Perineum

The perineum is the lowermost part of the pelvis, which corresponds to the pelvic outlet. If we look at the true pelvis (i.e. the region between pelvic brim/pelvic inlet and the pelvic outlet), there a muscle called the pelvic floor could be seen subdividing the true pelvis into two sub-regions. The region below the pelvic floor (i.e. between pelvic floor and the skin) is called the ‘perineum’. Also please note that the region above the pelvic floor (i.e. between pelvic floor and pelvic brim) is called pelvic cavity proper.

Perineum is a diamond-shaped region, which is divided into two sub-triangles, namely the urogenital triangle (in front) and the anal triangle (at the back). The triangles are separated from each other by a hypothetical plane between two ischial tuberosities. The urogenital triangle is bounded by pubic symphysis, ischiopubic rami and ischial tuberosities. It contains openings for urogenital organs. While the anal triangle is bounded by ischial tuberosities and sacrotuberous ligaments. The latter connect the ischial tuberosities with the sacrum and the coccyx behind.

The perineum houses the erectile tissue and muscles and distal continuations of urogenital structures. Vertically, the perineum is further sub-divided into superficial and deep perineal pouches with the help of a perineal membrane. This membrane is a fibrous, triangular structure that fills in the gap between ischiopubic rami and lies below the pelvic diaphragm.

The superficial perineal pouch lies inferior to the perineal membrane (i.e. between membrane and the skin) and contains erectile tissue, including corpora cavernosa and corpus spongiosum, and associated muscles, i.e. ischiocavernosus, bulbospongiosus and superficial transverse perineal muscles.

The corpora cavernosa are attached to the ischiopubic rami at the root of the penis (or clitoris), while their distal ends are free and contribute to the body of the penis (or clitoris). Similarly, the proximal part of corpus spongiosum is attached on the underside of perineal membrane and perineal body and distal end again contributes to the body of penis (or clitoris). With regards to the erectile muscles, the ischiocavernosus muscle originates form the ischial tuberosity and inserts into the corpus cavernosum, at the root. While the bulbospongiosus muscle arises from the perineal body and gets inserted into the bulb or proximal part of corpus spongiosum.

The deep perineal pouch lies above the perineal membrane (i.e. between the membrane and the pelvic floor) and contains structures including the membranous part of urethra, bulbourethral glands, sphincter urethrae muscle and deep transverse perineal muscles.