thus, when injecting local anesthetic for a pudendal nerve block, the ischial spine serves an identifiable landmark (Chap. 25, Pudendal Block). The pudendal nerve then runs beneath the sacrospinous ligament and above the sacrotuberous ligament as it reenters the lesser sciatic foramen to course along the obturator internus muscle. Atop this muscle, the nerve lies within the pudendal canal, also known as Alcock canal, which is formed by splitting of the obturator internus investing fascia (Shafik, 1999). In general, the pudendal nerve is relatively fixed as it courses behind the sacrospinous ligament and within the pudendal canal. Accordingly, it may be at risk of stretch injury during downward displacement of the pelvic floor during childbirth (Lien, 2005).
Pudendal nerve and vessels. (From Corton, 2012, with permission.)
The pudendal nerve leaves this canal to enter the perineum and divides into three terminal branches. Of these, the dorsal nerve of the clitoris runs between the ischiocavernosus muscle and perineal membrane to supply the clitoral glans (Ginger, 2011b). The perineal nerve runs superficial to the perineal membrane (Montoya, 2011). It divides into posterior labial branches and muscular branches, which serve the labial skin and the anterior perineal triangle muscles, respectively. The inferior rectal branch runs through the ischioanal fossa to supply the external anal sphincter, the anal mucosa, and the perianal skin (Mahakkanukrauh, 2005). The major blood supply to the perineum is via the internal pudendal artery, and its branches mirror the divisions of the pudendal nerve.