Perineum is a diamond shaped space that overlies the pelvic outlet (inferior aperture of pelvis) and has the same boundaries as the pelvic outlet. Perineum is divided into two triangles by an imaginary transverse line passing just in front of ischeal tuberosities: -
- anterior urogenital triangle and
- posterior anal triangle.
Urogenital triangle is bounded on either side by conjoint ischiopubic rami, anteriorly by lower border of pubic symphysis and arcuate pubic ligament, and posteriorly by an imaginary line passing through ischial tuberosities separating it from anal triangle. Urogenital triangle is divided into: -
- Superficial perineal pouch, and
- Deep perineal pouch.
Superficial perineal pouch (space)
- This space is bounded above (deep) by perineal membrane and below (superficial) by Colle's fascia (membranous layer of superficial perineal fascia).
- Important contents of superficial perineal space are:-
- Root of penis (bulb and crura) and clitoris in females.
- Muscles:-Ischiocavernosus, bulbospongiosus, superficial transverse perinei
- Nerves: -(A) Branches from perineal nerve: - posterior scrotal nerve in male (or posterior labial nerve in female), branch to bulb of penis (in male) or bulb of vestibule (in female), muscular branches; (B) Long perineal nerve from posterior cutaneous nerve of thigh.
- Vessels:-(A) Branches from perineal artery: - posterior scrotal in male (or posterior labial in females), and transverse perineal; (B) in male four branches from artery of penis: artery to bulb of penis, urethral artery, deep and dorsal arteries of penis; (C) in females four branches from artery of clitoris: - artery to bulb of clitoris, urethral artery, deep and dorsal arteries of clitoris.
- Glands and ducts:-Great vestibular (Bartholin's) gland in females, and duct of bulbo-urethral glands in males.
Deep perineal pouch (space)
It is the space between perineal membrane (below) and superior fascia of urogenital diaphragm (above). Important contents of deep perineal space are:-
- Membranous urethra and in females part of vagina.
- Muscles:- Sphincterurethrae, deep transverse perinei, compressor urethrae (in females), sphincter urethrovaginalis (in females).
- Artery:- Dorsal artery of penis (or clitoris); stem of origin of four arteries from the artery of penis/clitoris passing to the superficial perineal space.
- Glands:- Bulbourethral glands in males (no gland in females).
- Nerves: - Dorsal nerve of penis (or clitoris) and muscular branches of perineal nerve.
- Anal triangle is the posterior subdivision of perineum which slopes downwards and forwards. It is traversed by anal canal and its center is supported on either side by fat filled space ischiorectal (ischeoanal) fossa. Ischiorectal (ischeoanal) fossa is wedge shaped space on either side of anal canal. It is separated from pelvis by pelvic diaphragm. It contains a cushioning, pad of fat which allows the distension of the canal during defection. Its boundaries are :-
- Base :- Skin
- Apex: Line where obturator fascia meets the inferior fascia of pelvic diaphragm (anal fascia).
- Anteriorly: -Posterior border of perineal membrane.
- Posteriorly - Gluteus maximus muscle, sacrotuberous ligament.
- Laterally i- Ischial tuberosity and obturator internus.
- Medially:-Sphincter ani externus (external anal sphincter) and pelvic diaphragm (levator ani).
- There are two extensions of ischiorectal (ischeoanal) fossa beyond its boundaries.
- Anterior recess: - Extends forwards above the perineal membrane and is related to prostate or vagina.
- Posterior recess: - Extends deep to the sacrotuberous ligament.
- Both ischiorectal fossae are connected to each other posteriorly behind anal canal through a horse-shoe recess.
- Contents of ischiorectal (ischeoanal fossa) are- (I) Fat pad; (2) inferior rectal nerve and vessels; (3) pudendal canal with its contents (pudendal nerve and internal pudendal vessels); (4) posterior scrotal (in males) or posterior labial (in females) nerve and vessels; (5) perineal branch of S4; and (6) perforating cutaneous branches of S2, S3.
- Pudendal canal (Alcock's canal) is a fascial canal in the lateral wall of ischiorectal (ischio-anal fossa), enclosing pudendal nerve and internal pudendal vessels (artery and vein).
Pudendal nerve is a mixed nerve (motor and sensory). Pudendal nerve is the chief nerve of the perineum and of the external genitalia. It arises from sacral plexus from ventral division of S2, S3, and S4. It originates in the pelvis, enters gluteal region through greater sciatic notch, and leaves it through lesser sciatic notch to enter the pudendal canal. In pudendal canal it gives a branch inferior rectal nerve, and then divides into two terminal branches, the perineal nerve and the dorsal nerve of penis or clitoris. The perineal nerve terminates in to: (i) Medial and lateral posterior scrotal or labial nerves, and (ii) Muscular branches.
Internal pudendal artery
This is the chief artery of the perineum and of external genital organs. It is smaller terminal branch of anterior division of internal iliac artery. It is smaller in females. It accompanies pudendal nerve and course is same as pudendal nerve. Its branches are: -
- Inferior rectal artery;
- Posterior artery which gives two branches: transverse perineal and posterior scrotal or posterior labial; and (3) Artery of pen is or clitoris which gives four branches:
- artery to the bulb,
- urethral artery,
- deep artery of penis or clitoris,
- dorsal artery of penis or clitoris.
It is a musculo-fascial partition across the pubic arch and separates the pelvic cavity from anterior part of pelvic outlet. It consists of two muscles (sphincter urethrae and deep transverse perinei, also called transversus perinei profundus, and two fasciae (Inferior fascia of urogenital diaphragm, also called perineal membrane and superior fascia of urogenital diaphragm). Both muscles of urogenital diaphragm are supplied by muscular branches of perineal nerve, a branch of pudendal nerve.
Structures piercing the urogenital diaphragm are urethra and in female vagina (behind urethra).
Perineal body is the central point of the perineum. It is a fibromuscular node situated in the median plane, about 1.25 cm in front of the anal margin and close to the bulb of penis. Ten muscles of the perineum converge and interlace in the perineal body -
- Two unpaired :-
- External anal sphincter,
- Fibres of longitudinal muscle coat of anal canal.
- Four paired:-
- Superficial transverse perinei,
- Deep transverse perinei,
- Levator ani.
In females, sphincter urethrovaginalis is also attached here