The boundaries of the diamond or somewhat trapezoidal shaped inferior pelvic outlet are formed by the symphysis pubis in front, the coccyx behind, on each side the inferior pubic ramus and the ramus of the ischium, the ischial tuberosity and the sacrotuberous ligament.
There are 2 different muscular layers closing the inferior pelvic outlet:
- Pelvic Diaphragm (Deep): The pelvic diaphragm or floor is a myofascial hammock which spans the inferior opening. It is composed of the composite Levator Ani and the Coccygeus or Ischiococcygeus, collectively known as the pelvic floor muscles (PFM). Complete and help this layer the Obturator Internus and the Piriformis, which are muscles of the lower extremity but have a continuous fascia with the pelvic floor muscles.
- The Perineum (Superficial): An imaginary line drawn transversely in front of the ischial tuberosities divides the region into two triangular parts. The posterior part contains the anus and is known as ‘the anal triangle’, while the anterior part is the ‘urogenital triangle’ containing the external urogenital organs. The muscles of the perineum may therefore be divided into two groups:
- Muscles of the Anal region of Perineum.
- Those of the urogenital region: these muscles are different in females (Muscles of the Genital region of Perineum – Female, picture 1 and 2) and males (Muscles of the Genital region of Perineum – Male, picture 3 and 4).