Helen E O'Connell, Kalavampara V Sanjeevan
Historical, sociocultural, and religious factors appear to have affected the scientific study of female sexual anatomy and physiology. Throughout history, the clitoris was discovered and rediscovered, previous knowledge presumably having been lost or hidden.1 Famous anatomists argued passionately against the very existence of the clitoris.2 Moore and Clarke studied anatomic descriptions and diagrams across the twentieth cen- tury.3 Labeling evident in diagrams early in the twentieth century was absent in subsequent editions of the same textbooks. Until the work of De Graaf in the seventeenth century, there was no comprehensive description of female genital anatomy. There was confusion regarding the distinction between the clitoris and labia because of the effects of translation of ancient texts and imprecision of terminology in relation to female genital structures. De Graaf emphasized the importance of using the term “clitoris”.4 Modern textbooks typically provide a complete account of male anatomy only and then highlight the differences as a means of describing female anatomy.5-7 In the anatomic textbooks, the dorsal nerve of the clitoris is not described but is noted to be “very small and supplies the clitoris”.5 This is an example of the inaccuracy present even in distinguished anatomic textbooks. The dorsal clitoral nerves were visible to the naked eye in every cadaver we studied (i.e. these are not small structures).8 Near-complete omission of the anatomy of the clitoris has also occurred.9 In recent years, there have been some dissection and magnetic resonance imaging (MRI) studies of the female genitalia, facilitating a more accurate understanding of the anatomy.8,1^12 This clarified the accuracy of the work by Kobelt and De Graaf.13
For descriptive purposes, reproductive organs lying within the body cavity such as ovaries, uterus, and fallopian tubes are grouped as internal genitalia, and sexual or reproductive and adjacent structures outside the body cavity are grouped as external genitalia (Fig. 4.1.1).
The external genitalia in women include the structures surrounding the urogenital cleft, referred to as the vulva. The vulva is comprised of the mons pubis, clitoris, and labia majora and minora. The mons pubis forms the anterosuperior limit of the cleft with the labia majora on both sides and ending posteriorly at the anterior margin of the perineal body (Fig. 4.1.2). Surface anatomy varies considerably. In some women, only the mons and labia majora may be seen unless the labia are parted, while, in others, the labia minora and clitoral glans and/or the hood of the clitoral prepuce may also been seen. The area between the labia minora is called the vestibule, although in the literature at times the term “vestibule” has been used synonymously with “introitus” or “vaginal opening”.
The mons pubis is a hair-covered mound overlying the pubic symphysis (Fig. 4.1.2). Deep to the fat of the mons pubis lies a “fan-shaped” fibrofatty layer that converges downward from the mons pubis to gain attachment along the body of the clitoris up to its glans. This large superficial component of the suspensory ligament of clitoris also extends into the labia majora, and attaches to the crura and bulbs of the clitoris as well. It appears to provide stability to the clitoris and labia.14 The body of the clitoris projects into the fat of the mons pubis, as is well seen on MRI studies in the sagittal plane.15
The labia majora are the two prominent lateral boundaries of the urogenital cleft. Anteriorly, they meet, together forming the anterior commissure in front of the glans of the clitoris; posteriorly, they meet, together forming a low ridge of skin called the posterior commissure, which is about an inch in front of the anus. Each labium has external pigmented hairy skin, which may be slightly wrinkled in the nonaroused state, and a smooth internal surface. Multiple large, sebaceous follicles line the shiny internal surface. Dissections reveal the highly vascular content of the labia, with the vessels mostly derived from the external pudendal vasculature (Fig. 4.1.3). The labia minora are composed of supple, elastic skin, without any subcutaneous fat but rich in sebaceous glands, flanking the vaginal opening from the clitoral glans anteriorly to the posterior limit of vaginal opening. The size and appearance of the labia minora vary widely. Anteriorly, the labia minora split into two layers, the anterior or upper layer forming the hood of the clitoral prepuce and gaining attachment circumferentially to the base of the glans of clitoris. The posterior or lower layer encircles partially the ventrum of the clitoris to meet the similar layer from the other side to form the frenulum of clitoris, which gains attachment to the posteroinferior aspect of glans of the clitoris in the midline.
