Lab Manual - Rectum, Anal Canal, & Pelvic Floor

Assignments:

Learning Objectives:

Upon completion of this session, the student will be able to:

  1. Recognize the features of the rectum that differentiate it from the colon.
  2. Describe the point at which the anal canal begins.
  3. Describe the internal features of the anal canal, and determine the point at which its lining changes from cutaneous to mucosal.
  4. Recall the lymph node groups that drain the rectum and anal canal.
  5. Organize blood and nerve supply to the rectum and anal canal.
  6. Describe the formation of the two sciatic foramina. List the muscles, nerves, and vessels which pass through each.
  7. Demonstrate the origins of the piriformis and obturator internus muscles and describe two specializations of the obturator fascia.
  8. Identify the pelvic diaphragm and differentiate its components.
  9. Demonstrate the formation of the sacral plexus, its relationship to the piriformis muscle and gluteal vessels, and its pelvic splanchnic nerves.
  10. Identify and describe the inferior hypogastric (pelvic) plexus and its connections to the superior hypogastric plexus via the hypogastric nerves.
  11. Identify and describe the sacral sympathetic trunks and the sacral sympathetic nerves.
  12. Trace the sympathetic and parasympathetic nerve supply to any pelvic organ, listing the location of the preganglionic cell body, the course of preganglionic fibers, the location of the postganglionic cell body, and the course of postganglionic fibers.
  13. Trace the skeletal and ligamentous boundaries of the perineum, and define the anal and urogenital triangles.
  14. Describe the position and boundaries of the ischioanal fossa.
  15. Describe the structure, contents, and course of the pudendal canal.
  16. Differentiate between the internal and external anal sphincters in structure and function.

Procedure:

1. Review the bony landmarks. Examine the peritoneal relationships in the pelvis. (Play movie; View images: N 248, 354, 356, 357, 358, 360, 361, 362, 363, 371, 380, 486, TG 5-03,3-04, 6-05A, 6-05B, 6-05CD, 6-05EF, 6-07A, 6-07B, 6-08A, 6-08B, 6-11, 6-13, 6-21A, 6-21B, 6-23, 6-24A, 6-24B)

On the skeleton define the bony landmarks of the perineum: pubic symphysis, pubic arch, inferior ramus of the pubis and the ramus of the ischium (together known as the ischiopubic ramus), the ischial tuberosities, and the coccyx. Define the urogenital triangle and the anal triangle. Note that the two triangles do not lie on the same plane.

Although it is not well seen at this time and will be studied in detail later, understand that the pelvic viscera are supported by the muscles comprising the pelvic diaphragm.

Examine male and female bony pelves and determine different characteristics of each. Note the difference between male and female in the subpubic angle, the angle formed by the pubic arch. Other sex differences in the pelvic skeleton? Now continue with dissection procedures for either male or female, but be sure to review all structures on a cadaver of the opposite sex as well.

Bony structures of the pelvis

Observe the peritoneal relationships in the pelvis and identify the uterus, uterine tubes, and ovaries in the female and rectum in both sexes. Notice how these structures and the associated peritoneum form the rectouterine and vesicouterine pouches in the female and rectovesical pouch in the male.

The peritoneal reflection onto the uterus, uterine tubes, and ovaries from the lateral pelvic wall is called the broad ligament. Observe its three component parts: the mesosalpinx associated with the uterine tubes, the mesovarium associated with the ovary, and the mesometrium contacting the uterus.

2. Make a midsagittal section of the pelvis of the cadaver, removing the right pelvis and lower limb at the 4th lumbar vertebra. (Play movie; View images: N 360, 361, TG 6-08A, 6-08B)

Make a careful midsagittal section of the pelvis. Cut all soft tissues, including all genital organs, with a sharp scalpel as close to the actual midline as possible, and use the saw to cut the pubic symphysis, sacrum and last two lumbar vertebrae. On the right side cut half way through the disk between the third and fourth lumbar vertebrae to meet the sagittally-sectioned pelvis. This will keep the posterior abdominal wall intact and continuous with the left half of the pelvis and one lower limb. Thoroughly wash all fecal material out of the rectum on both sides.

3. Examine the internal features of the rectum. (Play movie; View images: N 307, 311, 360, 361, 393, 394, 409, 410, 412, TG 5-14, 5-28, 6-08A, 6-08B, 6-15A, 6-15B, 6-15C, 6-16, 6-19A, 6-19B)

Rectum: Thoroughly wash out the specimen. Define the rectum. Observe its continuity with the sigmoid colon and the anal canal. Note relations to other pelvic organs in both male and female. Examine characteristics of rectum: the longitudinal musculature, the transverse rectal folds, the ampulla. Trace the superior rectal vessels. Be careful not to destroy the inferior hypogastric plexus, which lies on the side of the rectum.

Barium enema

4. Identify the structures of the anal canal. (Play movie; View images: N 393, 394, 398, 399, 402, 406, 407, 408, 410, 411, TG 5-28, 6-16, 6-17A, 6-17B, 6-19A, 6-19B, 6-30, 6-33, 6-34)

Anal canal: Define and note the flexure between rectum and anal canal. What muscle assists this flexure? In the mucosa, define the anal columns, pectinate line (significance? ), anal valves, anal sinuses. On the cut surface of the sectioned anal canal, note the internal anal sphincter. Trace fibers of the longitudinal muscle through the external anal sphincter to end in the skin. Consider various sources of the blood supply, the collaterals between portal and caval venous drainage, and the different functions of the autonomic and somatic innervation. Consider the pattern of lymphatic drainage and the node groups that receive lymphatic drainage from the rectum and anal canal.

