Perineum

The perineum is the region of the body inferior to the pelvic diaphragm and between the legs. The actual limits are specific and will be described. First we will take a look at the living perineum and point out its items of interest.

The Living Perineum

These two images compare the structure of the male and female perinei.

In the male, identify:
  • scrotum
  • bulb of penis
  • perineal body (or central tendon of the perineum) - not as obvious as in the female perineum and not a clinically important structure in the male as it is in the female
  • anus


In the female, identify:
  • clitoris
  • labia majorum
  • labia minorum
  • posterior fornix
  • vagina
  • urethral opening
  • perineal body (central tendon of the perineum)
  • anus
         


         

Boundaries of the Perineum

The boundaries of the perineum are:
  • anterior - pubic symphysis (PS)
  • posterior - coccyx (C)
  • lateral - ischial tuberosities (IT)
  • anterolateral - ischiopubic ramus (IPR)
  • posterolateral - sacrotuberous ligament


A line drawn between the two tuberosities separates the diamond shaped perineum into an anterior urogenital triangle (UG) and a posterior anal triangle (A)

Contents of the Anal Triangle

Both the male and female anal triangles are similar so we will just describe one.
Starting by removing the skin and superficial fascia, we can identify the anus in the midline and two fat filled areas adjacent to the anus represented by the yellow. This fat fills a space known as the ischiorectal fossa.


After the ischiorectal fat has been removed, you can identify the makeup of the anal triangle.
Starting from the center and working out identify the:
  • anus
  • external anal sphincter
  • levator ani muscle
  • obturator internus muscle
    The vessels and nerves are:
  • perineal branch of the posterior femoral cutaneous nerve
  • posterior scrotal or labial nerve and artery from the pudendal nerve and internal pudendal artery
  • inferior rectal nerve and artery from the pudendal nerve and the internal pudendal artery


The walls of the ischiorectal fossa are as follows:
  • medial - external anal sphincter
  • lateral - obturator internus muscle
  • superior - levator ani
  • inferior - superficial fascia and skin


Another way of looking at the anal triangle is to examine what it looks like on a frontal section of the region.
On section, identify the:
  • hip bone
  • obturator internus muscle on the inside of the hip bone
  • levator ani muscle suspended from the obturator fascia
  • anus
  • external anal sphincter


Also note that a small canal is formed in the obturator fascia along the lateral wall of the ischiorectal fossa. This canal is the pudendal canal and it carries the pudendal nerve, internal pudendal artery and vein on their way to the perineum.


Male Urogenital Triangle

As we move forward in the perineal region, we find the urogenital triangle. After the skin and Camper's (fatty) superficial fascia are removed, the first thing to identify is the continuation of Scarpa's fascia into the perineum where it is now called Colles' fascia. This fascia is firmly attached to the ischiopubic ramus along the side of the UG triangle and to the posterior free margin of the urogenital diaphragm, which can not be seen now. This layer is represented in this image as a white fascia.
When Colles' fascia is removed, you have opened up a small compartment called the superficial perineal space. This space is found between Colles' fascia and the inferior fascia of the urogenital diaphragm, represented in this image as the perineal membrane.

You can now identify the:
  • ischiocavernosus muscle covering the crus of the penis
  • bulbospongiosus muscle covering the bulb of the penis
  • superficial transverse perineus muscle
If we remove the crus of the penis and its muscle, the bulb and its muscle, we can view the inferior fascial layer of the urogenital diaphragm. In the image, the dashed lines represent where these structures attached to this layer of fascia. This fascial layer is continuous around the anterior and posterior borders of the urogenital diaphragm with the superior fascia of the diaphragm.
Removal of the inferior fascia of the UG diaphragm opens up the deep perineal space with its contents:
  • deep transverse perineus muscle
  • urethral sphincter
  • bulbourethral gland (Cowper's gland)
  • terminal branches of the internal pudendal artery to the bulb of the penis as well as the deep artery of the penis


The fascia over the anterior border of the UG diaphragm is thickened and is called the transverse perineal ligament. Anterior to this ligament is a gap between it and the arcuate pubic ligament which serves as passage for the:
  • deep dorsal vein of the penis
  • dorsal artery of the penis
  • dorsal nerve of the penis

Frontal Section of the Male UG Diaphragm

In this image, you can appreciate the relationships of the different structures in the UG triangle from superficial to deep and medial to lateral.

First identify the hip bone and notice that it has been cut at the obturator foramen which is covered by the obturator membrane. Then identify the UG diaphragm itself.

Starting from inferior and working your way upward identify:
  • Colles' fascia - continuation of Scarpa's fascia of abdomen
  • structures in the superficial perineal space:
    • ischiocavernosus muscles
    • crura of the penis
    • bulbospongiosus
    • bulb of penis
    • perineal membrane
  • UG diaphragm
    • inferior fascia
    • deep perineal space
    • muscle in the space
    • urethra
    • Cowper's gland
    • superior fascia
    • anterior extension of icshiorectal fossa
    • obturator internus muscle
    • levator ani muscle
    • prostate - a pelvic structure above the UG diaphragm
    • prostatic urethra
    • membranous urethra - that part within the deep perineal space

Female Urogenital Triangle

Just as in the male urogenital triangle, we will examine the female triangle from superficial to deep. In the first image, the skin and fatty layer (Camper's) of the superficial fascia has been removed to expose the membranous (Scarpa's) layer of superficial fascia, now called Colles' fascia. The items to identify at this stage are:
  • clitoris
  • urethral opening
  • vaginal opening
After Colles' fascia is removed, identify the structures in the superficial perineal space:
  • ischiocavernosus muscle overlying the crus of the clitoris
  • bulbospongiosus muscle overlying the bulb of the vestibule
  • perineal body (central tendon)
  • urethral opening
  • vaginal opening
By reflecting the muscles of the perineum, you can identify the:
  • crus of the clitoris
  • bulb of the vestibule
  • greater vestibular gland (deep to the bulb)

Frontal Section of Female Urogenital Triangle

Finally, take a look at the female urogenital triangle from a different view point and identify the same structures that we have just covered.

Orient yourself and identify the:
  • uterus
  • vagina
  • obturator internus muscle with its fascia
  • levator ani muscle and its fascia
  • deep transverse perineus muscle within the urogenital diaphragm
      structures in the superficial perineal space
    • ischiocavernosus muscle
    • crus of clitoris
    • bulbospongiosus
    • bulb of the vestibule
    • greater vestibular gland
  • labia majora
  • labia minora
  • anterior extension of the ischiorectal fossa
Again, pretend that you are a surgeon and you want to enter the pelvic cavity by going through the urogenital triangle. Here are the layers you will have to go through to get there:
  • skin
  • fatty superficial fascia (Camper's layer)
  • membranous fascia (Scarpa's layer or Colles' fascia in perineum)
  • superficial perineal space
  • inferior fascia of urogenital diaphragm
  • deep transverse perineus muscle
  • superior fascia of urogenital diaphragm
  • anterior extension of ischiorectal fossa
  • inferior fascia of levator ani
  • levator ani
  • superior fascia of levator ani
  • pelvic fascia
  • peritoneum
  • pelvic cavity


Pelvis


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