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Chapter 38: The perineal region and external genitalia

Perineal region (fig. 38-1)

The perineal region is that part of the trunk below the pelvic diaphragm. It is diamond shaped and has the same boundaries as the pelvic outlet. It is divided into an anterior, urogenital triangle and a posterior, anal triangle. The term "perineum" in obstetrics and gynecology is generally limited to the area between the anal and vaginal orifices.

The tendinous center, or perineal body, is a median, fibromuscular mass between the urogenital and anal triangles. Several muscles and fasciae are anchored to it, including the levator ani and the external anal sphincter. The perineal body may be damaged during parturition. To avoid injury, the opening for the passage of the fetal head may be enlarged by incising the posterior wall of the vagina and the nearby part of the perineum (episiotomy).

Urogenital region (fig. 38-1A, C, and F)

The male urogenital region is pierced by the urethra, whereas the female urogenital region is pierced by the vagina as well as the urethra and also contains the female external genitalia.

In the male, the region (fig. 38-2) consists of (1) skin, (2) superficial perineal fascia (which consists of fatty and membranous layers of the subcutaneous tissue), (3) deep perineal fascia, (4) the superficial perineal space (which contains the root of the penis and the superficial perineal muscles), (5) inferior fascia of the urogenital diaphragm, (6) the deep perineal space (which contains the urogenital diaphragm, membranous urethra, and bulbourethral glands), and (7) superior fascia of the urogenital diaphragm (recently disputed).

In the female, the urogenital region comprises the seven components listed above for the male, but the superficial perineal space transmits the distal vagina and contains the greater vestibular glands.

Superficial Perineal Muscles (figs. 38-1D and 38-2).

Three muscles are found bilaterally. In the male, the bulbospongiosus arises from the perineal body and the fibrous raphe on the bulb of the penis and is inserted into the superior aspect of the corpus spongiosum. It aids in expelling urine or semen. The ischiocavernosus arises from the ischial ramus and is inserted on the crus penis. It helps to maintain erection by compressing the veins in the crus. The superficial transverse perineal muscle arises from the ischial ramus and is inserted into the perineal body. All three superficial muscles are supplied by the pudendal nerve.

In the female, the bulbospongiosus is separated from the contralateral muscle by the vagina. It arises from the perineal body, passes around the vagina, and is inserted into the clitoris. The ischiocavernosus is inserted on the crus clitoridis (see fig. 38-1C).

Deep Perineal Muscles (figs. 38-1F and 38-2).

The urogenital diaphragm, which is pierced by the urethra, consists of two muscles bilaterally: the deep transverse perineal and the sphincter urethrae. The deep transverse perineal muscle arises from the ischial ramus and is inserted into the perineal body, which it supports. In the male, the sphincter urethrae, which may be fused with the deep transverse perineal muscle, arises from the inferior pubic ramus and passes medially to meet the muscle of the opposite side and surround the membranous urethra. * It constricts the membranous urethra and is said to expel the last drops of urine. Both of the deep muscles are supplied by the pudendal nerve (through the dorsal nerve of the penis).

In the female, the urogenital diaphragm is pierced by the vagina as well as the urethra. The sphincter urethrae is inserted mostly on the lateral wall of the vagina. The sphincters of the two sides do not surround the urethra and hence, despite their name, do not act as a sphincter urethrae (see fig. 38-1E).

Anal region (see fig. 38-1A)

The anal triangle is the posterior part of the perineal region, and it contains the external anal sphincter muscle. The subcutaneous tissue ascends on each side of the anus as the ischioanal pad of fat, which fills the ischioanal (ischiorectal) fossa. The ischioanal fossa is the space between the skin of the anal region that marks its inferior extent and the pelvic diaphragm on the superior side. It is sometimes the site of an abscess, which may communicate with the rectum or anal canal. The ischioanal fossa, triangular in coronal sections (figs. 38-2 and 38-3), is situated between (1) the obturator internus muscle and its fascia laterally and (2) the pelvic diaphragm and its inferior fascia, as well as the external anal sphincter, medially. The pudendal canal, which contains the internal pudendal vessels and pudendal nerve, is a fascial compartment on the lateral wall of the ischioanal fossa (fig. 38-3).

