Dissector Answers - Perineum & External Genitalia

Learning Objectives:

Upon completion of this session, the student will be able to:

1. Trace the skeletal and ligamentous boundaries of the perineum, and define the anal and urogenital triangles.
2. Describe the position and boundaries of the ischioanal fossa.
3. Describe the structure, contents, and course of the pudendal canal.
4. Trace the branching pattern of the internal pudendal vessels and the pudendal nerve.
5. Differentiate between the internal and external anal sphincters in structure and function.
6. Differentiate between male and female urethrae.
7. Identify the components of the external genital organs and give the homologues in each of both sexes.
8. Describe structure and function of the erectile bodies.
9. Trace the nerve and blood supply to the external genital organs.
10. Trace the lymphatic drainage of the perineum.

Learning Objectives and Explanations:

1. Trace the skeletal and ligamentous boundaries of the perineum, and define the anal and urogenital triangles. (W&B 519, N 379A, 379B, TG 6-24A, 6-24B)

Officially, the perineum is the outlet of the pelvis. (Used more loosely, it can refer to the area of skin between the anus and the posterior part of the external genitalia.) It is diamond-shaped, and can therefore be divided into two isosceles triangles by a line drawn between the ischial tuberosities. The anterior, or urogenital triangle has as its apex the pubic symphysis, with the ischiopubic rami as equal sides, and our imaginary line as the base. The posterior, or anal triangle is upside-down, with our line again as the base, the sacrotuberal ligaments as the equal sides, and the coccyx as the apex. (peri + inan ("to empty out" in Greek))

You really need to think about this in 3-D because, although drawn two-dimensionally from an inferior point of view it looks like the coccyx, anus, vagina, and pubic symphysis are all coplanar, they are not.

2. Describe the position and boundaries of the ischioanal fossa. (W&B 531-532)

The ischioanal fossa lies lateral to the anal canal and inferior to the pelvic diaphragm. Its boundaries are as follows:

• Superomedial: pelvic diaphragm (anterior recess extends superior or deep to the sphincter urethrae muscle)
• Medial: external anal sphincter muscle and anal canal
• Lateral: obturator internus fascia and ischial tuberosity
• Posterolateral: sacrotuberous ligament and gluteus maximus muscle (posterior recess extends superior to the gluteus maximus muscle)

3. Describe the structure, contents, and course of the pudendal canal. (W&B 524-526, N 404, 405, 411, 413, TG 6-28A, 6-28B)

The pudendal canal extends from the lesser sciatic foramen, where its contents enter the perineum, to the posterior edge of the perineal membrane. It contains the internal pudendal artery, internal pudendal vein, and the pudendal nerve.

4. Trace the branching pattern of the internal pudendal vessels and the pudendal nerve. (W&B 524-526, N 404, 405, 411, 413, TG 6-29A, 6-29B, 6-30A, 6-30B)

The internal pudendal artery gives off the following branches. The internal pudendal vein receives analogous tributaries.

• Within pudendal canal:
• inferior rectal artery: supply to lower rectum and anus
• perineal artery: supply to bulbospongiosus muscle and ischiocavernosus muscles, as well as posterior scrotal or posterior labial artery to supply the skin of the respective structures
• Within urogenital triangle:
• artery of the bulb of the vestibule (in females) or artery of the bulb of the penis (in males): supply to respective structures
• deep (central) artery of clitoris (in females) or deep (central) artery of the penis (in males) - within corpus cavernosum of appropriate structure
• dorsal artery of clitoris (in females) or dorsal artery of penis (in males): runs entire length of appropriate structure, sending branches to corpus cavernosa and terminate in branches to glans and prepuce.

The pudendal nerve has the following branches:

• Within pudendal canal:
  • inferior rectal nerves: supply to external sphincter ani muscle and skin of anus
  • perineal nerve: gives off posterior labial nerve (in female) or posterior scrotal nerve (in male), which supplies the skin of the perineum - also gives off the deep perineal nerve, which supplies motor innervation to all of the muscles of the urogenital triangle.
• Within urogenital triangle:
  • dorsal nerve of clitoris (in female) or dorsal nerve of penis (in male): supply to appropriate structure

5. Differentiate between the internal and external anal sphincters in structure and function. (W&B 530, 537, N 391A, 391B, 392, 393, TG 6-15A, 6-15B, 6-16)

• internal anal sphincter muscle: involuntary. It surrounds the superior 2/3 of anal canal. The internal anal sphincter muscle is a thickening of circular muscle that has parasympathetic innervation. It is contracted most of the time to prevent leakage of fluid and/or feces.
  
• external anal sphincter muscle: voluntary. It surrounds the superior 2/3 of anal canal as well, wrapped around the internal anal sphincter muscle. It constricts anal canal, but can be relaxed on command for the purpose of defecation.

6. Differentiate between male and female urethrae. (W&B 542-543, N 369, 379, 384, 385, TG 6-08A, 6-08B, 6-09A, 6-09B, 6-10A, 6-10B)

The female urethra is about four centimeters long and is entirely "membranous urethra". The male urethra is longer than the female urethra, and is divided into three parts: membranous, prostatic, and penile (spongy) urethra. (See Pelvis & Pelvic Viscera Dissector Answers.)

