Schedule  

Lab 9 - Perineum

Primary Lab Objectives:

Cadavers with a penis

  1. Remove the skin near the base of the penis, extending to the pubic rami and upper parts of the inner thigh.
  2. Identify the bulbospongiosus, ischiocavernosus, and superficial transverse perineal muscles. Understand the structures they cover and their role in erection and ejaculation.
  3. Examine the dorsal surface of the penis and examine a cross-section of the midshaft.  Locate the deep fascia and the three columns of erectile tissue. Understand where blood and urine flow through these sections.
  4. Expose the urogenital diaphragm from below and locate the central tendon of the perineum.
  5. Bisect the pelvis.
  6. Explore the ischioanal fossa and identify pelvic diaphragm from below.
  7. Make sure you look at a cadaver with a clitoris.

Cadavers with a clitoris

  1. Examine the external genitalia and then remove the skin covering the external genitalia.
  2. Identify the bulbospongiosus, ischiocavernosus, and superficcial transverse perineal muscles. Understand the structures these muscles cover.
  3. Expose the urogenital diaphragm from below and locate thecentral tendon of the perineum.
  4. Bisect pelvis.
  5. Explore ischioanal fossa and identify pelvic diaphragm from below.
  6. Make sure you look at a cadaver with a penis.

 

The external genitals arise from homologous structures in all individuals. The structures of the penis are relatively more expanded, but essentially homologous to the clitoris and its associated structures. In cadavers with a clitoris, the corpus spongiosum is represented by the bulbs of the vestibule on either side of the vagina.  The clitoris is made up of the corpora cavernosa, and terminates in a glans covered by a prepuce, just as in the phallus. The crura are attached along the pubic rami in all individuals.

 

DISSECTION OF THE UROGENITAL TRIANGLE IN CADAVERS WITH A PENIS

1. EXTERNAL GENITALIA

  1. Examine the external genitalia (MOVIE - courtesy of UMich, note: this clip shows both female and male genitalia).
  2. Make a skin incision around the proximal end of the shaft of the penis (following the dashed blue lines in the image below). 
  3. Make an incision along the midline between the penis and the anus.
  4. Extend the dissection latteraly (following the red dashed lines in the image below).
  5. Remove the skin from the space between the thighs that you have just cut. If you have not already done so, remove the skin of the scrotum.

Atlas Images:

FUNCTIONAL ANATOMY:

The perineum is a diamond-shaped area that is inferior to the muscular pelvic floor. The anterior portion of the perineum, the urogenital triangle, is between the ischial tuberosities and the pubic symphysis. The majority of the urogenital triangle is covered by a thick layer of fascia, the perineal membrane, which serves as a support for the pelvic diaphragm on the deep (superior) side as well as an anchor for the external genitalia on the superficial (inferior) side. The perineal membrane defines the openings of the urethra and the vagina. The perineal membrane is anchored at the midline by the perineal body, which is thick connective tissue where many muscles attach including: the external anal sphincter, urethral, superficial transverse perineal, deep transverse perineal muscle, and bulbospongiosus.

 

2. PERINEUM

FUNCTIONAL ANATOMY:

The perineum is often described as having pouches.  These pouches represent layers of structures and fascia separating them.  They can be of clinical significance with cancer and injury, especially with tears of the urethra where urine can enter specific pouches. The structures located in the superficial perineal pouch are: 3 paired muscles (the superficial transverse perineal, bulbospongiosus, and ischiocavernosus), the crura of the penis and the bulb of the penis as well as the vessels and nerves that serve these structures.

  1. Use blunt dissection to find and identify the bulbospongiosus muscle that covers over the bulb of the penis (this is the dilated proximal end of the corpus spongiosum).
  2. Palpate the ischiopubic rami to identify the ischiocavernosus muscle. The ischiocavernosus covers over the crura (singular: crus), the proximal ends of the corpus cavernosum.
  1. Identify the perineal body located between the bulb of the penis and the anus.
  2. Identify the superficial transverse perineal muscle running between the perineal body and the ischiopubic ramus.
  3. Use a probe to dissect between the three muscles of the superficial perineal pouch (superficial transverse perineal, bulbospongiosus, and ischiocavernosus) to find the thin perineal membrane.

