Lab 14 navigation
Station 2: Pelvic Cavity and Pelvic Floor; Anal Triangle of Perineum
Complete anatomy
Muscles of the pelvic cavity
Pelvic Cavity
The pelvic cavity is below the pelvic brim, within the confines of the pelvic skeleton. It is continuous with the abdominal cavity above = the abdominal and pelvic cavities communicate via the pelvic inlet. The pelvic cavity is closed below by the muscular pelvic diaphragm.
Walls of Pelvic Cavity
- Anterior = Bone (pubic bones and pubic symphysis)
- Posterior = Bone (sacrum)
- Postero-lateral = Muscles (obturator internus and piriformis)
- Floor = Muscle (pelvic diaphragm)
Does the pelvic cavity have a roof?
Muscles in the Postero-Lateral Wall
Study the muscles of the pelvic wall and the pelvic diaphragm on the models and hemisected prosection.
Piriformis muscle
Originates from the anterior sacrum
Exits the pelvic cavity through the greater sciatic foramen; inserts on the greater trochanter of the femur
What is the function of the piriformis?
Obturator internus muscle
Fan-shaped muscle; has a broad origin from the pelvic surface of the pubic bone and from the obturator membrane
Narrows to a tendon that leaves the pelvic cavity through the lesser sciatic foramen; Inserts on greater trochanter of the femur
Only the upper half of the muscle is visible in the lateral pelvic wall (it is visible above the tendinous arch of the levator ani muscle); the lower half is in the lateral wall of the perineum.
What is the function of the obturator internus?
Floor of the Pelvic Cavity = Pelvic Diaphragm
Pelvic Diaphragm: has two parts = Levator ani and Coccygeus (see Figures 13 and 14).
Levator ani muscle
Three named portions: Pubococcygeus, iliococcygeus, and puborectalis
The levator ani is like a hammock in the pelvic floor—it slopes downward from lateral to medial, where its left and right halves meet in the midline.
Its lateral bony attachments are to the spine of the ischium posteriorly and the pubic bone anteriorly.
Between these bones, the levator ani attaches along a curved line of pelvic fascia called the tendinous arch. The tendinous arch crosses the obturator internus muscle.
Coccygeus muscle—stretches from the spine of ischium to the coccyx
- The pelvic diaphragm is the floor of the pelvic cavity
- The pelvic diaphragm is the roof of the perineum
- The perineum is therefore outside and below the pelvic cavity.
Question
What are the functions of the pelvic diaphragm? How does the orientation of the pelvic diaphragm relate to its function? What could happen if the pelvic diaphragm were weak?
The puborectalis muscle forms a sling around the anorectal junction and its contraction produces the anorectal (perineal) flexure. Two consequences:
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- Closure of the anorectal junction aids in fecal continence—preventing defecation.
- Contraction during defecation regulates the size of the feces passed.
Anal Triangle
Note
The anal triangle is the posterior part of the perineum.
Complete anatomy
Anal triangle
Review
The perineum is diamond-shaped. It has two triangles. What are their names?
We will view the anal triangle in a prone donor. It is helpful to have a model of the perineum handy and to place the model in the same position.
Contents of the Anal Triangle
Anal canal and Anus—the anal canal is the terminal part of the alimentary canal; the anus is its external orifice.
External anal sphincter—is this composed of skeletal or smooth muscle?
Ischio-anal fossae (left and right)—see Figure 16.
Boundaries of the ischio-anal fossae:
- Roof: The sloping inferior surface of the pelvic diaphragm
- Floor: Skin
- Medial: Anal canal and external anal sphincter
- Lateral: Obturator internus muscle
- Anterior: Communicates with the deep perineal pouch in the urogenital triangle
- Posterior: Gluteus maximus muscle and sacrotuberous ligament
The ischio-anal fossae contain large fat pads. What is their function?
In the lateral wall of the ischio-anal fossa are the pudendal nerve and internal pudendal vessels. These traverse the ischio-anal fossa in a fascia-lined tunnel called the pudendal canal (Alcock’s canal). See Figure 17.
Question
Which spinal nerves give rise to the pudendal nerve?
Question
Where does the internal pudendal artery arise? Where does the internal pudendal vein drain?
Coronal section = The ischio-anal fossae (filled with fat) are indicated with asterisks.
Checklist, Lab #14
Checklist items at each of the five stations are indicated by checkboxes.