Figure 4.1.1. Composite picture of midsagittal section of female pelvis showing the internal and external genitalia and the clitoro-urethro-vaginal complex. Left inset shows the three-dimensional appearance of clitoris. Right inset shows the vulva and its relation to the clitoris. Reproduced with permission from Williamson S, Nowak R. The truth about women. New Scientist 1998; 159: 34-5.
The clitoris is a complex erectile structure, of which only a tiny portion, the clitoral glans, is appreciable from the surface (Figs 4.1.1 and 4.1.4). The glans projects variably between the anterior bifurcated ends of the labia minora. The glans clitoris is the distal extremity of the clitoral body, although the two structures are separate. The body, corpus clitoris, is comprised of paired corpora that are joined in the sagittal plane by an incomplete septum. The deep extensions of the corpora diverge under the pubic arch as the crura. In postmenopausal cadavers, the body is 0.5-1.0cm wide, 1-2 cm high, and 2-4 cm long. The body of the clitoris is boomerang-shaped in sagittal section, the bend being maintained by the deep component of the suspensory ligament.15 In anatomic textbooks, it appears to be flat, but an oblique image reveals the depth of the clitoral body. The body is surrounded by a thick tunica albuginea and has a midline septum between its constituent corpora. Large nerve trunks run along its anterior surface (although they are named the “dorsal” nerves of the clitoris). The nerve trunks run with the corresponding artery and veins on either side of a midline deep dorsal vein of the clitoris.
Figure 4.1.2. Vulva in a young virgin. Adapted with permission from Dickinson RL. Atlas of Human Sex Anatomy. Williams & Wilkins, 1949.
The bulbs of the clitoris lie deep to the thin sheet of bul- bospongiosus muscle, the fibers of which extend from the perineal body to the body of the clitoris around the vagina and urethra. They are covered by a delicate membrane which is markedly different from the tough capsule, the tunica albuginea, which surrounds the corpus clitoris. The bulbs are posteroinferior to the body of the clitoris. They lie lateral to the urethra and superficial to the crura. Their posterior extent varies considerably, but, in younger women, will reach as far posteriorly as the perineal body. In cadaver studies the bulbs are 3-7 cm long, crescentic or triangular in shape, fill the space between the crus and body of the clitoris, and flank the corresponding lateral wall of urethra and distal vagina to a variable extent.8 Dissection and MRI studies reveal them very clearly (Figs 4.1.4—4.1.6). The bulbs do not relate consistently to the vestibule, their constant relationship being with the clitoral body, crura, and glans. The bulbs have had a variety of names throughout the literature, most of which do not acknowledge their constant relationship to the rest of the clitoris, urethra, and vagina.
The paired clitoral bulbs appear to be more extensive than the male counterpart. We have progressed very little in the understanding of their physiology from the time of De Graaf, who suggested their role to be “the constriction of the penis” during intercourse. They contain erectile tissue of a different consistency from the body and crura.16
The crura, 5-9 cm long and narrower than the body, attach to the deep aspect of the undersurface of the ischiopubic rami on either side and are covered by ischiocavernosus muscles. They are composed of typical erectile tissue, like that of the body, and lie adjacent to the dorsal clitoral neurovascular bundle, although no branches appear to surround or perforate the crura of the clitoral body.
Figure 4.1.3. Vasculature of labium majus revealing the predominant vasculature from the branches of external pudendal artery and some supply from the perineal branches of internal pudendal artery. The highly vascular nature of the labia is not usually observed in anatomy textbooks.
