Anal Triangle



the diamond-shaped area inferior to the pelvic diaphragm that includes the anus and external genitalia

Perineal boundaries and divisions

  • Limited above by the pelvic diaphragm (the fibromuscular pelvic floor formed by levator ani and coccygeus muscles)
  • Lateral walls: medial surface of the inferior pubic and ischial rami (anteriorly) and the obturator internus muscle below the attachment of the levator ani
  • Divided into two triangles by a transverse line between the ischial tuberosities:
    • A posterior anal triangle (including the anus, anal canal, and ischioanal fossae)
    • An anterior urogenital triangle (including perineal pouches, external genitalia, and urethra)
  • Inferiorly, presents a free surface covered by skin
  • Key landmarks in the perineum
    sites of convergence of several perineal muscles
    • Perineal body (the central tendon of the perineum)
      A thickened, midline condensation of fibrous tissue
      Located near the center of the perineum, between vagina and anus in females and between urethra and anus in males
    • Coccygeal body
      A midline condensation of fascia between coccyx and anus

Anal triangle

  1. Anal canal and anus
    Length about 4 cm, directed posteriorly and downward, beginning at the level of the levator ani
    Interior divided into 3 regions:
    • Superior
      Anal valves and columns: 6-10 folds that encircle the inner anal canal
      Anal sinuses: pockets of space formed by valves; site of openings of
      Anal glands
    • Transitional (intermediate)
      Characterized by pecten, a smooth hairless band ~2 cm wide
      Pectinate line: marks the transition between 1) endodermal and ectodermal mucosal epithelia and 2) inferior and superior blood supply, venous and lymphatic drainage
      Limited below by the anocutaneous verge (intersphincteric groove) [described by Hilton in 1863 as the white line, although it is not white in either the living or dead]
    • Cutaneous
      pigmented skin, hairs, glands
  2. Ischioanal fossa (IAF)
    • Wedge-shaped spaces lateral to the anal canal
      Vertical lateral walls formed by obturator internus
      Filled with adipose and fibrous tissue
      Permits distention of the anal canal
    • Pudendal canal (Adcock's canal)
      Found on the lateral walls of the IAF
      Contains the pudendal nerve and internal pudendal vessels
      Extends from the lesser sciatic notch to the posterior edge of the UG diaphragm

Urogenital triangle

Divided into 2 pouches (deep and superficial), sandwiched between 3 fascial layers
The 3 fascias fuse posteriorly to merge with the perineal body