External genitalia

Male external genitalia

The male external genitalia are the penis and scrotum.

The penis.

The penis (see figs. 34-1 and 38-4), or male organ of copulation, becomes erect and enlarged as a result of engorgement with blood. The penis is attached to the linea alba and pubic symphysis by fundiform and suspensory ligaments. It consists of a root and a body.

The root, or attached portion, is situated in the perineum, is covered by the bulbospongiosus, and consists of three masses of erectile tissue: a bulb and two crura. The bulb is pierced by the urethra, which continues anteriorward in it, and it is covered by the bulbospongiosus muscles. At its anterior extent, the bulb becomes the corpus spongiosum. Each crus of the penis is attached to the corresponding ischial ramus and is covered by the ischiocavernosus muscles (see fig. 38-2). The two crura meet and turn inferiorward as the corpora cavernosa.

The body of the penis is the free portion covered by skin. The surface that is continuous with that of the anterior abdominal wall is termed the dorsum of the penis. The other, or urethral, surface is characterized by a median raphe continuous with that of the scrotum. The body of the organ contains the corpus spongiosum and the paired corpora cavernosa.

Erectile tissues

The corpus spongiosum contains the urethra and ends in an expansion, the glans. The glans penis is limited circumferentially by a neck, adjacent to which is a rim known as the corona. Preputial glands, which secrete smegma, are present on the neck and corona. The external urethral orifice is a median slit near the tip of the glans. The prepuce, or foreskin, is a double layer of skin that extends from the neck to cover the glans. A median fold, the frenulum of the prepuce, is present near the urethral opening.

The corpora cavernosa, which constitute the bulk of the body of the penis, form the dorsum and sides of the organ. They end in blunt projections covered by the glans.

Vasculature and innervation.

The penis is supplied by branches (the artery of the bulb and the dorsal and deep arteries of the penis) of the internal pudendal artery. The penis is innervated by branches of the pudendal, perineal, ilio-inguinal, and cavernous nerves. The cavernous nerves (from the prostatic plexus) contain parasympathetic fibers that cause vasodilatation in the erectile tissue. The flow of blood into the cavernous spaces causes distension of the corpora cavernosa and the corpus spongiosum. Venous return is limited by pressure on the veins that drain the corpora. At the end of ejaculation, sympathetic vasoconstriction of the arteries allows blood to enter the veins.

The scrotum.

The scrotum (see fig. 34-1) is a cutaneous pouch that contains the testes and epididymides. The left half of the scrotum is usually at a slightly more inferior level than the right. A median raphe indicates the subdivision of the scrotum by a septum into right and left compartments. Smooth muscle, the dartos, is firmly attached to the overlying skin. Loose connective tissue underlying the dartos allows free movement.

Female external genitalia

The female external genitalia (fig. 38-5), known as the pudendum or vulva, are the mons pubis, labia majora and minora, vestibule of the vagina, bulb of the vestibule, vestibular glands, and clitoris.

The mons pubis is a fatty eminence in front of the pubic symphysis. Two elongated folds, the labia majora (labium majus in the singular), pass posteriorward from the mons. They are usually united in anteriorly (by an anterior commissure) but not posteriorly, although the forward projection of the perineal body may give the appearance of a posterior commissure. The round ligaments of the uterus end in the deep surface of the labia majora. Developmentally, the labia majora are homologus with the scrotum of the male.

The labia minora and vestibule.

The labia minora (labium minus in the singular) are two cutaneous folds situated between the labia majora and on either side of the vaginal opening. In their anterior extent, they unite anterior to the glans clitoris to form a prepuce, and posterior to it to form a frenulum. The labia minora join the labia majora in their posterior extent and may be united by a transverse fold known as the frenulum of the labia, or fourchette.

The vestibule of the vagina is the space between the labia minora. It has the openings of the urethra, vagina, and ducts of the greater vestibular glands. The external urethral orifice is posterior to the clitoris. The vaginal orifice, immediately posterior to the urethral orifice, varies with the condition of the hymen.

The bulb of the vestibule consists of two elongated, erectile masses covered by the bulbospongiosus muscles. The masses are united anteriorly at the glans. The bulb of the vestibule is homologous with the bulb of the penis and the adjoining part of the corpus spongiosum (see fig. 38-4).

