Anal Triangle

Perineum

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

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
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

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
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
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
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

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
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)

Mons pubis
Rounded fatty elevation anterior to the pubic eminence
Consists mainly of a pad of fatty connective tissue deep to the skin
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
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)
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
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
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
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

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
Portions of perineal skin supplied by perineal branches of the posterior femoral cutaneous, ilioinguinal, and genitofemoral nerves

Perineal blood supply and venous drainage

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
Branches:
- 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
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
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