Perineal Region

Perineal Region

I. Perineum

• Is a diamond-shaped space that has the same boundaries as the inferior aperture of the pelvis.
• Is bounded by the pubic symphysis anteriorly, the ischiopubic rami anterolaterally, the ischial tuberosities laterally, the sacrotuberous ligaments posterolaterally, and the tip of the coccyx posteriorly.
• Has a floor that is composed of skin and fascia and a roof formed by the pelvic diaphragm with its fascial covering.
• Is divided into an anterior urogenital triangle and a posterior anal triangle by a line connecting the two ischial tuberosities.

II. Urogenital Triangle

A. Superficial perineal space (pouch)

• Lies between the inferior fascia of the urogenital diaphragm (perineal membrane) and the membranous layer of the superficial perineal fascia (Colles' fascia).
• Contains the superficial transverse perineal muscle, the ischiocavernosus muscles and crus of the penis or clitoris, the bulbospongiosus muscles and the bulb of the penis or the vestibular bulbs, the central tendon of the perineum, the greater vestibular glands (in the female), branches of the internal pudendal vessels, and the perineal nerve and its branches.

1. Colles' fascia

• Is the deep membranous layer of the superficial perineal fascia and forms the inferior boundary of the superficial perineal pouch.
• Is continuous with the dartos tunic of the scrotum, with the superficial fascia of the penis,and with the Scarpa's fascia of the anterior abdominal wall.

2. Perineal membrane

• Is the inferior fascia of the urogenital diaphragm that forms the inferior boundary of the deep perineal pouch and the superior boundary of the superficial pouch.
• Lies between the urogenital diaphragm and the external genitalia, is perforated by the urethra, and is attached to the posterior margin of the urogenital diaphragm and the ischiopubic rami.
• Is thickened anteriorly to form the transverse ligament of the perineum, which spans the subpubic angle just behind the deep dorsal vein of the penis.

3. Muscles of the superficial perineal space

• Ischiocavernosus muscles
• Arise from the inner surface of the ischial tuberosities and the ischiopubic rami.
• Insert into the corpus cavernosum (the crus of the penis or clitoris).
• Are innervated by the perineal branch of the pudendal nerve.
• Maintain erection of the penis by compressing the crus and the deep dorsal vein of the penis, thereby retarding venous return.
• Bulbospongiosus muscles
• Arise from the perineal body and fibrous raphe of the bulb of the penis in the male and the perineal body in the female.
• Insert into the corpus spongiosum and perineal membrane in the male and the pubic arch and dorsum of the clitoris in the female.
  
• Are innervated by the perineal branch of the pudendal nerve.
  
• Compress the bulb in the male, impeding venous return from the penis and thereby maintaining erection. Contraction (along with contraction of the ischiocavernosus) constricts the corpus spongiosum, thereby expelling the last drops of urine or the final semen in ejaculation.
• Compress the erectile tissue of the vestibular bulbs in the female and constrict the vaginal orifice.

• Superficial transverse perineal muscle
• Arises from the ischial rami and tuberosities.
• Inserts into the central tendon (perineal body).
• Is innervated by the perineal branch of the pudendal nerve.
• Stabilizes the central tendon.

4. Perineal body (central tendon of the perineum)

• Is a fibromuscular mass located in the center of the perineum between the anal canal and the vagina (or the bulb of the penis)
• Serves as a site of attachment for the superficial and deep transverse perineal, bulbospongiosus, levator ani, and external anal sphincter muscles.

5. Greater vestibular (Bartholin's) glands

• Lie in the superficial perineal space deep to the vestibular bulbs in the female.
• Are homologous to the bulbourethral glands in the male.
• Are compressed during coitus and secrete mucus that lubricates the vagina. Ducts open into the vestibule between the labium minora below the hymen.

B. Deep perineal space (pouch)

• Lies between the superior and inferior fasciae of the urogenital diaphragm.
• Contains the deep transverse perineal muscle and sphincter urethrae, the membranous part of the urethra, the bulbourethral glands (in the male), and branches of the internal pudendal vessels and pudendal nerve.