The terminal divisions of the pudendal neurovascular bundle are the dorsal clitoral and perineal neurovascular bundles. The dorsal clitoral neurovascular bundle rises along the crus on each side plastered to the periosteum of the ischiopubic ramus (Fig. 4.1.7). This neurovascular bundle supplies the crus, body, and glans of the clitoris, and superficial tissues of the labia. The bulbs and urethra are supplied by the perineal neurovascular bundle. The dorsal nerve of clitoris runs anterosupero- medially from its formation underneath the ischiopubic ramus, and runs along the superior surface of the corpus clitoris, not in the 12 o’clock position, to enter the deep aspect of the glans clitoris. The perineal neurovascular bundle runs medially and more horizontally after its formation, to reach the posterolateral aspect of the bulb. The cavernosal neurovascular bundle runs lateral to the urethra and lies directly on the pelvic aspect of the anterior vaginal wall. The cavernous nerves originate from the vaginal nerve plexus occupying the 2 and 10 o’clock positions on the anterolateral vagina and travel at the 5 and 7 o’clock positions along the urethra.10 These nerves are microscopic and appear to be a network of fibers rather than discrete nerves.
Figure 4.1.4. Anteroposterior view of a dissected specimen of clitoris in a postmenopausal woman. This reveals the relationship between the bulbs, crura, and urethra. The glans is apparent superior to the urethra. The body is best revealed in the sagittal plane, its shape and size not being apparent in the anteroposterior view. n.v.b. = neurovascular bundle.
The clitoris is supported externally by the superficial and deep suspensory ligaments. The deep ligament arises from the undersurface of ischiopubic rami and pubic symphysis. Internally, the clitoris is supported by the urethra and vagina and their attachments to the pelvic floor and bony pelvis.
The urethral orifice (external meatus) is located between the body of the clitoris and the vaginal opening in the midline, about an inch posteroinferior to the glans clitoris. The appearance of the meatus may vary widely from a tiny, inconspicuous, vertical slit, through crescentic or stellate forms to a round aperture, with a small bulge or dimple. Surrounding the distal urethra, slightly proximal to the meatus, are several paraurethral glands (Skene’s glands), which express prostate-specific antigen (PSA)17,18 and prostate-specific acid phosphatase (PSAcP).18 Female ejaculation is reported in some women with orgasmic expulsion of fluid, different from urine, which could be from the periurethral glands, as it contains high levels of PSA.19 A sexually sensitive area in the anterior vaginal wall is termed the Grafenberg spot, or G- spot.20 Ultrasound studies have demonstrated that the site of greatest sensitivity corresponds to the location of the external urethral sphincter. There is no consensus, however, regarding “female ejaculation” or presence of the G-spot.21 It is likely that the increased area of sensitivity reported in some women in relation to the urethra is due to the fact that the urethra is surrounded by erectile tissue. The sexual role of the urethra and the physiology of the clitoris are topics in their infancy.
Figure 4.1.5. Coronal cross-section through the midbody of clitoris. Note the bulk of clitoral body, supraurethral portion of the clitoral bulbs and the relationship of clitoral bulb to the lateral walls of the urethra and vagina.
The vaginal orifice (introitus) is a vertical opening inferior to the urethral meatus. It is irregular at the level of the hymen, proximal to which the anterior and posterior walls of the vagina meet together, separating the lateral walls, and forming an H- or W-shaped slit.
The vagina extends from the vestibule to the uterine cervix and posterior fornix. This fibromuscular tube is described as having four walls, the anterior, posterior, and right and left lateral walls, the anterior wall measuring 7.5 cm in length and the posterior wall, 9 cm in cadavers. Vaginal to clitoral dimensions are likely to be greater in the live state, but such data are lacking. The uterine cervix projects into the proximal end of the vagina, forming a circular recess, or fornix, described as anterior, posterior, or lateral. The anterior wall of the vagina covers the urethral wall. The posterior wall is related to peritoneum lining the rectouterine pouch proximally, connective tissue in the midhalf, and perineal body distally, which separates the vagina from the anal canal. The vagina is attached laterally to the pelvic walls forming a single divider in the middle of the pelvis.22 The connective tissue supports of the vagina, of extreme importance to pelvic organ support and pelvic organ prolapse, are beyond the scope of this chapter.