5. Examine the bony pelvis. (Play movie; View images: N 248, 352, 486, 503, TG 3-04, 3-05, 3-07, 6-03, 6-06A, 6-06B)

Examine the bony pelvis and identify the obturator foramen and groove, the greater sciatic notch (foramen), spine of the ischium, the lesser sciatic notch (foramen), ischial tuberosity. Locate the sacroiliac joint. Review how the sacrotuberal and sacrospinal ligaments help to form the greater and lesser sciatic foramina.

6. Identify the nerves of the pelvis. Identify the muscles of the pelvic wall and floor. (Play movie; View images: N 367A, 367B, 368, 369, 370, 402, 403, 409, 410, 412, 415, 416, 417, 497, 499A, 499B, 502, 503, TG 3-25A, 3-25B, 3-26, 3-28, 6-04, 6-15A, 6-15B, 6-18A, 6-18B, 6-17, 6-18A, 6-18B, 6-19A, 6-19B, 6-21A, 6-21B, 6-22A, 6-22D, 6-23A, 6-23B, 3-27)

Locate the first four sacral nerves as they emerge from the anterior (pelvic) sacral foramina. Locate the lumbosacral trunk; note its course, the relation of the superior gluteal artery to it and the first sacral nerve. Examine the formation of the sacral plexus.

Pick up the superior hypogastric plexus on the bifurcation of the aorta and trace it into the pelvis. Note its division into hypogastric nerves and their continuity into the inferior hypogastric (pelvic) plexus. Locate the sympathetic trunk entering the pelvis along the medial border of the pelvic sacral foramina. Note ganglia (number? ), gray rami communicantes, sacral splanchnics. Now return to the sacral nerves and identify the pelvic splanchnic nerves. How many are there? Trace them to the inferior hypogastric plexus and search for branches passing up into the sigmoid mesocolon and to the descending and sigmoid colon. Consider subsequent distribution to rectum, vagina, uterus, bladder, prostate. Organize your knowledge of the autonomic (sympathetic and parasympathetic) supply to the pelvic viscera.

Reflect or mobilize the pelvic viscera medially on one side only. Locate the obturator internus muscle and define its fascia. Locate its thickening, the arcus tendineus levator ani. Determine the parts of the pelvic diaphragm and trace each. Distinguish between the levator ani and the coccygeus muscles. What is the puborectalis muscle; significance? Define the urogenital hiatus. What does it transmit?

Examine the piriformis muscle and note its attachment to the sacrum, as well as its relation to the sacral nerves, to the sacral plexus and to the superior and inferior gluteal arteries.

Lymphangiogram of iliac lymph nodes and vessels

7. Skin the ischioanal fossa and remove the fat. Identify the sacrotuberal and sacrospinal ligaments and the inferior rectal nerves and vessels. (Play movie; View images: N 353, 357, 358, 359, 377, 379, 380, 381, 395, 402 403, 404, 405, 411, 413, 502, 503, TG 3-07, 3-26, 6-06, 6-15A, 6-15B, 6-15C, 6-23A, 6-23B, 6-24A, 6-24B, 6-26A, 6-26B, 6-28, 6-29A, 6-29B, 6-30A, 6-30B)

Remove the skin from the anal triangle. Care should be taken when skinning around the anus because the external anal sphincter muscle is in contact with the skin. Clean and identify the external anal sphincter.

Using blunt dissection through the fat of the ischioanal (ischiorectal) fossa, locate the inferior rectal vessels and nerves, supplying the external anal sphincter. On the lateral side, follow the walls of the ischioanal fossa and remove all of the fat within the fossa. To do this, identify the inferior border of the gluteus maximus muscle and the ischial tuberosity and follow these surfaces deeply within the ischioanal fossa until the fat wedge is free. Note how gluteus maximus overhangs the ischioanal fossa. Reflect the fibers of gluteus maximus that originate from the sacrotuberal ligament. Define all boundaries of the ischioanal fossa. Note that the lateral wall is the fascia of the obturator internus muscle, and cut the sacrotuberal ligament from its attachment on the ischial tuberosity. Beneath the sacrotuberal ligament, identify the sacrospinal ligament and the greater and lesser sciatic foramina.

8. Identify the pudendal canal and pudendal neurovascular bundle. (Play movie; View images: N 402, 403, 404, 405, 410, 411, 413, 497, 499, 503, TG 3-25A, 3-25B, 3-26, 6-17A, 6-17B, 6-19, 6-29A, 6-29B, 6-30)

Identify the pudendal neurovascular bundle passing around the sacrospinal ligament. Pick up the inferior rectal vessels and nerves, and gently pull toward the midline. This tension will elevate the wall of the pudendal canal so that it may be visualized. Open the canal and identify the pudendal nerve, the internal pudendal artery and veins. Note how far (deep) this canal is from the tip of the ischial tuberosity.