Urogenital Diaphragm and Ishiorectal Fossa

The pelvis | Main Anatomy Index | Kidneys and ureters

Last updated 30 March 2006

Urogenital Diaphragm and Ischiorectal Fossa

The Urogenital Diaphragm

• This is a thin sheet of striated muscle stretching between the two sides of the pubic arch.
• It covers the anterior part of the inferior pelvic aperture (pelvic outlet).

• The most anterior and posterior fibres of the urogenital diaphragm (deep transverse perineal muscle) run transversely.
• The middle fibres (sphincter urethrae muscle) surround the membranous urethra.

Sphincter Urethrae Muscle

• This muscle is attached to the medial surface of the inferior pubic ramus.
• Its fibres pass medially toward the urethra, where they meet the fibres from the opposite side.

• Some fibres encircle the membranous urethra in the male and form a true voluntary sphincter that compresses the urethra.
• It also extends to the base of the bladder and invests the prostate anteriorly and anterolaterally.

• In the female, the inferior 1/2 of the sphincter urethrae blends with the anterolateral walls of the vagina, forming a urethrovaginal sphincter that compresses the urethra and vagina.

• Innervation: perineal nerve, a branch of the pudendal nerve (S2, S3 and S4).

• This voluntary sphincter of the urethra constricts the membranous urethra in the male and compresses the urethra and vagina in the female.

Deep Transverse Perineal (Transversus Perinei) Muscle

• This is a narrow slip of muscle that is attached to the medial surface of the ischial ramus.
• It runs transversely to insert into the perineal body (central perineal tendon).
• In the female, some fibres also insert into the vaginal wall.

• Innervation: perineal nerve, a branch of the pudendal nerve (S2, S3 and S4).

• This muscle steadies the perineal body, thereby contributing to the general supportive role of the urogenital diaphragm for the pelvic floor and viscera.

The Perineal Body

• The tendinous centre of the perineum or perineal body is a small wedge-shaped mass of fibrous tissue located at the centre of the perineum.

• The perineal body is the landmark of the perineum, where several muscles converge: the transverse perineal, bulbospongiosus, levator ani and some fibres of the external anal sphincter.

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The Perineal Fascia

• The urogenital diaphragm is surrounded by deep fascia.
• The perineal fascia consists of two sheets, the inferior and superior fasciae of the urogenital diaphragm.

The Inferior Fascia of Urogenital Diaphragm

• This is usually referred to as the perineal membrane.
• It is continuous with the superior fascia of the urogenital diaphragm.
• It is also attached to the pubic rami.

The Superior Fascia of the Urogenital Diaphragm

• The membranous layer of the subcutaneous connective tissue of the perineum (Colles' fascia) and inferior part anterior abdominal wall are continuous.

• The attachments of the superior perineal fascia are:

1. The fascia lata enveloping the thigh muscles;
2. The pubic arch;
3. And the posterior edge of the perineal membrane.

• Anteriorly, the superficial perineal fascia is prolonged over the penis and scrotum, thereby forming a membranous covering for the testes and spermatic cords (in males).
• In the female, this fascia is prolonged over the clitoris and labia majora.

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The Superficial Perineal Space

• This is the space between the superficial perineal fascia and the perineal membrane.
• The superficial perineal fascia attaches medially to the superior border of the pubic symphysis and laterally to the body of the pubis.

Contents of the Superficial Perineal Space

• In the male, this space contains:

1. The root of the penis and the muscles associated with it;
2. The contents of the scrotum;
3. The proximal part of the spongy urethra;
4. The superficial perineal muscles;
5. The branches of the internal pudendal vessels;
6. And the pudendal nerves.

• In the female, this space contains:

1. The root of the clitoris;
2. The bulbs of the vestibule;
3. The superficial perineal muscles;
4. The related vessels and nerves;
5. And the great vestibular glands.

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The Deep Perineal Space

• This is the fascial space enclosed by the superior and inferior fasciae of the urogenital diaphragm.

• The two layers of fascia are attached laterally to the pubic arch and blend with each other anteriorly at the apex and posteriorly at the base of the urogenital diaphragm.
• The deep dorsal vein of the penis enters the pelvis between its anterior edges (transverse perineal ligament) and the arcuate pubic ligament.

Contents of the Deep Perineal Space

• In the male, the deep perineal space contains:

1. The membranous urethra;
2. The sphincter urethrae muscle;
3. The bulbourethral glands;
4. The deep transverse perineal muscles;
5. And related vessels and nerves.

• In the female, the deep perineal space contains:

1. Part of the urethra;
2. The sphincter urethrae muscles;
3. The deep transverse perineal muscle;
4. And related vessels and nerves.

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The Ischiorectal (ischioanal) Fossae

• This is a large, fascia-lined, wedge-shaped space on each side of the rectum or anal canal.
• It is located between the skin of the anal region and the pelvic diaphragm.

• The apex of each ischiorectal fossa lies superiorly, at the point where the levator ani muscle arises from the obturator fascia.
• The apex of each fossa is located about 6 cm superior to the ischial tuberosity.

• The base of each fossa is formed by perianal skin.

• Because the two levator ani muscles are shaped like a funnel, the ischiorectal fossae are wide inferiorly and narrow superiorly.

• Anteriorly, the ischiorectal fossae continue superior to the urogenital diaphragm, where they form the anterior recesses of the ischiorectal fossae.
• These spaces are filled with loose connective tissue.

• There are also posterior recesses where the gluteus maximus muscle overhangs the ischiorectal fossa.

• The 2 ischiorectal fossae communicate over the anococcygeal ligament.
• Posteriorly, each fossa is also continuous with the lesser sciatic foramen, superior to the sacrotuberous ligament.

Boundaries of the Ischiorectal Fossae

• Each fossa is bounded:

• Laterally: the ischium and the inferior part of the obturator internus muscle.

• Medially: the anal canal to which the levator ani and external anal sphincter are applied.

• Posteriorly: the sacrotuberous ligament and gluteus maximus muscle

• Anteriorly: the base of the urogenital diaphragm and its fasciae.

Contents of the Ischiorectal Fossae

• These wedge-shaped fascial spaces are filled with soft fat, called the ischiorectal pads of fat.
• They are traversed by many tough, fibrous bands and septa.

• These fibrofatty pads support the anal canal, but they can be readily displaced to permit the anal canal to expand when faeces are present.

• The ischiorectal fossae also contain the internal pudendal artery and vein and the pudendal nerve.
• These structures run on the lateral walls of the fossae in the fibrous canals called the pudendal canals.
• Posteriorly, these vessels and the pudendal nerve give off the inferior rectal vessels and nerves.
• These structures become superficial as they pass toward the surface to supply the external anal sphincter and the perianal skin.

• Two other cutaneous nerves, the perforating branch of S2 and S3 nerves and the perineal branch of S4 nerve also pass through the ischiorectal fossae.

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