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The perineum is the diamond-shaped region inferior to the pelvic diaphragm (Figure 12-1A). The pubic symphysis, pubic arches, ischial tuberosities, and coccyx bound the perineum. An imaginary line between the ischial tuberosities divides the perineum into urogenital and ischioanal triangles. The pudendal nerve (S2–S4) and internal pudendal artery primarily supply the perineum. The urogenital triangle extends between the paired ischiopubic rami. The ischioanal triangle is the fat-filled area (ischioanal fossa) surrounding the anal canal and situated below the pelvic diaphragm.
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The perineum consists of fascial layers (like other areas of the trunk) that divide the perineum into different spaces (Figure 12-1B-E). The perineal fascia is as follows:
Superficial perineal fascia. The subcutaneous tissue in the perineal region. This tissue is similar to the superficial fascia of the lower anterior abdominal wall, which consists of a superficial fatty layer and deeper membranous layer.
Fatty layer of superficial perineal fascia. Continuous into the labia majora and the mons pubis in females; diminished in males, being replaced in the penis and scrotum with smooth (dartos) muscle and fascia.
Membranous layer of superficial perineal fascia (Colles’ fascia). Attached to the perineal membrane (posterior margin), perineal body, fascia lata of the superomedial thigh, and Scarpa's fascia of the abdomen. In males, the membranous layer is continuous with dartos fascia in the scrotum and in females helps form the labia majora.
Deep perineal fascia (investing or Gallaudet's fascia). Attached laterally to the ischiopubic rami and invests the ischiocavernosus, bulbospongiosus, and superficial transverse perineal muscles; continuous with the deep fascia covering the external oblique muscle and rectus sheath.
Perineal membrane. Thick triangular fascia; attached to the ischiopubic rami in the horizontal plane; has a free posterior margin.
Perineal spaces. The perineal membrane forms a boundary between the deep perineal space (situated superiorly to the perineal membrane) and the superficial perineal space (situated inferiorly to the perineal membrane).
Support. The perineal membrane and associated structures in the deep perineal space contribute to the pelvic floor and support elements of the urogenital system in the pelvic cavity. The perineal membrane and adjacent ischiopubic rami provide attachment for the roots of the external genitalia and superficial perineal muscles.
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The urogenital (UG) triangle is the anterior region of the perineum and is oriented in the horizontal plane. The boundaries of this triangle are the pubic symphysis, ischiopubic rami, ischial tuberosities, and perineal body. The UG triangle is associated with the superficial and deep perineal spaces (Figure 12-1A-E).