NOTE – The structures and tissue in this area are very delicate, so use caution while dissecting.

NOTE: The bulbospongiosus and ischiocavernosus muscles are smaller in individuals with a clitoris than in individuals with a penis. The bulbospongiosus, superficial transverse perineal muscles, levator ani, and the external anal sphincters are all attached to a fibromuscular mass, the perineal body.

The perineal body will orient you to the dividing line between the urogenital and anal triangles.

Atlas Images:

 

3.  DISSECTION OF THE PENIS

NOTE: In the anatomical position, the penis is erect. The surface of the penis that is closest to the anterior abdominal wall is the dorsal surface.

The dorsal surface of the penis contains important structures. In order to preserve these structures, make a cut down the ventral suface of the penis and then carefully peel the skin laterally to reveal the dorsal surface.

  1. Use a scalpel to make a midline incision down the ventral surface of the penis (black dashed line in the image below).

 

  1. Use a blunt probe to pull off the skin laterally and eventually remove all of the skin from the entire shaft. Do not skin the glans.
  2. Use a probe to dissect and identify the superficial dorsal vein of the penis.
  3. Carefully cut through the deep fascia (Buck's fascia) of the penis to find and identify the deep dorsal vein and arteries of the penis, and the dorsal nerves of the penis.

Atlas Images:

  1. Make a cross-section of the penis by making a transverse cut at mid-shaft.
  2. Identify the three components of erectile tissue: two paired columns called corpus cavernosum, and the ventral corpus spongiosum surrounding the penile (spongy) urethra.
  3. Examine the spongy erectile tissue of the corpus cavernosum and attempt to find the deep artery of the penis near the center.
  4. Insert a probe into the urethra and trace its path through the penis.
  5. Determine if there is a prepuce (foreskin) and identify the glans.

Atlas Images:

 

 

 

HEMISECTION OF THE PELVIS

1. TRANSECTING THE BODY -click here to see a video summarizing this procedure (courtesy of UMich)

  1. The first step in bisection is to separate the upper and lower halves of the body.
  2. With the cadaver on its back, use a scalpel to cleanly cut through the soft tissues connecting the upper and lower halves of the body at the level of the L3 vertebrae (or just above the iliac crest).
  3. Use a hand saw to saw through the vertebral body at the L3 level and complete the soft tissue transection as necessary.

NOTE – you will be cutting primarily through abdominal wall muscle. Make sure you cut as close to the vertebra as you can in order to make the final cut cleanly.

 

 

 

2. HEMISECTION OF THE PELVIS

  1. The next step is to separate the right and left halves of the body.
  2. Cut the superior portion of the bladder along the midline.
  3. Identify the internal urethral orifice and insert a probe.
  4. Use this probe as a guide to incise and divide the urethra and the prostate gland.
  5. Finish dividing the GI tract and other viscera along the midline. If there is a significant length of sigmoid colon, it may be removed leaving just the rectum to be divided. Be sure to tie off the sigmoid colon if you decide to remove it prior to the hemisection.

NOTE – Remember to cut these soft tissue structures from above and below. You want to make sure that the midline cuts with the scalpel are complete before you use the saw on the bones. Otherwise, it will be difficult to identify structures when you have completed the hemisection.

 
  1. Saw through the pubic symphysis.
  2. Saw vertically through the sacrum, the coccyx, and the remaining lumbar vertebrae at the midline.
  3. After the pelvis is hemisected, clean out the contents of the rectum and anal canal. Do this in a large steel sink (not a hand sink). Avoid dripping water and feces on the lab floor.
 