1. Muscles of the deep perineal space

• Deep transverse perineal muscle
• Arises from the inner surface of the ischial rami.
• Inserts into the medial tendinous raphe and the perineal body; in the female, it also inserts into the wall of the vagina.
• Is innervated by the perineal branches of the pudendal nerve.
• Stabilizes the perineal body and supports the prostate gland or the vagina.
• Sphincter urethrae
  
  • Arises from the inferior pubic ramus.
  • Inserts into the median raphe and perineal body.
  • Is innervated by the perineal branch of the pudendal nerve.
  • Encircles and constricts the membranous urethra in the male.
  • Has an inferior part that is attached to the anterolateral wall of the vagina in the female, forming a urethrovaginal sphincter that compresses both the urethra and vagina.

2. Urogenital diaphragm

• Consists of the deep transverse perineal muscle and the sphincter urethrae and is invested by superior and inferior fasciae.
• Stretches between the two pubic rami and ischial rami, but does not reach the pubic symphysis anteriorly.
• Has inferior fascia that provide attachment to the bulb of the penis.
• Is pierced by the membranous urethra in the male and by the urethra and the vagina in the female.

3. Bulbourethral (Cowper's) glands

• Lie among the fibers of the sphincter urethrae in the deep perineal pouch in the male, on the posterolateral sides of the membranous urethra. Ducts pass through the inferior fascia of the urogenital diaphragm to open into the bulbous portion of the spongy (penile) urethra.

III. Anal Triangle

A. Ischiorectal (ischioanal) fossa

• Is the potential space on either side of the anorectum and is separated from the pelvis by the levator ani and its fasciae.
• Contains ischioanal fat , which allows distention of the anal canal during defecation; the inferior rectal nerves and vessels , which are branches of the internal pudendal vessels and the pudendal nerve; and perineal branches of the posterior femoral cutaneous nerve (which communicates with the inferior rectal nerve).
• Contains the pudendal (Alcock's) canal on its lateral wall. This is a fascial canal formed by a split in the obturator internus fascia and transmits the pudendal nerve and internal pudendal vessels.
• Is occasionally the site of an abcess that can extend to other fossa by way of the communication over the anococcygeal raphe.
• Has the following boundaries:
• Anterior: the sphincter urethrae and deep transverse perineal muscles
• Posterior: the gluteus maximus muscle and the sacrotuberous ligament
• Superomedial: the sphincter ani externus and levator ani muscles
• Lateral: the obturator fascia covering the obturator internus muscle
• Floor: the skin over the anal triangle

B. Muscles of the anal triangle

1. Obturator internus

• Arises from the inner surface of the obturator membrane.
• Has a tendon that passes around the lesser sciatic notch to insert into the medial surface of the greater trochanter of the femur.
• Is innervated by the nerve to the obturator.
• Laterally rotates the thigh.

2. Sphincter ani externus

• Arises from the tip of the coccyx and the anococcygeal ligament.
• Inserts into the central tendon of the perineum.
• Is innervated by the inferior rectal nerve.
• Closes the anus.

3. Levator ani muscle

• Arises from the body of the pubis, the arcus tendineus of the levator ani (a thickened part of the obturator fascia), and the ischial spine.
• Inserts into the coccyx and the anococcygeal raphe or ligament.
• Is innervated by the branches of the anterior rami of sacral nerves S3 and S4 and the perineal branch of the pudendal nerve.
• Supports and raises the pelvic floor.
• Consists of the puborectalis, pubococcygeus, and iliococcygeus.
• Has as its most anterior fibers, which are also the most medial, the levator prostatae or pubovaginalis.

4. Coccygeus

• Arises from the ischial spine and the sacrospinous ligament.
• Inserts into the coccyx and the lower part of the sacrum.
• Is innervated by branches of the fourth and fifth sacral nerves.
• Supports and raises the pelvic floor.

◄.....Go back to the Gross Anatomy homepage