Dissector Answers - External Female 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. Identify the superficial features of the external genitalia.
  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 male and female urethrae.
  6. Identify the components of the external genital organs and give the homologues in each of both sexes.
  7. Describe structure and function of the erectile bodies.
  8. Identify the muscles and fasciae of the perineum and their functions.
  9. Trace the nerve and blood supply to the external genital organs and the muscles of the perineum.
  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. Identify the superficial features of the external genitalia. (W&B 519-522, N 351, 377, 382, 387, 390, 398, TG 6-02, 6-25A, 6-25B, 6-31)

Sorry to cop out and paste in the tables, but there is little connectivity or functionality that needs to be explained. It is pretty much a "these are the things you should know about" situation.


Female:
Organ Location/Description Notes
clitoris combination of 3 erectile bodies: glans clitoris and 2 corpora cavernosa clitoris clitoris is covered by smooth skin
glans clitoridis expanded distal end of the corpus spongiosum which caps the ends of the corpora cavernosa  
prepuce fold of smooth skin extending over the glans clitoris the anterior divisions of the labia minora combine to form the prepuce of the clitoris
hymen thin, usually incomplete septum at the inferior vaginal orifice the hymen is the remnant of the urogenital septum of the embryo, and the hymeneal caruncles are remnants of the hymen
labia majora fat-filled elevations of hair-covered skin lying on either side of the vestibule of the vagina labia majora are joined across the midline by the anterior and posterior labial commissures; they blend into each other anteriorly to form the mons pubis
labia minora paired folds of hairless skin located medial to the labia majora and extending posteroinferiorly from the clitoris the anterior end of each labium minus divides: the anterior division of each labium forms the prepuce of the clitoris and the posterior division of each labium forms the frenulum of the clitoris
vagina part of the female genital canal located between the vestibule and the cervix vaginal fornices surround the cervix; the word "vagina" is derived from the Latin word for sheath
vestibule of the vagina region inferior to the hymen/hymeneal caruncles and between the labia minora vestibule of the vagina contains the external urethral orifice, the opening of the vagina and the openings of the greater vestibular glands
uterus hollow muscular organ within which the fertilized ovum develops; normally lies anteverted posterosuperior to the bladder uterus is anteflexed (curved anteroinferiorly) at the uterine isthmus; the fundus is the part superior to the entrance of the uterine tubes and it normally lies anterosuperiorly; the body meets the cervix at a constricted area called the isthmus; the cervix projects into the vagina
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.

The pudendal nerve has the following branches:

5. 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.)

6. 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
7. 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.

8. Identify the muscles and fasciae of the perineum and their functions. (W&B 524-525, N 404, 405, 411, 413, TG 6-29, 6-30A, 6-30B)

The perineal membrane stretches between the two ischiopubic rami and serves as an anchor for the erectile bodies and their associated muscles. It also divides the perineum into superficial and deep spaces, sometimes referred to as "pouches". The perineal membrane is set at an angle (as are the ischiopubic rami), so that the superficial perineal pouch is anteroinferior to the perineal membrane while the deep perineal pouch is posterosuperior to the perineal membrane. The best way to consider these structures is to look at the tables:

perineal membrane (N379, N381, TG6-27A, TG6-27B) membrane stretching across the urogenital triangle attaching to both ischiopubic rami perineal membrane is pierced by the urethra, vagina and branches of the pudendal neurovascular bundle
perineal body (N379, TG6-27A) fibrous connective tissue in the midline between the vestibule of the vagina and the anus many of the muscles of the perineum attach at or to the perineal body, including the transverse perineal muscles, bulbospongiosus, and external anal sphincter; the male version is often called the central tendinous point

Muscles
Muscle Origin Insertion Action Innervation Artery Notes Image
bulbospongiosus, in female (N379, N382, TG6-27A, TG6-27B) perineal body and fascia of the bulb of the vestibule perineal membrane and corpus cavernosum of the clitoris compresses the vestibular bulb and constricts the vaginal orifice deep branch of the perineal nerve (from pudendal nerve) perineal a. skeletal muscle
ischiocavernosus (N379, N382, TG6-27A, TG6-27B) medial surface of the ischial tuberosity and the ischiopubic ramus corpus cavernosum and crus of the penis/clitoris compresses the corpus cavernosum deep branch of the perineal nerve (from pudendal nerve) perineal a. ischiocavernosus m. is closely applied to the surface of the crus penis/clitoris
sphincter urethrae, in female (N379, N382, N385, TG6-28A, TG6-28B) encircles the urethra encircles urethra and vagina; extends superiorly along the urethra as far as the inferior surface of the bladder compresses urethra and vagina deep branch of perineal nerve from pudendal nerve internal pudendal a. skeletal muscle (Greek, sphincter = that which binds tight)
superficial transverse perineus medial surface of the ischial ramus contralateral muscle and the perineal body/central tendinous point fixes and stabilizes perineal body/central tendinous point deep branch of perineal nerve from pudendal nerve perineal a. superficial and deep transverse perineus muscles are separated by the perineal membrane
deep transverse perineus medial surface of the ischial ramus contralateral muscle and perineal body/central tendinous point fixes and stabilizes the perineal body/central tendinous point deep branch of perineal nerve from pudendal nerve internal pudendal a. superficial and deep transverse perineus muscles are separated by the perineal membrane

9. Trace the nerve and blood supply to the external genital organs and the muscles of the perineum. (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, which is sensory for the clitoris/penis, and also gives off the perineal nerve, which in turn gives off posterior labial or posterior scrotal nerves (sensory) and the deep perineal nerve supplying all of the muscles of the urogenital triangle.

The autonomic supply to the erectile bodies arrives via the cavernous nerves from the inferior hypogastric plexus, which descend lateral to the prostate (male) or membranous urethra (female) to reach the erectile tissue of the corpus cavernosum and corpus spongiosum. The parasympathetic fibers are responsible for relaxing the vascular smooth muscle of the deep arteries and arteries of the bulb, allowing blood to fill the erectile tissue, resulting in erection.

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)

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 Large Intestine - The Urinary Bladder - The Female Urethra - The Male Urethra - 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

Questions and Answers:

1a. Note the difference between male and female in the subpubic angle, the angle formed by the subpubic arch. What are other sex differences in the pelvic skeleton? (W&B 571-573, N 354, TG 6-05A, 6-05B, 6-05CD, 6-05EF)

Structure/Section Female Male
pelvic inlet oval and rounded heart-shaped
pelvic outlet large small
pubic arch and subpubic angle wide narrow
iliac wings flared less flared
2a. Explore the female urethra, noting length, sphincter muscle, and relation to vagina. Note specifically the relation of the orifice to the anterior vaginal wall. What is the significance? (W&B 549, N 383, 379, TG 6-08, 6-10)

The urethra is about four centimeters long. It is homologous to male prostatic/membranous urethra. Its orifice is within the vestibule of the vagina, immediately in front of the vaginal orifice.

3a. 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.
4a. 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.
5a. 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.

6a. 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.