 

 

4. EXPLORING THE ISCHIOANAL FOSSA - We will examine the bisected pelvis in detail in the next lab.

  1. Find the posterior edge of the perineal membrane in your cadaver.
  2. Probe the fat-filled area that is posterior to this edge.
  3. The ischioanal fossa is a fat-filled area that is inferior to the levator ani and extends anteriorly to lie superior to the perineal membrane.

The deep perineal pouch contains several muscles: the external urethral sphincter and deep transverse perineal (in all individuals), the compressor urethrae and sphincter urethrovaginalis in individuals with a clitoris. It also contains branches of the internal pudendal vessels and pudendal nerve that run through to supply the muscles and structures of the perineum and external genitalia.

NOTE - Do not attempt to dissect the ischioanal fossa in detail. It is time-consuming and relatively unproductive (though clinically relevant). Click here to see a video demonstration of perineal structures using a prosected specimen.

Atlas Images:

 

 

DISSECTION OF THE UROGENITAL TRIANGLE IN CADAVERS WITH A CLITORIS

The external genitals arise from homologous structures in all individuals. The structures of the penis are relatively more expanded, but essentially homologous to the clitoris and its associated structures. In cadavers with a clitoris, the corpus spongiosum is represented by the bulbs of the vestibule on either side of the vagina.  The clitoris is made up of the corpora cavernosa, and terminates in a glans covered by a prepuce, just as in the phallus. The crura are attached along the pubic rami in all individuals.

1. EXTERNAL GENITALIA

Examine the external genitalia (MOVIE - courtesy of UMich, note: this clip shows both female and male genitalia).

Identify the following structures: mons pubis, labia majora, clitoris, prepuce of clitoris, labia minora, vestibule of the vagina, external urethral orifice, vaginal orifice

Atlas Images:

  1. Make a skin incision along the midline between the labia minora and the anus.
  2. Make an incision from above the labia minora toward the abdomen.
  3. Make an incision that follows the medial surface of the labia majora and connects to the midline incisions.
  4. Reflect the skin laterally at the level of the mons pubis and just above the anus.
  5. Remove these skin flaps.
  6. Note that the posterior labial nerve and vessels enter the urogenital triangle by passing lateral to the external anal sphincter muscles.

NOTE: There is a lot of fatty tissue in this area. The structures that you are looking for are relatively small and delicate and lie beneath this layer of fat.

Atlas Images:

FUNCTIONAL ANATOMY:

The perineum is often described as having pouches.  These pouches represent layers of structures and fascia separating them. They can be of clinical significance with cancer and injury, especially with tears of the urethra where urine can enter specific pouches. The contents of the superficial perineal pouch are: 3 paired muscles (superficial transverse perineal, bulbospongiosus, ischiocavernosus), the crura of the clitoris, and the bulb of the vestibule as well as the vessels and nerves that serve these structures.

 

 

3. PERINEUM

FUNCTIONAL ANATOMY:

The perineum is a diamond-shaped area that is inferior to the levator ani. The anterior portion of the perineum, the urogenital triangle, is between the ischial tuberosities and the pubic symphysis. The majority of the urogenital triangle is covered by a thick layer of fascia, the perineal membrane, which serves as a support for the pelvic diaphragm on the deep (superior) side as well as an anchor for the external genitalia on the superficial (inferior) side. The perineal membrane defines the openings of the urethra and the vagina. The perineal membrane is anchored at the midline by the perineal body, which is thick connective tissue where many muscles attach including: the external anal sphincter, levator ani, urethral, superficial transverse perineal, deep transverse perineal muscle, and bulbospongiosus.

  1. Use blunt dissection to find and identify the bulbospongiosus muscle covering over the bulb of the vestibule, which is lateral to the labia minora.
  2. Identify the ischiocavernosus muscle covering each crus of the clitoris along the ischiopubic rami.
  3. Identify the perineal body located between the bulb of the vestibule and the anus.
  4. Identify the superficial transverse perineal muscle running between the perineal body and the ischiopubic ramus.
  5. Use a probe to dissect between the three muscles of the superficial perineal pouch (superficial transverse perineal, bulbospongiosus, and ischiocavernosus) to find the thin perineal membrane.

