#29 - Perineum Demonstration

Because of the difficulty in dissecting the structures in the perineum, the area will be demonstrated by your laboratory instructor.

  1. The relation of the pelvic (N345) and urogenital diaphragms (395/N361) will be shown as well as the parts of the pelvic diaphragm that are more evident in a specimen in which the pelvic viscera have been removed:

    1. Coccygeus muscle (N345) - lies above the sacrospinous ligament, which is degenerated coccygeus muscle.
    2. Levator ani (N345) consist of puborectalis, iliococcygeus and pubococcygeus fibers (N345). Attachment of the medial part of the diaphragm to organs in the midline will be evident. The funnel shape of the pelvic diaphragm can be appreciated in these specimens. The attachment of the iliococcygeus is not to pelvic bone but a thickening of the fascia of the obturator internus muscle (N345).
  2. The boundaries of the perineum and division into anal and urogenital triangles (389/N363) will be shown:

    5. Sacrum (N340) and coccyx (N363).
  3. Important in understanding the relations of the perineum are the following bony landmarks and ligaments (N340):

    1. Ischial spine (N340).
    4. Sacrospinous ligaments (N340) deep to sacrotuberous ligament.
    Thinking of the pelvis in the anatomic position is crucial for appreciating terms of position and direction in the perineum. In the anatomic position the pelvis can be thought of as resting on its ischiopubic rami.
  4. A dissection of the anal triangle in which the fat has been removed from the ischioanal (ischiorectal) fossa (395/N373) will be shown. Structures which can be identified include:

    1. Obturator internus and levator ani (N373) - walls of ischioanal fossa.
    2. External anal sphincter (N373) - skeletal muscle.
    3. Internal anal sphincter (N373) - smooth muscle in the wall of the anal canal can only be demonstrated microscopically.
    4. Pudendal canal (N373) - a canal for the internal pudendal artery (N373) and branches of the pudendal nerve (N373) is formed by a reduplication of fascia on the medial surface of the obturator internus.
    5. The inferior rectal vessels and nerves (N391) - nerve supply for the external anal sphincters. Exit from pudendal canal. Despite their supply of the anal canal, they are named inferior rectal.
  5. Dissection of the urogenital triangle (N361) will be demonstrated in male and female specimens. The membranous layer of superficial fascia continuous into the perineum and scrotum (Colles') from the anterior abdominal wall (Scarpa's) has been removed to open the superficial perineal pouch (N353) to view . Erectile bodies are covered with muscle and occupy the superficial pouch. The urogenital (UG) diaphragm (396/N360) stretched from the ischiopubic ramus to the other, limits the superficial pouch superiorly. The pouch is closed by attachment of the membranous fascia to ischiopubic rami and posterior edge of UG diaphragm. The UG diaphragm consists of a fibrous membrane, the perineal membrane (inferior fascia) (N360) inferiorly, a thin layer of skeletal muscle, including sphincter urethrae, sphincter vagina, deep transverse perineus, (N361) and a superior fascia (N360) covering those muscles. The interval between the perineal membrane and the superior fascia has been called the deep perineal space or pouch (N360), although it is not truly a potential space. The bulbourethral gland, a male counter part of the female greater vestibular gland, is located within this perineal pouch or UG diaphragm. It is very difficult to locate.
  6. The structures in the male superficial pouch (N376) include the following (406-411):

    1. Erectile bodies covered with skeletal muscle.
      1. Crura of penis (N365) covered with ischiocavernosus muscle (N376) which continue into the body of the penis as corpora cavernosa (N364).
    2. Extending transversely parallel to the posterior edge of the UG diaphragm is the small superficial transverse perineus muscle (N376). It is usually lost in dissection.
    3. The central tendon (N376) is a midline union of all muscles in the perineum except the ischiocavernosus muscles.
  7. The structures in the female superficial pouch (N361) include the following (411-417):

    1. Erectile bodies covered with skeletal muscle.
      1. Crura of clitoris (N361) covered with ischiocavernosus muscle (N361).
      2. Bulb of the vestibule (N361) covered with bulbospongiosus muscle (N361).
    2. Extending transversely parallel to the posterior edge of the UG diaphragm is the small superficial transverse perineus muscle (N361). It usually is lost in dissection.
    3. The greater vestibular glands (N361), the female counterpart of the bulbourethral gland in the male, are located on each side of the vestibule of the vagina, posterior to the bulb of the vestibule.
    4. The perineal body (central tendon) (N361) is a midline union of all muscles in the perineum except the ischiocavernosus muscles. The perineal body is more than muscle; it is fibromuscular in structure. It is difficult in dissection to distinguish the difference between the bulb and the gland.
  8. The pudendal nerve and internal pudendal artery and vein (N391) leave the pelvic cavity to enter the gluteal region through the greater sciatic foramen below the level of the piriformis muscle. They hook posteriorly around the ischial spine or sacrospinous ligament and re-enter the bony pelvis through the lesser sciatic foramen. The major branches of these neurovascular structures will be demonstrated and are listed below:

    1. The first branches given off in the ischioanal fossa are the inferior rectal nerves and vessels (N391).
    2. The neurovascular structures enter the pudendal canal (N391) formed by the fascia of the obturator internus muscle and reach the superior surface of the UG diaphragm near its posterior edge.
    3. The perineal nerve (N391), a terminal branch of the pudendal nerve, pierces the posterior edge of UG diaphragm and then ramifies in the superficial perineal space. Branches are given to the skin of the scrotum or labium majus. Innervating nerves to the musculature of the urogenital triangle all come from the perineal nerve.
    4. The remaining terminal branch, dorsal nerve of penis or clitoris (N391), enters the urogenital diaphragm (deep perineal space).
    5. The internal pudendal vessels (366/N385) pierce the posterior edge of the UG diaphragm to enter the deep perineal space. The branches to the bulb, bulb of vestibule, corpra cavernosa and crura of clitoris branch off within the deep perineal space and pierce the perineal membrane (inferior fascia) to reach their destinations. Their course within the UG diaphragm will be described but probably not demonstrated.
    6. After traversing the UG diaphragm, the emergence of the dorsal nerve and artery (N366) from the urogenital diaphragm to the dorsum of penis or clitoris will be shown. The accompanying deep dorsal vein (N366) will be shown entering the pelvic cavity between the pubis and the anterior edge of the urogenital diaphragm.
    7. The key landmarks to perform a pudendal nerve block (N393) will be demonstrated.
  9. A section of the penis (406/N364) will be shown to display:

    1. The corpus cavernosum penis (406/N364) - covered with tunica albuginea.
    2. The corpus spongiosum penis (406/N364) - contains the urethra.
    3. The fascia penis (Buck's fascia) (406/N364) - derived from fascia of ischiocavernosus and bulbospongiosus muscles. Wraps all erectile bodies.
    4. The deep artery (406/N364) of the penis. Lies within #1.
    5. The dorsal artery and nerve (406/N364). Lie deep to fascia penis. Nerve lies lateral to artery. Deep dorsal vein is in midline.
    6. The superficial dorsal vein (406/N364). Lies outside of fascia penis.
    10. Homologous structures in the female perineum will be demonstrated (413).

Perineum Alternative Dissection

  1. The anal triangle of the perineum should be dissected first. Place the cadaver in the prone position and elevate the pelvis with blocks. Incise the skin in the midline over the sacrum and coccyx, around the anus to the central point of the perineum (between anus and scrotum or labia majora) and then laterally to the thigh. Reflect the skin flaps laterally.
  2. Remove fat and fascia to expose the inferior border of the gluteus maximus. The area between the anus and the inferior border of the gluteus maximus is the fat-filled ischioanal (ischiorectal) fossa. The only structures traversing the fat-filled cavity are inferior rectal nerves and vessels. Try to preserve them as you remove the fat from the fossa to reveal the levator ani muscle medially and the obturator internus muscle laterally. The inferior rectal vessels and nerve should be followed to the pudendal canal- a fascial canal on the medial surface of the obturator internus muscle extending forward from the lesser sciatic foramen. Slit the fascial covering of the canal and identify the branches of the pudendal nerve, the perineal nerve and the dorsal nerve of penis or clitoris, and the internal pudendal artery.
  3. The urogenital triangle will be approached with the cadaver in supine position. It is difficult to obtain enough room for this dissection. The lower limbs should be spread as far as possible and the skin and superficial fascia of the medial thigh may also be removed.
  4. The external genitalia should be studied using a standard text book diagram. Remove the skin from the urogenital triangle.
  5. Attempt to identify the membranous layer of fascia in the perineum which attaches to the ischiopubic ramus and the posterior edge of the urogenital diaphragm by following this layer into the perineum from the anterior abdominal wall. If you can see skeletal muscle in the urogenital triangle the membranous layer of fascia has been removed. Vessels and nerves encountered superficial to the membranous layer of fascia are posterior scrotal or posterior labial branches of the perineal nerve or internal pudendal artery.
  6. Expose the muscles of the superficial pouch (area between membranous layer of fascia and the urogenital diaphragm.) The ischiocavernosus muscles lie along the ischiopubic rami. Bulbospongiosus muscles are fused in a midline raphe in males and separated in females. The superficial transverse perinei muscles are insignificant and often very difficult to display. If the ischiocavernosus and bulbospongiosus muscle, which are thin, are reflected, the cavernous tissue of the crura, bulb of penis or bulb of the vestibule will be displayed. Deep fascial coverings of these muscles continue onto the shaft of the penis as the fascia penis.
  7. Clean the perineal membrane, the inferior layer of the urogenital diaphragm by pushing the muscles and cavernous tissue to one side or the other. Removal of the perineal membrane will reveal muscle fibers of the sphincter of the urethra and the deep transverse perineus muscles. In addition, in female bodies some of those fibers constitute the sphincter of the vagina.
  8. By bluntly dissecting in the sphincteric muscle just medial to the ischiopubic ramus it may be possible to demonstrate the dorsal nerve of penis or clitoris and the internal pudendal artery.
  9. Follow the crura of the penis or clitoris into the shaft of the penis or body of the clitoris. On the dorsum of the penis or clitoris locate the deep dorsal vein, the dorsal arteries and the dorsal nerves. These structures lie deep to the fascia penis. A similar layer is not evident on the clitoris. Follow the dorsal nerve proximally and by reflecting the shaft of the penis or body of the clitoris away from the symphysis pubis observe its penetration of the perineal membrane.
  10. Cut the shaft of the penis in order to display:
    1. Corpus cavernosum - covered with tunica albuginea.
    2. Corpus spongiosum - contains the urethra.
    3. Deep artery of penis - lies in #1.
    4. Dorsal vessels and nerve - lie deep to fascia penis.
    5. Fascia penis - derived from muscular fascia, wraps all erectile bodies and neurovascular structures.

  11. Study the funnel shaped pelvic diaphragm on the pelvis in which the viscera have been removed. Blending of muscle fibers with viscera in the midline is apparent.
    1. Coccygeus muscle- lies above the sacrospinous ligament.
    2. Levator ani- consists of puborectal, iliococcygeus and pubococcygeus fibers.