Perineum The perineum is situated at the lower end of the trunk between the buttocks. Its bony boundaries include the lower margin of the pubic symphysis anteriorly, the tip of the coccyx posteriorly, and the ischial tuberosities laterally. These landmarks correspond to the boundaries of the pelvic outlet. The diamond shape of the perineum is customarily divided by an imaginary line joining the ischial tuberosities immediately in front of the anus, at the level of the perineal body, into an anterior urogenital and a posterior anal triangle (Fig. 5.19).
Urogenital Triangle The urogenital triangle includes the external genital structures and the urethral opening (Fig. 5.19). These external structures cover the superficial and deep perineal compartments and are known as the vulva (Figs. 5.20 and 5.21).
The mons pubis is a triangular eminence in front of the pubic bones that consists of adipose tissue covered by hair-bearing skin up to its junction with the abdominal wall.
The labia majora are a pair of fibroadipose folds of skin that extend from the mons pubis downward and backward to meet in the midline in front of the anus at the posterior fourchette. They include the terminal extension of the round ligament and occasionally a peritoneal diverticulum, the canal of Nuck. They are covered by skin with scattered hairs laterally and are rich in sebaceous, apocrine, and eccrine glands.
The labia minora lie between the labia majora, with which they merge posteriorly, and are separated into two folds anteriorly as they approach the clitoris. The anterior folds unite to form the prepuce or hood of the clitoris. The posterior folds form the frenulum of the clitoris as they attach to its inferior surface. The labia minora are covered by hairless skin overlying a fibroelastic stroma rich in neural and vascular elements. The area between the posterior labia minora forms the vestibule of the vagina.
Figure 5.19 Vulva and perineum.
Figure 5.20 Superficial perineal compartment.
Figure 5.21 Deep perineal compartment.
The clitoris is an erectile organ that is 2 to 3 cm in length. It consists of two crura and two corpora cavernosa and is covered by a sensitive rounded tubercle (the glans).
The vaginal orifice is surrounded by the hymen, a variable crescentic mucous membrane that is replaced by rounded caruncles after its rupture. The opening of the duct of the greater vestibular (Bartholin) glands is located on each side of the vestibule. Numerous lesser vestibular glands are also scattered posteriorly and between the urethral and vaginal orifices.
The urethral orifice is immediately anterior to the vaginal orifice and about 2 to 3 cm beneath the clitoris. The Skene (paraurethral) gland duct presents an opening on its posterior surface.