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 the obturator membrane and surrounding bones and its tendon passes through the lesser sciatic foramen to attach to the medial surface of the greater trochanter. The muscular fascia of the perineal surface of this muscle is split, as the pudendal canal or Alcock’s canal, and transmits the pudendal nerve and internal pudendal vessels. These structures enter it by first leaving the gluteal region via the lesser sciatic foramen. From the pudendal canal, inferior rectal n, a, v are given off and course within the fat-filled ischiorectal fossa to supply the external anal sphincter and skin of the anal triangle. Q. What are the boundaries of the anal triangle?

The remainder of the internal pudendal artery and pudendal nerve course anteriorly toward the urogenital triangle and will be discussed further in the dissection of the Urogenital Triangle.

Investigate the anus. The musculature of the external anal sphincter, pelvic diaphragm and rectum form a functional unit known as the anorectal musculature. The components of the anorectal musculature are fused and consequently not easily separated from each other.

Osteology. Identify and review on skeletal materials or pelvic models: • Bony Pelvis. Composed of 2 hipbones, sacrum, and coccyx . Hip bones (os coxae) - ilium, ischium, pubis  Ilium . ASIS, AIIS, PSIS, PIIS . Iliac crest . Greater sciatic notch . Iliac fossa . Arcuate line  Ishium . Body . Ischial spine . Ischial tuberosity . Lesser sciatic notch . Ischial ramus  Pubis . Body . Superior ramus (pubic crest, pubic tubercle, pectin pubis) . Inferior ramus . Pubic arch . Obturator foramen . Acetabulum . Sacrum . Sacral promontory . Alae . Anterior sacral foramina

. Pelvic brim (superior pelvic aperture/pelvic inlet) - marks the boundary between the false pelvis and true pelvis. It represents a circumferential line connecting the pubic symphysis, pubic crest, pecten pubis (pectineal line), arcuate line (of ilium), ala and promontory of sacrum • Ligaments  Sacrotuberous ligament - attaches from the lateral border of the sacrum to the ischial tuberosity and, in conjunction with the sacrospinous ligament, converts the lesser sciatic notch into the lesser sciatic foramen  Sacrospinous ligament - attaches from the lateral border of the sacrum to the ischial spine. It converts the greater sciatic notch into the greater sciatic foramen

PEER TEACHING GUIDE: 1. Demonstration of the course of the pudendal nerve and internal pudendal vessels out of the pelvis, temporarily in the gluteal region, and subsequently into the perineum. 2. Demonstration of the sacrotuberous and sacrospinous ligaments. 3. Demonstration of the boundaries of the ischiorectal fossa, the external anal sphincter, inferior rectal nerve and vessels, and the undersurface of the pelvic diaphragm. 4. Demonstration of relevant osteology.