The vagina is composed of mucosa (stratified squamous epithelium), a lamina propria, and a muscularis consisting of an inner layer of circular smooth muscle and outer, thicker layer of longitudinal smooth muscle.23 In the upper third of the vagina, the smooth muscle is clearly separate from the urethra and distinct from the lower two-thirds of the anterior vaginal wall from which the urethra is inseparable. On the lateral aspect of the vaginal muscularis is a venous plexus of variable thickness (Fig. 4.1.8). In younger women, the venous plexus on either side can be over 1 cm in diameter. In older women, and possibly related to hormonal factors, the lateral wall thickness may be less than 0.5 cm. Anterolaterally and distally, the bulbs of the clitoris lie directly deep to the vaginal mucosa.
Branches of the superior and inferior vesical arteries supply the upper and middle thirds of the vagina. These branches anastomose with branches of the uterine artery. In continuity with the urethra, the lower third is supplied by branches of the bulbar artery (branch of the perineal division of the terminal part of the internal pudendal artery). The dorsal clitoral arteries also supply the distal vagina where it is adherent to the urethra and bulbs.
Clarification of vaginal neuroanatomy has been achieved recently.10 Autonomic nerves form a dense network extending from the rectum to the lateral aspects of the proximal and midvagina. On the inner aspect of the pelvis, this vaginal plexus supplies the cavernosal nerves to the clitoris at the level of the proximal urethra. These nerves stain immunohistologically for neuronal nitric oxide synthase (nNOS).
The hymen vaginae is a thin fold of mucous membrane, seen just within the vaginal orifice, that varies greatly in appearance. It may be absent, may or may not rupture with sexual activity, or may rupture in athletics or in certain other physical activities unrelated to sexual activity. Its remnants after its rupture are the small round carunculae hymenales. While many misconceptions prevail regarding the anatomy of the external genitalia, more has been written about the hymen than any other structure, highlighting the socio-legal implications of female anatomy. Gaping of the hymen can be seen even in sexually nonabused preschool girls.24
Figure 4.1.6. Magnetic resonance imaging of the clitoris and its components in axial plane: the bulbs, crura, and corpora are well demonstrated. These structures lie ventral and lateral to the urethra and vagina as a cluster or complex. Reproduced with permission from Journal of Urology''".
The greater vestibular (Bartholin’s) glands are variably depicted throughout anatomic textbooks. Dissections reveal that these glands lie deep to the bulbs; that is, between those structures and the lateral or outer aspect of the distal vaginal wall. A recent MRI study reported the presence of small circular areas of decreased density that presumably represented the greater vestibular glands in all 21 healthy women studied, at the 4 and 8 o’clock positions on the posterolateral aspects of introitus.11 The greater vestibular glands are tubuloacinar secretory glands that are thought to play a role in lubrication during sexual arousal. Their ducts open on each side into the posterolateral aspect of the vaginal introitus between the labium minora and the hymen.
Figure 4.1.7. Lateral view of the dissected specimen of clitoris with its neurovascular bundle. The bundle is clearly seen ascending along the right crus to pass along the superior surface of the clitoral body. n.v.b. = neurovascular bundle.
Figure 4.1.8. Axial section through vagina showing the bulk of vaginal muscular wall and paravaginal plexus of veins in a premenopausal cadavar.
In recent years, there has been development in the knowledge and review of female genital anatomy. The historical descriptions of Kobelt and De Graaf were comprehensive and, with few exceptions, accurate. Clarification of clitoral anatomy in particular, by dissection and MRI, has led to a new understanding of the relationships between the urethra, distal vagina, and clitoris.
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