Anatomy of the female pelvis

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Anatomy of the female pelvis

Caroline de Costa

Learning outcomes

After studying this chapter you should be able to:

Knowledge of the major features of the female pelvis is essential to the understanding of the processes of reproduction and childbearing and to the effect that various pathological processes may have on the pelvic organs and on the health of the woman.

The structure and function of the genital organs vary considerably with the age of the individual and her hormonal status, as will be apparent in chapter 16, which covers the changes that take place in puberty and the menopause. This chapter aims to outline the major structures comprising the female pelvis, predominantly in the sexually mature female.

The bony pelvis

The bony pelvis consists of the paired innominate bones (each consisting of ilium, ischium and pubis) and the sacrum and coccyx (Fig. 1.1).

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Fig. 1.1 Bony pelvis.

The innominate bones are joined anteriorly at the symphysis pubis and each articulates posteriorly with the sacrum in the sacroiliac joints. All three joints are fixed in the non-pregnant state, but during pregnancy there is relaxation of the joints to allow some mobility during labour and birth. The sacrum articulates with the fifth lumbar vertebra superiorly and the coccyx inferiorly.

The bony pelvis is divided into the false pelvis and the true pelvis by the pelvic brim. The true pelvis is divided into three sections: the pelvic inlet (bounded anteriorly by the superior surface of the pubic bones and posteriorly by the promontory and alae of the sacrum); the mid-pelvis (at the level of the ischial spines); and the pelvic outlet (bounded anteriorly by the lower border of the symphysis, laterally by the ischial tuberosities and posteriorly by the tip of the sacrum).

imageThe ischial spines are easily palpable on vaginal examination during labour and provide the reference point for assessing the descent of the fetal head during labour and birth.

The external genitalia

The term vulva is generally used to describe the female external genitalia, and includes the mons pubis, the labia majora, the labia minora, the clitoris, the external urinary meatus, the vestibule of the vagina, the vaginal orifice and the hymen (Fig. 1.2).

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Fig. 1.2 External genital organs of the female.

The mons pubis, sometimes known as the mons veneris, is composed of a fibrofatty pad of tissue that lies above the pubic symphysis and, in the mature female, is covered with dense pubic hair. The upper border of this hair is usually straight or convex upwards and differs from the normal male distribution. Pubic hair generally begins to appear between the ages of 11 and 12 years.

The labia majora consist of two longitudinal cutaneous folds that extend downwards and posteriorly from the mons pubis anteriorly to the perineum posteriorly. The labia are composed of an outer surface covered by hair and sweat glands and an inner smooth layer containing sebaceous follicles. The labia majora enclose the pudendal cleft into which the urethra and vagina open.

Posterior to the vaginal orifice, the labia merge to form the posterior commissure and the area between this structure and the anterior verge of the anus constitutes the obstetric perineum.

The labia majora are homologous with the male scrotum.

The labia minora are enclosed by the labia majora and are cutaneous folds that enclose the clitoris anteriorly and fuse posteriorly behind the vaginal orifice to form the posterior fourchette or posterior margin of the vaginal introitus. Anteriorly, the labia minora divide to enclose the clitoris, the anterior fold forming the prepuce and the posterior fold the frenulum. They are richly vascularized and innervated and are erectile. They do not contain hair but are rich in sebaceous glands.

The clitoris is the female homologue of the penis and is situated between the anterior ends of the labia minora. The body of the clitoris consists of two corpora cavernosa of erectile tissue enclosed in a fibrous sheath. Posteriorly, these two corpora divide to lie along the inferior rami of the pubic bones. The free end of the clitoris contains the glans, composed of erectile tissue covered by skin and richly supplied with sensory nerve endings and hence very sensitive. The clitoris plays an important role in sexual stimulation and function.

The vestibule consists of a shallow depression lying between the labia minora. The external urethral orifice opens into the vestibule anteriorly and the vaginal orifice posteriorly. The ducts from the two Bartholin’s glands drain into the vestibule at the posterior margin of the vaginal introitus and the secretions from these glands have an important lubricating role during sexual intercourse.

Skene’s ducts lie alongside the lower 1 cm of the urethra and also drain into the vestibule. Although they have some lubricating function, it is minor compared to the function of Bartholin’s glands.

The bulb of the vestibule consists of two erectile bodies that lie on either side of the vaginal orifice and are in contact with the surface of the urogenital diaphragm. The bulb of the vestibule is covered by a thin layer of muscle known as the bulbocavernosus muscle.

The external urethral orifice lies 1.5–2 cm below the base of the clitoris and is often covered by the labia minora, which also function to direct the urinary stream. In addition to Skene’s ducts, there are often a number of paraurethral glands without associated ducts and these sometimes form the basis of paraurethral cysts.

The vaginal orifice opens into the lower part of the vestibule and, prior to the onset of sexual activity, is partly covered by the hymenal membrane. The hymen is a thin fold of skin attached around the circumference of the vaginal orifice. There are various types of opening within the hymen and the membrane varies in consistency. Once the hymen has been penetrated, the remnants are represented by the carunculae myrtiformes, which are nodules of fibrocutaneous material at the edge of the vaginal introitus.

