A thorough understanding of pelvic, perineal, and anterior abdominal wall anatomy is essential for obstetric practice and surgery. Although anatomic consistencies can be expected, marked variation may be encountered among women and in individual women as pregnancy advances. This is especially true for major blood vessels and genitourinary structures.
The anterior abdominal wall plays several roles during pregnancy. It confines the abdominopelvic viscera; contributes muscular action for respiration, elimination, and parturition; and stretches to accommodate the expanding uterus. For cesarean delivery, the anterior abdominal wall must be divided to gain surgical access to the internal reproductive organs. Thus, a comprehensive knowledge of its layered structure is required for safe and effective entry into the peritoneal cavity. The layers of the anterior abdominal wall include the skin and subcutaneous layer, which receive blood supply from the femoral artery, and the muscles and fascia, which are supplied by branches of the external iliac artery (Fig. 3-1).
Langer lines correspond to the natural orientation of collagen fibers within the skin and are generally parallel to the orientation of the underlying muscle fibers. In the anterior abdominal wall, they are mostly arranged transversely. As a result, vertical skin incisions sustain greater lateral tension compared with low transverse incisions such as the Pfannenstiel, and thus generally develop wider scars.
The subcutaneous layer can be separated into a superficial, predominantly fatty layer known as Camper fascia, and a deeper, more fibrofatty layer known as Scarpa fascia. Camper fascia continues onto the perineum to provide fatty substance to the mons pubis and labia majora. Scarpa fascia continues inferiorly onto the perineum as Colles fascia, which is also known as the superficial perineal fascia (p. 31). Thus, blood or infection within the subcutaneous layer of the anterior abdominal wall can extend to the perineum, and vice versa.
Clinically, Scarpa fascia is better developed in the lower abdomen, and during surgery it can be best identified in the lateral portions of a low transverse incision. In contrast, this fascia is rarely recognized during midline vertical incisions and may be absent at the umbilicus (Martin, 1984).
The anterior abdominal wall muscles consist of the midline rectus abdominis and pyramidalis muscles as well as the more lateral external and internal oblique and transversus abdominis muscles. These last three muscles, often called the flank muscles, contain a lateral muscular portion and a medial fibrous aponeurotic portion. The aponeuroses of these muscles contribute to the primary fascia of the anterior abdominal wall and form the important rectus sheath. In the midline, these aponeurotic layers fuse to create the linea alba, which extends from the xiphoid process to the symphysis pubis.
This anatomy is clinically relevant. First, surgically, because the aponeuroses of the internal oblique and transversus abdominis fuse in the lower abdomen, only two layers are identified laterally during low transverse incision creation. Also, in the lower abdomen, transition from muscle to aponeurosis for the internal oblique and transversus abdominis muscles takes place at a more medial site than that for the external oblique muscles. Accordingly, during low transverse incisions, muscle fibers of the internal oblique muscle are often noted below the aponeurotic layer of the external oblique muscle.
The lowermost portion of the aponeurosis of the external oblique ends in a tendinous border known as the inguinal ligament. This ligament extends from the anterior superior iliac spine to the pubic tubercle. The superficial inguinal ring represents a narrow opening of the inguinal ligament near the pubic tubercle and serves as the exit site for the round ligament and one or two nerve branches. These are the inguinal branch of the ilioinguinal nerve and genital branch of the genitofemoral nerve (see Fig. 3-1).
This is formed by the aponeuroses of the external and internal oblique and transversus abdominis muscles (Fig. 3-2). This sheath surrounds and holds the position of the rectus muscles. The composition of this sheath varies above and below the arcuate line, also known as the semicircular line of Douglas. This transverse line is a curved, tendinous boundary in the posterior layer of the rectus sheath. It typically lies midway between the umbilicus and symphysis pubis. Cephalad to the arcuate line, the rectus sheath wraps both anterior and posterior to the rectus abdominis muscles. At this level, the anterior rectus sheath is formed by the aponeurosis of the external oblique and the split aponeurosis of the internal oblique muscle. The posterior rectus sheath is formed by the split aponeurosis of the internal oblique and aponeurosis of the transversus abdominis muscle. Caudad to the arcuate line, all aponeurotic layers pass anterior to the rectus abdominis muscles. Thus, in the lower abdomen, the posterior surfaces of the rectus muscles are in direct contact with the transversalis fascia. This transition of rectus sheath composition can be best appreciated during midline vertical abdominal incisions.