The two greater vestibular glands are situated posterior to the bulb of the vestibule. The duct of each gland opens into the vestibule between the vaginal orifice and the labia minora. The greater vestibular glands are homologus to the bulbo-urethral glands of the male (see fig. 38-4).

The clitoris.

The clitoris, which is homologous with the penis, consists of erectile tissue but is not traversed by the urethra. It is posterior to the anterior commissure and partly hidden by the labia minora. Each crus clitoridis (see fig. 38-4) is attached to the corresponding ischial ramus and is covered by the ischiocavernosus. The two crura meet and turn downward as the corpora cavernosa. The corpora constitute the body of the clitoris and end as the glans clitoridis (see fig. 38-4), which consists of erectile tissue and is highly sensitive. The clitoris is attached to the pubic symphysis by a suspensory ligament.

The blood supply and innervation of the female external genitalia are similar to those of the corresponding structures in the male.

Questions

38-1 What is the perineum?

38-1 The term perineum in obstetrics and gynecology is generally limited to the area between the anal and vaginal orifices. Anatomically, the perineal region is that part of the trunk below the pelvic diaphragm. It underlies the outlet of the pelvis (see fig. 38-1A).

38-2 What is the perineal body?

38-2 The perineal body (see fig. 38-1B), or tendinous center of the perineum, is a median, fibromuscular node situated at the convergence of several muscles, including the levator ani and the exteal anal sphincter. The perineal body may be injured during parturition.

38-3 What is the urogenital diaphragm?

38-3 The urogenital diaphragm, which is pierced by the urethra and the vagina, includes the deep transverse perineal and the sphincter urethrae muscles bilaterally (see fig. 38-1F). However, the sphincter urethrae is not horizontal, and "the concept of a urogenital diaphragm is not borne out" (T. M. Oelrich, Am. J. Anat., 158:229, 1980).

38-4 What is the ischioanal fossa?

38-4 The ischioanal fossa (see fig. 38-2) is the fat-filled space between the obturator internus laterally (overlying the ischium) and the external anal sphincter and pelvic diaphragm medially (overlying the anal canal and rectum; hence the name ischioanal). The base of the fossa is the skin of the anal region, which allows surgical access to the fossa, e.g., to drain an ischiorectal abscess. Anorectal abscesses may be found 1) in the region of the subcutaneous part of the external sphincter (perianal abscess), (2) in the ischioanal fossa (ischiorectal abscess), (3) beneath the mucosa of the upper part of the anal canal (submucous abscess), and (4) above the pelvic diaphragm (supralevator abscess). Anorectal abscesses are important because they may give rise to a fistula-in-ano, i.e., an open passage between the anal canal or rectum and the perianal skin.

38-5 What is excision of the prepuce termed?

38-5 The prepuce is a double layer of skin that covers the glans (see fig. 38-4), and its excision is termed circumcision (L., cutting around).

38-6 What is the pudendum?

38-6 The pudendum consists of the external genitalia. The term female pudendum is more or less synonymous with vulva.

Figure legends

Figure 38-1 A, Boundaries and subdivisions of the perineal region, inferior view. H, Fasciae of the male perineal region. The superficial fascia has been removed at the right. C, Muscles of the superficial perineal space in the female after removal of the superficial and deep perineal fasciae. D, Similar view in the male. E, Muscles of the deep perineal space in the female. The inferior fascia of the urogenital diaphragm has been removed at the right. F, Similar view in the male.

Figure 38-2 Fasciae of the male pelvis and urogenital region, as seen in a coronal section through the prostatic part of the urethra.

Figure 38-3 The ischioanal fossae and pelvic diaphragm.

Figure 38-4 The male and female external genitalia. M., membranous urethra.

Figure 38-5 The female external genitalia.

* The usual account has been disputed. According to T. M. Oelrich (Am. J. Anat., 158:229-246, 1980). (1) "there is no distinct superior fascia of the so-called urogenital diaphragm." (2) the sphincter urethrae is not horizontal ("the concept of a urogenital diaphragm is not borne out"), and (3) the sphincter urethrae (at least developmentally) is part of a continuous sheet that extends from the bladder to the perineal membrane and that lies within the pelvic cavity.

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