7. Identify the components of the external genital organs and give the homologues in each of both sexes. (W&B 543-551, 403-413, N 379A, 379B, 382A, 382B, 418A, 418B, TG 6-25A, 6-25B, 6-27A, 6-27B)

Female	Male
vestibular bulbs	corpus spongiosum
greater vestibular glands	bulbourethral glands
urethral and paraurethral glands	prostate gland
glans clitoris	glans penis
prepuce of clitoris	prepuce of penis
corpus of clitoris	corpus (shaft) of penis
labia minora	penoscrotal raphe
labia majora	scrotum

8. Describe structure and function of the erectile bodies. (W&B 520-523, 527-528, N 379A, 379B, 381, 382A, 382B, TG 6-27A, 6-27B, 6-32)

The three primary erectile bodies of the penis are the two corpora cavernosa and the corpus spongiosum. These structures are surrounded by a dense tunica albuginia such that, when they are engorged with blood, the penis becomes erect. The glans penis, the expanded cap of the corpus spongiosum, remains more malleable during erection because it has a much thinner tunica albuginea than the rest of the components of the penis.

The corpora cavernosa are rooted in the perineum by two crura of the penis. Each crus is attached to the ischiopubic ramus. The corpus spongiosum is rooted as the bulb of the penis, which is attached to the perineal membrane, and receives the membranous urethra as it transverses the membrane.

There are homologous structures in the female. The clitoris contains corpora cavernosa as its erectile tissue. Like in the male, each crus is attached to the ischiopubic ramus and perineal membrane. Instead of a corpus spongiosum, the female has two vestibular bulbs, which lie along the sides of the vestibule, and also expand as the glans clitoris to cap the distal ends of the corpora cavernosa.

9. Trace the nerve and blood supply to the external genital organs. (W&B 524-525, N 404, 405, 411, 413, TG 6-29, 6-30A, 6-30B)

The nerve supply to the external genital organs is via the pudendal nerve, which gives off the dorsal clitoral or penile nerve, and also gives off the perineal nerve, which in turn gives off posterior labial or posterior scrotal nerves and the deep perineal nerve supplying all of the muscles of the urogenital triangle.

The internal pudendal artery gives off the dorsal artery of the clitoris or penis and the deep artery of the clitoris or penis as terminal branches. It also gives off an artery to the bulb of the vestibule (in females) or an artery to the bulb of the penis (in males). The perineal artery gives off a posterior labial or posterior scrotal artery.

10.Trace the lymphatic drainage of the perineum. (N 406, 407, 408A, 408B, TG 6-33, 6-34)

• perineum and external genitalia: drain to superficial inguinal nodes
• anal canal: superior part drains to internal iliac nodes, inferior part drains to superficial inguinal nodes
• testes: lymphatic vessels run with the spermatic cord, terminating in the lumbar nodes

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Cultural enrichment: Check out these sections from the 1918 version of Gray's Anatomy of the Human Body! Some of the terms are (of course) out-of-date, but the illustrations are timeless.

The Pelvis - The Muscles and Fasci¾ of the Pelvis - Female Organs of Reproduction 1, 2, 3, 4, 5 - Male Organs of Reproduction 1, 2, 3, 4, 5, 6 - Surface Anatomy of the Perineum - Surface Markings of the Perineum

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Questions and Answers:

11. Do you find muscular (deep) branches of the perineal nerves? (N 411, 413, TG 6-30A, 6-30B)

The deep perineal nerve innervates all of the muscles of the urogenital triangle via slender branches that may be difficult to locate.

12. What is the source and drainage of the deep dorsal vein of the clitoris/penis and the dorsal veins and arteries of the clitoris/penis? (N 265, 383, 359, 404, 405, 381, TG 5-34, 6-10, 6-23, 6-27, 6-29)

The deep dorsal veins drain into the vesical venous plexus. The "normal" dorsal veins drain into the superficial external pudendal vein. The dorsal arteries come from the internal pudendal arteries.

13. What is the function of the perineal membrane? (N 404, 405, 411, 413, TG 6-27A, 6-27B, 6-28A, 6-28B)

This membrane covers the anterior part of pelvic outlet. It aids in support of the pelvic viscera and as an attachment for perineal structures. It is pierced by the arteries of the erectile bodies and the dorsal arteries and nerves of the clitoris or penis.

14. What is the source of the deep (central) artery of the clitoris/penis? (N 404, 405, 6-29A, 6-29B)

This artery is a branch of the internal pudendal artery.

15. What gland is embedded in the sphincter urethrae muscle in males? (N 385A, 384B, 379A, 379B)

In males, the bulbourethral glands (Cowper's glands) are embedded in the external urethral sphincter muscle. In females, the greater vestibular gland is the homologous structure, but it is not associated with the sphincter urethrae muscle. (The greater vestibular glands lie posterior to the vestibular bulb, under the cover of the bulbospongiosus muscle.)