Bartholin’s glands are a pair of racemose glands located at either side of the vaginal introitus and measuring 0.5–1.0 cm in diameter. The ducts are approximately 2 cm in length and open between the labia minora and the vaginal orifice. Their function is to secrete mucus during sexual arousal. Cyst formation is relatively common but is the result of occlusion of the duct, with fluid accumulation in the duct and not in the gland.

Although it does not strictly lie within the description of the vulva, the perineum as described in relation to obstetric function is defined as the area that lies between the posterior fourchette anteriorly and the anus posteriorly; it lies over the perineal body, which occupies the area between the anal canal and the lower one-third of the posterior vaginal wall.

The internal genital organs

The internal genitalia include the vagina, the uterus, the Fallopian tubes and the ovaries. Situated in the pelvic cavity, these structures lie in close proximity to the urethra and urinary bladder anteriorly and the rectum, anal canal and pelvic colon posteriorly (Fig. 1.3).

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Fig. 1.3 Sagittal section of the female pelvis showing the relationship of the pelvic organs with surrounding structures.

The vagina

The vagina is a muscular tube some 6–7.5 cm long in the mature female. It is lined by non-cornified squamous epithelium and is more capacious at the vault than at the introitus. In cross-section, the vagina is H-shaped and it is capable of considerable distension, particularly during parturition when it adapts to accommodate the passage of the fetal head. Anteriorly, it is intimately related to the trigone of the urinary bladder and the urethra. Posteriorly, the lower part of the vagina is separated from the anal canal by the perineal body. In the middle third, it lies in apposition to the ampulla of the rectum and in the upper segment it is covered by the peritoneum of the rectovaginal pouch (pouch of Douglas).

The uterine cervix protrudes into the vaginal vault. Four zones are described in the vaginal vault: the anterior fornix; the posterior fornix; and the two lateral fornices. The lateral fornices lie under the base of the broad ligament in close proximity to the point where the uterine artery crosses the ureter.

The pH of the vagina in the sexually mature non-pregnant female is between 4.0 and 5.0. This has an important antibacterial function that reduces the risk of pelvic infection. The functions of the vagina are copulation, parturition and the drainage of menstrual loss.

The uterus

The uterus is a hollow, muscular, pear-shaped organ situated in the pelvic cavity between the bladder anteriorly and the rectum and pouch of Douglas posteriorly. The size of the uterus depends on the hormonal status of the female. In the sexually mature female, the uterus is approximately 7.5 cm long and 5 cm across at its widest point. The uterus normally lies in a position of anteversion such that the uterine fundus is anterior to the uterine cervix. In about 10% of women, the uterus lies in a position of retroversion in the pouch of Douglas. The uterus may also be curved anteriorly in its longitudinal axis, a feature that is described as anteflexion, or posteriorly, when it is described as retroflexion.

It consists of a body or corpus, an isthmus and a cervix.

The corpus uteri consists of a mass of smooth muscle cells, the myometrium, arranged in three layers. The external layers contain smooth muscle cells that pass transversely across the uterine fundus into the lateral angles of the uterus, where their fibres merge with the outer layers of the smooth muscle of the Fallopian tubes and the ovarian and round ligaments. The muscle fibres in the middle layer are arranged in a circular manner and the inner layer contains a mixture of longitudinal, circular and oblique muscle fibres.

The cavity of the uterus is triangular in shape and is flattened anteroposteriorly so that the total volume of the cavity in the non-pregnant state is approximately 2 mL. It is lined by endometrium that consists on the surface of mucus-secreting columnar epithelium. The nature of the endometrium depends on the phase of the menstrual cycle. Following menstruation, the endometrium in the proliferative phase is only 1–2 mm thick. By the second half (secretory phase) of the cycle the endometrium has grown to a thickness of up to 1 cm.

The endometrial cavity is in contact with the vaginal cavity inferiorly via the cervical canal and superiorly with the peritoneal cavity through the Fallopian tubes.

The cervix is a barrel-shaped structure extending from the external cervical os, which opens into the vagina at the apex of the vaginal portion of the cervix, to the internal cervical os in its supravaginal portion. The internal os opens into the uterine cavity through the isthmus of the uterus. In non-parous women the external os is round or oval, but it becomes transverse following vaginal birth and this can be noted in clinical examination when a speculum is passed, for example, when taking a Pap smear.

The cervical canal is fusiform in shape and is lined by ciliated columnar epithelium that is mucus-secreting. The transition between this epithelium and the stratified squamous epithelium of the vaginal ectocervix forms the squamocolumnar junction. The exact site of this junction is related to the hormonal status of the woman. Some of the cervical glands in the endocervical lining are extensively branched and mucus-secreting. If the opening to these glands becomes obstructed, small cysts may form, known as nabothian follicles.

The cervix consists of layers of circular bundles of smooth muscle cells and fibrous tissue. The outer longitudinal layer merges with the muscle layer of the vagina.

The isthmus of the uterus joins the cervix to the corpus uteri and in the non-pregnant uterus is a narrow, rather poorly defined, area some 2–3 mm in length. In pregnancy, it enlarges and contributes to the formation of the lower segment of the uterus, which is the normal site for the incision of caesarean section. In labour it becomes a part of the birth canal but does not contribute significantly to the expulsion of the fetus.

Supports and ligaments of the uterus

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