THE BONY PELVIS AND ANAL TRIANGLE
(Grant's Dissector [16th Ed.] pp. 141-145)
TODAY’S GOALS:
1. Identify relevant bony features/landmarks on skeletal materials or pelvic models.
2. Identify the sacrotuberous and sacrospinous ligaments.
3. Describe the organization and divisions of the perineum into two triangles: anal triangle
and urogenital triangle
4. Dissect the ischiorectal (ischioanal) fossa and define its boundaries.
5. Identify the inferior rectal nerve and artery, the pudendal (Alcock’s) canal and the
external anal sphincter.
DISSECTION NOTES:
The perineum is the diamond-shaped area between the upper thighs and below the inferior
pelvic aperture and pelvic diaphragm. It is divided anatomically into 2 triangles: the anal
triangle and the urogenital (UG) triangle (Dissector p. 142, Fig. 5.2). The anal triangle is
bounded by the tip of the coccyx, sacrotuberous ligaments, and a line connecting the right and
left ischial tuberosities. It contains the anal canal, which pierced the levator ani muscle portion
of the pelvic diaphragm. The urogenital triangle is bounded by the ischiopubic rami to the
inferior surface of the pubic symphysis and a line connecting the right and left ischial
tuberosities. This triangular space contains the urogenital (UG) diaphragm that transmits the
urethra (in male) and urethra and vagina (in female).
A. Anal Triangle
Turn the cadaver into the prone position. Make skin incisions as on page 144, Fig. 5.4 of the
Dissector. Reflect skin and superficial fascia of the gluteal region in one flap to expose the large
gluteus maximus muscle. This muscle has proximal attachments to the posteromedial surface
of the ilium, posterior surfaces of the sacrum and coccyx, and the sacrotuberous ligament.
Exercise caution when removing this muscle from the underlying sacrotuberous ligament. Its
distal attachment is the iliotibial tract and gluteal tuberosity of the femur. The muscles and
nerves exposed during reflection of this muscle will be examined later when we study the
Gluteal Region.
Ischiorectal (Ischioanal) Fossa: This is a fat-filled wedged-shaped area on either side of
the anus and is bounded laterally by the obturator internus muscle, medially by the
inferior surface of the pelvic diaphragm, and inferiorly by skin. See Dissector p. 145,
Fig. 5.6 Remove skin and subcutaneous fat around the anus using blunt dissection and
expose the external anal sphincter. In the fat-filled space between the lateral wall of
the ischiorectal fossa and the sphincter palpate the inferior rectal nerve and inferior
rectal artery and vein. These are important branches, respectively, of the pudendal
nerve and internal pudendal artery and vein. Remember, these neurovascular
structures emerge from the pelvis through its greater sciatic foramen and then course
over the sacrospinous ligament to enter the lateral wall of the ischiorectal fossa by
passing through the lesser sciatic foramen. Push your finger through the lesser sciatic
foramen into the ischiorectal fossa. Your finger runs in the same direction as pudendal
nerve and internal pudendal vessels. The lateral wall of the ischiorectal fossa contains a
muscle, the obturator internus muscle. This muscle arises from the internal surface of