  1. Urogenital (UG) diaphragm (deep perineal pouch)
    A transverse sheet of muscles spanning the triangular space between ischiopubic rami
    Covered above and below by superior and inferior fascias of the UG diaphragm, which fuse around the anterior free edge to form the transverse perineal ligament
    The superior and inferior fascias of the UG diaphragm also fuse posteriorly, along with the posterior edge of the superficial perineal fascia
    Completely closed; does not communicate with other perineal or pelvic spaces
    • Muscular components (deep muscles of the perineum) completely fill the pouch:
      • Deep transverse perineal muscle
      • Urethrovaginal sphincter (in females): medial fibers that encircle both urethra and vagina
      • Sphincter urethrae (in males): medial fibers that encircle the membranous urethra
    • Other contents
      • Branches of the pudendal nerve and internal pudendal vessels run along the lateral walls
      • In females, pierced by the urethra and vagina
      • In males, contains bulbourethral glands (Cowper's glands) whose ducts pierce the inferior fascia to terminate in the spongy urethra
      • In males, pierced by the membranous urethra (the shortest, thinnest, and narrowest [except for the external orifice] part of the urethra)
        begins at the apex of the prostate and ends at the bulb of the penis
  2. Perineal membrane (inferior fascia of the UG diaphragm)
    Thick sheet of fascia that serves as both the inferior fascia of the UG diaphragm (deep perineal pouch) and the superior fascia of the superficial perineal pouch
    Provides attachment for external genitalia
  3. Superficial perineal pouch
    Space enclosed between the perineal membrane and superficial perineal fascia
    Continuous anterosuperiorly with superficial abdominal wall
    Limited laterally by attachments of the superficial perineal fascia to ischiopubic rami
    Limited posteriorly by the posterior margin of the UG diaphragm
    • In males:
      • Surrounds the penis and scrotum
      • Divided into two compartments by deep perineal fascia (Buck's fascia): superficial compartment continuous with the superficial tissue space of the anterior abdominal wall while deep space is closed anterosuperiorly
    • In females: split by the vestibule and confined to each side of labia majora
    • Contents:
      • Erectile cavernous masses
        • Corpus spongiosum
        • Corpora cavernosa
      • Superficial perineal muscles
        • Superificial transverse perineal: extends from the ischial tuberosity to the perineal body, anterior to the anus
        • Bulbospongiosus
        • Ischiocavernosus
      • Branches of the pudendal nerve and internal pudendal vessels supply genitalia
      • In males, contents of the scrotum and the spongy urethra
      • In females, vestibule of the vagina, greater vestibular glands, urethra, and paraurethral glands
  4. Superficial perineal fascia (Colles' fascia)
    Layer of subcutaneous connective tissue deep to perineal skin
    Can be divided into outer fatty and inner membranous layers
    Attached to ischiopubic rami laterally, pubic rami anteriorly, and the margin of the UG diaphragm posteriorly
    Continuous with Scarpa's fascia of the anterior abdominal wall
    • In males:
      Surrounds scrotum and penis; regions have specific names (superficial fascia of the penis and tunica dartos of the scrotum)
      Deep fascia (Buck's fascia) closely surrounds superficial muscles of the penis; does not descend into the scrotum
    • In females, split by the vestibule and confined to each side of labia majora

External Genitalia - Male

  1. Penis
    • Root of the penis
      made up of 3 masses of erectile tissue
      • The bulb is situated in the midline, attached to the perineal membrane, traversed by the spongy urethra, and covered on its outer surface by bulbospongiosus muscles (which form a sphincter, compressing the bulb to expel urine and/or semen)
      • The left and right crura are attached to the sides of the pubic arch and are covered by ischiocavernosus muscles (which force blood from the crura into the distal penis to increase tumescence)
    • Body of the penis
      Enclosed by a dense white fibrous capsule, the tunica albugenia, surrounded by deep fascia (Buck's fascia) of the penis
      The 2 crura converge anteriorly and come to lie side by side to form the corpora cavernosa
      The bulb projects forward to form the corpus spongiosum, which expands distally to form the glans penis
      The urethra dilates distally as the fossa navicularis then opens onto the glans as the external urethral meatus
      The prepuce (foreskin), a hoodlike fold of skin that covers the glans, is connected to the glans below the urethral orifice by a fold, the frenulum
  2. Scrotum
    Contents (testes and coverings) described with the abdomen
    A loose cutaneous fibromuscular sac situated posteroinferior to the penis and inferior to the pubic symphysis
    Consists of skin and dartos (smooth) muscle
    Homologue of the female labia majora

External genitalia - Female

organs known collectively as the vulva (pudendum)

