Pelvic Inlet and Outlet

The pelvic inlet is defined by the sacral promontory (S1 vertebra) and the linea terminalis. The linea terminalis includes the pubic crest, iliopectineal line, and arcuate line.

It divides the pelvic cavity into two parts:
Major (false) pelvic cavity - this lies above the pelvic inlet between the iliac crests and is actually part of the abdominal cavity.
Minor (true) pelvic cavity - this lies below the pelvic inlet and extends to the pelvic outlet.

The pelvic inlet is oval shaped in females and heart shaped in males.

Perineum

The perineum is part of the pelvic outlet located inferior to the pelvic diaphragm, it is diamond shaped and can be divided by a line passes through the ischial tuberosities into two triangles: the urogenital triangle and the anal triangle.

1. Urogenital Triangle

• Deep Perineal Space

  • The deep perineal space is a space that lies between the superior fascia of the urogenital diaphragm and inferior fascia of the urogenital diaphragm (perineal membrane)

  • It contains a number of structures that completely occupy it as seen in the table below

  • one of those structures is the urogenital diaphragm, which consists of the deep transverse perineal muscle and the sphincter urethrae muscle.

2. Superficial perineal space

• lies between the inferior fascia of the urogenital diaphragm (perineal membrane) and superficial perineal fascia (colles fascia)

Anal Triangle

1. Ischiorectal fossa

   • The ischiorectal fossa is located on either side of the anorectum and is separated from the pelvic cavity by the levator ani muscle

   • This fossa contains ischiorectal fat, inferior rectal nerves, inferior rectal artery and vein, perineal branches of the posterior femoral cutaneous nerve, and the pudendal canal, which transmits the pudendal nerve and internal pudendal artery and vein.

2. Muscles of the anal triangle

   • Obturator internus, external anal sphincter, levator ani, and coccygeus muscles.