The paired small triangular pyramidalis muscles originate from the pubic bones, insert into the linea alba, and then lie ventral to the rectus abdominis muscles but beneath the anterior rectus sheath. This muscle may be absent in approximately 10 percent of women.
Similar to skin fibers, the flank muscles and rectus sheath fibers are oriented primarily transversely. Thus, suture lines placed in a vertical fascial incision must withstand more tension than those in a transverse incision. As a result, vertical fascial incisions are more prone to dehiscence and hernia formation. Thus, during physical examination, an abnormally wide separation of the rectus muscles may suggest diastasis recti or hernia.
The transversalis fascia is the thin fibrous tissue layer that lies between the inner surface of the transversus abdominis muscle and the peritoneum. It serves as part of the general fascial layer that lines the entire abdominal cavity (Memon, 1999). Surgically, this layer is best recognized as the tissue that is bluntly or sharply dissected off the anterior surface of the bladder during entry into the abdominal cavity. Between the transversalis fascia and the peritoneum in the anterior abdominal wall lies a layer of extraperitoneal loose connective tissue often called preperitoneal fat.
That portion of the peritoneum that lines the inner surface of the abdominal wall is termed parietal peritoneum. In the anterior abdominal wall, there are five elevations of parietal peritoneum that are raised by different structures (see Fig. 3-2). All five converge toward the umbilicus and are known as umbilical ligaments.
The single median umbilical ligament is formed by the urachus, an obliterated tube that extends from the apex of the bladder to the umbilicus. In fetal life, the urachus, which is a fibrous remnant of the allantois, extends from the umbilical cord to the urogenital sinus, which gives rise to the bladder. The paired medial umbilical ligaments are formed by the obliterated umbilical arteries that connected the internal iliac arteries to the umbilical cord in fetal life. The paired lateral umbilical ligaments contain the inferior epigastric vessels.
Surgically, transection of a rare, patent urachus can result in extravasation of urine into the abdominal cavity. In addition, the differential diagnosis of a midline anterior abdominal wall cyst includes urachal cyst, urachal sinus, and urachal diverticulum.
The umbilical ligaments serve as valuable surgical landmarks. First, the inferior epigastric vessels can be injured during Maylard incisions (Hurd, 1994). Also, direct visualization of the inferior epigastric vessels within the lateral umbilical folds can prevent injury to these vessels during placement of accessory laparoscopic ports (Rahn, 2010). Second, the medial umbilical ligaments, if followed proximally, can guide a surgeon to the internal iliac artery. This may aid identification of the uterine artery’s origin to assist with uterine artery ligation.
The superficial epigastric, superficial circumflex iliac, and external pudendal arteries arise from the femoral artery just below the inguinal ligament within the femoral triangle (see Fig. 3-1). These vessels supply the skin and subcutaneous layers of the anterior abdominal wall and mons pubis. Of surgical importance, with low transverse skin incisions, the superficial epigastric vessels can usually be identified at a depth halfway between the skin and the anterior rectus sheath, just above Scarpa fascia, and several centimeters from the midline. During laparoscopic procedures, these vessels may be identified by transillumination in thin patients (Chap. 15, p. 254).
The inferior “deep” epigastric vessels and deep circumflex iliac vessels are branches of the external iliac vessels. They supply the muscles and fascia of the anterior abdominal wall. Of surgical relevance, the inferior epigastric vessels initially course lateral to, then posterior to the rectus abdominis muscles, which they supply. These vessels then pass ventral to the posterior rectus sheath, course between the sheath and the rectus muscles, and provide muscular branches. Near the umbilicus, these vessels anastomose with the superior epigastric artery and veins. The surgical importance of the inferior epigastric vessels is noted in the preceding section and in Chapter 4 (p. 53). Also, these vessels rarely may rupture following abdominal trauma leading to a rectus sheath hematoma (Tolcher, 2010).