  1. Mons pubis
    Rounded fatty elevation anterior to the pubic eminence
    Consists mainly of a pad of fatty connective tissue deep to the skin
  2. Labia majora
    2 symmetrical folds of skin filled with subcutaneous fat and connective tissue
    Site of termination of round ligament
    Lateral surface is hairy; medial surface studded with sebaceous glands and encloses the pudendal cleft
    Join anteriorly and posteriorly to form indistinct anterior and posterior commissures
    Homologue of the male scrotum
  3. Labia minora
    Thin, delicate folds of vascular, fat-free, hairless skin located in the pudendal cleft
    Enclose the vestibule of the vagina
    Divides anteriorly to form upper and lower folds that fuse around the clitoris: an upper prepuce of the clitoris and a lower frenulum of the clitoris
    United posteriorly by a fold, the frenulum (or fourchette)
  4. Vestibule of the vagina
    Space enclosed by labia minora
    Anteriorly, receives the external urethral orifice (and associated paraurethral glands [Skene's glands], homologues of the male prostate)
    Posteriorly, receives the orifice of the vagina and the ducts of the greater vestibular glands
  5. Clitoris
    Consists of 2 crura, a body, and a glans
    Corpora cavernosa commence as 2 crura attached to ischiopubic rami, then unite in the midline to form the body of the clitoris
    Terminates as the glans clitoris
    Connected to the symphysis pubis by a suspensory ligament
    Ischiocavernosus muscles run from ischial rami to insert on the crura of the clitoris
    Bulbospongiosus muscle runs from the perineal body, passes around vagina, and inserts into the clitoris
  6. Bulbs of the vestibule
    2 large, elongated (~3 cm) masses of erectile tissue underlying each labium minus
    Taper anteriorly to join one another and attach to the undersurface of the clitoris
    Homologous to half the bulb of the penis and the posterior part of corpus spongiosum
  7. Greater vestibular glands (Bartholin's glands)
    Deep to the posterior extension of the bulbs of the vestibule
    Secrete a lubricating mucus into the vestibule of the vagina
    Homologues of the bulbourethral glands

Perineal innervation

  1. Pudendal nerve
    Formed by ventral rami of S2-S4
    Exits the pelvis via the greater sciatic foramen, enters the perineal space through the lesser sciatic foramen, and traverses the pudendal canal
    Accompanies pudendal vessels
    Sole somatic motor nerve of the perineum; also supplies most perineal skin
    The inferior rectal nerve branches off just above the ischial tuberosity
  2. Portions of perineal skin supplied by perineal branches of the posterior femoral cutaneous, ilioinguinal, and genitofemoral nerves

Perineal blood supply and venous drainage

  1. Internal pudendal artery
    Branches off the anterior division of the internal iliac artery
    Enters the perineum through the lesser sciatic foramen, passes downward and forward through the IAF, and runs along the ischiopubic ramus in the deep perineal pouch
    Terminates by dividing into deep and dorsal arteries of the penis or clitoris before reaching the transverse perineal ligament
    • Inferior rectal a.
      Runs medially across the IRF to supply anal canal and perianal skin
      Anastomoses freely with middle and superior rectal arteries
    • Perineal a.
      Gives off muscular branches then divides into transverse perineal and posterior scrotal or labial branches
    • Artery of the bulb
      Arises within the deep perineal space lateral to the bulb; runs medially to pierce the perineal membrane
      In males, supplies bulb of the penis and bulbourethral glands
      In females, supplies bulb of the vestibule and greater vestibular glands
    • Urethral a.
      In males, pierces the perineal membrane, enters corpus spongiosum, and anastomoses with branches of the dorsal artery
      In females, absent or inconspicuous
    • Deep artery of the penis or clitoris
      Leaves the deep perineal pouch by piercing the perineal membrane and entering the crus; continues along the axis of the corpus cavernosum
      Not accompanied by a vein
    • Dorsal artery of the penis or clitoris
      Pierces the perineal membrane more anteriorly and medial to the crus
    • Deep and dorsal arteries anastomose freely and pass along the corpora cavernosum and the corpus spongiosum, on either side of the dorsal vein
  2. External pudendal artery
    Supplies skin and superficial fascia of the external genitalia
    Branches from the femoral artery and anastomoses freely with branches of the internal pudendal
    2 on each side, a deep and superficial external pudendal artery
  3. Venous drainage
    • Venous tributaries accompany all of the branches of the internal and external pudendal arteries
    • Deep dorsal vein of the penis or clitoris
      Drains the cavernae and glans
      Unpaired; runs between the 2 dorsal arteries of the penis or clitoris
      Leaves the perineum through the gap anterior to the transverse perineal ligament
      Communicates with tributaries of internal pudendal veins