The bulbospongiosus and ischiocavernosus muscles are smaller in individuals with a clitoris than individuals with a penis and may be difficult to distinguish. The bulbospongiosus, superficial transverse perineal muscles, levator ani, and the external anal sphincters are all attached to a fibromuscular mass, the perineal body.

NOTE: The perineal body will orient you to the dividing line between the urogenital and anal triangles.

HINT – The structures and tissue in this area are very delicate, so use caution while dissecting. Your probe is a good tool to use

Atlas Images:

 

 

 

HEMISECTION OF THE PELVIS

1. TRANSECTING THE BODY -click here to see a video summarizing this procedure (courtesy of UMich)

  1. The first step in bisection is to separate the upper and lower halves of the body.
  2. With the cadaver on its back, cleanly sever the soft tissues connecting the upper and lower halves of the body at the level of the L3 vertebrae (or just above the iliac crest).
  3. Saw through the vertebral body at the L3 level and complete the soft tissue transection as necessary.

NOTE – you will be cutting primarily through abdominal wall muscle. Make sure you cut as close to the vertebra as you can in order to make the final cut cleanly.

 
  1. The next step is to separate the right and left halves of the body.
  2. Cut the uterus along its median plane.
  3. Extend the cut through the cervix and the fornix of the vagina.
  4. Cut the superior portion of the bladder along the midline.
  5. Using a probe as a guide, incise and divide the urethra.
  6. Finish by dividing the GI tract and other viscera along the midline (if there is a significant length of sigmoid colon, it may be removed leaving just the rectum to be divided).

NOTE – Remember to cut these soft tissue structures from above and below. You want to make sure that the midline cuts with the scalpel are complete before you use the saw on the bones. Otherwise, it will be difficult to identify structures when you have completed the hemisection.

 

 

 

3. HEMISECTION OF THE PELVIS

  1. Saw vertically through the sacrum, the coccyx, and the remaining lumbar vertebrae at the midline.
  2. Saw through the pubic symphysis.
  3. Using a scalpel, make a mid­ sagittal cut through all the soft tissues: viscera, pelvic and urogenital diaphragms, nerves, and vessels in the midline. Clean out the contents of the rectum and anal canal. Do this in a large steel sink (not a hand sink).
  4. Avoid dripping water and/or feces on the lab floor.
 

 

 

4. EXPLORING THE ISCHIOANAL FOSSA - We will examine the bisected pelvis in detail in the next lab.

  1. Find the posterior edge of the perineal membrane in your cadaver.
  2. Probe the fat-filled area that is posterior to this edge.
  3. The ischioanal fossa is a fat-filled area that is inferior to the pelvic diaphragm and extends anteriorly to lie superior to the perineal membrane.
  4. Find the pudendal nerve and vessels on the lateral wall of of the ischioanal fossa.

The deep perineal pouch contains several muscles: the external urethral sphincter and deep transverse perineal (in both sexes), the compressor urethrae and sphincter urethrovaginalis in females. It also contains branches of the internal pudendal vessels and pudendal nerve that run through to supply the muscles and structures of the perineum and external genitalia.

Click here to see a video demonstration of perineal structures using a prosected specimen.

Atlas Images:

 

 

CLINICAL CORRELATIONS

Unilateral rupture of the spongy (penile) urethra into the perineal cleft (the potential space between Colle’s and Gallaudet’s fascia) will first result in urine collection in the perineum and the scrotal sac on the same side as the rupture. Since the scrotal septum is not completely fused, continued urine collection will eventually spill into the perineum and scrotal sac on the other side as well.

Compare the image on the left showing spongy (penile) urethral rupture with the image on the right showing an intact urethra. 

 

 

 

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