On each side of the lower anterior abdominal wall, Hesselbach triangle is the region bounded laterally by the inferior epigastric vessels, inferiorly by the inguinal ligament, and medially by the lateral border of the rectus abdominis muscle. Hernias that protrude into the abdominal wall through Hesselbach triangle, and thus medial to the inferior epigastric vessels, are termed direct inguinal hernias. These are generally acquired. In contrast, indirect inguinal hernias enter the deep inguinal ring, which lies lateral to this triangle and thus lateral to the inferior epigastric vessels. Although infrequent, an indirect hernia may extend medially within the inguinal canal, exit through the superficial inguinal ring, and reach the ipsilateral labium majus.
The anterior abdominal wall is innervated by intercostal nerves (T7-11), the subcostal nerve (T12), and the iliohypogastric and the ilioinguinal nerves (L1) (see Fig. 3-1). Of these, the intercostal and subcostal nerves are ventral rami of the thoracic spinal nerves and run along the lateral and then anterior abdominal wall between the transversus abdominis and internal oblique muscles. This space is termed the transversus abdominis plane (Fig. 19-3, p. 314). Near the lateral borders of the rectus abdominis muscle, these nerve branches pierce the posterior rectus sheath, rectus muscle, and then anterior rectus sheath to reach the skin. Therefore, these nerve branches may be severed during a Pfannenstiel incision when the overlying anterior rectus sheath is separated from the rectus muscle (Fig. 4-5, p. 51).
In contrast, the iliohypogastric and ilioinguinal nerves originate from the ventral ramus of the first lumbar spinal nerve and often receive contributions from T12. They emerge at a point lateral to the psoas muscle and course retroperitoneally. Their path continues ventrally in an inferomedial line. At a site 2 to 3 cm medial to the anterior superior iliac spine, the nerves then pierce through the internal oblique muscle and course superficial to it and toward the midline (Whiteside, 2003). The iliohypogastric nerve perforates the external oblique aponeurosis near the lateral rectus border to provide sensation to the skin over the suprapubic area. The ilioinguinal nerve supplies the skin of the lower abdominal wall and upper portion of the labia majora and medial portion of the thigh through its inguinal branch. The inguinal branch enters the inguinal canal and courses along the round ligament.
The ilioinguinal and iliohypogastric nerves can be severed during a low transverse incision or entrapped during closure. This is especially true if incisions extend beyond the lateral borders of the rectus muscle (Rahn, 2010). During laparoscopy, these nerves can also be injured by accessory trocar insertion through the lower abdominal wall. Preventively, these risks can be minimized if lateral trocars are placed superior to the anterior superior iliac spines and low transverse fascial incisions are not extended beyond the lateral borders of the rectus muscle. These nerves carry sensory information only, and injury leads to loss of sensation within the areas supplied. Rarely, chronic pain may develop.
The T10 dermatome approximates the level of the umbilicus. Regional anesthesia for cesarean delivery or for puerperal sterilization ideally blocks T10 through L1 levels. In addition, a transversus abdominis plane (TAP) block can provide broad blockade to the nerves that traverse this plane (Chap. 19, p. 314). It may be placed following cesarean delivery to lessen analgesia requirements (Mishriky, 2012). There are also reports of rectus sheath block or ilioinguinal-iliohypogastric nerve block to decrease postoperative pain (Mei, 2011; Sviggum, 2012; Wolfson, 2012).
The external female genitalia, collectively known as the vulva or pudendum, lie over the pubic bones and extend posteriorly toward the perineal body. They include the mons pubis, labia majora and minora, clitoris, vestibule, vestibular bulbs, greater vestibular (Bartholin) glands, lesser vestibular glands, Skene and paraurethral glands, and the urethral and vaginal orifices (Fig. 3-3).
