Perineum and External Genitalia

A diamond-shaped region below the pelvic diaphragm, sharing its outer bony boundaries with the pelvic outlet: pubic symphysis (anteriorly), ischiopubic rami and ischial tuberosities (anterolaterally), sacrotuberous ligaments (posterolaterally), and the coccyx (posteriorly).

• Divided by an imaginary transverse line connecting the two ischial tuberosities into an anterior urogenital triangle and a posterior anal triangle.

• A fibromuscular node located in the midline, at the junction between the urogenital and anal triangles (between the vaginal/urethral opening and the anus in females; between the scrotum/bulb of the penis and the anus in males).
• Serves as the central point of convergence and attachment for numerous perineal muscles: bulbospongiosus, superficial and deep transverse perineal muscles, the external anal sphincter, and fibres of levator ani, functioning as a critical structural keystone for overall pelvic floor/perineal integrity.
• Clinically vital given its role in supporting the posterior vaginal wall and its vulnerability to tearing during childbirth (see Clinical Anatomy).

Ischioanal fossae (paired):

• Wedge-shaped, fat-filled spaces on either side of the anal canal, bounded medially by the anal canal/sphincter complex and the inferior fascia of the pelvic diaphragm (levator ani), laterally by the ischial tuberosity and obturator internus fascia (containing the pudendal canal), and roofed by the levator ani itself, the apex points superiorly where the medial and lateral walls converge.
• The two fossae communicate posteriorly (via the deep postanal space, above the anococcygeal ligament), an important anatomical fact, since infection originating in one ischioanal fossa can track around to the contralateral side via this posterior communication, producing a "horseshoe" abscess/fistula.
• The fat filling these spaces allows the anal canal to expand during defecation, and provides a relatively avascular but infection-prone space (see Clinical Anatomy).

Pudendal canal (Alcock's canal):

• A fascial tunnel within the obturator internus fascia, on the lateral wall of the ischioanal fossa, transmitting the internal pudendal vessels and the pudendal nerve from the lesser sciatic foramen forward toward the urogenital triangle.
• The pudendal nerve gives off its inferior rectal branch within (or near the entry to) this canal.

Inferior rectal nerves and vessels:

• Branches of the pudendal nerve and internal pudendal vessels respectively, arising within the pudendal canal (or just proximal to it), crossing the ischioanal fossa to supply the external anal sphincter (motor) and the perianal skin/lower anal canal (sensory), and the corresponding vascular territory.

Perineal membrane:

• A tough fibrous sheet spanning the anterior half of the pelvic outlet (the urogenital triangle), attached to the ischiopubic rami on each side, provides a firm anchor point for the erectile tissues (crura of the penis/clitoris) and the muscles of the superficial perineal pouch, and is perforated by the urethra (and vagina, in females).

Superficial perineal pouch:

• The space between the perineal membrane (deep boundary) and the superficial perineal (Colles') fascia (superficial boundary), containing the erectile tissue bodies (crura, bulb) and their associated muscles (ischiocavernosus, bulbospongiosus), the superficial transverse perineal muscle, and the proximal (bulbar) urethra.

Deep perineal pouch:

• The space between the perineal membrane (superficial/inferior boundary) and the superior fascia of the pelvic diaphragm (deep/superior boundary), historically often described as bounded by a "deep perineal fascia" superiorly, though modern anatomical descriptions vary on the precise superior boundary; functionally the space containing the external urethral sphincter, part of the membranous urethra (males), and (variably described) the deep transverse perineal muscle.
• Contains the external urethral sphincter, and in males, the bulbourethral (Cowper's) glands (embedded within the sphincter, their ducts opening into the spongy urethra), the female equivalent, the greater vestibular (Bartholin's) glands, lies instead within the superficial pouch, opening into the vestibule.

Ischiocavernosus:

• Origin: Ischial tuberosity and ischial ramus.
• Insertion: Crus of the penis/clitoris.
• Innervation: Perineal branch of the pudendal nerve.
• Action: Compresses the crus, helping maintain penile/clitoral erection by impeding venous outflow.

Bulbospongiosus:

• Origin: Perineal body (midline raphe in males, covering the bulb of the penis).
• Insertion: In males, covers the bulb of the penis and the proximal corpus spongiosum, also inserting into the perineal membrane/dorsal penile fascia. In females, covers the bulb of the vestibule and greater vestibular gland, inserting into the pubic arch/clitoral body region.
• Innervation: Perineal branch of the pudendal nerve.
• Action: In males, assists erection (compressing the bulb, contributing to engorgement of the spongy urethra/glans), and provides forceful contraction during ejaculation (expelling semen along the urethra) and at the end of micturition (expelling residual urine). In females, assists in engorgement of the bulb of the vestibule/clitoris, and weakly constricts the vaginal opening.

