1.4: The Perineum

1.4.1 Boundaries of the Perineum

The perineum is a diamond-shaped area on the inferior surface of the trunk between the symphysis pubis and the coccyx. It has the following boundaries:

• Anterior: symphysis pubis & pubic arch
• Lateral: pubic and ischial rami, ischial tuberosity, sacrotuberous ligament
• Posterior: sacrum and coccyx, and Roof: the pelvic diaphragm

The perineum is divided into two triangles by drawing an imaginary line between the two ischial tuberosities, the anterior urogenital (UG) triangle and the posterior anal triangle. (Figure 1.19)

Figure 1.19 Cartoon drawing shows the layout and the general attachments of the urogenital and anal triangles.
Image Source: Michael F. Dauzvardis, Ph.D. Assistant Professor Emeritus, Loyola University; use of image with permission.

1.4.2 Urogenital Triangle

The Urogenital Triangle is anterior to a line connecting the ischial tuberosities and includes:

• Perineal Membrane
• Superficial Perineal Pouch (Female)
• Deep Perineal Pouch (Female)

Before going over the urogenital triangle, consider the fascial planes:

Through the three fascial layers, the urogenital triangle contains two pouches: superficial and deep perineal pouches. From superficial to deep, here is the arrangement of fascia and pouches of the female perineum:

• Skin
• Membranous layer of superficial fascia.
• Superficial perineal pouch.
• Perineal Membrane (inferior fascia of urogenital Diaphragm (UG diaphragm).
• Deep perineal pouch (the space between the superior and inferior fascia of the UG diaphragm).
• Superior fascia of UG Diaphragm.
• Deep fascia of Pelvic Diaphragm.

1.4.3 Perineal Membrane

The perineal membrane forms the base for clitoral musculature and clitoris in females (penile in males). The space deep to the perineal membrane is the deep pouch, and the space superficial to the perineal membrane is the superficial pouch.

1.4.4 Superficial Perineal Pouch

Urogenital structures: Root of Clitoris (formed of two crura), splitting of the bulb of the vestibule around the urethral and vaginal openings, and the greater vestibular glands.

1.4.4.1 Functional Features of the Superficial Perineal Pouch

• Muscles

  • Ischiocavernosus muscles attach to ischiopubic rami and cover the crura of clitoris
  • Bulbospongiosus muscles split to cover the bulb of the vestibule
  • Superficial transverse perinei muscles

• Vessels

Perineal artery (a branch of the internal pudendal), which feeds dorsal and deep arteries of the clitoris, in addition to labial branches and muscular branches.

• Nerves

Perineal n. (a branch of pudendal n), dorsal nerve of clitoris and labial branches.

Figure 1.20 Cartoon drawing shows the contents and the muscles of the superficial perineal pouch.
Image Source: Michael F. Dauzvardis, Ph.D. Assistant Professor Emeritus, Loyola University; use of image with permission^.

1.4.5 Deep Perineal Pouch

1.4.5.1 Functional Features of the Deep Perineal Pouch

• Muscles

Sphincter urethrae and deep transverse perinei muscles. Note that the UG diaphragm comprises these two muscles (deep transverse perinei and sphincter urethrae). (Figure 1.21)

• Vessels

The internal pudendal artery, which gives the following branches:

• Artery of the bulb of the vestibule pierces the perineal membrane to enter the bulb of the clitoris
• Dorsal & deep arteries of the clitoris pierces the perineal membrane to reach the dorsum of the penis or clitoris
• Muscular branches

• Nerves

Dorsal nerve of the clitoris.

Figure 1.21 Cartoon drawing shows the content and the muscles of the deep perineal pouch.
Image Source: Michael F. Dauzvardis, Ph.D. Assistant Professor Emeritus, Loyola University; use of image with permission.

For more information

For more information about the superficial perineal pouch and related clinical applications, please visit:

Anatomy, Abdomen, and Pelvis: Superficial Perineal Space - StatPearls - NCBI Bookshelf (nih.gov) (Fahmy, Sanganeria, and Munakomi 2022)

Anatomy, Abdomen, and Pelvis: Deep Perineal Space - StatPearls - NCBI Bookshelf (nih.gov) (Bolla, Hoare, and Varacallo 2023)

1.4.6 Anal Triangle

The I-Anal Triangle is posterior to a line connecting the ischial tuberosities and includes:

• Anal Canal
• Anal Sphincters
• Ischiorectal Fossa
• Pudendal Canal

1.4.6.1 Anal Canal

The anal canal is ia continuation of rectum beyond the puborectalis m.