FIGURE 3-3
Vulvar structures (left) and subcutaneous layer of the anterior and posterior perineal triangles (right). Note the continuity of Colles and Scarpa fasciae. Inset: Vestibule boundaries and openings onto vestibule. (Reproduced with permission from Corton MM: Anatomy. In Hoffman BL, Schorge JO, Bradshaw KD, et al (eds): Williams Gynecology, 3rd ed. New York, McGraw-Hill Education, 2016.)
The mons pubis is the rounded fat pad that lies ventral to the pubic symphysis. The labia majora are two prominent folds that extend inferiorly from the mons pubis toward the perineal body. Embryologic homologues of the male scrotum, the labia majora are generally 7 to 8 cm in length, 2 to 3 cm in width, and 1 to 1.5 cm in thickness. The round ligaments and obliterated processus vaginalis, which is also termed the canal of Nuck, exit the inguinal canal and attach to the adipose tissue or skin of the labia majora. Posteriorly, the labia majora taper and merge into the area overlying the perineal body to form the posterior commissure.
Hair generally covers the skin of the mons pubis and labia majora. In addition, apocrine, eccrine, and sebaceous glands are abundant. The inner surface of the labia majora, however, lacks hair. Beneath the skin, the labia majora contain 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, these veins commonly develop varicosities, especially in parous women, from the increased venous pressure generated by the enlarging uterus. These varicosities appear as engorged tortuous veins or as small grapelike clusters, but are typically asymptomatic.
The subcutaneous layer of the mons and labia majora consists of a superficial fatty layer that is similar to and continuous with Camper fascia and a deeper membranous layer, which is Colles fascia.
The labia minora are two cutaneous folds that lie between the labia majora. In males, its homologue forms the ventral shaft of the penis. Anteriorly, each labium minus separates to form two folds that surround the glans of the clitoris. The prepuce or hood is the anterior fold that overlies the glans, and the frenulum is the fold that passes below the clitoris. Posteriorly, the labia minora end at 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 contain connective tissue with numerous vessels, elastin fibers, and scarce smooth muscle fibers. They are supplied with many nerve endings and are extremely sensitive (Ginger, 2011a). The epithelium of the labia minora varies with location. Thinly keratinized stratified squamous epithelium covers the outer surface of each labium. On the inner surface, the lateral portion is covered by this same epithelium up to a demarcating line—Hart line. Medial to Hart line, each labium is covered by squamous epithelium that is nonkeratinized. The labia minora lack hair follicles, eccrine glands, and apocrine glands. However, they contain many sebaceous glands (Wilkinson, 2011).
This 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 (Fig. 3-4). 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 (p. 36).
FIGURE 3-4
Superficial space of the anterior and posterior perineal triangles. On the image’s left are the structures noted after removal of Colles fascia. On the image’s right are the structures noted after removal of the superficial perineal muscles. (Reproduced with permission from Corton MM: Anatomy. In Hoffman BL, Schorge JO, Bradshaw KD, et al (eds): Williams Gynecology, 3rd ed. New York, McGraw-Hill Education, 2016.)
This is the functionally mature female structure derived from the embryonic urogenital sinus. In adult women, it is an almond-shaped area that is enclosed by Hart line laterally, the hymen medially, the clitoral frenulum anteriorly, and the fourchette posteriorly (see Fig. 3-3 inset). The vestibule is usually perforated by six openings: the urethra, the vagina, two Bartholin gland ducts, and the two ducts of the largest paraurethral glands—the Skene glands. It also contains the numerous openings of the lesser vestibular glands. The posterior portion of the vestibule between the fourchette and the vaginal opening is called the fossa navicularis. It is best observed in nulliparas.
The Hart line is clinically relevant when choosing incision sites for Bartholin gland drainage or marsupialization. That is, in attempts to recreate near-normal gland duct anatomy following these procedures, incisions are ideally placed between the hymen and Hart line (Kaufman, 1994).
