Pelvis

The pelvis is the lower part of the trunk, between the abdomen and the thighs (a.k.a. pelvic region of the trunk).

The bony framework of the pelvis comprises mainly of two pelvic (or hip) bones. Posteriorly, we can find the lumbar, sacral vertebrae and the coccyx. The pelvic bones interconnect with each other anteriorly at the pubic symphysis, while posteriorly they articulate with the lumbar vertebrae. Together, these bones constitute the bony pelvis (a.k.a. pelvic skeleton).

The space enclosed between the two pelvic bones is called the pelvic cavity. In fact, the space between the iliac fossae of both pelvic bones (a.k.a. false pelvis), that lies above the pelvic brim, is considered to be a part of the abdominal cavity. The actual pelvic cavity (a.k.a. true pelvis) is the space situated below the pelvic brim. It extends from pelvic brim (or pelvic inlet) above, to pelvic outlet below.

The pelvic cavity is further sub-divided into, with the help of the pelvic floor muscles, into a ‘pelvic cavity proper’ and a ‘perineum’. The pelvic cavity proper lies above the pelvic floor and contains urogenital organs, including the bladder, uterus, cervix and rectum. The part which is below the pelvic floor is called ‘perineum’.

Perineum is a diamond-shaped region, which corresponds to the pelvic outlet and 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, while the anal triangle is formed by ischial tuberosities and sacrotuberous ligaments connecting the ischial tuberosities with the sacrum and the coccyx behind.

The perineum houses the erectile tissue and muscles and distal continuations of urogenital structures. In a vertical plane, 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 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.