1.4.6.2 Functional Features of the Anal Canal

• Arterial supply

Superior rectal artery above the pectinate line and inferior rectal artery below the pectinate line

• Venous drainage

Superior rectal vein above the pectinate line and inferior rectal vein below the pectinate line

• Innervation

Autonomic, by the inferior hypogastric plexus, somatic by the inferior rectal nerve

1.4.6.3 Anal Sphincters

• Internal Anal Sphincter (smooth muscle) innervated by the inferior hypogastric nerve plexus. It is the thickened lower end of the inner circular muscle of the rectum.
• External Anal Sphincter (skeletal muscle) innervated by the inferior rectal branch of the pudendal nerve. It has three parts:
  • Deep: encircles the anal canal and is fused with puborectalis m.
  • Subcutaneous: complete circle just deep to the skin
  • Superficial: attached posteriorly to the coccyx, anteriorly to the perineal body.

1.4.6.4 Ischiorectal Fossa

The ischiorectal fossa is the fat-filled space lateral to the anal triangle, used as dead space for the anus to expand. It has the following boundaries:

• Lateral wall = fascia of obturator internus muscle, sacrotuberous ligament, and ischial tuberosity.
• Medial wall = anal canal and levator ani muscle.
• Posterior = gluteus maximus and sacrotuberous ligament.
• Anterior = transverse perineal muscles.

The pudendal canal is formed by the fascia of the obturator internus, which splits to enclose the pudendal neurovascular bundle and connects the lesser sciatic foramen to the posterior perineal membrane. Perineal Body: lies at the front of the anal canal; it is composed of interdigitating fibers of pubovaginalis (puboprostaticus in males), superficial external anal sphincter, and transverse perineal muscles.

1.4.6.5 Functional Features of the Pudendal Canal

• Blood supply

The internal pudendal artery and vein feed the following branches, including the inferior rectal artery and the perineal artery, that supply the superficial pouch and continue into the deep pouch.

• Innervation

The pudendal nerve, which branches into the inferior rectal nerve (posterior) and the dorsal nerve of the penis (clitoris), and the perineal nerve (anterior) near the anterior end of the pudendal canal.

For more information

For more information about the pudendal nerve, its branches, and the clinical application of pudendal nerve block please visit Anatomy, Abdomen and Pelvis, Pudendal Nerve - StatPearls - NCBI Bookshelf (nih.gov) (Kinter and Newton 2023)

For more information about the anal triangle and the clinical application of ischiorectal fossa, please visit Anatomy, Abdomen and Pelvis: Anal Triangle - StatPearls - NCBI Bookshelf (nih.gov) (Nauman and Samra 2023)

Videos

Video 1.3 Let's build a perineum

Glossary Terms

 

Coccyx

(tailbone) - The tailbone (coccyx) is the small bone at the lower tip of the spine. The most common cause of injury to the tailbone is a backward fall onto a hard surface, such as a slippery floor or ice

Levator ani

(muscle) - muscle that makes up the levator ani along with the iliococcygeus

Obturator internus

muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip

Pelvic diaphragm

muscular sheet that comprises the levator ani and the ischiococcygeus

Sacrotuberous ligament

ligament that spans the sacrum to the ischial tuberosity of the hip bone

Sacrum

single bone located near the inferior end of the adult vertebral column that is formed by the fusion of five sacral vertebrae; forms the posterior portion of the pelvis

Sphincter

A ring-shaped muscle that relaxes or tightens to open or close a passage or opening in the body. Examples are the anal sphincter (around the opening of the anus) and the pyloric sphincter (at the lower opening of the stomach)

Symphysis pubis

joint formed by the articulation between the pubic bodies of the right and left hip bones

Image Acknolwedgements

Dauzvardis, Michael F., Ph.D. Assistant Professor Emeritus, Loyola University; used with permission