Superficial transverse perineal muscle:

• Origin: Ischial ramus.
• Insertion: Perineal body.
• Innervation: Perineal branch of the pudendal nerve.
• Action: Stabilises/fixes the perineal body, providing a stable central point for the other perineal muscles to act against.

External urethral sphincter:

• Voluntary skeletal muscle within the deep perineal pouch, surrounding the membranous urethra (males) or the urethra as it passes through the perineal membrane (females).

Deep transverse perineal muscle (where applicable):

• Origin: Ischial ramus.
• Insertion: Perineal body (and blends with the external urethral sphincter/vaginal wall in females).
• Innervation: Perineal branch of the pudendal nerve.
• Action: Supports the pelvic floor/perineal body, working with the external urethral sphincter.
• Note: this muscle is more consistently and distinctly described in males; in females, its existence as a discrete separate muscle (versus being effectively incorporated into the broader "compressor urethrae/urethrovaginal sphincter" complex around the urethra and vagina) is a point of some anatomical description variation across textbooks.

Penis

• Root: The fixed, proximal part, within the superficial perineal pouch, comprising the two crura (attached to the ischiopubic rami, continuous with the corpora cavernosa) and the bulb (attached to the perineal membrane, continuous with the corpus spongiosum).
• Body (shaft): The mobile, pendulous part, composed of the three erectile bodies bound together by deep (Buck's) fascia and covered by skin/superficial (dartos) fascia.
• Crura (×2): Proximal continuations of the corpora cavernosa, diverging to attach firmly to the ischiopubic rami, anchoring the penis's base and contributing to erectile rigidity.
• Bulb: The proximal expansion of the corpus spongiosum, attached to the perineal membrane in the midline, surrounded by bulbospongiosus muscle; transmits the bulbar (proximal spongy) urethra.
• Corpora cavernosa (×2, paired): The dorsal/lateral pair of erectile bodies, forming the bulk of the erectile tissue responsible for penile rigidity during erection, surrounded by the tough tunica albuginea.
• Corpus spongiosum (single, midline/ventral): Surrounds the entire length of the spongy urethra, expanding proximally as the bulb and distally as the glans penis: its erectile tissue is less rigid than the cavernosal bodies (a deliberate anatomical feature, preventing urethral compression/obstruction during erection).
• Glans penis: The distal expansion of the corpus spongiosum, capping the ends of the corpora cavernosa, containing the external urethral meatus at its tip and covered (in the uncircumcised state) by the prepuce (foreskin).

Female External Genitalia (Vulva)

• Labia majora: Paired, hair-bearing skin folds, the female homologue of the scrotum (from the same embryological genital swellings), containing fat and the termination of the round ligament of the uterus.
• Labia minora: Paired, hairless skin folds medial to the labia majora, surrounding the vestibule, meeting anteriorly to form the prepuce and frenulum of the clitoris, the female homologue of the (ventral) penile shaft skin/spongy urethra region (from the urogenital folds).
• Clitoris: The female homologue of the penis, though purely erogenous (no urinary function, given the separate urethral opening), composed of paired crura (attached to the ischiopubic rami, continuous with the corpora cavernosa) converging on a body, capped by the glans clitoris (richly innervated, the most sensitive part); lacks a corpus spongiosum equivalent structure directly analogous to the male's (the vestibular bulbs, are considered the embryological homologue instead).
• Vestibule: The cleft between the labia minora, into which the urethra, vagina, and the ducts of the greater vestibular glands open.
• Bulbs of the vestibule (×2, paired): The female homologue of the male bulb/proximal corpus spongiosum (though structurally divided into two separate masses in females, flanking the vaginal opening, rather than fused in the midline as in the male bulb), composed of erectile tissue, covered by bulbospongiosus muscle.
• Greater vestibular glands (Bartholin's glands, ×2, paired): Located posterolateral to the vaginal opening, within the superficial perineal pouch, deep to the posterior part of the bulbs of the vestibule, their ducts open into the vestibule, providing lubrication (the female homologue of the male bulbourethral/Cowper's glands, though anatomically positioned in the superficial rather than deep pouch).

• The internal pudendal artery and vein, and the pudendal nerve, are the dominant neurovascular supply to the entire perineum (both triangles) and external genitalia (exiting via the greater sciatic foramen, crossing the ischial spine, re-entering via the lesser sciatic foramen, running within the pudendal canal).
• Within the perineum, each gives off: the inferior rectal branch (to the anal triangle, external anal sphincter, perianal skin), the perineal branch (to the superficial perineal pouch structures, muscles, posterior scrotum/labium skin), and the dorsal nerve/artery of the penis or clitoris (the terminal branch, running within the deep perineal pouch before emerging onto the dorsum of the penis/clitoris, providing the primary sensory innervation and blood supply to these structures).
• This shared internal pudendal origin for essentially all perineal neurovascular supply (in both sexes) is the key unifying principle of this section, mirroring the male/female organ homology (crura, bulb, corpus spongiosum/vestibular bulbs) as parallel embryological themes running through perineal anatomy.