These are homologues to the male penile bulb and corpus spongiosum. They are two elongated, approximately 3-cm long, richly vascular erectile masses that surround the vaginal orifice (see Fig. 3-4). Their posterior ends are in contact with the Bartholin glands. Their anterior ends are joined to one another and to the clitoris. Their deep surfaces are in direct contact with the perineal membrane. Their superficial surfaces are partially covered by the bulbospongiosus muscles, previously known as the bulbocavernosus muscles.
Clinically, the proximity of the Bartholin glands to the vestibular bulbs accounts for the significant bleeding often encountered with Bartholin gland excision. Following vulvar trauma, laceration of these bulbs or the clitoral crus may lead to sizable hematomas as discussed in Chapter 30 (p. 484).
These major glands measure 0.5 to 1 cm in diameter. They are the homologues of the male bulbourethral or Cowper glands. On their respective side, each lies dorsal to the vascular vestibular bulb and deep to the inferior end of the bulbospongiosus muscle. The duct from each gland 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.
The glands contain columnar cells that secrete clear or whitish mucus with lubricating properties. These glands are stimulated by sexual arousal. Contraction of the bulbospongiosus muscle, which covers the superficial surface of the gland, stimulates gland secretion.
Following trauma or infection, either duct may swell and obstruct to form a cyst or if infected, an abscess, which typically requires surgical drainage. This is illustrated in Chapter 12 (p. 193). Symptomatic or recurrent cysts may require marsupialization or gland excision. In contrast, the minor vestibular glands are shallow glands lined by simple mucin-secreting epithelium and open along Hart line.
The external urethral opening or meatus lies in the midline of the vestibule, 1 to 1.5 cm below the pubic arch, and a short distance above the vaginal opening. The dorsal surface of the urethra lies on the ventral surface of the anterior vaginal wall.
The paraurethral glands are a collective arborization of small glands whose multiple small ducts open predominantly on the dorsal and lateral aspect along the entire urethral length. The two largest are called Skene glands, and their ducts typically lie distally and near the urethral meatus and open at the vestibule. Clinically, inflammation and duct obstruction of the paraurethral glands can lead to urethral diverticulum formation or a Skene gland cyst or abscess. A Skene gland cyst or abscess can generally be differentiated from a urethral diverticulum in that it deviates the external urethral opening to the contralateral side.
The distal portion of the vagina and hymen embryologically derive from the urogenital sinus, whereas the proximal vagina derives from the paramesonephric ducts. In adult women, the hymen is a thin border around the vaginal opening. It contains elastic and collagenous connective tissue, and both outer and inner surfaces are covered by nonkeratinized stratified squamous epithelium. 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 caruncles.
Proximal to the hymen, the vagina is a musculomembranous tube that extends to the uterus (Fig. 3-5). Ventrally, the vagina is separated from the bladder and upper part of the urethra by loose connective tissue—the vesicovaginal space. The distal third of the urethra and vaginal wall are fused. Dorsally, between the mid vagina and the rectum, loose connective tissue forms the rectovaginal space. The lower third of the posterior vaginal wall is separated from the anus by the perineal body. The upper fourth of the vagina is separated from the rectum by the rectouterine pouch, also called the posterior cul-de-sac or pouch of Douglas.
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 by the cervix into anterior, posterior, and two lateral fornices. 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 level of the hymen, the perineal membrane attaches to the lateral walls of the vagina and is believed to aid distal support. Just above this point, the pubovaginalis component of the levator ani muscles also attach to the lateral vaginal walls. The constant tone of these muscles keeps the distal vagina closed, especially prior to parturition. During vaginal birth, trauma to the levator ani muscles can lead to a wider vaginal introitus.
At the midportion of the vagina, its lateral walls are attached to the pelvis by visceral connective tissue. These lateral attachments blend into the investing fascia of the levator ani muscles.
The vaginal lining is composed of nonkeratinized stratified squamous epithelium and underlying lamina propria. In premenopausal women, this lining is arranged 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). Following epithelial birth trauma and healing, fragments of stratified epithelium are occasionally 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.
