Urogenital triangle

The urogenital triangle is an area in the anterior perineum defined by a theoretical triangle which is bounded posteriorly by an imaginary line drawn between the ischial tuberosities (known as the interischial line). It contains external genitalia and the urethra.

The corners of this triangle are the pubic symphysis anteriorly and the ischial tuberosities posterolaterally.

The borders of the urogenital triangle are formed by the following structures:

Anterolateral borders - by the ischial and pubic rami;
Posterior border - by the transverse perineal muscles.

In females, the urogenital triangle contains the urethral orifice and the external genitalia, including the vaginal orifice, labia majora, labia minora, Bartholini's glands, and the clitoris, as well as the posterior labial nerves, blood vessels and lymphatics, and perineal muscles.

A tough layer of a deep fascia called the perineal membrane separates the urogenital triangle into the superficial pouch and deep pouch. The perineal membrane is perforated by the urethra and the vagina in females.

Anal triangle

The anal triangle is a theoretical triangle located posterior to the urogenital triangle. It is bounded by the interischial line anteriorly, and the sacrotuberous ligaments posterolaterally.

The corners of the anal triangle are defined by the ischial tuberosities anterolaterally and the coccyx posteriorly.

Structures that form boundaries of the anal triangle are the following:

Anterior border - posterior margin of the perineal membrane;
Posterolateral borders - sacrotuberous ligaments;
Superior border - levator ani muscles.

The contents of the anal triangle include the external anal sphincter and the anal aperture (opening of the anal canal) and two ischiorectal fossae - spaces located laterally on each side of the anal canal.

The ischiorectal fossae are defined by the following boundaries:

Anteriorly - the urethral sphincter and the deep transverse perineal muscles;
Posteriorly - the gluteus maximus muscles and the sacrotuberous ligaments;
Superomedially - the external anal sphincter and the levator ani muscles;
Laterally - the obturator fascia;
Inferiorly - the skin.

The inferior rectal nerve, arteries and veins are contained within the ischorectal fossae, as well as the ischioanal fat that provides support for the anal canal and allows the anal canal to expand during defecation.

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Urogenital triangle and anal triangle