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.
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 middle rectal artery may contribute 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 roughly follows the course of the arteries. Lymphatics from the lower third of the vagina, 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.
In the supine position with thighs abducted, the perineum represents the diamond-shaped area between the thighs and has boundaries that mirror those of the 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.
Also called the central tendon of the perineum, the perineal body is a fibromuscular mass of tissue that lies between the distal part of the posterior vaginal wall and the anus, at the junction between the anal and urogenital triangles (see Figs. 3-3 and 3-5). The perineal body provides significant distal support for the vagina and anus and serves as the point of attachment for several structures in both the superficial and deep urogenital compartments (Shafik, 2007; Woodman, 2002). Superficially, the bulbospongiosus, superficial transverse perineal, and external anal sphincter muscles converge on the perineal body. More deeply, the perineal membrane, urethrovaginal sphincter muscles, portions of the pubococcygeus muscle, and internal anal sphincter muscle contribute (Corton, 2005; Larson, 2010). In the absence of prior trauma to the perineal body, its extent between the vagina and anus and its depth each measures approximately 3 to 4 cm. The perineal body is incised during an episiotomy and is torn with second-, third-, and fourth-degree lacerations (Chap. 20, p. 321).
The anterior perineal triangle can be further divided into a superficial and a deep compartment (pouch or space) by the perineal membrane. The superficial perineal space lies superficial to the perineal membrane and the deep space lies deep to the membrane. Perineal membrane has replaced the terms urogenital diaphragm or inferior fascia of the urogenital diaphragm (Federative Committee on Anatomical Terminology, 1998; Oelrich, 1983). It attaches laterally to the ischiopubic rami, medially to the distal third of the urethra and vagina, and posteriorly to the perineal body (Figs. 3-4 and 3-6).
FIGURE 3-6
Deep space of the anterior perineal triangle. On the image’s right lie structures noted after removal of the perineal membrane. Inset: Striated urogenital sphincter muscles. Also shown are all structures that attach to perineal body: bulbospongiosus, superficial transverse perineal, external anal sphincter, and puboperinealis muscles, perineal membrane, and urethrovaginal sphincter. (Reproduced with permission from Corton MM: Anatomy. In Hoffman BL, Schorge JO, Bradshaw KD, et al (eds): Williams Gynecology, 3rd ed. New York, McGraw-Hill Education, 2016.)
The perineal membrane consists of two histologically and probably functionally distinct portions that span the opening of the anterior pelvic triangle (Stein, 2008). The dorsal or posterior portion is a dense fibrous tissue sheet that attaches laterally to the ischiopubic rami and medially to the distal third of the vagina and to the perineal body. The ventral or anterior portion of the perineal membrane is intimately associated with the compressor urethrae and urethrovaginal sphincter muscles (see Fig. 3-6 inset). The deep or superior surface of the perineal membrane appears to have direct connections to the levator ani muscles, and the superficial or inferior surface of the membrane fuses with the vestibular bulb and clitoral crus.
Clinically, the perineal membrane attaches to the lateral walls of the vagina at the level of the hymen. It provides support to the distal vagina and urethra by attaching these structures to the bony pelvis. In addition, its attachments to the levator ani muscles suggest that the perineal membrane may play an active role in support.
This space of the anterior perineal triangle is bounded deeply by the perineal membrane and superficially by Colles fascia. This is an enclosed compartment except for the extension of Colles fascia with Scarpa fascia of the anterior abdominal wall. Colles fascia has firm attachments to the ischiopubic rami and fascia lata of the thigh laterally and to the superficial transverse perineal muscles and the perineal membrane posteriorly. These attachments prevent the spread of most fluid, blood, or infection from the superficial perineal space to the thighs or posterior perineal triangle. However, in the mid-anterior region, Colles fascia has no attachments to the pubic bones and is therefore continuous with the anterior abdominal wall (Martin, 1984). This continuity may allow the spread of fluids between the superficial perineal space and the abdominal wall.
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